palaeomagnetism, geochronology and …...palaeomagnetism, geochronology and geochemistry of the...

Palaeomagnetism geochronology and geochemistry of the

Palaeoproterozoic Rabbit Creek and Powder River dyke swarms

implications for Wyoming in supercraton Superia

TAYLOR M KILIAN1 WOUTER BLEEKER2 KEVIN CHAMBERLAIN3

DAVID A D EVANS1 amp BRIAN COUSENS4

1Department of Geology and Geophysics Yale University

210 Whitney Ave New Haven CT 06511 USA2Geological Survey of Canada 601 Booth Street Ottawa ON Canada K1A 0E8

3Department of Geology and Geophysics University of Wyoming

Laramie WY 82071-3006 USA4OttawandashCarleton Geoscience Centre Department of Earth Sciences Carleton University

1125 Colonel By Drive Ottawa ON Canada K1S 5B6

Corresponding author (e-mail taylorkilian1gmailcom)

Abstract It is likely that Archaean cratons of Laurentia had different palaeogeographic historiesprior to their amalgamation New palaeomagnetic geochronological and geochemical evidencesupports a reconstruction of the Wyoming craton adjacent to the southern margin of the Superiorcraton at 216 Ga before rifting (c 21ndash20 Ga) and eventual reamalgamation after the HudsonianOrogeny (c 18 Ga) UndashPb ages (TIMS on baddeleyite) from five dykes yield two groups of ages atc 2164 and 2155 Ma The younger group of ages defines the Rabbit Creek swarm at 2161ndash2152 Ma and precisely dates its palaeomagnetic pole Two large and differentiated dykes(100 m) in the Bighorn and Wind River uplifts are geographically related to the Rabbit Creekswarm but have slightly different orientations and yield slightly older ages at 2171ndash2157 MaThese dykes may be parts of a single intrusion (the lsquoGreat Dyke of Wyomingrsquo) that spans over200 km between uplifts possibly representing a different magmatic event This older event doesnot have enough distinct intrusions to provide a correctly averaged palaeomagnetic pole but cor-relates with magmatism in the Superior craton and has a palaeomagnetic remanence comparable tothe Rabbit Creek dykes With minor tilt corrections the palaeomagnetic data from the Rabbit Creekswarm and Powder RiverndashSouth Pass dykes support a reconstruction of the southeastern Wyomingcraton against the southern Superior craton This fit juxtaposes the Palaeoproterozoic Huronian andSnowy Pass Supergroups along two passive margins that experienced a prolonged period of maficmagmatism (100 myr) and rift basin development Although there are slight geochemical vari-ations across the Rabbit Creek swarm all dykes fit into two distinct groups that are independentlydated and internally consistent

Supplementary material Supporting figures and locality tables are available at wwwgeolsocorgukSUP18824

The Archaean cratons scattered around the globeoften contain collisional belts ancient rifted mar-gins and multiple generations of dyke swarms sug-gestive of horizontal plate motions and continentalamalgamations Supercontinent Nuna was hypoth-esized by recognizing the worldwide distributionof collisional belts in the 2100ndash1800 Ma interval(Hoffman 1997 aka Columbia Rogers amp Santosh2002 Zhao et al 2002) and depended on a majorfocus on the Proterozoic history of Laurentia (Hoff-man 1988 Ansdell et al 2005 St-Onge et al 2006)These same tectonic markers also suggest that many

of these Archaean fragments were connected evenearlier (Bleeker 2003 Bleeker amp Ernst 2006) butperhaps in different configurations This studyfocuses mainly on dyke swarms the intrusive rem-nants of large igneous provinces which are excel-lent tools for testing Precambrian continentalreconstructions (Bleeker amp Ernst 2006) Maficdyke swarms signal possible rifting events and hot-spot magmatism while they also contain the idealmineralogical assemblage for palaeomagnetic andgeochronological studies In addition when dykeswarms intrude two landmasses in the process

From Li Z X Evans D A D amp Murphy J B (eds) 2016 Supercontinent Cycles Through Earth HistoryGeological Society London Special Publications 424 15ndash45First published online June 4 2015 httpdoiorg101144SP4247 2016 The Author(s) Published by The Geological Society of London All rights reservedFor permissions httpwwwgeolsocorgukpermissions Publishing disclaimer wwwgeolsocorgukpub_ethics

by AJS on May 1 2016httpsplyellcollectionorgDownloaded from

of rifting the orientations of the dykes often con-verge on a piercing point along the rifted margins(Ernst amp Bleeker 2010) The piercing point providesadditional insight into possible palaeogeographiccorrelations and also aids in the refinement ofpalaeomagnetically derived reconstructions

The Wyoming craton hosts a large number ofdyke swarms that occur in high densities throughoutalmost every region of exposed ArchaeanProtero-zoic rock (Snyder et al 1989) Although a hand-ful of magmatic events have been studied datingfrom the Neoarchaean Era through the Neoprotero-zoic Era it is difficult to establish conclusivelywhether or not these magmatic events can be corre-lated across the craton since the limited and spora-dic exposures of Precambrian crust reveal as littleas 10 of the whole craton Precambrian basementexposures resulted largely from differential upliftduring the Laramide Orogeny (80ndash55 Ma) thrust-ing large basement blocks up and past kilometresof Phanaerozoic sedimentary rocks This created ahandful of windows to view the craton (Love ampChristiansen 1985 Fan amp Carrapa 2014) hencethe Wyoming craton requires several importantand somewhat unique questions to be addressedbefore using mafic dyke swarms to test reconstruc-tions (1) Is the basement geology of the differentLaramide uplifts coherent enough to allow a craton-wide approach In other words can we correlatedyke events and palaeomagnetic data betweenuplifts in detail and correct for minor rotations orhas Laramide tectonics made the problem intract-able (2) Are enough dykes of each swarmexposed to average out secular variation of theEarthrsquos magnetic field and to provide a statisticallysignificant palaeomagnetic pole After satisfyingthose concerns to which Archaean blocks can wecompare our data with and which pieces may havebeen adjacent to Wyoming

The best approach in solving these questions is toestablish precise ages and primary palaeomagneticpoles for regional dyke swarms correlate thesebetween uplifts and apply minor inter-uplift struc-tural corrections where necessary thereby esta-blishing a detailed magmatic barcode and apparentpolar wander path for Wyoming craton as a wholeprior to its incorporation into Laurentia In this studywe also determined the whole-rock compositionof some of the dykes with two aims in mind to dis-tinguish each swarm geochemically and to identifygeochemical trends within swarms

Herein we present a palaeomagnetic pole forthe Rabbit Creek dyke swarm at 2152ndash2161 Mawhich includes independent age analyses fromthree different dykes exposed in the Bighorn Moun-tains We also compare results with those of Harlanet al (2003) refining the age of the large intrusion inthe Wind River Range and connecting it to a very

similar intrusion more than 200 km away in theBighorn Mountains These two magmatic eventsprobably represent individual pulses of a protractedinterval of magmatism along the margin of Wyo-ming craton (2170ndash2000 Ma) correlating wellwith mafic dyke swarms from southern Superiorand other cratons worldwide

Background geology

Wyoming in Laurentia

The Wyoming craton arrived at its current positionduring the amalgamation of Laurentia at c 1800ndash1700 Ma (eg St-Onge et al 2006) The dating ofthe sutures surrounding Wyoming suggests multipleepisodes of metamorphism making it more difficultto identify the exact order of collisions and whetherany pieces were contiguous with Wyoming priorto collision (Dahl et al 1999 Mueller et al 20022005) Most importantly all of the available agesfor the suture zones surrounding Wyoming areyounger than c 1900 Ma indicating that beforeincorporation into Laurentia Wyoming was eitherby itself or with other fragments that subsequentlyrifted away

Some attempts to recognize prior connectionsto Wyoming (before c 1800 Ma) have focusedon the Snowy Pass Supergroup passive marginsequence in the Medicine Bow Mountains of theSE Wyoming craton (Graff 1979 Bekker amp Eriks-son 2003) Many scientists have recognized simi-larities between the Snowy Pass Supergroup andthe Huronian Supergroup of the southern Superiorcraton (Roscoe 1969) especially the distinct glacio-genic strata found in both sections (Blackwelder1926) and their possible relationships to other gla-ciogenic sediments in North America (Young 1970Graff 1979 Houston 1993 Young et al 2001)Some workers have suggested that deposition couldhave taken place in separate locations on Earth(Karlstrom et al 1983) especially if the glacial epi-sodes were worldwide in nature Other studieshypothesized that with Wyoming and Superior intheir current configuration deposition could havetaken place along the same contiguous southernmargin of Laurentia (Houston 1993 Aspler amp Chiar-enzelli 1998) Roscoe amp Card (1993) qualitativelydefined a reconstruction of southeastern Wyo-ming against southern Superior placing palaeocur-rent data from both sequences into agreementwith stratigraphic correlations The intrusion of theNipissing diabase sills at c 2215 Ma into the Huro-nian Supergroup (Noble amp Lightfoot 1992) con-strains the age of the reconstructions which maycorrelate with metamorphosed intrusions withinthe Snowy Pass Supergroup in Wyoming Our

T M KILIAN ET AL16


newly recognized Wyoming events postdate theNipissing magmatic event and are probably relatedto subsequent circum-Superior rifting possiblyfrom the creation of another nearby rift basin orfrom distal continental-scale stresses Multipledyke swarms within the Superior craton recordsimilar stresses and magmatism during this broadtime interval providing many possible correlativeevents along Superiorrsquos southern margin

Geology of Wyoming

The Archaean framework of the Wyoming craton orprovince can be divided into four tectonic subpro-vinces (Fig 1) (1) the Montana metasedimentaryterrane (MMT) (2) the Bighorn subprovince (3)the Sweetwater subprovince and (4) the southernaccreted terranes (SAT) (Chamberlain et al 2003Frost et al 2006b Mueller amp Frost 2006 Chamber-lain amp Mueller 2007) The core of the craton isdominated by the 2900ndash2800 Ma plutons in theBighorn and Sweetwater subprovinces includinghigh-Na tonalitendashtrondhjemitendashgranodiorite andhigh-K calcndashalkaline magmatism (Frost et al1998 2006a Chamberlain et al 2003 Frost ampFanning 2006) A handful of plutonic and supracrus-tal ages greater than 3000 Ma also exist (Muelleret al 1996 Grace et al 2006 Chamberlain ampMueller 2007) The MMT is a smaller terrane onthe NW margin of Wyoming that contains some ofthe oldest exposed rocks in the craton (Mogk et al1992) Meta-supracrustal rocks dominate the MMTin contrast to the rest of the Wyoming provinceHowever the enriched 207Pb204Pb isotopic signa-tures measured in rocks throughout the MMTBighorn and Sweetwater subprovinces are fairlyunique in Archaean cratons and suggest the samesource region of crustal extraction (Mueller ampFrost 2006) The simplest interpretation is that allthree subprovinces grew as part of a single largercontinental province yet they experienced differenthistories until their unification in the NeoarchaeanEra at c 27 Ga (Mogk et al 1988 1992) OtherArchaean cratons that show the same enriched isoto-pic signature include the North Atlantic Slave Zim-babwe and Yilgarn cratons (Luais amp Hawkesworth2002 Kamber et al 2003)

In contrast the SAT are interpreted as isotopi-cally juvenile arc terranes accreted to the Archaeancore of the Wyoming craton along the OregonTrail structural belt at c 2650ndash2615 Ma in present-day southern Wyoming (Chamberlain et al 2003Frost et al 2006b Grace et al 2006) They mayrepresent a long-lived active margin with wide-spread arc magmatism from 2720 to 2620 Ma(Frost et al 1998 Wall 2004 Bowers amp Chamber-lain 2006) and associated supracrustal rocks (Frostet al 2006b)

Since c 2730 Ma the Archaean rocks exposedin the Bighorn Mountains have remained at rela-tively high crustal levels (summarized in Frost ampFanning 2006) and avoided Proterozoic defor-mations that took place far along the cratonrsquosboundaries They are intruded by a number of unme-tamorphosed Proterozoic mafic dyke swarms thatare the result of far-field stresses from extensionalevents and coeval mantle-derived magmatism

Prior to this research KndashAr and RbndashSr data(Condie et al 1969 Heimlich amp Armstrong 1972Stueber et al 1976) constrained the ages of theseProterozoic Bighorn Mountain dykes Potential cor-relative mafic intrusions from other regions of theWyoming craton have UndashPb ages of 2480ndash1870 Ma (see Fig 1 for details and references)These repeated pulses of Proterozoic mafic magma-tism may be due to the progressive rifting andbreakup of Wyomingrsquos ancestral landmass TheSuperior craton contains numerous mafic magmaticevents during this same interval including manywith robust palaeomagnetic data the Matachewandykes at c 2450 Ma (Halls 1991) the Nipissingsills and Senneterre dykes at c 2215 Ma (Buchan1991 Buchan et al 1993 2000) the Biscotasingdykes at c 2170 Ma (Buchan et al 1993 Halls ampDavis 2004) the Marathon dykes at c 2125 and2105 Ma (Buchan et al 1996 Halls et al 2008)the Fort Frances and Lac Esprit dykes at c2070 Ma (Halls 1986 Buchan et al 1996 2007)and the Minto dykes at c 2000 Ma (Buchan et al1998) among others (Evans amp Halls 2010) Thelarge number of similarly aged magmatic pulses inSuperior support a proximal relationship toWyoming (Ernst amp Bleeker 2010)

Tilt corrections can be calculated locally usingbedding orientations of overlying Phanaerozoic sedi-mentary rocks or structural studies of the thrustedblocks The uplifts have varying amounts of reliefand outcrop style depending on rock type exhuma-tion history and possibly lower crustal structures(Barnhart et al 2012) Recent glaciations in thehigh mountains (eg the central Bighorn Moun-tains) provide widespread pavement exposures ofbasement rocks but in regions such as South Pass(Wind River Range) the foothill shrub-lands andsagebrush steppe provide little relief or exposureother than recently incised river valleys

Methodology

UndashPb geochronology

Samples for geochronology were taken from thesame outcrops and dykes as palaeomagnetic sam-ples (Fig 1) In most cases samples were takenfrom the freshest region of the coarsest rock often

WYOMING CRATON RABBIT CREEK DYKES 17


Fig 1 Regional geological map with localities sampled for this study Inset shows the location of the map (dottedyellow rectangle) with respect to state borders including estimated extent of Wyoming craton (light grey) and outcropof Precambrian rock (dark grey) (Foster et al 2006) HU-B Hartville upliftndashBlack Hills subprovince GFTZ GreatFalls Tectonic zone other subdivisions of the Wyoming craton are explained in the text Inset also plots locationsand trends of other dated mafic magmatic events in the early Proterozoic the c 2480 Ma Blue Draw metagabbro(blue dot) (Dahl et al 2006) the South Pass dyke of the Wind River Range at c 2164 Ma (red line) (Harlan et al 2003

T M KILIAN ET AL18


from the centre of spheroids resistant to weatheringThe geochronological sample (BNB-09-WY-202b)for site T09BH14 was collected c 13 m from theeastern margin of the c 40 m-wide dyke Sample(BNB-09-WY-204) for T09BH15 was collectedwithin a linear meadow defined by the dyke 10ndash12 m from the southern margin of the c 40 m-widedyke For dyke T09BH24 a sample (BNB-09-WY-207b) was collected 10ndash15 m from the northernmargin of the c 50 m-wide dyke Sample (BNB-09-WY-208a) for T09BH22 was taken c 500 m NE ofthe palaeomagnetic sampling area along a road-cutin the coarse diabasic portion of the 100 m-widedyke In the Wind River Range the dyke of Harlanet al (2003) was sampled (BNB-09-WY-210b) inits centre along the Sweetwater River

All samples were crushed and separated usingthe Wilfley table technique of Soderlund amp Johans-son (2002) at the Institute of Geology University ofLund Sweden All three 40ndash50 m-wide RabbitCreek dykes from the Bighorn Mountains (202b204 207b) yielded sufficient baddeleyites allowingfor the selection of high-quality grains Four frac-tions were analysed for each sample each fractionconsisting of one to nine grains or crystal fragments(Table 1) Samples from large intrusions in boththe Wind River Range (210b) and the BighornMountains (208a) yielded baddeleyite crystals thatwere mostly clear A subset of grains had a slightlymottled appearance on the surfaces these wereavoided for all but two of the analyses Four frac-tions were analysed for each of these particularsamples comprising one to five crystals or crystalfragments

Selected baddeleyite grains were spiked with amixed 205Pbndash233Undash235U tracer (ET535) and dis-solved in 6 N hydrochloric acid in precleanedteflon microbombs at 180 8C for 30 h Hydrochloricacid was used rather than hydrofluoric acid to mini-mize dissolution of any undetected zircon alterationrims (eg Rioux et al 2010) Dissolved sampleswere evaporated and loaded onto Re filaments usingH3PO4 and silica gel without any further chemicalprocessing Isotopic compositions of Pb and U (run

as UO2) were determined by peak-switching in aDaly-photomultiplier single collector on a Micro-mass S54 thermal ionization mass spectrometer atthe University of Wyoming Pb procedural blanksaveraged 1ndash3 pg and all measured common Pbwas assigned to blank U procedural blanks wereconsistently less than 02 pg Mass discriminationof 0198 + 010amu for Pb was determined byreplicate analyses of NIST SRM 981 U fraction-ation was determined internally for samples spikedwith ET535 Concordia coordinates interceptsweighted means and uncertainties were calculatedusing MacPBDAT and ISOPLOT programs (basedon Ludwig 1988 1991) and the decay constants rec-ommended by Steiger amp Jager (1977)

Geochemistry

The vast majority of sites consist of eight samplestaken from the same outcrop of a single coolingunit (dyke) Specimens for geochemical analyseswere taken from one of the eight samples used forpalaeomagnetic measurements and were chosenfrom regions approximately one-third of a dykewidth across in order to minimize effects from dif-ferentiated interiors or chilled margins (Table 2)

Rock samples were slabbed crushed in a BicoChipmunk jaw crusher and ground to a fine powderin an agate ring mill Whole-rock major and traceelement contents were determined by fused-discX-ray fluoresence spectrometry and solution-modeinductively coupled plasma mass spectrometry atthe Ontario Geological Survey Geochemical Lab-oratories Sudbury Ontario The precisions of thedata based on replicate analyses of samples andblind standards along with representative analysesare listed in Table 2 A subset of samples wasselected for SmndashNd isotopic analysis utilizing aThermoFisher Triton T1 thermal ionization massspectrometer at Carleton University (techniques ofCousens 2000) Nd isotope ratios are normalizedto 146Nd144Nd frac14 072190 Analyses of the USGSstandard BCR-1 yield 143Nd144Nd frac14 0512668 +20 (n frac14 4) Thirty runs of an internal Nd metal

Fig 1 (Continued) and this study) c 2110 Ma Bear Mountain dyke in the Ferris Mountains (orange line) (Bowers ampChamberlain 2006) c 2090 Ma metagabbro in the Sierra Madre (orange dot) (Premo amp Van Schmus 1989) c 2060 Madyke in the Tobacco Root Mountains (pink dot) (Brady et al 2004 Harms et al 2004 Mueller et al 2004) the c2010 Ma Kennedy dyke swarm in the Laramie Mountains (yellow lines) (Cox et al 2000 Hark et al 2008) and c1870 Ma sills and dykes in the Black Hills (green dot) (Redden et al 1990) In the main figure Precambrian mafic dykesare shown as red (Rabbit Creek dykes and Powder River Pass dyke) and blue (unknown age) linear polygons withpalaeomagnetic sites indicated by text labels (eg BH26) and yellow circles sites that yielded reliable palaeomagneticresults are shown in bold Toothed lines indicate the main thrusts that delineate the eastern Bighorn Mountains (teeth onhanging-wall side) while grey dashed lines indicate other faults Roads are indicated with black (ie Route 16) anddotted black lines (US Forest Service roads) A Granitic gneiss Archaean Aum pyroxenite Pz Palaeozoic rocks(undifferentiated) Jurassic and Triassic rocks (665ndash725 m thickness) K Lower Cretaceous rocks Ku UpperCretaceous rocks T Tertiary sediments and Q Quaternary glacial deposits The figure is based on field mapping fromthis study and existing maps (Heimlich et al 1973 Hinrichs et al 1990)



Table 1 UndashPb TIMS baddeleyite data for samples BNB-09-WY-208a -210b -202b -204 and -207b

Corrected atomic ratios Ages

Mass U Sample Pb Pbc PbPbc ThU 206Pb204Pb

206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho

Sample (mg) (ppm) (ppm) (pg) (pg) (rad) err (rad) err (rad) err Ma Ma Ma err disc

BNB-09-WY-208a Powder River Pass dyke (44154532588888N 107055516988888W) Baddeleyite crystallization age frac14 21639+71 Ma (MSWD frac14 24) 4 point upper intercept dateWY208a p6 dk 4g 032 101 40 13 14 9 004 599 82 03993 +052 74581 +09 01355 +063 2166 2168 2170 +110 073 022WY208a p3 4g 12 152 60 72 47 15 015 993 23 03907 +021 72467 +049 01345 +038 2126 2142 21576 +67 068 17WY208a p2 5g 18 80 34 61 54 11 072 633 4 03608 +026 66326 +075 01333 +06 1986 2064 21424 +105 069 849WY208a p8 mt 1g 033 65 24 8 09 9 009 570 37 03629 +043 67346 +108 01346 +085 1996 2077 21587 +149 067 876

BNB-09-WY-210b Wind RiverndashSouth Pass dyke (42415443688888N 108514509288888W) Baddeleyite crystallization age frac14 21641+73 Ma (MSWD frac14 15) 3 point upper intercept dateWY210b p6 2g 077 170 67 51 21 24 007 1538 53 03965 +021 73423 +043 01343 +032 2153 2154 21551 +56 068 013WY210b p7 3g 077 62 25 19 1 19 016 1223 21 03936 +02 73214 +05 01349 +04 2139 2151 21629 +70 065 127WY210b p5 3g 077 359 138 106 58 18 022 1139 15 03708 +022 68148 +054 01333 +043 2033 2088 21419 +75 066 592WY210b p4 4g 077 112 37 29 17 17 009 1117 33 03336 +045 60458 +071 01314 +049 1856 1982 21172 +85 074 1419

BNB-09-WY-202b lsquoFrench Creekrsquo diabase dyke (448888820752primeN 1068888853721primeW) Baddeleyite crystallization age frac14 21559+29 Ma (MSWD frac14 011) 4 point weighted mean 207Pb206Pb dateWY202b p1 4g 06 261 100 60 3 19 006 1253 53 03881 +014 71942 +038 01344 +031 2114 2136 21567 +54 067 232WY202b p2 4g 06 306 119 71 12 6 006 399 56 03911 +031 72562 +115 01346 +095 2128 2143 21585 +165 075 167WY202b p3 8g 06 351 137 83 7 11 009 732 36 03909 +021 72437 +064 01344 +052 2127 2142 21563 +90 069 16WY202b p5 4g 068 108 43 29 1 27 011 1758 32 03931 +023 72806 +035 01343 +022 2137 2146 21552 +39 077 098

BNB-09-WY-204 lsquoRabbit Creekrsquo diabase dyke (44888881423primeN 1068888855945primeW) Baddeleyite crystallization age frac14 21542+22 Ma (MSWD frac14 038) 4 point weighted mean 207Pb206Pb dateWY204 p2 4g 09 268 104 94 10 10 008 649 44 03913 +019 72641 +07 01346 +058 2129 2144 21592 +101 074 165WY204 p4 6g 12 61 24 29 1 24 012 1528 30 03962 +021 73352 +037 01343 +027 2152 2153 21546 +47 071 017WY204 p6 6g 181 42 17 30 1 21 008 1403 42 03962 +021 73306 +039 01342 +029 2151 2153 21536 +50 069 012WY204 p7 5g 12 81 31 38 1 28 006 1783 58 03861 +015 71455 +026 01342 +018 2105 2130 21538 +32 072 266

BNB-09-WY-207b lsquoDike Crsquodagger near Hazelton Peak (44888884182primeN 1068888859978primeW) Baddeleyite crystallization age frac14 21577+36 Ma (MSWD frac14 008) 4 point weighted mean 207Pb206Pb dateWY207b p3 9g 271 88 34 93 20 5 005 321 68 03946 +032 73198 +143 01345 +119 2144 2151 2158 +207 081 075WY207b p4 6g 12 38 15 18 2 11 014 701 26 03963 +027 73405 +072 01343 +057 2152 2154 21555 +100 067 02WY207b p5 6g 181 47 19 33 1 26 008 1677 44 03982 +017 73856 +033 01345 +025 2161 2159 21579 +44 068 2015WY207b p7 6g 181 14 5 10 1 10 014 645 25 03931 +043 7296 +081 01346 +058 2137 2148 21589 +101 072 117

Locations in WSG84 coordinates All date errors are reported at a 95 confidence interval p Pick identifier dk dark mt mottled g number of grains Sample Pb sample Pb (radiogenic + initial) correctedfor laboratory blank Pbc total common Pb (blank plus sample initial Pb) PbPbc radiogenic Pb to total common Pb (blank + initial) corrected atomic ratios measured 206Pb204Pb corrected for massdiscrimination and tracer all others corrected for blank mass discrimination tracer and initial Pb err 2s errors in per cent 207Pb206Pb errors (err) are+2s Rho 206Pb238U vs 207Pb235U error correlationcoefficient disc per cent discordant Baddeleyite dissolution and chemistry were adapted from methods developed by Krogh (1973) Parrish et al (1987) and Rioux et al (2010) One dissolution of eachsample was processed through ion exchange columns to separate Hf but the others were evaporated and loaded without chemical processing All of the common Pb was assigned to blank although measuredblanks were between 2 and 06 pg Isotopic composition of the Pb blank was estimated as 1875+1 15652+06 and 3881+02 for 206204 207204 and 208204 respectively U blanks were consistentlyless than 02 pg Initial Pb isotopic compositions were estimated from Stacey amp Kramers (1975) model The decay constants used by PBDAT and MacPBDAT are those recommended by the IUGS Subcom-mission on Geochronology (Steiger amp Jager 1977) 0155125 times 1029year for 238U 098485 times 1029year for 235U and present-day 238U235U frac14 13788Excluded from linear regressiondaggerlsquoDike Crsquo refers to the original designation of this dyke by Heimlich et al (1973)

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Table 2 Geochemical results for dykes from the Bighorn Mountains

BH13 BH14 BH15 BH16 BH17 BH21 BH22 BH24 BH25 BH58 BH59 Int Std 1sd w 3175 930 440 0729 1748 1567 770 + 240 + 2540 413 520

SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005

MnO 023 021 022 024 023 023 023 020 021 020 021 024 0MgO 571 525 615 598 658 599 656 673 711 589 719 402 005CaO 969 1016 989 984 1003 988 985 1107 1111 1098 1121 747 011Na2O 212 237 206 204 202 203 211 210 202 213 213 316 005K2O 067 040 055 056 066 045 081 064 028 057 020 185 004P2O5 016 014 014 016 013 015 012 007 008 008 008 117 001LOI 098 069 082 074 055 054 054 077 038 118 019 162 033Total 9966 9999 9962 9955 9958 9928 9988 9996 9978 9913 9977 9953 ndash

Mg 0434 0431 0461 0446 0483 0442 0479 0519 0513 0491 0525 0397 0V 311 323 335 336 297 348 317 284 297 285 284 337 13Cr 161 157 156 135 172 143 156 185 195 135 200 19 5Co 516 497 528 538 53 524 554 502 546 464 544 29 3Ni 76 75 91 85 95 79 94 100 110 76 116 12 3Zn 1368 1294 1268 1308 1248 1254 1356 967 1068 1092 1030 133 7Rb 283 109 219 239 239 161 320 226 112 138 36 40 3Sr 128 140 133 128 124 115 135 167 137 162 138 408 5Y 314 284 294 302 278 303 266 176 192 173 180 47 4Zr 115 106 108 112 98 107 92 60 66 59 62 149 5Nb 60 55 55 59 52 57 48 31 33 30 31 8 1Ba 120 979 1811 1606 1074 1107 1119 1007 884 913 415 2202 143La 938 85 86 87 792 86 758 411 446 406 421 2668 055

(Continued)

WY

OM

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CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

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Table 2 Continued


Ce 2265 205 2055 2118 1911 2092 1836 103 1109 1027 1088 5855 021Pr 323 3 298 31 278 306 264 16 179 163 168 841 008Nd 1523 1382 1391 1448 1293 1437 1242 825 877 797 839 3939 042Sm 428 38 392 401 362 405 352 251 281 248 261 947 006Eu 138 13 13 134 122 132 117 1 106 099 103 382 01Gd 51 469 475 485 438 492 426 317 344 312 322 992 035Tb 0869 0772 0782 0818 0746 0835 0723 0524 0578 052 0531 145 002Dy 568 514 528 54 489 556 479 339 365 327 344 881 016Ho 117 106 108 112 103 117 099 068 073 067 07 176 001Er 355 317 319 336 308 345 303 195 21 19 198 494 001Tm 0508 046 0466 0484 0444 0496 0443 0273 0295 0266 0273 068 001Yb 333 301 303 318 292 321 290 175 189 171 174 433 006Lu 05 045 045 047 044 049 043 025 027 025 026 064 002Rb 2829 1091 2191 2393 2392 1612 3196 2257 1116 1377 36 4174 046Sr 128 140 133 128 124 115 135 167 137 162 138 41461 94Nb 596 55 553 586 521 566 481 307 331 297 312 758 022Cs 096 036 053 048 063 107 06 086 083 065 021 173 004Hf 305 285 289 302 265 293 249 171 183 168 172 409 005Ta 04 04 04 04 03 04 03 02 02 02 02 047 003Th 108 102 097 102 091 104 09 037 041 037 04 408 032U 028 025 026 026 023 029 022 009 009 01 01 152 011Sc 395 368 411 405 394 417 414 343 349 323 333 376 05Pb 17 18 32 29 13 12 32 1 34 08 06 695 007

Major oxides by XRF in wt trace elements by ICP-MS in weight ppm and LOI loss on ignition Mg Mg(Mg + Fe2+) Int Std Standard basalt from Lake Tahoe California submitted as blind standardPrecision is shown in final column 1s (SD) d Estimated distance (in metres) of sample from the closest margin of the dyke w total width of dyke (in metres)

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standard average 143Nd 144Nd frac14 0511823 + 12corresponding to a La Jolla value of 0511852based on comparative runs (May 2008ndash2011) Iso-topic data are listed in Table 3

Palaeomagnetism

Sampling was accomplished in the field using aportable gasoline-powered core drill Cylindricalsamples were oriented using both a magnetic and asun compass except in the case of sites T09BH18and T10BH59 where only magnetic measurementswere possible and a regional declination correctionwas used

Only one baked-contact test was performedbecause exposures of the baked zones of countyrock are very limited and often non-existent Expo-sures of dyke intersections (for the purpose ofbaked-contact tests) also are extremely rare Whereintersections are mapped there are often wetlandsor lakes both in the alpine and conifer forests Inthe field dykes are typically either slightly moreresistant forming minor topographic highs orare devoid of trees forming long meadows withsmall resistant diabase knobs protruding throughthe soil

Samples were prepared and measured at the Yalepalaeomagnetic facility which is equipped with a2G EnterprisesTM SQuID rock magnetometer andan automatic sample changer system (Kirschvinket al 2008) with a background noise sensitivity of5 times 10212 A m2 per axis The samples were storedwithin a magnetically shielded room (100 nTresidual field) and were demagnetized and measuredwith additional magnetic shielding After measure-ment of the natural remanent magnetization thesamples were first demagnetized using liquid nitro-gen immersion (Halgedahl amp Jarrard 1995) greatlyreducing the masking remanence of multi-domainmagnetite by cycling through magnetitersquos Verweycrystallographic transition at 77 K (Muxworthy ampWilliams 2006) in a near-zero magnetic field Sub-sequently the samples were thermally demagne-tized in small temperature increments within a

magnetically shielded furnace and controlled nitro-gen atmosphere Results were analysed using thefree computer software packages described byJones (2002) and Cogne (2003) which use principalcomponent analysis to calculate least-squares fits ofmagnetic components (Kirschvink 1980) andpalaeomagnetic poles

Structural corrections

Large-scale thrust faults during the LaramieOrogeny carried exposures of the Wyoming cratonto the surface In most cases small outliers of Pha-naerozoic strata still remain on top of the upliftedblocks and can be used to assess for basic tilt correc-tions In the Bighorn Mountains Phanaerozoicstrata indicate that tilt corrections are small(108) especially when sampling is restrictedtowards the stable interior of the uplift (Osterwald1978 Hoy amp Ridgway 1997) Large rotations aremainly restricted to the steep frontal limbs ofuplifts where fold trends curve (Weil et al 2009)The Wind River Range has both Phanaerozoic out-liers and structural studies of its main thrust fault(Smithson et al 1979 1980) and at its core requiresno more than 158 of tilt correction Still thesampling localities of Harlan et al (2003) derivefrom the far-SE portion of the uplift where the dra-matically straight but tilted NE boundary of theWind River Range curves 308 to the north Thissuggests an additional assessment of some verti-cal-axis rotations resulting from different amountsof fault displacement along the Wind River thrustwhich decreases towards the NW and SE ends ofthe uplift We assume that any large vertical-axisrotations are restricted to the edges of thrustedblocks while the central cores of the large base-ment uplifts experienced only small horizontal-axis tilting modelled well by Laramide structuraland seismic studies (Brown 1993) Palaeomagneticdirections obtained from the Triassic ChugwaterFormation along the margin of multiple upliftsin Wyoming agree remarkably well after onlyminor tilt corrections according to local bedding

Table 3 Isotopic data for selected Bighorn dykes

Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965

m Measured pres present day T ratio at time of crystallization at 2155 Ma based on UndashPb ages (this study) Tdm depleted mantlemodel age



(Collinson amp Runcorn 1960) suggesting that nomajor uplift-scale vertical-axis rotations occurred inthe basement hanging walls throughout Wyoming

In general dykes intrude along sub-verticalplanes into the crust and are good indicators bythemselves of major systematic tilting (unlesstilting occurred perpendicular to the trend of a ver-tical dyke) Nonetheless each palaeomagnetic siteis assessed for tilting independently taking intoaccount local structural studies or nearby outliersof sedimentary rock In addition apatite (UndashTh)He thermochronology of the Bighorn Mountainsconfirms that relative differences in exhuma-tion rates across the range were small with onlyminor regional tilting of the thrust block (Crowleyet al 2002)

Results


The northernmost dyke at French Creek (BNB-09-WY-202b palaeomagnetic site BH14) yields aweighted mean 207Pb206Pb age of 21559 + 29Ma (all uncertainties are 2s unless stated other-wise) The more northeasterly trending RabbitCreek dyke (2204 palaeomagnetic site BH15)yields a 207Pb206Pb age of 21542 + 22 Mawhile the southern dyke (2207b palaeomagneticsite BH24) in the Hazelton Peak area yields a207Pb206Pb age of 21577 + 36 Ma (Fig 2) Giventhese uncertainty envelopes all three dated dykesappear to belong to the same magmatic eventand we cannot currently resolve whether the NE-trending Rabbit Creek dyke is slightly younger orolder than the more NNE-trending dykes Cross-cutting relationships are difficult to obtain becausemost locations where dykes intersect are moredeeply eroded and covered by wetlands or lakesTherefore we group all three dykes (202b 204 and207b) into the same event possibly lasting from2161 to 2152 Ma in multiple pulses with slightlydifferent trends

Geochronological results from the Powder RiverPass dyke in the Bighorn Mountains (2208a) andfrom the South Pass dyke in the Wind River Range(2210b) are overlapping in error with upper inter-cept ages of 21639 + 71 and 21641 + 73 Marespectively (Fig 3) If we assume that these twodykes are part of the same intrusion then theirdata may be combined to calculate a single age of21640 + 51 Ma (Fig 4) Comparing this with the207Pb206Pb ages calculated for the smaller RabbitCreek dykes from the Bighorn Mountains (202b204 207b) we can see that at a 95 confidenceinterval some of the individual ages overlap butare different enough to suggest that there were at

least two distinct pulses of magmatism in the timeinterval from 2152 to 2171 Ma This is particularlyevident when comparing the individual ages ofBH15 (2204) and BH22 (2208a) which are distinctat a 95 confidence interval For that reason wename the younger group of dykes the RabbitCreek swarm after the drainage along the northernmargin of BH15 (2204)

Four of the five dated samples had at least oneanalysis that overlapped concordia within error(Table 1) Results for the three smaller Bighorndykes are all 3 discordant and form relativelytight clusters with zero age lower intercepts (Table1 Fig 2) so both upper intercept and weighted-mean 207Pb206Pb dates yield the same meanswithin error The weighted-mean 207Pb206Pb datesare interpreted as the best estimates of crystalliza-tion ages for these three samples Results from the100 m-wide dykes from Powder River (2208a)and South Pass (2210b) display a range of discor-dance from 0 to 14 (Table 1 Fig 3) and lowerintercepts of 340 and 480 Ma respectively Forthese samples the upper intercept dates are inter-preted to be the best estimates of magmatic agesas they include most of the analyses One analysisfrom the South Pass dyke (p6) does not lie on thechord defined by the other three analyses and wasexcluded from the linear regression even though itis the least discordant Our new data for the SouthPass dyke are significantly less discordant thanthose of Harlan et al (2003) They lead to a moreprecise date and permit this intrusion to be linkedto the Powder River Pass dyke

Geochemistry

All samples are basaltic in composition with SiO2

contents (recalculated to a volatile-free basis)between 487 and 493 (weight per cent) Thedyke samples can be subdivided into at least twogroups referred to below by their locations withinthe swarm southeastern and northwestern Sam-ples BH24 BH25 BH58 and BH59 from the south-eastern dyke set have the highest MgO (72ndash68)and lowest SiO2 TiO2 P2O5 and incompatible traceelement abundances (eg Th Nb REE Fig 5)Samples BH13 to BH17 as well as BH21 and BH22from the northwestern dyke set have lower MgO(66ndash53) and higher SiO2 TiO2 P2O5 andincompatible trace elements (Table 3) In the lat-ter group concentrations of most incompatibletrace elements are negatively correlated with Mg(atomic Mg(Mg + Fe2+)) The geochemical dis-tinction between the southeastern and northwesterndyke sets is evident in the primitive mantle-normalized incompatible element plot of Figure 6There is no overlap between patterns from the twogroups and the patterns are different for each group

T M KILIAN ET AL24


The southeastern dykes have lower LaSmpmn

(pmn frac14 primitive mantle-normalized) and a slightpositive Eu anomaly and lack the enrichment in

Th and U relative to Nb that is evident in the north-western dykes Both dyke sets are depleted in P rela-tive to the REE

0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160

Intercepts at31plusmn390 amp 21545plusmn35 Ma

MSWD = 055

2150

BNB-09-WY-204 (T09BH15)

data-point error ellipses are 2

206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166

2170Mean = 21542plusmn22 (95 conf)

Wtd by data-pt errs onlyMSWD = 038 probability = 077

(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736

Intercepts at-291plusmn1300 amp 21542plusmn72 Ma

MSWD = 0051 data-point error ellipses are 2

2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)

Mean = 21559plusmn29 (95 conf)Wtd by data-pt errs only

MSWD = 0109 probability = 095 (errorbars are 2 )

2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180

Mean = 21577plusmn36 (95 conf) Wtd by data-pt errs only

MSWD = 0082 probability = 097 (error bars are 2 )

BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7

Fig 2 UndashPb geochronological results for samples BNB-09-WY-202b -204 and -207b (a) (c) and (e) show concordiadiagrams for each sample including upper intercept ages bracketed precisions include decay constant uncertaintiesMean square weighted deviations (MSWD) are normalized chi-squared probability tests with expectation values of c 1(b) (d) and (f ) show 207Pb206Pb ages of the different baddeleyite crystal fractions analysed for each dyke includingtheir weighted means which are preferred because of their agreement with intercept ages and more precise errorinterval All error intervals are shown as 2s



The two dyke groups are also distinct isotopi-cally Two southeastern dykes have 1Nd valuesbetween +18 and +22 whereas two northwesterndykes have 1Nd of +11

Palaeomagnetism

We sampled 19 sites in 15 dykes that were probablypart of the Rabbit Creek swarm according to sim-ilar dyke orientations and proximity (Table 4) 13sites yielded stable palaeomagnetic data (shown inbold font Fig 1) Six sites yielded data that werescattered and inconsistent because magnetiza-tions either were unstable even at low temperatures(300 8C) or had been remagnetized by a light-ning strike The Rabbit Creek palaeomagneticpole is calculated from nine sites (or coolingunits) that produced internally consistent thermaldemagnetization behaviour (Figs 7 amp 8)

yielding a mean thermoremanent magnetization(TRM) confirmed to be primary through a positivebaked-contact test (Everitt amp Clegg 1962Schwarz et al 1985) This test was performed onthe Sheep Mountain dyke intruding a probablyArchaean NW-trending mafic dyke (Figs 9 amp 10)The Sheep Mountain dyke is not precisely amember of the Rabbit Creek swarm rather it corre-lates to the slightly older South Pass dyke None-theless this positive baked-contact test verifies theprimary nature of the Rabbit Creek dyke polebecause their magnetizations and ages are so closeto each other Four sites from the east-centralportion of the study area (BH13 BH16 BH17BH59) yielded samples possessing only one direc-tion of magnetization that persisted until theunblocking temperature of magnetite was surpas-sed All other sites produced a mixture of samplebehaviour including frequent removal of magneti-zations at temperatures below 500 8C that showlittle consistency within or between sites (Fig 8)When compared these components below c 500 8Cappear to be uniformly distributed on an equal-area plot therefore they are probably the result ofmulti-domain magnetite that acquired viscous rema-nent magnetization (VRM) At two sites (BH22and BH24) a component was demagnetized below200 8C that appears very similar to the presentlocal field which is probably a chemical remanentmagnetization held by goethite

The primary magnetization typically unblockedbetween 550 and 575 8C Some samples unblockedthrough a wider range of temperatures (from 500 to565 8C) allowing a least-squares fit using more datapoints (Fig 8) In some cases smaller unblockingtemperature ranges are the result of overlappingmagnetization components from a wider range ofmagnetite grain sizes with small VRM componentsunblocking below c 550 8C This may be the resultof different sampling locations across the width of adyke in which grain sizes can vary greatly Grainsize variation can be even more prevalent in widerdykes which may be the cause of unblocking temp-erature variation in the Rabbit Creek swarm wheredyke widths vary from 3 to 50 m and sampling islimited to certain portions of dykes that arebest exposed

The high temperature magnetization is proved asprimary with a positive baked-contact test sampledinto an older Archaean() dyke (SHM2) near thesummit of Sheep Mountain (Figs 1 amp 9) Thermaldemagnetization of samples adjacent (SHM1) tothe Sheep Mountain dyke (SHM3) probably a con-tinuation of the Powder River Pass dyke reveal twodistinct components a lsquobakedrsquo TRM indistinguish-able from that obtained from site SHM3 and astable lsquounbakedrsquo magnetization farther away fromthe Sheep Mountain dyke Sample SHM2-5 is

035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200



BNB-09-WY-208a (T09BH22)Powder River Pass Bighorn Mtns

206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15

BNB-09-WY-210bWind River-South Pass qtz diorite dyke


206 P

b23

8 U

(b)

Fig 3 UndashPb geochronological results for samples (a)BNB-09-WY-208a and (b) BNB-09-WY-210b are up to14 discordant and rely on upper intercept fits formagmatic crystallization ages A concordant analysisfrom BNB-09-WY-210b (pale grey) is excluded from theage estimation because it plots off the linear regression ofthe other three analyses

T M KILIAN ET AL26


completely baked by the Sheep Mountain dyke atc 5 m away with no indication of any remainingArchaean magnetization (Fig 10) Samples takenat c 65 m (SHM1-7) c 9 m (SHM1-8) and c15 m (SHM1-9) show a hybrid zone behaviour inwhich the older lsquounbakedrsquo magnetization was notcompletely overprinted by heat from the SheepMountain dyke (Fig 10) resulting in two com-ponents that coexist in the same sample withindifferent temperature ranges Sample SHM1-7 con-tains a high-temperature Archaean() magnetiza-tion (568ndash573 8C) in addition to a baked partialTRM that unblocked at a lower temperature range(500ndash565 8C) Sample SHM1-8 also contains bothmagnetizations however the high-temperature(unbaked) component extends into lower tempera-tures (560ndash576 8C) the result of being c 25 mfarther away from the heat source (Fig 10) Sam-ple SHM1-9 retained a baked magnetization atsimilar temperatures to sample SHM1-8 Any indi-cation of a baked component disappears at c 20 mfrom the Sheep Mountain dyke in sample SHM1-10 which records only an older unbaked compo-nent The baked zone where temperatures exceededc 530 8C in samples SHM1-9 extends aboutone-third of a dyke width from the Sheep Mountain

dyke within the predicted range of cooling models(Delaney 1987) The survival of the unbaked mag-netization in sample SHM1-7 indicates thatc 65 m away from the Sheep Mountain dyke temp-eratures never exceeded c 568 8C This is unex-pected considering the large size of the SheepMountain but this phenomenon can be explainedin a couple of ways The ambient host rock tempera-tures could have been fairly low (shallow in thecrust) shifting contact temperatures lower andeffectively shrinking the size of the baked zoneAnother possibility has to do with the samplelocality and outcrop on Sheep Mountain Owing tolimited exposure the estimated distances of sam-ples from the Sheep Mountain dyke margin (Fig10) were measured from the projected contact ofthe dyke (Fig 9) It is possible that the dyke is nota perfect plane as assumed and that it changes direc-tion near SHM1 causing our length estimates toshift by metres A dextral fault could have the sameeffect if it were to pass between the Sheep Mountaindyke and site SHM1 making SHM1 appear closer tothe margin of the Sheep Mountain dyke This seemsplausible considering that the intrusion is nextexposed over 700 m to the SE making it difficultto judge if there has been any offset Regardless

Fig 4 Summary of UndashPb ages for the Rabbit Creek and Powder RiverndashSouth Pass dykes Sample IDs are given interms of geochronological sample numbers with corresponding palaeomagnetic site for the same dyke in parenthesesMean ages are calculated hypothesizing that 202b and 207b are a single dyke and 208a (Bighorn Mountains-PowderRiver Pass dyke) and 210b (Wind River Range-South Pass dyke) are similarly from a single dyke Also shown is aweighted mean of all Rabbit Creek age estimates



the identification of baked hybrid and unbakedzones adjacent to the Sheep Mountain dyke com-prise a complete and positive baked-contact testaffirming that the remanence was acquired at thetime of dyke emplacement and cooling

The unbaked direction is similar to magnetiza-tions obtained from other likely Archaean dykes inthe southern Bighorn Mountains though thisunbaked magnetization is considerably differentfrom that derived from the 271 Ga Stillwatercomplex (Selkin et al 2008) The unbaked directionmay be somewhat younger than the Stillwatercomplex and possibly a thermoviscous remanentmagnetization recorded through regional uplift andcooling during late Archaean accretion and pluton-ism on the southern margin of the craton

Two sites from the large and differentiated Pow-der River Pass dyke (BH22 and SHM3) yieldedstable palaeomagnetic data Site BH22 was col-lected from the diabasic portion of the dyke nearsample BNB-09-WY-208a along a quarried sectionof the dyke Site SHM3 was collected in coarse leu-cogabbro near the summit of Sheep MountainBecause this intrusion is of such unique size andmineralogy we judge that it is very likely to bethe only c 2164 Ma dyke sampled in the Bighornswith all other sites belonging to the Rabbit Creek

swarm Although its age is significantly differentfrom the Rabbit Creek dykes the Powder RiverPass dyke has a very similar high-temperature mag-netization to the rest of the dykes in the SE BighornMountains For that reason the baked-contact teston the Powder River Pass dyke can be applied to theRabbit Creek magnetization especially because theintrusion slightly predates the Rabbit Creek dykes


Assessment for small structural corrections arenecessary on both local and regional scales toaccount for the slight rotations of blocks thatoccurred during the Laramide Orogeny Models ofdeformation during uplift (Hoy amp Ridgway 1997)indicate that the basement was vertically thrust up8ndash10 km on the eastern flank of the Bighorn Moun-tains The same models also suggest that the core ofthe uplift remained sub-horizontal with negligibletilting of less than 58 Towards the margin of theuplift near the main basement thrust fault (lessthan 2 km away) more significant tilt correctionsare required (Table 5) In the two areas wheresamples were collected 2ndash3 km from thrust faults(Sisters Hill and Clear Creek Thrusts sites BH58BH59 and BH14) we apply a 108 correction

Fig 5 Th concentration LaSmpmn ThLa and 1Nd v Mg (Mg(Mg + Fe2+) for Rabbit Creek dykes Symbolsdistinguish dykes by location within the swarm

T M KILIAN ET AL28


Palaeomagnetic results from two dykes in theBighorn Mountains (BH14 and BH24) are separatedby c 708 of declination from the mean results of allother Rabbit Creek dykes After tilt corrections areapplied to the palaeomagnetic dataset the concen-tration and confidence interval of the Rabbit Creekmean direction are both improved except for theoutlying data from BH14 and BH24 which comeinto better agreement with each other not themean (Fig 7) Along with their similar ages thissuggests that BH14 and BH24 might be the samedyke Despite their difference geochemically aswell as the difficulty in tracing them to each otherin the field they are approximately the same widthand trend and are nearly along strike of each otherWe exclude the palaeomagnetic data from thesesites in the calculation of the Rabbit Creek palaeo-magnetic pole because they are completely differentfrom all other sites even though they share similarand precise UndashPb ages along with stable palaeo-magnetic behaviour The primary palaeomagneticremanence from dykes BH14 and BH24 probablyrecords an excursion of the magnetic field that isnot representative of Earthrsquos dipolar field

Results from the South Pass region (Harlanet al 2003) require a tilt correction according to

nearby Palaeozoic sediments to the north (Densonamp Pipiringos 1974 Hausel 1991) In addition thearea near the dyke is sheared and faulted caus-ing the overall trend of the intrusion to appearmore northerly We propose that a lateral thrustramp underlies the far southeastern edge of theWind River thrust fault causing c 308 of counter-clockwise (CCW) rotation of the basement accom-modated mostly by coplanar shearing of verticalArchaean metasediments The angle of vertical-axisrotation that the South Pass region experienced isestimated from the change in strike of the northernboundary of the Wind River Range (Fig 11)When the South Pass dyke is restored accordingto all Laramide rotations it points towards thesouthern Bighorn Mountains nearly along-strikewith the Powder River Pass dyke Between theSouth Pass and Powder River Pass dykes is the DePass region of the Owl Creek Mountains where asimilarly orientated and faulted large dyke intru-sion cuts through a small window of the basement(Thaden 1980) This mafic intrusion may be anoutcrop of the same dyke (or network of dextrallyoffset dykes) as the South Pass and Powder RiverPass dykes however no samples were collectedfrom that locality

Fig 6 Primitive mantle-normalized (Sun amp McDonough 1989) trace element patterns for Rabbit Creek dykes Onlyimmobile elements are plotted Filled symbols indicate the southeastern group of dykes



Table 4 Results from palaeomagnetic sites before and after tilt corrections

Site latitude(N)

Site longitude(W)

Before correction After correction

Site ID Width(m)

Trend(deg)

(deg) (min) (deg) (min) Dec(deg)

Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)

BH13 75 50 44 14720 106 56371 2229 2661 24 605 3195 35 88 ndash ndash ndash ndash ndashBH14 40 15 44 20752 106 53721 1227 2583 4 477 1766 5 88 1369 2648 601 1832 57BH15 40 50 44 14380 106 55945 2094 2705 72 684 3036 115 58 ndash ndash ndash ndash ndashBH16 29 50 44 14743 106 56376 2078 2612 77 698 3389 104 67 ndash ndash ndash ndash ndashBH17 48 55 44 14802 106 56368 2289 2644 27 561 3221 38 88 ndash ndash ndash ndash ndashBH18 67 25 44 16133 106 56920 2013 262 115 746 3394 158 4(1)7 ndash ndash ndash ndash ndashBH21 67 35 44 16104 106 57202 2104 2679 34 686 3141 53 88 ndash ndash ndash ndash ndashBH22 100 40 44 9019 107 3522 2328 2482 33 459 3444 35 88 ndash ndash ndash ndash ndashBH24 50 20 44 4182 106 59978 1414 2611 98 620 1728 132 4(1)8 1231 2649 509 1878 142BH25 40 30 44 3344 107 0722 2132 2414 148 562 88 141 4(2)8 2158 2511 595 3545 165BH58 13 58 44 15616 106 51600 1771 2573 217 833 931 271 3(3)8 1916 2548 774 243 258BH59 20 55 44 15630 106 51389 1819 2464 46 734 673 48 78 1915 2435 689 431 45SHM3 70 45 44 11464 107 0343 2395 2539 149 440 3339 174 3(1)8 ndash ndash ndash ndash ndashQDmean 100 10 42 26dagger 108 31dagger 3549 646 46 846 2112 66 8 369 597 625 3328 6WR-58a NA 20 42 2493 108 30876 3429 552 63 752 1404 76 9(3)10 227 518 695 31 71

Bold sites used for Rabbit Creek swarm mean calculation a95 95 percent confidence interval Dec declination Inc inclination Plat and Plong palaeolatitude and palaeolongitude virtual geomagnetic polecoordinates respectively n Number of samples ( p plane-fits) used in mean N number of samples collected and processedUses data from Harlan et al (2003) with tilt correction of data about bedding first (323888 NE) then about a vertical axis (308 clockwise)daggerSee Harlan et al (2003) for sampling locality information

T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from

Because of the age difference between thePowder RiverndashSouth Pass dykes (c 2164 Ma) andthe Rabbit Creek dykes (c 2155 Ma) the palaeo-magnetic data from Harlan et al (2003) and the

Powder River Pass dyke (BH22 and SHM3) areexcluded from the Rabbit Creek palaeomagneticpole (Table 6) In the case that all of the dykes inBighorn Mountains represent a mixture of RabbitCreek and Powder River dykes we have also cal-culated an alternative palaeomagnetic pole forall Bighorn intrusions (excluding sites BH14 andBH24) that applies to a slightly longer time interval2171ndash2152 Ma This age range is also assigned tothe mean calculated for the South PassndashPowderRiver Pass and Rabbit Creek dykes If South Passdata are corrected according to both horizontal andvertical-axis rotations (Table 4) the virtual geomag-netic poles agree very well with the Rabbit Creekpole (Fig 7) The pole calculated from all correcteddata from all ranges changes the mean coordinatesby only a few degrees and reduces the error intervalby less than a degree Statistically these three meansare indistinct possibly a result of under samplingthe intrusions from the South PassndashPowder RiverPass event (Table 6) We prefer to use the inclusiveRabbit CreekndashPowder RiverndashSouth Pass dykespalaeomagnetic pole (excluding BH14 and BH24)in our palaeogeographic analysis because it hasthe highest likelihood of fully averaging secularvariation from 2171 to 2152 Ma

Discussion

The 2171ndash2152 Ma event is well suited to palaeo-geographic reconstructions with a positive baked-contact test increased precision after tilt correctionsand multiple high-precision UndashPb ages How-ever subtle differences in some results within theswarm prompt questions about the spatial and tem-poral extent of the Rabbit Creek event and itsrelationship to the Powder RiverndashSouth Pass dykes

The Rabbit Creek swarm most likely intrudedin less than 10 myr in the Bighorn Mountains butpossibly in different generations (or pulses) of mag-matism that are difficult to distinguish with geo-chronology because Rabbit Creek ages all overlapwithin uncertainty Slight variations in trendsamong dykes are probably the result of the oldershear fabric in the central and southern gneiss ter-rain (Frost amp Fanning 2006) and are not reflectedin geochemical and palaeomagnetic variations inthe Bighorn Mountains Dykes BH14 and BH24have similar trends (0108 and 0158 respectively)age and palaeomagnetic directions but they aredifferent geochemically It appears that trend doesnot necessarily have a correlation with geochemis-try considering that dykes BH58 and BH59 havetrends (0508) very different from BH24 (0108) butshare similar major element REE and Nd isotopiccharacteristics Sites BH14 BH58 and BH59 arethe closest localities to the edge of the uplift

0

90

180

270

(b) Harlan et al (2003) data(tilt corrected)

Rabbit Crk-Powder Riv-South Pass mean

(tilt corrected N = 13)

Bighorn Mountains data (tilt corrected)

excluded(BH14 and 24)

0

90

180

270

(a)

Harlan et al (2003) dataSouth Pass - Wind River Range


(uncorrected N = 15)

Bighorn Mountains data(this study)

Fig 7 Site mean data (local coordinates) for the RabbitCreek swarm and Powder River Pass dykes (greyellipses) along with data from South Pass dykes (orangeellipses) (Harlan et al 2003) on equal-area plots Filledellipses indicate lower hemisphere directions (positiveinclinations) whereas open ellipses indicate upperhemisphere directions (negative inclinations) (a) Beforestructural corrections are applied to data the overallmean is composed of fairly scattered site means All dataare included in the Rabbit CreekndashPowder RiverndashSouthPass uncorrected mean (red ellipse) also plotted in thelower hemisphere (open red ellipse) (b) Structurallycorrected data in local coordinates with an improvedRabbit CreekndashPowder RiverndashSouth Pass mean (redellipses) Sites BH14 and BH24 are judged as excursionsand excluded from the mean calculation



Fig 8 The Rabbit Creek dykersquos demagnetization behaviour is exemplified by samples from three different dykes Eachpanel (andashc) displays measurements for the same samples on different diagrams (from left to right) orthogonal plotequal-area plot and JJ0 intensity diagrams Natural remanent magnetization indicates the initial measurement beforeany demagnetization LN2 indicates the measurement taken after liquid nitrogen immersion and subsequent labelsindicate the temperature step of thermal demagnetization J0 is the initial intensity of the sample magnetization Openred circles on the equal-area plots indicate negative inclinations and blue symbols indicate positive inclinations On theorthogonal plot red squares indicate inclination direction and blue circles represent declination Orange vectorsrepresent the least-squares fit of the primary magnetizations whereas green vectors represent spurious magnetizationsremoved during the earlier stages of demagnetization The lower-temperature components (green vectors) have similardirections in (b) and (c) but are not consistent enough in either dyke (or the region) to be of statistical significance

T M KILIAN ET AL32


nonetheless it is very unlikely that structuralrotations explain the discrepancies in trend andpalaeomagnetism Sites BH24 and BH25 are verynear to each other geographically (18 km) andhave similar REE concentrations yet they havedifferent trends (0158 and 0308) and palaeomagneticdirections suggesting that geochemical differencesare most strongly correlated with the location of adyke within the swarm Sites BH24 BH25 BH58and BH59 are the farthest SE localities sampledwithin the Bighorn Mountains and group well geo-chemically distinct from all other sites from thenorthwestern dyke set The lower MgO and incom-patible element abundances in the southeasterngroup low ThLa and higher Nd isotope ratiosare consistent with them representing a mantle-derived magma that has undergone some fractionalcrystallization with minimal crustal contamination

The northwestern group may be related to thesoutheastern dyke magmas by a combination offractional crystallization and crustal assimilationbased on the roughly parallel incompatible elementpatterns but lower MgO higher ThLa and lower1Nd in the northwestern dykes The range of compo-sitions exhibited by the northwestern dyke set is notgeographically controlled BH17 and BH22 are very

alike geochemically but are widely spaced andadjacent samples BH13 BH16 and BH17 span theentire range of northwestern dyke compositionsThe overlap in UndashPb ages between the two dykegroups indicates that all of these magmas wereavailable to be emplaced throughout the history ofthe dyke swarm The petrological variations overthis time period are consistent with existence of azoned magma reservoir feeding the swarm Thesoutheastern group magmas were derived eitherfrom the less differentiated core of the reservoir orfrom dyke-fed magmas that missed the reservoirand proceeded directly to the surface The northwes-tern dykes tapped more fractionated and contami-nated magmas from the margins of the reservoir

The orientation of dykes within the Rabbit Creekswarm varies by c 408 (010ndash0508) but there is notmuch evidence to support a correlation betweentrend and palaeomagnetic direction As mentionedthe only sites that support a trend v palaeomagneticcorrelation are BH14 and BH24 Other intrusionsoften change their orientations along the length ofthe intrusion for example site BH11 was sampledin a dyke that appears to bifurcate or change trendsdrastically over a short distance (Fig 1) The dykesin this swarm may have simultaneously intruded inmultiple orientations during the entire magmaticevent implying that even if there were discrete sub-swarms over a 15 myr interval they may not beeasily defined by their orientations (ie there couldhave been a rapidly changing stress field) It is mostlikely that antecedent faults and structures in thebasement gneiss of the southern Bighorns mayrelate to preferences in trend Most of the rivervalleys in the SE Bighorn region are controlled bylarge NE-trending faults that extend for tens ofkilometres The faults often contain mylonitic zoneswith foliation and coplanar cleavage parallel tothe fault orientations These roughly NE-trendingfaults cross-cut Archaean dykes and have identicalvariations in orientation (010ndash0508) to the RabbitCreek dykes intimating that the swarm preferen-tially intruded along these faults with offshoots ofslightly different trends Faults with more northerlyorientations (0108) are mapped in the vicinity ofboth BH14 and BH24 and they possibly served aspre-existing fractures that allowed dykes to jog orjump before continuing along the dominant NEfault plane orientations A later generation of ESE-trending faults offset some Rabbit Creek dykesbut their minimum age is unknown and they maybe related to the Trans-Hudson Orogeny the Lara-mide Orogeny or anything in between

Great Dyke of Wyoming

The large (100 m-wide) dykes that crop out in thesouthern Bighorn Mountains (Powder River Pass

Fig 9 Map of the Sheep Mountain dyke intersectionwith an Archaean() dyke palaeomagnetic samplingsites are indicated Surface exposures of the dykes areshown by solid black line polygons dashed black linesindicate the inferred extent of intrusions Sites in redindicate the locality where the Sheep Mountain dyke wassampled (SHM3) green sites identify the unbakedsample locality for the Archaean dyke and grey sitesindicate the baked-contact test samples for the SheepMountain dyke A baked-contact zone is shown as a redgradient which is estimated from the demagnetizationcomponents isolated in samples SHM1-7 SHM1-8SHM1-9 and similar to length scales of half-spacecooling models proposed in Delaney (1987)



Fig 10 Data from the baked-contact test for the Sheep Mountain dyke (SHM3) showing (a) an equal-area plot with allmagnetization components isolated from SHM1 SHM2 and SHM3 excluding a small number of samples that wereprobably struck by lightning Red circles indicate baked components and blue squares indicate unbaked components(from SHM1 and SHM2) Site means are shown for the unbaked direction (SHM1 and SHM2) the intruding SheepMountain dyke (SHM3) and baked components from SHM1 samples close to the intrusion Sample directions with boldsymbols correspond to the vectors shown in (bndashf ) (b) Sample SHM1-5 is shown in an orthogonal plot to be completelybaked by the Sheep Mountain dyke at c 5 m away Remanence removed at lower temperatures is only slightly differentthan the baked direction with blue circles (red squares) representing declination (inclination) data (c) Demagnetizationdata from hybrid sample SHM1-7 shown in an orthogonal plot Orange vectors indicate the lower-temperature bakedmagnetization and blue vectors indicate the high-temperature unbaked remanence direction from grains not heated longenough to reset their magnetization (d) An orthogonal plot for sample SHM1-8 shows the progression of the hybridzone away from the intruding dyke with a baked component that is removed at c 535 8C (e) An orthogonal plot forsample SHM1-9 showing a small baked component up to 530 8C with an unbaked component at high temperatures thatdominated the sample magnetization (f ) An orthogonal plot of sample SHM1-10 which has no perceivable bakedcomponent and a very strong unbaked magnetization The decrease in magnitude of the baked component (orangevectors) and increase in unbaked remanence magnitude (blue vectors) correspond to the dissipation of a partial TRMwith increasing distance from the heat source (Sheep Mountain dyke) In both (c) and (d) certain areas of the plot havebeen magnified to better display magnetic components

T M KILIAN ET AL34


dyke sites BH22 BH26 and SHM3) and in SouthPass (the dyke of Harlan et al (2003) and sampleBNB-09-WY-210b) are very probably the sameintrusion that spans more than 200 km in lengthThere are many distinct characteristics linking theseintrusions to each other For instance both dykes areunique in their large size and reach over 100 mwidth at both localities They each show 10ndash15 mof inward-coarsening diabase on either marginwithin single outcrops with a narrow transitionto a leucogabbro (to quartzndashdiorite) core In theBighorn Mountains South Pass and Owl CreekMountains large offshoot dykes of diabase sur-round each intrusion with orientations 10ndash208clockwise from the main intrusions and right-stepping en echelon offsets along their lengthsThe features of these intrusions are unique withinthe Wyoming craton providing a strong geologicalcorrelation supported by indistinguishable UndashPbages If faults in the South Pass region are restoredand the SE region of the Wind River uplift is cor-rected for a vertical-axis rotation then outcrops ofthe hypothesized Great Dyke of Wyoming appearnearly along the same NE-trending line (Fig 11)

Additional supporting evidence for the existenceof a continuous lsquoGreat Dyke of Wyomingrsquo is thepresence of a similarly large intrusive body mappedin the eastern Owl Creek Mountains (Thaden 1980)This dyke appears almost directly between the twoother outcrops in the Bighorn Mountains andWind River Range but it is oriented more northerlySuch an orientation may be due to block rotations

during the Laramide Orogeny when major faultingand transport of nearby Palaeozoic rocks occurredleaving fault gouges along some dyke marginsThe orientations of the South Pass and De Passdykes may be emblematic of one large intrusionthat has a NNE-trend and right-stepping enechelon offsets along its length giving an apparentNE-trend to the intrusion on a regional scale

There is also a significant possibility that thePowder River Pass dyke and the South Pass dykeare from separate magmatic pulses The lack of con-tinuous outcrop and the intrinsic uncertainty inradiometric dating do not provide a strong enoughbasis to affirm that they are parts of the same intru-sive body The palaeomagnetic results highlight thisambiguity because they are of different magneticpolarities Thermal and alternating field demagneti-zation results from the South Pass dyke (Harlanet al 2003) have a positive inclination whereas allresults from the Bighorn Mountains (this study)including the wide Powder River Pass dyke (BH22and SHM3) have negative inclinations The posi-tive baked-contact test confirms the primary natureof the negative inclination while the results fromSouth Pass are almost identical to the present localfield direction (before tilt correction) Given thisscenario there are a few possible interpretationswith varying amounts of likelihood (1) Both palaeo-magnetic results could be primary TRMs from thelong-term cooling of one large intrusion (or distinctpulses into the same intrusion) a dyke of this size(100 m) could take over 500 years to cool below

Table 5 Tilt-correction parameters for each palaeomagnetic site

Nearby beddingattitude (RHD)

Site ID Strike Dip Tilt correction reference

BH24 110 10 Hoppin et al (1965)BH14 350 10 Hoppin et al (1965)BH25 110 10 Hoppin et al (1965)BH22 No significant correction Hoy amp Ridgway (1997)BH18 No significant correction Hoy amp Ridgway (1997)BH16 No significant correction Hoy amp Ridgway (1997)BH21 No significant correction Hoy amp Ridgway (1997)BH15 No significant correction Hoy amp Ridgway (1997)BH13 No significant correction Hoy amp Ridgway (1997)BH17 No significant correction Hoy amp Ridgway (1997)BH58 350 10 Hoy amp Ridgway (1997)BH59 350 10 Hoy amp Ridgway (1997)SHM3 No significant correction Hoy amp Ridgway (1997)Qdmean 323 8 Hausel (1988)WR-58a 323 8 Hausel (1988)

References indicate the source of bedding data or estimations RHD refers to thedip direction which is 908 clockwise from the strike directionRequires an additional vertical-axis correction see Table 4 and text



300 8C at its centre (Delaney 1987) However this isnot likely because results were consistent across thewidth of each intrusion where one might expect tosee the effects of differential cooling rates (2) Con-sidering the time it took the large dyke to propagateover 200 km both results could be primary TRMswith different ages Again this contingency is alsounlikely given estimates of quite rapid dyke propa-gation over a period of days (Sigurdsson 1987 Keiret al 2011) (3) The palaeomagnetic data from theSouth Pass dyke (Harlan et al 2003) could be theresult of deuteric oxidation of FendashTi oxides dur-ing the late stages of dyke cooling which wouldstill be considered a primary magnetization for thepurposes of tectonic reconstructions (4) The out-crops in the Bighorns and South Pass could expose

different erosional levels of the dyke with eitheroutcrop having come from a deeper region of thecrust In this scenario crustal temperatures werehigher at the time of intrusion and could have hada profound impact on when magnetite acquired itsmagnetization possibly long after the intrusion ofmagma (Halls amp Zhang 2003) (5) It is also possiblethat the positive inclination of the dyke is a VRMacquired more recently though this is unlikelywith the magnetization being held by magnetite upto the 570 8C and an 40Ar39Ar amphibole age of2124 + 30 Ma determined by Harlan et al (2003)

Palaeomagnetic results from the Powder RiverPass dyke (BH22 and SHM3) and the two intru-sions from South Pass (Harlan et al 2003) are notenough to calculate a fully averaged palaeomagnetic

Fig 11 Regional map showing Precambrian rocks of three Laramide uplifts The inset map indicates the locationof this figure on a larger scale Yellow stars indicate the location of the Powder River Pass dyke in the BighornMountains and the South Pass dyke in the Wind River Range After correcting for the vertical-axis rotation of thesoutheastern limb of the Wind River uplift outcrops of the intrusions appear to line up with each other from South Passto the Bighorn Mountains (dashed red line is the inferred extent of the Great Dyke of Wyoming underneath Palaeozoiccover) The dashed red line may not resemble the actual igneous bodies which may be a network of NNE-trending dykesthat intruded with significant en echelon offsets along its length that were additionally displaced by later dextral faultsIn between uplifts are deep Phanaerozoic basins that completely obscure the Precambrian basement rocks The bluedashed lines indicate the approximate strike of the Wind River thrust that takes a 308 turn north of South Pass whichagrees with the attitude of Palaeozoic sediments along the northern side of the range and is the premise for needing anadditional vertical-axis correction in the SE Wind River Range Black lines indicate major faults with teeth indicatingthe hanging wall of thrust faults Map modified from Sims et al (2001)

T M KILIAN ET AL36


pole After tilt correction the VGPs are fairly closeto each other and not significantly different fromRabbit Creek dyke data but because they may bethe only cooling units of that age it is statisticallyproblematic to contrast them to the Rabbit Creekdata

The northern extent of the Powder River Passdyke is hard to follow in the field and on satelliteimagery Northeast of site BH22 the dyke appearsto bifurcate as a portion continues north whileanother branch continues through Sheep Mountain(Fig 1) The dyke then becomes thinner and lessprominent topographically as it is offset by multi-ple small sinistral faults As it is about to crossRoute 16 (NE of site BH15) the small ridge (pre-sumed to be the intrusion) disappears into a riverdrainage At first there was concern that the north-eastern extent of the Sheep Mountain (PowderRiver Pass) dyke may be related to dykes BH13BH16 and BH17 offset along a sinistral fault thatclearly cuts the nearby Rabbit Creek dyke How-ever the displacement of the fault can clearly bemeasured using the Rabbit Creek dyke which isoffset by c 100 m The fault would have to offsetthe Powder River Pass dyke over 400 m to alignwith BH13 which is c 75 m wide and very fine-grained We conclude that the Powder River Passdyke ends somewhere in the wetlands betweensites BH15 and BH13 shrinking to such a smallsize that it is impossible to follow

The intrusions we suspect to be parts of thelsquoGreat Dyke of Wyomingrsquo are distinct from a mag-netic anomaly to the east of the Bighorn Mountainsthat was called by the same name (Gay 2003) Thatsubsurface body is tens of kilometres wide and isinterpreted by others as an Archaean domain

boundary (Sims et al 2001) but its affinity andcomposition remain unknown

Global age comparisons

The Rabbit Creek event has few direct age matcheswith known events in other cratons The SouthPass dyke was previously assigned a slightly olderage of 2170 + 8 Ma the result of UndashPb analyseswith 20ndash30 discordance (Harlan et al 2003)Our new dates for the South PassndashPowder Riverevent permit a correlation with the Biscotasingevent of southern Superior which has an age spanobtained from multiple dykes of 2165ndash2176 Ma(Halls amp Davis 2004) within error of both Harlanet al (2003) and our age estimates yet distinctfrom the younger Rabbit Creek intrusions (202b204 and 207b) Neither of these new ages sup-port a plausible correlation to the SW Slave Mag-matic Province of the Slave craton which has anolder age range of 2175ndash2193 Ma (Bleeker ampHall 2007)

Globally the c 2155 Ma magmatic age is rela-tively rare One of the few magmatic events that isnearly coeval is the Riviere du Gue swarm in north-eastern Superior dated at 2149 + 3 Ma (Mauriceet al 2009) If Wyoming is reconstructed to south-ern Superior as in Roscoe amp Card (1993) then theRabbit Creek and Riviere du Gue swarms couldbe distally related to the same event Other nearage matches include the Avayalik dykes of theNorth Atlantic craton (Nain portion) at 2142 +2 Ma (Connelly 2001) and the Hengling swarm ofthe North China craton at 2147 + 5 Ma (Penget al 2005) Both of these events are slightlyyounger than the Rabbit Creek swarm and currently

Table 6 Palaeomagnetic poles from this study before and after structural corrections

Palaeomagnetic pole Age range(Ma)

Plat(8N)

Plong(8E)

a95 N k

Before correctionsRabbit Creek Powder River and South

Pass dykes (all sites)2171ndash2152 784 3320 131 15 95

Rabbit Creek Powder River and SouthPass dykes (except BH(14 and 24))

2171ndash2152 719 3388 109 13 155

Rabbit Creek dykes (except SHM3 andBH(14 22 and 24))

2161ndash2152 720 3385 106 9 243

After correctionsRabbit Creek Powder River and South



2171ndash2152 655 3392 76 13 305

Rabbit Creek and Powder River dykes(except BH(14 and 24))

2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36



have no palaeomagnetic data available forcomparison

Palaeogeography

The Superior craton has a palaeomagnetic datasetthat spans the time interval of the Rabbit Creekswarm providing a convenient test for the Roscoeamp Card (1993) fit of SE Wyoming against southernSuperior The Biscotasing swarm (Buchan et al1993) and the Marathon swarm (normal polarity)at 2126ndash2121 Ma (Buchan et al 1993 Halls et al2005) both have reliable palaeomagnetic data sup-ported by field tests Because the Wyoming cratonhas no other Rhyacian (2300ndash2050 Ma) palaeo-magnetic data there is complete freedom of Wyom-ingrsquos position in palaeolongitude

The Marathon dyke swarm was emplaced on thesouthern margin of the Superior craton in two dis-cernable pulses 2101ndash2106 and 2121ndash2126 Ma(Hamilton et al 2002) both postdating the RabbitCreek swarm The Biscotasing swarm is slightlyolder at 2165ndash2176 Ma and has large dykes thatextend from southern Superior to the NE por-tion of the craton where the centre of the event ishypothesized to exist with additional coeval dykeswarms and sills (Ernst amp Bleeker 2010) Althoughneither of these events matches the Rabbit Creekswarm in age they define an apparent polar wan-der path segment that includes the age of theRabbit Creek swarm for comparison (Fig 12) TheBiscotasing swarm age roughly agrees with agesfrom the South Pass and Powder River Pass dykesat c 2164 Ma suggesting that both swarms intrudedduring the same event while Superior and Wyom-ing were connected However it is unknown howmany other dykes in Wyoming belong to the slightlyolder 2164 Ma group limiting a palaeomagneticcomparison

We reconstructed Wyoming against Superior byplacing the Huronian and Snowy Pass Supergroupsin close proximity according to the correlations ofRoscoe amp Card (1993) and then rotated Wyo-ming counter-clockwise until the Rabbit Creekpole intersected with the Superior apparent polarwander path (Fig 12) The structurally correctedpalaeomagnetic pole of the Rabbit CreekndashPowderRiverndashSouth Pass dykes (6558N 33928E a95 frac14768) reconstructs very well to the correct age rangebetween the Biscotasing and Marathon swarmpoles from Superior with enough leeway to recon-struct Wyoming anywhere along southwesternSuperior Unfortunately with only one palaeomag-netic pole for comparison Wyomingrsquos relative pos-ition cannot be pinpointed (it is only possible toknow its palaeolatitude) and could instead lie any-where along the dashed grey small circle in Figure12 Nevertheless it is remarkable that the

palaeomagnetic data allow a precise connectionbetween two Palaeoproterozoic sedimentary se-quences predicted as proximally related to an earlyProterozoic aulacogen (Karlstrom et al 1983) asshown in our reconstruction

The two dykes BH14 (202b) and BH24 (207b)are quite distinct geochemically but they are bothclosely related in age 21559 + 29 and 21577 +36 Ma and yield similar irregular palaeomagneticdirections These anomalous magnetizations areexcluded in calculating a palaeomagnetic pole butwhen reconstructed using the fit provided hereinthey fall beyond the Biscotasing pole from theSuperior craton (Fig 12) These unexpected direc-tions are probably due to a short-lived magneticexcursion caused by relatively stronger non-dipolecomponents (ie quadrupole and octupole) pos-sibly owing to a drop in intensity of the geocentricaxial dipole (Roberts 2008) This could be evalu-ated by conducting palaeointensity experiments tosee if the geomagnetic field was weaker duringthe intrusion of BH14 and BH24 It is unlikely thatthis excursion can be explained by a true polarwander event such as those known to occur laterin this era (Mitchell et al 2010) because rateswould probably exceed the maximum allowedeven for fast rate estimates for true polar wander(Tsai amp Stevenson 2007)

Wyoming and Superior are only two Archaeanfragments of many so it is difficult to make aconclusive case for the existence of either super-continent Kenorland (Williams et al 1991) or super-craton Superia (Bleeker 2003) based on this onereconstruction the SuperiorndashWyoming landmasscould have been part of either However the con-figuration presented in this study fits the model forsupercraton Superia described by Ernst amp Bleeker(2010) which includes the correlations of theKarelia Kola and Hearne cratons (Bleeker amp Ernst2006) based upon coeval magmatic events atc 2210 2110 and 2000 Ma The reconstruction cal-culated in this study refines the possible fit ofWyoming to southern Superior (Euler pole 488N2658E +1258 CCW) juxtaposing the Snowy Passand Huronian Supergroups along with a number ofmafic magmatic events temporally close to theRabbit Creek swarm Magmatism was concentratedin southern Superior for 100 myr from the c 2170Biscotasing dykes until the c 2070 Ma FortFrances dykes Magmatism in Wyoming occursover nearly the same interval from 2165 to2010 Ma but no ages in southern Superior matchwith the Rabbit Creek swarm One of the manydykes that remain undated in southern Superiormay provide a link to Rabbit Creek magmatism inWyoming

A failed rift basin that formed between Superiorand Wyoming during the Matachewan event

T M KILIAN ET AL38


(Heaman 1997) could have persisted through thePalaeoproterozoic finally becoming a successfulrift sometime between 2100 and 2000 Ma Strati-graphic correlations (Bekker et al 2005) matchingsoutheastern Wyoming to southern Superior andother possible cratons support this prospectKarelia has multiple large mafic events with broadage ranges that fit well with WyomingSuperiorat c 2000 2110 and 2210 Ma (Vuollo amp Huhma

2005) leading some to reconstruct it next to bothWyoming and Superior during this time (Bleeker2003 Ernst amp Bleeker 2010) The Hearne cratoncontains the Griffin gabbros at c 2110 Ma (Heamanamp LeCheminant 1993 Aspler et al 2002) a mag-matic age found on all cratons mentioned Thiscould be an indication that Wyoming SuperiorKarelia and Hearne were all part of the same largerifting event maybe even involving Nain where a

Fig 12 Reconstruction of the Wyoming craton to the Superior craton at c 2160 Ma Palaeomagnetic poles for Superiorare filled with blue and Wyoming and its poles are coloured red The Rabbit CreekndashPowder RiverndashSouth Pass pole isdisplayed with two other poles (opaque red ellipses) calculated for Wyoming one using solely the Rabbit Creek dataanother excluding South Pass dyke data The eastern Superior craton has present day coordinates with western Superiorrotated accounting for late Palaeoproterozoic Kapuskasing Zone deformation (Bates amp Halls 1991 Evans amp Halls2010) The applied Euler rotation (Wyoming to eastern Superior 488N 2658E 1258CCW) brings the 2171 to 2152 Mapole in between data from the Biscotasing and Marathon swarms along the Superior apparent polar wander path (dashedblue curve) which extends back in time towards the c 2215 Ma Nipissing palaeomagnetic pole (not visible in figure)There is complete freedom for Wyoming to rotate in palaeolongitude (ie in space about the Rabbit Creek pole) withpossible locations defining the grey small circle centred on the Rabbit Creek pole but the preferred reconstruction alsoaligns the Snowy Pass and Huronian Supergroups (basins shown as connected in light blue after Roscoe amp Card(1993)) There is also enough flexibility in the palaeomagnetic fit to move Wyoming slightly to the east whilemaintaining the stratigraphic correlation The two magnetic excursion results (BH14 and BH24) are shown in the bottomright (open red ellipses) falling earlier along Superiorrsquos apparent polar wander path The inset shows a magnified viewof the SuperiorndashWyoming boundary in this proposed fit Relevant dyke swarms are in various colours (red RabbitCreek swarm yellow Biscotasing swarm green Marathon swarm and the Bear Mountain dyke of Wyoming (Bowersamp Chamberlain 2006) purple Fort Frances swarm and metamorphosed intrusions of Wyoming (Mueller et al 2004)and orange Powder RiverndashSouth Pass dykes) These dyke swarms all fall along a possible long-lived rift zone withcolours of dyke swarms corresponding to the outlines of palaeomagnetic poles West Superior and its palaeomagneticdata have been rotated according to Evans amp Halls (2010)



diabase dyke has an age of 2121 + 15 Ma (Hamil-ton et al 1998)

Conclusion

The Rabbit Creek swarm provides new palaeo-magnetic and geochemical data during a key inter-val in Earth history The WyomingndashSuperior linkis possible if not probable additional palaeomag-netic results from already dated mafic dykeswarms will be useful to test synchronous motionof the WyomingndashSuperior (Superia) landmassThe Rabbit Creek event has no direct age matcheswith other blocks but it occurs during an intervalof frequent and voluminous magmatism overmuch of the southern Superior craton In our recon-struction of Wyoming (Fig 12) the Rabbit Creekdykes do not point directly north towards themargin of Superior and are therefore not expectedto continue into the adjacent craton However theRabbit Creek dykes do point towards a hypothesizedplume or rift region (Halls et al 2008) on the marginof Superior that occurs soon afterwards defined bythe radiating pattern of the Marathon and FortFrances dykes (2125 and 2070 Ma) Our fit pointsthe Bear Mountain dyke (c 2110 Ma) in Wyomingtowards the Marathon dykes in Superior and directlyjuxtaposes the Snowy Pass and Huronian Super-groups as part of a single basin possibly an aulaco-gen from early Palaeoproterozoic rifting thatwas reactivated during the late Palaeoproterozoic(Karlstrom et al 1983) The Powder RiverndashSouthPass dykes overlap in age with the Biscotasingswarm but they do not line up along-strike in ourreconstruction (Fig 12) This could be the resultof the stress field that existed during the intrusionof these dykes defining the orientations at whichfractures formed and propagated Stress fields arebroadly consistent on continental scales but cancurve significantly near and across major tec-tonic structures accounting for differences in trendbetween dyke swarms on separate yet adjacentcratons

The Powder RiverndashSouth Pass dykes could alsobe related to coincidental magmatism on anotheradjacent block or perhaps to another centre of mag-matism on SW Superior related to the Biscotasingdykes (Dahl et al 2006) The latter is supportedby a UndashPb age from the NNE-trending MargotLake dyke in central Superior (Fig 12) at21746 + 32 Ma (Hamilton amp Stott 2008) whichhas a significantly different trend from the Biscotas-ing dykes even after correcting for late Palaeopro-terozoic rotation between east and west Superior(Evans amp Halls 2010) Like the Rabbit Creekdyke magnetization the Margot Lake dyke is of pur-portedly lsquoreversedrsquo magnetic polarity (Hamilton amp

Stott 2008) whereas the rest of the Biscotasingswarm is of lsquonormalrsquo polarity (lower hemispheredirections) This array of c 2170 Ma dyke trendsproduces a piercing point on the southern marginof Superior far different from another possible pier-cing point in northwestern Superior defined by theBiscotasing dykes Labrador trough magmatism(Rohon et al 1993) and Payne River dykes (Ernstamp Bleeker 2010)

Previously it was unknown whether thrustingduring the Laramide Orogeny resulted in vertical-axis rotations of regional-scale basement blocksmaking a craton-wide reference frame intractableOur results suggest that the cores of Precambrianuplifts are buttressed by the craton in the subsur-face requiring only small tilt corrections Rotatedbasement blocks on the margins of uplifts (eg theSouth Pass region) are more susceptible to verti-cal and horizontal rotations but could be recovera-ble after structural restorations (Fig 7) Though amore comprehensive statistical comparison is nec-essary between exposures of the Wyoming cratonby establishing the first primary palaeomagneticpole for this time interval our results create arobust Proterozoic reference frame for the core ofthe Wyoming craton in the Bighorn uplift In addi-tion we put forth strong evidence for the lsquoGreatDyke of Wyomingrsquo which spans almost half ofthe Wyoming craton

Financial support for part of this work was providedby NSF grants EAR-1019739 (palaeomagnetic workat Yale) and collaborative NSF grant EAR-1019595(geochronological work at University of Wyoming)This is publication no 40 of the Large Igneous Provincesndash Supercontinent Reconstruction IndustryndashGovernmentndashAcademia Consortium Project (wwwsupercontinentorgNSERC CRDPJ 419503-11 CAMIRO Project 08E03)We thank R Mitchell R Ernst J Panzik and A LeChemi-nant for insightful discussions and assistance with field-work and also H Halls and an anonymous reviewer forhelping to greatly improve this manuscript

References

Ansdell K M amp Heaman L M et al 2005 Corre-lation chart of the evolution of the trans-HudsonorogenndashManitobandashSaskatchewan segment CanadianJournal of Earth Sciences 42 761 httpdoiorg101139e05-004

Aspler L B amp Chiarenzelli J R 1998 Two Neoarch-ean supercontinents Evidence from the Paleoprotero-zoic Sedimentary Geology 120 75ndash104

Aspler L B Cousens B L amp Chiarenzelli J R2002 Griffin gabbro sills (211 Ga) Hurwitz BasinNunavut Canada long-distance lateral transport ofmagmas in western Churchill Province crust Precam-brian Research 117 269ndash294 httpdoiorg101016S0301-9268(02)00090-6

T M KILIAN ET AL40


Barnhart K R Mahan K H Blackburn T JBowring S A amp Dudas F O 2012 Deep crustalxenoliths from central Montana USA implicationsfor the timing and mechanisms of high-velocitylower crust formation Geosphere 8 1408ndash1428httpdoiorg101130ges007651

Bates M P amp Halls H C 1991 Broad-scale Protero-zoic deformation of the central Superior Provincerevealed by paleomagnetism of the 245 Ga Matache-wan dyke swarm Canadian Journal of Earth Sciences28 1780ndash1796 httpdoiorg101139e91-159

Bekker A amp Eriksson K A 2003 A Paleoproterozoicdrowned carbonate platform on the southeasternmargin of the Wyoming Craton a record of the Kenor-land breakup Precambrian Research 120 327ndash364httpdoiorg101016s0301-9268(02)00165-1

Bekker A Kaufman A J Karhu J A amp ErikssonK A 2005 Evidence for Paleoproterozoic cap carbon-ates in North America Precambrian Research 137167ndash206 httpdoiorg101016jprecamres200503009

Blackwelder E 1926 Pre-Cambrian geology of theMedicine Bow Mountains Geological Society ofAmerica Bulletin 37 615ndash658

Bleeker W 2003 The late Archean record a puzzle inc 35 pieces Lithos 71 99ndash134

Bleeker W amp Ernst R 2006 Short-lived mantle gen-erated magmatic events and their dyke swarmsthe key to unlocking Earthrsquos paleogeographic recordback to 26 Ga In Hanski E Mertanen SRamo T amp Vuollo J (eds) Dyke SwarmsmdashTimeMarkers of Crustal Evolution Taylor amp FrancisLondon 3ndash26

Bleeker W amp Hall B 2007 The Slave Craton geologyand metallogenic evolution In Goodfellow W D(ed) Mineral Deposits of Canada A Synthesis ofMajor Deposit-Types District Metallogeny the Evol-ution of Geological Provinces and ExplorationMethods Geological Association of Canada MineralDeposits Division Special Publications 5 849ndash879

Bowers N E amp Chamberlain K R 2006 Precambrianhistory of the eastern Ferris mountains and Bear moun-tain south-central Wyoming Province CanadianJournal of Earth Sciences 43 1467ndash1487

Brady J B Mohlman H K Harris C CarmichaelS K Jacob L J amp Chaparro W R 2004 GeneralGeology and Geochemistry of Metamorphosed Proter-ozoic Mafic Dikes and Sills Tobacco Root MountainsMontana Geological Society of America BoulderCO Special Papers 377 89ndash104 httpdoiorg1011300-8137-2377-989

Brown W G 1993 Structural style of Laramidebasement-cored uplifts and associated folds Geologyof Wyoming Geological Survey of Wyoming Memoir5 312ndash371

Buchan K L 1991 Baked contact test demonstratesprimary nature of dominant (N1) magnetisation ofNipissing intrusions in Southern Province CanadianShield Earth and Planetary Science Letters 105492ndash499

Buchan K L Mortensen J K amp Card K D 1993Northeast-trending Early Proterozoic dykes of south-ern Superior Province multiple episodes of emplace-ment recognized from integrated paleomagnetism

and UndashPb geochronology Canadian Journal ofEarth Sciences 30 1286ndash1296

Buchan K L Halls H C amp Mortensen J K 1996Paleomagnetism UndashPb geochronology and geochem-istry of Marathon dykes Superior Province and com-parison with the Fort Frances swarm CanadianJournal of Earth Sciences 33 1583ndash1595

Buchan K L Mortensen J K Card K D amp Perci-

val J A 1998 Paleomagnetism and UndashPb geochro-nology of diabase dyke swarms of Minto blockSuperior Province Quebec Canada CanadianJournal of Earth Sciences 35 1054ndash1069

Buchan K L Mertanen S Park R G PesonenL J Elming SA Abrahamsen N amp BylundG 2000 Comparing the drift of Laurentia andBaltica in the Proterozoic the importance of keypalaeomagnetic poles Tectonophysics 319 167ndash198

Buchan K L Goutier J Hamilton M A ErnstR E amp Matthews W A 2007 PaleomagnetismUndashPb geochronology and geochemistry of Lac Espritand other dyke swarms James Bay area Quebec andimplications for Paleoproterozoic deformation of theSuperior Province Canadian Journal of EarthSciences 44 643ndash664

Chamberlain K R amp Mueller P A 2007 Chapter 63oldest rocks of the Wyoming craton In Martin Jvan Kranendonk R H S amp Vickie C B (eds)Developments in Precambrian Geology ElsevierOxford 775ndash791

Chamberlain K R Frost C D amp Frost B R 2003Early Archean to mesoproterozoic evolution of theWyoming province Archean origins to modern litho-spheric architecture Canadian Journal of EarthSciences 40 1357ndash1374

Cogne J P 2003 PaleoMac a MacintoshTM applicationfor treating paleomagnetic data and making platereconstructions Geochemistry Geophysics Geosys-tems 4 1007ndash1014 httpdoiorg1010292001GC000227

Collinson D W amp Runcorn S K 1960 Polar wan-dering and continental drift evidence from paleomag-netic observations in the United States GeologicalSociety of America Bulletin 71 915ndash958 httpdoiorg1011300016-7606(1960)71[915pwacde]20co2

Condie K C Barsky C K amp Mueller P A 1969Geochemistry of precambrian diabase dikes fromwyoming Geochimica et Cosmochimica Acta 331371ndash1388

Connelly J N 2001 Constraining the timing of meta-morphism UndashPb and SmndashNd Ages from a transectacross the Northern Torngat Orogen LabradorCanada The Journal of Geology 109 57ndash77 httpdoiorg101086317965

Cousens B L 2000 Geochemistry of the Archean KamGroup Yellowknife Greenstone Belt Slave ProvinceCanada Journal of Geology 108 181

Cox D M Frost C D amp Chamberlain K R 2000201-Ga Kennedy dike swarm southeastern WyomingRecord of a rifted margin along the southern Wyomingprovince Rocky Mountain Geology 35 7ndash30 httpdoiorg1021133517

Crowley P D Reiners P W Reuter J M amp KayeG D 2002 Laramide exhumation of the Bighorn



Mountains Wyoming An apatite (U-Th)He ther-mochronology study Geology 30 27ndash30 httpdoiorg1011300091-7613(2002)0300027leotbm

20co2Dahl P S Holm D K Gardner E T Hubacher

F A amp Foland K A 1999 New constraints on thetiming of Early Proterozoic tectonism in the BlackHills (South Dakota) with implications for dockingof the Wyoming province with Laurentia GeologicalSociety of America Bulletin 111 1335ndash1349

Dahl P S Hamilton M A Wooden J L FolandK A Frei R McCombs J A amp Holm D K2006 2480 Ma mafic magmatism in the northernBlack Hills South Dakota a new link connecting theWyoming and Superior cratons Canadian Journal ofEarth Sciences 43 1579ndash1600

Delaney P T 1987 Heat Transfer during Emplacementand Cooling of Mafic Dykes Geological Association ofCanada St Johnrsquos Newfoundland Special Papers 3431ndash46

Denson N M amp Pipiringos G N 1974 Geologicmap amp sections showing areal distribution of Ter-tiary rocks near the southeastern terminus of theWind River Range Fremont amp Sweetwater countiesWyoming USGS Miscellaneous Investigations SeriesMap I-835

Ernst R amp Bleeker W 2010 Large igneous provinces(LIPs) giant dyke swarms and mantle plumes signifi-cance for breakup events within Canada and adjacentregions from 25 Ga to the Present Canadian Journalof Earth Sciences 47 695ndash739

Evans D A D amp Halls H C 2010 Restoring protero-zoic deformation within the superior craton Precam-brian Research 183 474ndash489

Everitt C W F amp Clegg J A 1962 A field test ofpalaeomagnetic stability Geophysical Journal Inter-national 6 312ndash319 httpdoiorg101111j1365-246X1962tb00354x

Fan M amp Carrapa B 2014 Late Cretaceous-earlyEocene Laramide uplift exhumation and basin subsi-dence in Wyoming crustal responses to flat slab sub-duction Tectonics 33 2012TC003221 httpdoiorg1010022012TC003221

Foster D A Mueller P A Mogk D W WoodenJ L amp Vogl J J 2006 Proterozoic evolution of thewestern margin of the Wyoming craton implicationsfor the tectonic and magmatic evolution of the northernRocky Mountains Canadian Journal of EarthSciences 43 1601ndash1619

Frost C D amp Fanning C M 2006 Archean geo-chronological framework of the Bighorn MountainsWyoming Canadian Journal of Earth Sciences 431399ndash1418

Frost C D Frost B R Chamberlain K R amp Hul-

sebosch T P 1998 The Late Archean history of theWyoming province as recorded by granitic magmatismin the Wind River Range Wyoming PrecambrianResearch 89 145ndash173

Frost C D Frost B R Kirkwood R amp Chamber-

lain K R 2006a The tonalitendashtrondhjemitendashgrano-diorite (TTG) to granodioritendashgranite (GG) transitionin the late Archean plutonic rocks of the centralWyoming Province Canadian Journal of EarthSciences 43 1419ndash1444

Frost C D Fruchey B L Chamberlain K R ampFrost B R 2006b Archean crustal growth bylateral accretion of juvenile supracrustal belts in thesouth-central Wyoming Province Canadian Journalof Earth Sciences 43 1533ndash1555 httpdoiorg101139e06-092

Gay S P 2003 The lsquoGreat Dike of Wyomingrsquo and satel-lite bodies a comparison to the Great Dyke of Rhode-siaZimbabwe In Horn M S (ed) 2002 FieldConference lsquoWyoming Basinsrsquo and 2003 Field Confer-ence Wyoming Geological Association Casper WY101ndash111

Grace R L B Chamberlain K R Frost B R ampFrost C D 2006 Tectonic histories of the Paleo- toMesoarchean Sacawee block and Neoarchean OregonTrail structural belt of the south-central Wyoming Pro-vince Canadian Journal of Earth Sciences 431445ndash1466

Graff P 1979 A review of the stratigraphy and uraniumpotential of early Proterozoic (Precambrian X) metase-diments in the Sierra Madre Wyoming Rocky Moun-tain Geology 17 149ndash157

Halgedahl S L amp Jarrard R D 1995 Low-temperature behavior of single-domain through multi-domain magnetite Earth and Planetary ScienceLetters 130 127ndash139

Halls H C 1986 Paleomagnetism structure and longi-tudinal correlation of Middle Precambrian dykes fromnorthwestern Ontario and Minnesota CanadianJournal of Earth Sciences 23 142ndash157 httpdoiorg101139e86-018

Halls H C 1991 The Matachewan dyke swarmCanada an early Proterozoic magnetic field reversalEarth and Planetary Science Letters 105 279ndash292httpdoiorg1010160012-821X(91)90137-7

Halls H C amp Davis D W 2004 Paleomagnetism andUndashPb geochronology of the 217 Ga Biscotasingdyke swarm Ontario Canada evidence for vertical-axis crustal rotation across the Kapuskasing ZoneCanadian Journal of Earth Sciences 41 255ndash269

Halls H C amp Zhang B 2003 Crustal uplift in thesouthern Superior Province Canada revealed by paleo-magnetism Tectonophysics 362 123ndash136 httpdoiorg101016S0040-1951(02)00634-0

Halls H C Stott G M amp Davis D W 2005 Paleo-magnetism Geochronology and Geochemistry ofSeveral Proterozoic Mafic Dike Swarms in Northwes-tern Ontario Ontario Geological Survey Open FileReport 6171

Halls H C Davis D W Stott G M Ernst R E ampHamilton M A 2008 The Paleoproterozoic Mara-thon Large Igneous Province New evidence for a21 Ga long-lived mantle plume event along thesouthern margin of the North American Superior Pro-vince Precambrian Research 162 327ndash353

Hamilton M A amp Stott G M 2008 The significanceof new UPb baddeleyite ages from two Paleoprotero-zoic diabase dikes in Northern Ontario OntarioGeological Survey Open File Report 6226 17-11ndash17-10

Hamilton M A Ryan A B Emslie R F amp Ermano-

vics I F 1998 Identification of Paleoproterozoicanorthositic and monzonitic rocks in the vicinity ofthe Mesoproterozoic Nain Plutonic Suite Labrador

T M KILIAN ET AL42


UndashPb evidence Geological Survey of CanadaCurrent Research 1998-F 23ndash40

Hamilton M A Davis D W Buchan K L amp HallsH C 2002 Precise UndashPb Dating of ReverselyMagnetized Marathon Diabase Dykes and Impli-cations for Emplacement of Giant Dyke Swarmsalong the Southern Margin of the Superior ProvinceOntario Report 15 Current Research 2002-F6 Radio-genic age and Isotopic Studies Geological Survey ofCanada

Hark J S Frei R Whitehouse M J amp Dahl P S2008 New evidence for 201 Ga rifting of the eastern-most Wyoming craton (Black Hills South Dakota)Implications for break-up of a supercraton (Superia)Geological Society of America Abstracts with Pro-grams 40 145

Harlan S S Geissman J W amp Premo W R2003 Paleomagnetism and geochronology of anEarly Proterozoic quartz diorite in the southern windriver range Wyoming USA Tectonophysics 362105ndash122

Harms T A Brady J B Burger H R amp CheneyJ T 2004 Advances in the Geology of the TobaccoRoot Mountains Montana and their Implications forthe History of the Northern Wyoming Province Geo-logical Society of America Boulder CO SpecialPapers 377 227ndash243 httpdoiorg1011300-8137-2377-9227

Hausel W D 1988 Geologic map of the Radium SpringsQuadrangle including the Lewiston gold districtFremont County Wyoming Wyoming State Geologi-cal Survey Map Series 26

Hausel W D 1991 Economic geology of the South PassGranitendashGreenstone Belt southern Wind River RangeWestern Wyoming Geological Survey of WyomingReport of Investigations 129

Heaman L M 1997 Global mafic magmatism at245 Ga remnants of an ancient large igneous pro-vince Geology 25 299ndash302

Heaman L M amp LeCheminant A N 1993 Paragen-esis and UndashPb systematics of baddeleyite (ZrO2)Chemical Geology 110 95ndash126 httpdoiorg1010160009-2541(93)90249-I

Heimlich R A amp Armstrong R L 1972 Variance ofPrecambrian KndashAr biotite dates Bighorn MountainsWyoming Earth and Planetary Science Letters14 75ndash78 httpdoiorg1010160012-821X(72)90083-0

Heimlich R A Nelson G C amp Gallagher G L1973 Metamorphosed mafic dikes from the southernBighorn Mountains Wyoming Geological Society ofAmerica Bulletin 84 1439ndash1450

Hinrichs E N Kent B H amp Pierce F W 1990Bedrock geologic map and coal sections in the Buffal30prime times 60prime Quadrangle Johnson and Campbell coun-ties Wyoming USGS Miscellaneous InvestigationsSeries I-1923-A

Hoffman P F 1988 United Plates of America the birthof a craton Early Proterozoic assembly and growth ofLaurentia Annual Reviews in Earth and PlanetaryScience 16 543ndash603

Hoffman P F 1997 Tectonic genealogy of NorthAmerica In Van der Pluijm B A amp Marshak S(eds) Earth Structure An Introduction to Structural

Geology and Tectonics McGraw-Hill New York459ndash464

Hoppin R A Palmquist J C amp Williams L O 1965Control by Precambrian basement structure on thelocation of the Tensleep-Beaver Creek fault BighornMountains Wyoming The Journal of Geology 73189ndash195 httpdoiorg10230730066390

Houston R S 1993 Late Archean and early Protero-zoic geology of southeastern Wyoming Geology ofWyoming Geological Survey of Wyoming Memoir 578ndash116

Hoy R G amp Ridgway K D 1997 Structural and sedi-mentological development of footwall growth syn-clines along an intraforeland uplift east-centralBighorn Mountains Wyoming Geological Society ofAmerica Bulletin 109 915ndash935 httpdoiorg1011300016-7606(1997)1090915sasdof23co2

Jones C H 2002 User-driven integrated software liveslsquoPaleomagrsquo paleomagnetics analysis on the MacintoshComputers and Geosciences 28 1145ndash1151

Kamber B S Collerson K D Moorbath S ampWhitehouse M J 2003 Inheritance of earlyArchaean Pb-isotope variability from long-livedHadean protocrust Contributions to Mineralogy andPetrology 145 25ndash46 httpdoiorg101007s00410-002-0429-7

Karlstrom K E Flurkey A J amp Houston R S1983 Stratigraphy and depositional setting of the Pro-terozoic Snowy Pass Supergroup southeasternWyoming record of an early Proterozoic Atlantic-typecratonic margin Geological Society of America Bulle-tin 94 1257ndash1274

Keir D Pagli C Bastow I D amp Ayele A 2011 Themagma-assisted removal of Arabia in Afar evidencefrom dike injection in the Ethiopian rift capturedusing InSAR and seismicity Tectonics 30 TC2008httpdoiorg1010292010TC002785

Kirschvink J L 1980 The least-squares line and planeand the analysis of palaeomagnetic data GeophysicalJournal International 62 699ndash718 httpdoiorg101111j1365-246X1980tb02601x

Kirschvink J L Kopp R E Raub T D Baumgart-

ner C T amp Holt J W 2008 Rapid precise andhigh-sensitivity acquisition of paleomagnetic androck-magnetic data development of a low-noise auto-matic sample changing system for superconduct-ing rock magnetometers Geochemistry GeophysicsGeosystems 9 Q05Y01 httpdoiorg1010292007gc001856

Krogh T E 1973 A low-contamination method forhydrothermal decomposition of zircon and extractionof U and Pb for isotopic age determinations Geochi-mica et Cosmochimica Acta 37 485ndash494 httpdoiorg1010160016-7037(73)90213-5

Love J D amp Christiansen A C 1985 Geologic map ofWyoming US Geological Survey Reston VA

Luais B amp Hawkesworth C J 2002 Pb isotope vari-ations in Archaean time and possible links to thesources of certain MesozoicndashRecent basalts InFowler C M R Ebinger C J amp HawkesworthC J (eds) The Early Earth Physical Chemicaland Biological Development Geological SocietyLondon Special Publications 199 105ndash124 httpdoiorg101144gslsp20021990106



Ludwig K R 1988 PBDAT for MS-DOS a computerprogram for IBM-PC compatibles for processing rawPbndashUndashTh isotope data version 124 US GeologicalSurvey Open-File Report 88-542

Ludwig K R 1991 ISOPLOT for MS-DOS a plottingand regression program for radiogenic-isotope datafor IBM-PC compatible computers version 275 USGeological Survey Open-File Report 91-445

Maurice C David J OrsquoNeil J amp Francis D 2009Age and tectonic implications of Paleoproterozoicmafic dyke swarms for the origin of 22 Ga enrichedlithosphere beneath the Ungava Peninsula CanadaPrecambrian Research 174 163ndash180

Mitchell R N Hoffman P F amp Evans D A D2010 Coronation loop resurrected Oscillatory appar-ent polar wander of Orosirian (205ndash18 Ga) paleo-magnetic poles from Slave craton PrecambrianResearch 179 121ndash134

Mogk D W Mueller P A amp Wooden J L 1988Archean Tectonics of the North Snowy Block Bear-tooth Mountains Montana The Journal of Geology96 125ndash141

Mogk D W Mueller P A amp Wooden J L 1992 Thenature of Archean terrane boundaries an examplefrom the northern Wyoming Province PrecambrianResearch 55 155ndash168 httpdoiorg1010160301-9268(92)90020-o

Mueller P A amp Frost C D 2006 The Wyoming Pro-vince a distinctive Archean craton in Laurentian NorthAmerica Canadian Journal of Earth Sciences 431391ndash1397 httpdoiorg101139e06-075

Mueller P A Wooden J L Mogk D W NutmanA P amp Williams I S 1996 Extended history of a35 Ga trondhjemitic gneiss Wyoming ProvinceUSA evidence from UndashPb systematics in zircon Pre-cambrian Research 78 41ndash52 httpdoiorg1010160301-9268(95)00067-4

Mueller P A Heatherington A L Kelly D MWooden J L amp Mogk D W 2002 Paleoproterozoiccrust within the Great Falls tectonic zone implicationsfor the assembly of southern Laurentia Geology 30127ndash130 httpdoiorg1011300091-7613(2002)0300127pcwtgf20co2

Mueller P A Burger H R Wooden J LHeatherington A L Mogk D W amp DrsquoArcyK 2004 Age and Evolution of the PrecambrianCrust of the Tobacco Root Mountains Montana Geo-logical Society of America Boulder CO SpecialPapers 377 181ndash202 httpdoiorg1011300-8137-2377-9181

Mueller P A amp Burger H R et al 2005 Paleopro-terozoic metamorphism in the Northern Wyoming Pro-vince implications for the assembly of Laurentia TheJournal of Geology 113 169ndash179 httpdoiorg101086427667

Muxworthy A R amp Williams W 2006 Low-temperature cooling behavior of single-domain magne-tite forcing of the crystallographic axes and inter-actions Journal of Geophysical Research SolidEarth 111 B07103 httpdoiorg1010292006JB004298

Noble S R amp Lightfoot P C 1992 UndashPb baddeleyiteages of the Kerns and Triangle Mountain intrusionsNipissing Diabase Ontario Canadian Journal of

Earth Sciences 29 1424ndash1429 httpdoiorg101139e92-114

Osterwald F W 1978 Structure and petrology of thenorthern Big Horn Mountains Wyoming The Geo-logical Survey of Wyoming Bulletin 48 1ndash47

Parrish R R Roddick J C Loveridge W D amp Sul-

livan R D 1987 Uraniumndashlead analytical tech-niques at the Geochronology Laboratory GeologicalSurvey of Canada Radiogenic age and isotopicstudies Report 1 Geological Survey of CanadaPaper 87-2

Peng P Zhai M Zhang H amp Guo J 2005 Geochro-nological constraints on the Paleoproterozoic evolutionof the North China Craton SHRIMP Zircon ages ofdifferent types of Mafic Dikes International GeologyReview 47 492ndash508 httpdoiorg1027470020-6814475492

Premo W R amp Van Schmus W R 1989 Zircon Geo-chronology of Precambrian Rocks in SoutheasternWyoming and Northern Colorado GeologicalSociety of America Boulder CO Special Papers235 13ndash32 httpdoiorg101130SPE235-p13

Redden J A Peterman Z E Zartman R E ampDeWitt E 1990 UndashThndashPb geochronology and pre-liminary interpretation of Precambrian tectonic eventsin the Black Hills South Dakota The Trans-Hudsonorogen Geological Association of Canada SpecialPaper 37 229ndash251

Rioux M Bowring S Dudas F amp Hanson R 2010Characterizing the UndashPb systematics of baddeleyitethrough chemical abrasion application of multi-step digestion methods to baddeleyite geochrono-logy Contributions to Mineralogy and Petrology160 777ndash801 httpdoiorg101007s00410-010-0507-1

Roberts A P 2008 Geomagnetic excursions knownsand unknowns Geophysical Research Letters 35L17307 httpdoiorg1010292008GL034719

Rogers J J W amp Santosh M 2002 Configuration ofColumbia a Mesoproterozoic supercontinent Gond-wana Research 5 5ndash22

Rohon M L Vialette Y Clark T Roger GOhnenstetter D amp Vidal P 1993 Aphebianmaficndashultramafic magmatism in the Labrador Trough(New Quebec) its age and the nature of its mantlesource Canadian Journal of Earth Sciences 301582ndash1593 httpdoiorg101139e93-136

Roscoe S M 1969 Huronian rocks and uraniferous con-glomerates in the Canadian shield Geological Surveyof Canada Ottawa Papers 68ndash40 205 httpdoiorg104095102290

Roscoe S M amp Card K D 1993 The reappearance ofthe Huronian in Wyoming rifting and drifting ofancient continents Canadian Journal of EarthSciences 30 2475ndash2480

Schwarz E J Buchan K L amp Cazavant A 1985Post-Aphebian uplift deduced from remanent magneti-zation Yellowknife area of Slave Province CanadianJournal of Earth Sciences 22 1793ndash1802 httpdoiorg101139e85-190

Selkin P A Gee J S Meurer W P amp HemmingS R 2008 Paleointensity record from the 27 Ga Still-water Complex Montana Geochemistry GeophysicsGeosystems 9 Q12023

T M KILIAN ET AL44


Sigurdsson H 1987 Dyke injection in Iceland a reviewIn Halls H C amp Fahrig W H (eds) Mafic DykeSwarms Geological Association of Canada StJohnrsquos Newfoundland Special Papers 34 55ndash64

Sims P K Finn C A amp Rystrom V L 2001 Prelimi-nary Precambrian basement map showing geologic ndashgeophysical domains Wyoming US GeologicalSurvey Open-File Report 01-199 9

Smithson S B Brewer J A Kaufman S OliverJ E amp Hurich C A 1979 Structure of the LaramideWind River uplift Wyoming from COCORP deepreflection data and from gravity data Journal of Geo-physical Research 84 5955ndash5972 httpdoiorg101029JB084iB11p05955

Smithson S B Brewer J A Kaufman S OliverJ E amp Zawislak R L 1980 Complex Archeanlower crustal structure revealed by COCORP crustalreflection profiling in the wind river rangeWyoming Earth and Planetary Science Letters 46295ndash305

Snyder G L Hughes D J Hall R P amp LudwigK R 1989 Distribution of Precambrian mafic intru-sions penetrating some Archean rocks of westernNorth America US Geological Survey Open-FileReport 89ndash125 36

Soderlund U amp Johansson L 2002 A simple way toextract baddeleyite (ZrO2) Geochemistry Geophy-sics Geosystems 3 1ndash7 httpdoiorg1010292001GC000212

Stacey J S amp Kramers J D 1975 Approximation of ter-restrial lead isotope evolution by a two-stage modelEarth and Planetary Science Letters 26 207ndash221httpdoiorg1010160012-821X(75)90088-6

Steiger R H amp Jager E 1977 Subcommission on geo-chronology convention on the use of decay constantsin geo- and cosmochronology Earth and PlanetaryScience Letters 36 359ndash362 httpdoiorg1010160012-821X(77)90060-7

St-Onge M R Searle M P amp Wodicka N 2006Trans-Hudson Orogen of North America and Hima-layandashKarakoramndashTibetan Orogen of Asia structuraland thermal characteristics of the lower and upperplates Tectonics 25 TC4006 httpdoiorg1010292005tc001907

Stueber A M Heimlich R A amp Ikramuddin M1976 RbndashSr ages of Precambrian mafic dikesBighorn Mountains Wyoming Geological Society of

America Bulletin 87 909ndash914 httpdoiorg1011300016-7606(1976)87909raopmd20co2

Sun S-s amp McDonough W F 1989 Chemical and iso-topic systematics of oceanic basalts implications formantle composition and processes In SaundersA D amp Norry A J (eds) Magmatism in OceanBasins Geological Society London Special Publi-cations 42 313ndash345 httpdoiorg101144gslsp19890420119

Thaden R E 1980 Geologic map of the De Pass quad-rangle Fremont and Hot Springs Counties WyomingUS Geological Survey Geological Quadrangle Map1526

Tsai V C amp Stevenson D J 2007 Theoretical con-straints on true polar wander Journal of GeophysicalResearch Solid Earth 112 B05415 httpdoiorg1010292005JB003923

Vuollo J amp Huhma H 2005 Paleoproterozoic maficdikes in NE Finland Developments in PrecambrianGeology 14 195ndash236

Wall E N 2004 Petrologic geochemical and isotopicconstraints on the origin of 26 Ga post-tectonic gran-itoids of the central Wyoming Province UnpublishedMS thesis University of Wyoming

Weil A B Yonkee A Statman-Weil Z amp Wicks D2009 Determining the 3-D kinematic History of theWyoming Laramide Foreland Preliminary Resultsfrom a Paleomagnetic Investigation of the TriassisChugwater Group Geological Society of AmericaAbstracts with Programs Portland OR 692

Williams H Hoffman P F Lewry J F MongerJ W H amp Rivers T 1991 Anatomy of NorthAmerica thematic geologic portrayals of the continentTectonophysics 187 117ndash134

Young G M 1970 An extensive early proterozoic gla-ciation in North America Palaeogeography Palaeo-climatology Palaeoecology 7 85ndash101 httpdoiorg1010160031-0182(70)90070-2

Young G M Long D G F Fedo C M amp NesbittH W 2001 Paleoproterozoic Huronian basinproduct of a Wilson cycle punctuated by glaciationsand a meteorite impact Sedimentary Geology 141ndash142 233ndash254

Zhao G Cawood P A Wilde S A amp Sun M 2002Review of global 21ndash18 Ga orogens implications fora pre-Rodinia supercontinent Earth-Science Reviews59 125ndash162



of rifting the orientations of the dykes often con-verge on a piercing point along the rifted margins(Ernst amp Bleeker 2010) The piercing point providesadditional insight into possible palaeogeographiccorrelations and also aids in the refinement ofpalaeomagnetically derived reconstructions

The Wyoming craton hosts a large number ofdyke swarms that occur in high densities throughoutalmost every region of exposed ArchaeanProtero-zoic rock (Snyder et al 1989) Although a hand-ful of magmatic events have been studied datingfrom the Neoarchaean Era through the Neoprotero-zoic Era it is difficult to establish conclusivelywhether or not these magmatic events can be corre-lated across the craton since the limited and spora-dic exposures of Precambrian crust reveal as littleas 10 of the whole craton Precambrian basementexposures resulted largely from differential upliftduring the Laramide Orogeny (80ndash55 Ma) thrust-ing large basement blocks up and past kilometresof Phanaerozoic sedimentary rocks This created ahandful of windows to view the craton (Love ampChristiansen 1985 Fan amp Carrapa 2014) hencethe Wyoming craton requires several importantand somewhat unique questions to be addressedbefore using mafic dyke swarms to test reconstruc-tions (1) Is the basement geology of the differentLaramide uplifts coherent enough to allow a craton-wide approach In other words can we correlatedyke events and palaeomagnetic data betweenuplifts in detail and correct for minor rotations orhas Laramide tectonics made the problem intract-able (2) Are enough dykes of each swarmexposed to average out secular variation of theEarthrsquos magnetic field and to provide a statisticallysignificant palaeomagnetic pole After satisfyingthose concerns to which Archaean blocks can wecompare our data with and which pieces may havebeen adjacent to Wyoming

The best approach in solving these questions is toestablish precise ages and primary palaeomagneticpoles for regional dyke swarms correlate thesebetween uplifts and apply minor inter-uplift struc-tural corrections where necessary thereby esta-blishing a detailed magmatic barcode and apparentpolar wander path for Wyoming craton as a wholeprior to its incorporation into Laurentia In this studywe also determined the whole-rock compositionof some of the dykes with two aims in mind to dis-tinguish each swarm geochemically and to identifygeochemical trends within swarms

Herein we present a palaeomagnetic pole forthe Rabbit Creek dyke swarm at 2152ndash2161 Mawhich includes independent age analyses fromthree different dykes exposed in the Bighorn Moun-tains We also compare results with those of Harlanet al (2003) refining the age of the large intrusion inthe Wind River Range and connecting it to a very

similar intrusion more than 200 km away in theBighorn Mountains These two magmatic eventsprobably represent individual pulses of a protractedinterval of magmatism along the margin of Wyo-ming craton (2170ndash2000 Ma) correlating wellwith mafic dyke swarms from southern Superiorand other cratons worldwide

Background geology

Wyoming in Laurentia

The Wyoming craton arrived at its current positionduring the amalgamation of Laurentia at c 1800ndash1700 Ma (eg St-Onge et al 2006) The dating ofthe sutures surrounding Wyoming suggests multipleepisodes of metamorphism making it more difficultto identify the exact order of collisions and whetherany pieces were contiguous with Wyoming priorto collision (Dahl et al 1999 Mueller et al 20022005) Most importantly all of the available agesfor the suture zones surrounding Wyoming areyounger than c 1900 Ma indicating that beforeincorporation into Laurentia Wyoming was eitherby itself or with other fragments that subsequentlyrifted away

Some attempts to recognize prior connectionsto Wyoming (before c 1800 Ma) have focusedon the Snowy Pass Supergroup passive marginsequence in the Medicine Bow Mountains of theSE Wyoming craton (Graff 1979 Bekker amp Eriks-son 2003) Many scientists have recognized simi-larities between the Snowy Pass Supergroup andthe Huronian Supergroup of the southern Superiorcraton (Roscoe 1969) especially the distinct glacio-genic strata found in both sections (Blackwelder1926) and their possible relationships to other gla-ciogenic sediments in North America (Young 1970Graff 1979 Houston 1993 Young et al 2001)Some workers have suggested that deposition couldhave taken place in separate locations on Earth(Karlstrom et al 1983) especially if the glacial epi-sodes were worldwide in nature Other studieshypothesized that with Wyoming and Superior intheir current configuration deposition could havetaken place along the same contiguous southernmargin of Laurentia (Houston 1993 Aspler amp Chiar-enzelli 1998) Roscoe amp Card (1993) qualitativelydefined a reconstruction of southeastern Wyo-ming against southern Superior placing palaeocur-rent data from both sequences into agreementwith stratigraphic correlations The intrusion of theNipissing diabase sills at c 2215 Ma into the Huro-nian Supergroup (Noble amp Lightfoot 1992) con-strains the age of the reconstructions which maycorrelate with metamorphosed intrusions withinthe Snowy Pass Supergroup in Wyoming Our

T M KILIAN ET AL16



Geology of Wyoming






Methodology






T M KILIAN ET AL18






Geochemistry









206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho








T

M

KIL

IAN

ET

AL

20

by AJS on M


Dow

nloaded from



SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

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KIL

IAN

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Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























Geology of Wyoming






Methodology






T M KILIAN ET AL18






Geochemistry









206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho








T

M

KIL

IAN

ET

AL

20

by AJS on M


Dow

nloaded from



SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

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Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























T M KILIAN ET AL18






Geochemistry









206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho








T

M

KIL

IAN

ET

AL

20

by AJS on M


Dow

nloaded from



SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44





























Geochemistry









206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho








T

M

KIL

IAN

ET

AL

20

by AJS on M


Dow

nloaded from



SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44




























206Pb208Pb

206Pb238U 207Pb235U 207Pb206Pb 206Pb238U

207Pb235U

207Pb206Pb

Rho








T

M

KIL

IAN

ET

AL

20

by AJS on M


Dow

nloaded from



SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44



























SiO2 4866 4894 4882 4864 4895 4869 4905 4857 4849 4816 4850 505 036TiO2 155 151 150 158 136 161 136 115 122 115 117 245 003Al2O3 1351 1506 1365 1344 1360 1305 1358 1495 1404 1537 1457 1364 009

Fe2O3T 1638 1526 1581 1633 1547 1666 1567 1370 1483 1342 1432 1341 005



(Continued)

WY

OM

ING

CR

AT

ON

R

AB

BIT

CR

EE

KD

YK

ES

21

by AJS on M


Dow

nloaded from

Table 2 Continued




T

M

KIL

IAN

ET

AL

22

by AJS on M


Dow

nloaded from


Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

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IAN

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Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44

























Table 2 Continued




T

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IAN

ET

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Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

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IAN

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Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























Palaeomagnetism








Sm(ppm)

Nd(ppm)

147Sm144Nd

143Nd144Ndm

2s 1Nd

pres

143Nd144Nd(T )

1Nd

(T)Tdm

(Ma)

BH13 425 1512 01698 0512312 0000014 2635 0509903 112 2757BH22 395 1394 01714 0512334 0000012 2592 0509892 112 2784BH24 248 798 01879 0512625 0000012 2025 0509954 220 2852BH58 241 774 01879 0512605 0000013 2065 0509938 181 2965






Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

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Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44



























Results






Geochemistry



T M KILIAN ET AL24





0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

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IAN

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Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44




























0382

0386

0390

0394

0398

705 715 725 735 745

2130

2140

2160


MSWD = 055

2150

BNB-09-WY-204 (T09BH15)


206 P

b23

8 U

207Pb235U

2146

2150

2154

2158

2162

2166



(error bars are 2 )

BNB-09-WY-204

207 P

b20

6 Pb

date

(Ma)

Fraction 2 6 4 7

0386

0388

0390

0392

0394

0396

712 716 720 724 728 732 736



2130

2140

2150

BNB-09-WY-202b (T09BH14)20

6 Pb

238 U

207Pb235U

(a) (b)

(c) (d)

(e) (f)



2135

2145

2155

2165

2175

BNB-09-WY-202b

207 P

b20

6 Pb

date

(Ma)

Fraction 1 2 3 5

0389

0391

0393

0395

0397

0399

0401

716 720 724 728 732 736 740 744 748

206P

b23

8U

207Pb235U

2140

2160


MSWD = 0117

BNB-09-WY-207b (T09BH24)

2150


2140

2150

2160

2170

2180



BNB-09-WY-207b

207 P

b20

6 Pb

date

(Ma)

Fraction 3 4 5 7





Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

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IAN

ET

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nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























Palaeomagnetism





035

036

037

038

039

040

041

63 65 67 69 71 73 75 77207Pb235U

2040

2120

2200




206 P

b23

8 U

(a)

031

033

035

037

039

041

56 60 64 68 72 76207Pb235U

1950

2050

2150


MSWD = 15



206 P

b23

8 U

(b)


T M KILIAN ET AL26














T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44





































T M KILIAN ET AL28









Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44
































Site latitude(N)

Site longitude(W)


Site ID Width(m)

Trend(deg)


Inc(deg)

a95

(deg)Plat(8N)

Plong(8E)

a95

(deg)n( p)N Dec

(deg)Inc

(deg)Plat(8N)

Plong(8E)

a95

(deg)



T

M

KIL

IAN

ET

AL

30

by AJS on M


Dow

nloaded from



Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44



























Discussion



0

90

180

270






0

90

180

270

(a)









T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























T M KILIAN ET AL32












T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44



































T M KILIAN ET AL34

















T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44








































T M KILIAN ET AL36











Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


































Plat(8N)

Plong(8E)

a95 N k




2171ndash2152 719 3388 109 13 155


2161ndash2152 720 3385 106 9 243




2171ndash2152 655 3392 76 13 305


2171ndash2152 652 3380 9 11 266


2161ndash2152 697 3375 87 9 36




Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44


























Palaeogeography








T M KILIAN ET AL38








Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44































Conclusion






References




T M KILIAN ET AL40






























































T M KILIAN ET AL42




























































T M KILIAN ET AL44





















































































T M KILIAN ET AL42




























































T M KILIAN ET AL44



















































































T M KILIAN ET AL44

























palaeomagnetism, geochronology and …...palaeomagnetism, geochronology and geochemistry of the...

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