geochemistry, petrogenesis and tectonic setting …

GEOCHEMISTRY, PETROGENESIS AND TECTONIC

SETTING OF IGNEOUS ROCKS OF THE HARTVILLE

UPLIFT, EASTERN WYOMING

A thesis

presented to

the Faculty of the Graduate School

at the University of Missouri-Columbia

In partial Fulfillment

of the Requirements for the Degree

Master of Science

by

ANTONIO MANJÓN-CABEZA CÓRDOBA

Dr. Peter I. Nabelek, Thesis Supervisor

MAY 2016

The undersigned, appointed by the dean of the Graduate School, have examined the

thesis entitled

GEOCHEMISTRY, PETROGENESIS AND TECTONIC SETTING OF IGNEOUS

ROCKS OF THE HARTVILLE UPLIFT, EASTERN WYOMING

presented by Antonio Manjón-Cabeza Córdoba ,

a candidate for the degree of master of science,

and hereby certify that, in their opinion, it is worthy of acceptance.

Professor Peter I. Nabelek

Professor Robert L. Bauer

Professor Mike D. Glascock

To those long video calls…

‘Así como don Quijote entró por aquellas montañas se le alegró el corazón, pareciéndole

aquellos lugares acomodados para las aventuras que buscaba.’

‘As Don Quixote entered between those mountains his heart was filled with happiness,

for he thought those places were suited for the adventures he was seeking.’

Don Quijote de la Mancha I, chapter 23.

- Miguel de Cervantes Saavedra -

ii

ACKNOWLEDGEMENTS

I would like to thank the people that made this work possible. First, my advisor,

Peter I. Nabelek, without whose assistance, teachings and comments I could not have

finished this Master’s thesis, and also for his patience and guidance in the field. The work

of Carol Nabelek with the ICP-OES (department of geosciences, University of Missouri)

and James Guthrie with the ICP-MS (University of Missouri, Research Reactor) where

crucial for the obtainment of the data and their contribution should not go unadvertised.

Last, thanks to Eric S. Nowariak, who not only did help with the preparation of the dilutions

for the analyses, but also spent valuable time with me in the field, in long discussions and

also as a friend.

I also would like to thank the Fulbright Comission of Spain, and the ‘fundación

REPSOL’ (REPSOL Foundation) of REPSOL S.A. for their finantial support. Without

their confidence and help, my studies at the University of Missouri that resulted in this

thesis would not have been possible.

Last, but not least, thanks to the kind landowners that allowed us to work on their

lands and that very kindly informed and assisted us.

iii

TABLE OF CONTENTS

AKNOWLEDGEMENTS …………………………………………………………

ii

LIST OF FIGURES ………………………………………………………………..

iv

ABSTRACT ……………………………………………………………………….

v

Section

1. INTRODUCTION …………………………………………………………

1

2. GEOLOGICAL CONTEXT ……………………………………………….

The Wyoming Province …………………………………………………..

The Cheyenne Belt ………………………………………………………..

The Trans-Hudson, Black Hills and Dakotan Orogens ………………..

The Hartville Uplift ………………………………………………………

6

6

7

8

9

3. IGNEOUS ROCKS OF THE HARTVILLE UPLIFT …………………….

Mafic Rocks ……………………………………………………………….

Granitoids …………………………………………………………………

12

12

17

4. ANALYTICAL METHODS ………………………………………………

27

5. RESULTS ………………………………………………………………….

Mafic Rocks ……………………………………………………………….

Granitoids …………………………………………………………………

29

29

33

6. DISCUSSION ……………………………………………………………..

Mafic Rocks ……………………………………………………………….

Granitoids …………………………………………………………………

38

38

42

7. CONCLUSIONS AND TECTONIC HISTORY ………………………….

46

APPENDIX

1. COMPLETE TABLES OF THE ANALYTICAL RESULTS AND

LOCATIONS ……………………………………………………………...

47

REFERENCES ……………………………………………………………………. 55

iv

LIST OF FIGURES

Figure Page

1. Regional Map showing the study area (Figures 2a and 2b) relative to other

Laramide Uplifts that expose Precambrian rocks

3

2. Map of the Hartville Uplift

a) Map of the southern part of the Hartville Uplift

b) Map of the northern part of the Hartville Uplift

4

5

3. Photographs of the Muskrat Canyon metabasalt

14

4. Photographs of the Mother Featherlegs metabasalt

16

5. Photographs of the metadiabase dikes.

18

6. Photographs of the Rawhide Buttes granite

21

7. Hand samples of the Twin Hills diorite showing stratification

24

8. Photographs of the Twin Hills diorite

25

9. Major elements vs. Mg# diagrams for the three basaltic suites

31

10. REE patterns for the three basaltic suites

32

11. Chondrite-normalized ‘Spider’ diagrams for the three mafic suites

32

12. Harker Diagrams for the granitoids of the Hartville Uplift

35

13. REE patterns for the granitoids and metapelites of the Hartville Uplift

36

14. Chondrite-normalized ‘Spider’ diagrams for the granitoids and

metapelites of the Hartville Uplift

37

15. Total Alkalis vs. Silica (TAS) diagram

38

16. Th/Yb vs. Nb/Yb diagram

40

17. Tectonic discrimination diagrams for the mafic suites

41

v

ABSTRACT

The location of the eastern margin of the Wyoming Archaean Province and its

Proterozoic evolution are still debated. Previous studies have attributed north-south and

east-west directed structures to the Proterozoic Black Hills and Central Plains orogenies,

respectively, but the tectonic details of these orogenies are unclear. I have studied

igneous rocks in the Laramide-age Hartville Uplift (HU), which exposed Precambrian

rocks. At least part of the NNE-trending HU is bisected along its length by the Hartville

Fault (HF) that juxtaposes high-grade metamorphic rocks on its eastern side against

lower-grade metamorphic rocks on its western side. The objective is to use the

geochemical features of the igneous rocks to infer the tectonic settings in which they

formed.

The oldest dated magmatic rocks are 2.6 Ga Archaean Rawhide Buttes and

Flattop Butte granites (SiO2 > 67 wt%; K2O /Na2O wt% > 1; ASI > 1.05). They crop out

only in the northern part of the HU and appear to be of crustal origin. The next magmatic

episodes involved basaltic volcanism. They are represented by the Mother Featherlegs

metabasalt on the eastern side of the HF and the Muskrat Canyon metabasalt on the

western side. Compositions of these basalts are attributed to rifting and a mantle plume,

respetively. The ages of the metabasalts are unknown, thus it is not certain if they are

coeval. However, they may correspond to the ~2 Ga Kennedy dike swarm in the Laramie

Range and amphibolites in the Black Hills that show similar extension and plume-related

chemical characteristics.

In the southern HU, Proterozoic, 1.74 Ga Twin Hills diorite and Haystack Range

granite crop out on the eastern side of the HF. The latter appears to be younger as its

vi

dikes cut the diorite. SiO2 of ~55 wt % and K2O /Na2O ratios of < 1 suggest a

lithospheric mantle origin for the Twin Hills diorite, whereas SiO2 >69 wt %, K2O/Na2O

> 1, and peraluminous composition indicate a crustal origin for the Haystack Range

granite, specifically melting of schist that occur in the HU.

I suggest that the Archaean granitoids may be related to accretion along the Oregon

Trail Structure in southern Wyoming. The region has subsequently undergone rifting, as

shown by the basalt suites, possibly related to the breakup of the proto-continent

Kenorland. The Twin Hills diorite and the Haystack Range granite appear to be related to

westward subduction and collision (respectively) during the Black Hills-Dakotan

collisional orogeny. Enigmatic migmatitic, tonalitic pods with an age of 1.715 Ga mark the

latest deformation event that is attributed to the terminal collision of Wyoming and

Superior cratonic provinces.

1

1. INTRODUCTION

The Archean core of the North American continent, Laurentia, was constructed by an

amalgamation of small cratons between 2.0 and 1.7 Ga (Hoffman, 1988; Whitmeyer and

Karlstrom, 2007). The Wyoming Province has been interpreted as one of these

independent Archean cratons (Chamberlain et al., 2003; Mueller and Frost, 2006). On its

eastern margin, the Wyoming Province is bordered by the Black Hills-Dakoran Orogens,

which resulted from the collision of the Wyoming Province with the Superior Province.

In addition, Proterozoic arcs were accreted to the southern margin of the Wyoming

Province along the Central-Plains Orogen to form the Yavapai Province (Hoffman,

1988).

Despite general agreement on broad aspects of the role of the Wyoming Province in

the construction of Laurentia, the scarcity of Archean and Proterozoic outcrops precludes

precise field data to unravel the Precambrian geologic history of the southernmost

Laurentia. The southern border of the Wyoming Province (Figure 1) is marked by the

Cheyenne Belt (Karlstrom and Houston, 1984), a structural lineament attributed to the

Medicine Bow Orogeny ca. 1.78 Ga (Chamberlain, 1998). The influence of the orogeny

on the development of the Wyoming Province is still debated, as evidenced by the

number of models proposed (Karlstrom and Houston, 1984; Chamberlain, 1998; Patel et

al., 1999; Resor and Snoke, 2005; Jones et al., 2010). To the east, the limit of the

Wyoming province remains unresolved (Chamberlain et al., 2003; Sims et al., 2001;

Worthington et al., 2016). Complex deformation associated with the Wyoming-Superior

collision, together with multiple distinct ages in the Black Hills, South Dakota, have

resulted in the differentiation of two events affecting the eastern Wyoming province: (1)

2

the Black Hills orogen, corresponding to initiation of metamorphism ca. 1.76 Ga

(Goldich et al., 1966; Dahl and Frei, 1998) and (2) the Dakotan orogen, corresponding to

felsic magmatism ca. 1.72 Ga (Krugh, 1997; Chamberlain et al., 2002). Nabelek et al.

(1999, 2001) proposed that the metamorphism and magmatism were the products of a

single, protracted collisional event.

To unravel the tectonic regimes associated with construction of Laurentia along the

eastern margin of the Wyoming Province, this work presents a major and trace element

geochemical study of the Precambrian igneous rocks of the Hartville Uplift (Figure 2), a

Laramide uplift (ca. 70 - 35 Ma) that lays near the eastern limit of the Wyoming Province

(Sims et al., 2001), and presumably in a zone that was influenced by the Medicine Bow

orogeny and the Black Hills-Dakotan orogeny.

The main objectives of this work are: (1) to describe the geochemistry of the igneous

mafic and felsic rocks of the Hartville Uplift, (2) to place these rocks within tectonic

regimes in which they were generated, (3) to find relationships between these rocks and

those in other uplifts, including the Laramie Mountains and the Black Hills, and (4) to

develop a tectonic model that can explain the igneous rocks within the context of

previously published work.

3

Figure 1. Regional Map showing the Study area (Figures 2a and 2b) relative to other

Laramide Uplifts that expose Precambrian rocks. After Chamberlain et al. (2003),

Karlstrom and Houston (1984). OTS = Oregon Trail Structure. BH-D O = Black Hills -

Dakotan Orogen.

4

Figure 2: Map of the Hartville Uplift. a) Map of the southern part of the Hartville Uplift.

After Sims and Day (1999).

5

Figure 2. b) Map of the northern part of the Hartville Uplift. Symbols as in Figure 2a.

6

2. GEOLOGICAL CONTEXT

The Wyoming Province

The Wyoming Province is one of the Archean cratonic terranes that were accreted

during the Paleoproterozoic era to form Laurentia (Hoffman, 1988; Whitmeyer and

Karlstrom, 2007). Detrital zircon U-Pb ages as old as 3.96 Ga (Mueller et al., 1992)

evidence the antiquity of this province. The study of the geologic history of the Wyoming

Province is complicated by scarcity of outcrops. The existing rock exposures generally

consist of Archean granites and gneisses as old as 3.5 Ga (Mueller et al., 2006) that are

overlain by supracrustal Archean to Proterozoic rocks (see Chamberlain et al., 2003;

Houston, 1993 for a review). Chamberlain et al. (2003) distinguished at least four domains,

which get younger toward the southeast. The domain boundaries are thought to represent

a series of arc accretion. These domains are distinguished by a series of geochronologically

distinct, high-strain lineaments. The most prominent of these lineaments is the Oregon

Trail Structure (Figure 1; Chamberlain et al., 2003). These linear structures are thought to

have played a key role in the localization of the Mesozoic Laramide deformation and uplift.

Only along the southern margin is the limit of the Wyoming Province well defined.

Here the border consist of the Cheyenne Belt (Karlstrom and Houston, 1984). The location

of the eastern margin and its Proterozoic evolution remains unsolved. Sims et al. (2001)

situated it slightly to the east of the Hartville Uplift, based on an aeromagnetic linear

anomaly. Other authors (e. g. Chamberlain et al., 2003; Worthington et al., 2016), interpret

the Hartville Uplift and the Black Hills together as a separated domain that was accreted to

the Wyoming Province during the Proterozoic Black Hills-Dakotan orogeny.

7

The Cheyenne Belt

The Cheyenne Belt (Figure 1) is the southern limit of the Wyoming Province

(Karlstrom and Houston, 1984). It represents the accretion front of Proterozoic arcs and

terranes to the Archean Wyoming craton during the Medicine Bow orogeny (Chamberlain,

1998). These Proterozoic terranes are part of the Yavapai and Matzatzal provinces

(Hoffman, 1988; Whitmeyer and Karlstrom, 2007), whereas the Medicine Bow orogeny

corresponds to the Central Plains orogeny that affected all the southern margin of Archean

Laurentia (Sims and Peterman, 1986; Hoffman, 1988; Whitmeyer and Karlstrom, 2007).

The collision that formed this suture is fairly well constrained at ca. 1.78-1.76 (Premo and

Van Schmus, 1989; Resor et al., 1996; Chamberlain, 1998).

The paleo-sense of subduction is thought to have been southward, based on the

absence of subduction-related Proterozoic granitoids north of the Cheyenne Belt

(Karlstrom and Houston, 1984) and the interpretation of the current lithospheric structure

from seismic profiles (Templeton and Smithson, 1994). However, Jones et al. (2010)

suggested an alternative model involving northward subduction based on geochronology

and isotopic compositions of igneous rocks.

Near the Cheyenne Belt the Wyoming Province consists of supracrustal Proterozoic

and Archean rocks that are well exposed in the Medicine Bow and Sierra Madre Mountains

and to a lesser extent in the Laramie Mountains (see Houston, 1993; Houston et al., 1993

for a review). These rocks consist of metasedimentary and metavolcanic sequences that

seem to reflect several episodes of convergence and rifting (Karlstrom and Houston, 1979;

Lanthier, 1979; Graff et al., 1982; Karlstrom et al., 1983; Houston et al., 1992).

8

Structurally, the deformation is localized in the form of discrete shear zones

between large, relatively undeformed blocks (Karlstrom and Houston, 1984, Resor and

Snoke, 2005). This style of deformation is well exposed in the Medicine Bow and Sierra

Madre Mountains and it continues at least as far north as the Laramie Peak Shear System

(Resor et al., 1996; Patel, et al., 1999, Resor and Snoke, 2005).

The Trans-Hudson, Black Hills and Dakotan Orogens

The Trans-Hudson orogen is the result of the Proterozoic collision between the

Superior and Hearne Cratons ca. 1.9-1.8 Ga (Hoffmann, 1988; Whitmeyer and Karlstrom,

2007). Because of the lack of continuous outcrop east of the Wyoming Province, the Trans

Hudson Orogen was assumed to continue southward and include the collision between the

Wyoming Province and the Superior Province (Hoffmann, 1988; Whitmeyer and

Karlstrom, 2007).

Geophysical studies supported this interpretation. The North American Central

Plains conductivity anomaly (NACP; Alabi et al., 1975) is a linear conductive feature that

extends from central Saskatchewan (Canada) to the south-eastern Wyoming going through

the Black Hills. This feature has been thought to represent a long stratigraphic marker

related to either sulfide or graphitic layers that continue along the Trans-Hudson Orogen

(e. g. Jones et al., 1997), but a mantellic origin has also been invoked (Alabi et al., 1975;

Jones et al., 2005). In addition, COCORP seismic profiles (Nelson et al., 1993; Baird et

al., 1996) seem to favor these interpretations. However, the profiles add a complication - a

buried micro-continent between the Superior and Wyoming Cratons.

9

The Black Hills mainly consist of Proterozoic deformed and metamorphosed

sedimentary rocks and leucogranites with minor Archean outcrops (Gosselin et al., 1988;

see Redden and DeWitt,2008; Allard and Portis, 2013, for a review). Dahl and Frei (1998)

showed that ages of metamorphism (ca. 1.76 Ga) in the Black Hills are systematically

younger than those of the Trans-Hudson orogen in Canada (ca. 1.83 Ga; Brickford et al.,

1990). To emphasize the younger ages in the Black Hills, Dahl and Frei (1998) proposed

the name of Black Hills orogeny for the collision of the Superior and Wyoming cratons.

Krugh (1997) and Chamberlain (2002) distinguished a younger orogeny circa 1.72 Ga

based on zircon and monzite ages of the Harney Peak granite (Redden et al, 1990) and

pegmatite segregation in metadiabase dikes in the Rawhide Buttes of the Hartville Uplift.

Two interpretations about these two sets of ages were proposed: (1) either the younger age

represents a re-activation of the older collision (Krugh, 1997), or (2) the older age

represents the docking of a microcontinent (maybe the one imaged in the COCORP

profiles) and the younger represents the final collision between the Wyoming and Superior

cratons (Krugh, 1997; Chamberlain, 2002). Nabelek et al. (1999, 2001) ascribed the two

sets of ages to a continuous orogenic process, here called the Black Hills-Dakotan orogeny.

The Hartville Uplift

The Hartville Uplift (Figure 2) is a Laramide-age uplift that exposes a complex

setting of Archean and Paleoproterozoic rocks crop out (Sims and Day, 1999). It is an

elongated north-northwest trending uplift that extends approximately from Guernsey

(Platte County, WY) to Lusk (Niobrara County, WY). The most important structure is the

Hartville Fault that crops out in Hell Gap (Goshen County, WY) and juxtaposes relatively

10

low-grade rocks on the western side against high-grade rocks of the eastern side. The

contrast in metamorphic grades has been used to interpret the fault as a major structure that

would extend the entire length of the uplift (Snyder, 1980; Snyder and Peterman, 1982;

Sims and Day, 1999).

Stratigraphically, the Precambrian rocks of the Hartville Uplift consist of a

supracrustal sequence, the Whalen Group (Figure 2; Smith, 1903), and of Archean and

Proterozoic granitoids (Snyder and Peterman, 1982; Sims and Day, 1999). The

metasedimentary rocks consist of metacarbonates and metapelites. They have been

interpreted as either Proterozoic (Sims et al., 1997; Bekker et al., 2003) or Archean (Snyder

and Peterman, 1982; Hoffman and Snyder, 1985; Sims and Day, 1999). Nd model ages for

the Silver Spring Schist are consistent with both interpretations with values between 2.9

and 3.2 Ga (Frost and Frost, 1993).

At least three episodes of deformation are recorded in the Hartville Uplift. The first

(D1) is recognized in overturned sedimentary beds, including overturned stromatolites

(Hoffman and Snyder, 1985; Sims and Day, 1999) near Guernsey. This deformation is

interpreted have produced recumbent folds. No hinges are exposed and the folds have been

traditionally interpreted as north-vergent (Krugh, 1997; Sims et al., 1997; Sims and Day,

1999) but E. S. Nowariak (University of Missouri, personal communication, 2016) has

proposed a west-vergent character based on minor parasitic folds. D2 appears only in the

southwestern part of the uplift, near Guernsey. The corresponding folds are tight to

isoclinal, with westerly striking axial planes that dip steeply to the north (Sims et al., 1997).

The third deformation (D3) appears in all the uplift, except for the zone were D2 is

dominant. To the west of the fault, D3 is very similar to D2 (vertical axial planes, near

11

horizontal hinges, symmetric limbs) except for the north-south strike of the axial planes.

To the east of the fault, these axial planes are also nearly vertical and north-south directed,

but the fold hinges dip variably from horizontal to vertical (Krugh, 1997; Sims and Day,

1999). In addition, discrete, narrow, north-south to northeast-southwest directed shear

zones usually mark the contact between granites and rocks of the Whalen group and among

rocks of this group, especially (but not only) in the northern part of the uplift.

The tectonic relationship between the Hartville Uplift, the Wyoming Province and the

Black Hills has not been resolved. Two hypotheses have been proposed: (1) the Hartville

Uplift and the Black Hills are part of an Archean block, independent of the Wyoming

Province, that was accreted during the Black Hills-Dakotan orogeny (Chamberlain et al.,

2003; Whorthington et al., 2016); or (2) the Hartville Uplift were part of the Wyoming

Province and correspond to the folds and thrusts of the Black Hills-Dakotan orogeny (Sims

and Day, 1999).

12

3. IGNEOUS ROCKS OF THE HARTVILLE UPLIFT

A preliminary study of the geochemistry of mafic rocks of the Hartville Uplift has already

been published by Sims and Day (1999). Additional key samples were analyzed in the

present work. For granitoids, the published geochemical data are very sparse (Snyder and

Peterman, 1982; Snyder et al. 1989), thus additional data were collected on well-

characterized samples. The collected major and trace element data are used to assess the

geodynamic settings during genesis of the igneous rocks.

Mafic Rocks

Sims and Day (1999) distinguished two metavolcanic suites within the Whalen

Group based on their trace element geochemistry. Previously, the mafic metavolcanics

were grouped together as Mother Featherlegs metabasalt (Snyder, 1980; Snyder and

Peterman, 1982). Sims and Day (1999) divided this group into the Muskrat Canyon

metabasalt and the Mother Featherlegs metabasalt. Although they argue that these two

groups are indistinguishable at the field or in hand samples, some characteristics

distinguish the two groups. In addition, at least two generations of Proterozoic metadiabase

dikes crosscut the rocks of the Hartville Uplift (Sims and Day, 1999).

Muskrat Canyon Metabasalt

The Muskrat Canyon metabasalt crops out only on the western side of the Hartville Fault.

It is characterized by slight enrichment in LREE’s relative to HREE’s (Sims and Day,

1999). In the field, the basalt occurs as volcanic and subvolcanic products together with

minor gabbroic sills. It usually presents very well preserved original volcanic and gabbroic

textures. Near the apparent base of the formation, at Muskrat Canyon, excellent pillow

13

lavas are found (Figure 3a), and in the southwestern part of the uplift, there are original

centimeter-scale volcanic layers (Figure 3b). The metamorphic grade ranges from the

greenschist facies to the lower amphibolite facies.

The Muskrat Canyon metabasalt is usually green and lighter in color than the

Mother Featherlegs metabasalt. It commonly presents micrometer to centimeter-scale calc-

silicate nodules (Figure 3c). They have been interpreted to be pseudo-primary (Krugh,

1997) and they point (together with the presence of pillow morphology) to a sub-aqueous

environment of deposition. These nodules are absent in the Mother Featherlegs metabasalt.

The metamorphic minerals in the nodules include grossular garnet, epidote and

chondrodite.

Although no age is available for this metabasalt, it is included in the Whalen group, and

thus Sims and Day (1999) interpreted it as Archean and younger than the Mother

Featherlegs Metabasalt.

Sims and Day (1999) mapped a metagabbro in southwestern Hartville Uplift. The unit, is

identical to the Muskrat Canyon metabasalt; therefore, this unit is grouped together with the

Muskrat Canyon metabasalt.

14

Figure 3. Photographs of the Muskrat Canyon metabasalt. a) Pillow lavas exposed at Muskrat

Canyon. b) Original magmatic layering exposed at the southwestern Hartville Uplift. c)

Microphotograph of a calc-silicate nodule in a sample of the Muskrat Canyon metabasalt

(crossed-polarized). Size of the field view = 5 mm.

Mother Featherlegs Metabasalt

This metavolcanic unit crops out on the western side of the Hartville Fault in the

Guernsey Quarry and on the eastern side of the fault in the northern part of the uplift. Sims

a) b)

c)

15

and Day (1999) report nearly flat REE patterns for this metabasalt. As the Muskrat Canyon

metabasalt, in the field the Mother Featherlegs metabasalt takes the form of basaltic flows

and occasional sills. Pillows and original textures are also commonly preserved in this

basalt (Figure 4a). This basalt has been metamorphosed everywhere to the amphibolite

grade.

The Mother Featherlegs metabasalt is usually darker than the Muskrat Canyon

metabasalt and carbonate nodules have not been found. However, micro to macro-scale

quartz-rich veins commonly crosscut the rocks in preferential directions (Figure 4b).

No age is available for this basalt unit, but it is thought to be Archean and older in age than

the Muskrat Canyon metabasalt (Sims and Day, 1999).

Metadiabase Dikes

Proterozoic metadiabases or Fe-tholeiites (Snyder and Peterman, 1982; Day et al.,

1999; Sims and Day, 1999) crosscut the whole uplift and are especially abundant and

evident in the two Rawhide Buttes granites on both sides of the Hartville Fault.

Interestingly, the orientation of the metadiabases is not the same on the two sides of the

fault. Whereas the metadiabases on the eastern side have a north to northeast strike, on the

western side they have a more northwesterly strike.

16

Figure 4. Photographs of the Mother Featherlegs metabasalt. a) Pillow Lavas textures of

the Mother Featherlegs metabasalt. b) Photomicrograph showing abundant quartzitic veins

along preferential directions in the Mother Featherlegs metabasalt. Size of fieldview = 5

mm.

The dikes are ubiquitously metamorphosed to the amphibolite grade and present

week to strong foliation. Some metadiabases present poikioblastic amphiboles and

neoformed chlorite that may indicate a complex history of deformation (Figure 5a). In the

Rawhide Buttes granite, the metadiabases commonly present tonalitic pods in gash fillings

(Figure 5c, see below) that have been dated at ca. 1715 Ma (Krugh, 1997). These gash

fillings have biotite-rich selvages (Figure 5d). Although the biotite crystals are mostly

parallel to foliation, occasionally they can be seen crossing it slightly, pointing toward the

tonalitic pods.

A precise date for these rocks is not published, but the aforementioned age of the

tonalitic pods places a minimum age for them. Because of their foliation and high grade,

they must be older than the metamorphic peak. Their orientation has led some authors

(Krugh, 1997; Sims and Day, 1999) to correlate them with the Kennedy dike swarm (Cox

a) b)

17

et al., 2000) which is ca. 2.01 Ga and crops out in the Laramie Range. However,

Chamberlain et al. (2003) do not include the Hartville Uplift in the area that was penetrated

by these dikes.

Most of my samples of this metadiabase come from the Rawhide Buttes, with minor

representation of outcrops in the Silver Spring Schist. Because the metadiabases appear as

thin dikes, most of the outcrops aren’t depicted in Fig. 2. A complete map of the

metadiabase dikes can be found in Day et al., (1999).

Granitoids

Two main groups of granitoids are found in the Hartville Uplift. Archean Flattop

and Rawhide Buttes granites, and Proterozoic Twin Hills and Haystack Range granites. In

addition, the aforementioned tonalitic fillings occur in the rocks of the Rawhide Buttes.

Also, an older grey gneiss in the Cassa Anticline, near the Hartville Uplift, is also described

in Day et al., 1999. This last rock won’t be an objective of this study.

Ages of these rocks are published in the literature and model ages of sources are also

available. However, comprehensive geochemical data hasn’t been published.

18

Figure 5. Photographs of the metadiabase dikes. a) Microphotograph showing

poikioblastic texture, relict minerals and abundant chlorite in a metadiabase of the

Rawhide Buttes. Size of the fieldview = 5 mm. b) Non-parallel directions of biotite in

metadiabase that evidences post-foliation crystallization. S c) Pegmatitic gash fillings in a

metadiabase of the Rawhide Buttes. d) Pegmatitic gash filling with a dark selvage

consisting mostly of biotite (courtesy of E. S. Nowariak, University of Missouri, 2015).

Rawhide Buttes Granite:

The Rawhide Buttes granite is a foliated to gneissic, light pink to red granite. Bodies

of this granite crop out in the areas of Rawhide Buttes (east of the Rawhide Fault), Rawhide

Creek and Lone Tree Hill (Snyder, 1980; Day et al., 1999, Sims and Day, 1999). In the

a) b)

c) d)

19

area of the Hell Gap, in the center of the uplift, there are granitic rocks at the headwall of

the Hartville Fault (Sims and Day, 1999). They were initially interpreted as part of this

granite (Snyder, 1980; Sims and Day, 1999) but a zircon age younger than 1750 Ga (K. R.

Chamberlain, University of Wyoming, personal communication, 2015) suggests that the

granite near Hell Gap is part of the nearby Haystack Range Granite (see below; Houston,

1993; Krugh, 1997).

Published Rb-Sr whole rock ages point to a 2.58-2.66 Ga age (Snyder and

Peterman, 1982; Day et al., 1999). A U-Pb zircon dating on Rawhide Creek outrops

yielded an age of 2.6-2.7 Ga (with some inheritance; K. R. Chamberlain, University of

Wyoming, personal communication, 2016). A Nd model age of 2.60 Ga (Day et al., 1999)

coincides roughly with the age of intrusion. The granite is mainly a biotite granite, although

it can also contain considerable amounts of muscovite, showing that it is peraluminous

(Snyder and Peterman, 1982).

On the eastern side of the Rawhide Fault, the granite is intensely deformed and

locally has migmatitic appearance (Figure 6a). In thin section, inclusion-rich feldspar

grains are abundant. Locally, sillimanite-rich, sheet-like portions of the granite parallel the

foliation (Figure 6b), especially on the western side of the Rawhide Buttes. These were

interpreted by Snyder and Peterman (1982) as high-grade (second-sillimanite) Silver

Spring Schist, which has been used as the criterion for adjudicating an Archean age to the

schists (Snyder and Peterman, 1982). In fact, in thin section these rocks present relict

oriented minerals (mainly biotite, Figure 6c) as inclusions in feldspars that are here

interpreted as a possible relict foliation, suggesting at least a complex deformational and

20

metamorphic history for these rocks. In addition, the granite is crosscut with the

metadiabase dikes described above, also parallel to foliation.

On the western side of the Hartville Fault (near Rawhide Creek), the contact with

rocks of the Whalen Group is marked by an intense shear zone. All our samples of granite

from Rawhide Creek come from that shear zone. An interesting feature is the appearance

of tourmaline (Figure 6d), which is completely absent in the granite at Rawhide Buttes.

- Grey gneiss of the Rawhide Buttes.

This rock crops out at the southwestern side of the Rawhide Buttes. Early

interpretations place this granite as part of the Rawhide Buttes granite (Snyder, 1980;

Snyder and Peterman, 1982). However, Krugh (1997) published U-Pb ages that show this

gneiss to be younger (2.41 Ga). It is a biotite granite and in hand sample it appears to have

a slightly weaker foliation than the surrounding Rawhide Buttes granite (although the

foliation in the latter is variable).

- Tonalitic pods

In the Rawhide Buttes, tonalitic pods in tension gash fillings appear within the

metadiabases (Figure 5b, c, d; Krugh, 1997). These fillings have been dated (U-Pb

zircon) as ca. 1714 Ma. This age has been interpreted as the age of D3 deformation

(Krugh, 1997). Pods consist of feldspar and quartz, with hornblende also present. Krugh

(1997) also reports biotite as an accessory mineral, but no biotite has been found in our

samples.

21

Figure 6. Photographs of the Rawhide Buttes granite. a) Migmatitic texture at the Rawhide

Buttes granite. b) Microphotograph showing sillimanite concordant with foliation

(crossed-polarized). Size of fieldview = 5mm. c) Microphotograph showing relict foliation

inside a big microcline crystal of the Rawhide Buttes granite (crossed-polarized). Size of

fieldview = 1 mm. d) Photomicrograph of a granitic rock from the Rawhide Creek body

showing tourmaline and static (?) biotite. Size of fieldview = 5 mm.

Flattop Butte Granite:

Unfortunately, no access to the Flattop Butte granite was obtained from

landowners. What follows is a summary of previously published descriptions. Samples for

this work were donated by K. R. Chamberlain (University of Wyoming, 2015). This granite

is found between the Hartville and Rawhide Faults and to the northwest of the Rawhide

Buttes granite. It is a two-mica granite with variable foliation (Day et al., 1999; Sims and

a) b)

c) d)

22

Day, 1999). The main criteriium to differentiate the Flattop Butte granite from the Rawhide

Butte granite was the near absence of metadiabase dikes in the former. In fact, only one

dike can be recognized here (Krugh, 1997). In addition, Snyder and Peterman (1982)

obtained an age of 1.98 Ga from an Rb-Sr isochron, but Day et al. has interpreted that age

as deformational and K. C. Chamberlain (University of Wyoming, personal

communication, 2015) has obtained a U-Pb zircon age of ca. 2.65 Ga, similar to that of the

Rawhide Butte granite.

The relationship between the Flattop Butte and the Rawhide Buttes granites is yet

to be clarified. For example, several authors (i. e. Snyder, 1980; Day et al., 1999)

distinguish the two bodies on the basis of their field contexts, but major elements and modal

proportions of minerals do not show clear differences (Snyder and Peterman, 1982; Day et

al., 1999). The Nd model age, between 2.8 and 3.2 Ga for the Flattop Butte granite (Day

et al., 1999), is nearly coincident with model ages for metapelites (Frost and Frost, 1993).

Twin Hills Diorite

The Twin Hills diorite outcrops only in the center of the uplift, on the eastern side

of the Hartville Fault exposure at Hell Gap. In the field it is variable in appearance and

deformation. It is a zoned intrusion (Figure 7), with more mafic composition near the base

and more silica rich (quartz-bearing) rocks at the top. The more dominant composition is

quartz diorite with abundant hornblende and biotite (Figure 8a; Snyder & Peterman, 1982).

It is widely crosscut by dikes of different compositions (from dioritic to granitic; Figure

8b). Deformation is rather localized (Krugh, 1997). This localization of deformation seems

to be especially evident in the Haystack Range granite dikes that cut through the diorite.

Sims and Day (1999) mapped a dike in the Twin Hills diorite as belonging to the Flattop

23

Butte granite. Although this precise dike was not sampled, the dikes are more similar to

the nearby Haystack Range granite that the Flattop Butte granite. The foliation is marked

by chlorite, which suggests a low grade and/or fluid-mediated recrystallization. The

crystals have suffered brittle deformation (Figure 8c). Undeformed pegmatitic dikes are

very abundant at the base of the body (Figure 8d).

Ages published for the Twin Hills diorite include U-Pb ages of ca. 1.74 Ga (Snyder

and Peterman, 1982) and ca. 1744 Ma (Krugh, 1997). Geist et al. (1989) have published a

Nd model age of 2.7 Ga for the source, which has been interpreted as underplated

Proterozoic crust (Geist et al., 1989; Houston, 1993).

24

Figure 7: Hand samples of the Twin Hills diorite showing stratification . Note the transition

from felsic to more mafic compositions from location 22 (a) to location 25 (d). At the

bottom (e) two samples of the Haystack Range granite dikes are shown: 25-3 is an

undeformed dike whereas 25-4 is a deformed dike

25

Figure 8. Photographs of the Twin Hills diorite. a) Microphotograph showing the most abundant

phase of the Twin Hills diorite (quartz diorite). Size of fieldview = 5 mm. b) Numerous dikes

crosscutting the Twin Hills diorite. c) Microphotograph showing chlorite and brittle deformation

in one of the Haystack Range granite dikes. Size of fieldview = 5 mm. d) Undeformed pegmatitic

dike in the Twin Hills diorite (courtesy of E. S. Nowariak, University of Missouri, 2015).

Haystack Range Granite

As in the case of the Flattop Butte granite, no access was obtained from the

landowners and the information on this granite comes from previously published data,

dikes in the Twin Hills diorite and a sample donated by K. R. Chamberlain (University of

Wyoming, 2015). It is a metaluminous to peraluminous biotite granite with occasional

minor muscovite (Snyder and Peterman, 1982; Krugh, 1997).

a) b)

c) d)

26

The main outcrop is located in the southern part of the Haystack Range, where the

granite forms a dome that warps the metapelites and metadiabases around it (Sims and Day,

1999). In the granite, the deformation is almost nonexistent. This granite crops out again

on the other side of the McCann Pass Fault. The deformation gets stronger until it becomes

completely sheared at Hell Gap. The existence of dikes in the Twin Hills diorite suggests

that the granite is younger but the occurrence of both, foliated and unfoliated dikes suggests

that their ages may overlap.

Published ages range considerably. Rb-Sr ages of the granite are ca. 1.72 Ga

(Snyder and Peterman, 1982). U-Pb ages from Krugh (1997) date the granite as ca. 1.68

Ga, but K. R. Chamberlain (University of Wyoming, personal communication, 2015) has

obtained an older ca. 1747 Ma for this granite. Geist (1989) has published a Nd model age

of 2.99 Ga.

27

4. ANALYTICAL TECHNIQUES

One-hundred fifty-five samples representative of the rocks of the Hartville Uplift

were collected. Of those, 34 were selected for whole rock major element analysis and 30

were additionally selected for trace element analysis. To insure representability, the amount

of sample that was powdered was proportional to the grain size of the hand sample.

Each sample was first trimmed of weathering and then crushed to chips. The chips

were cleaned with 5% nitric acid except for the ones containing carbonates. Then they were

powdered using a SPEX Shatterbox.

Approximately 1 g of the powder was heated in a furnace at 900 oC for 60 minutes.

The weight before and after this process was measured to calculate the LOI (lost on

ignition). Of the resulting powder, 200 mg were mixed with 600 mg of lithium metaborate

(LiBO2) and molten at 1050 oC for 20 minutes. After quenching, the glass was digested in

50 ml of 20% reagent grade nitric acid. Distilled water was added to make the volume of

solution to be 100 ml.

For major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K), the solution was once again

diluted in a proportion of 1 to 20. The diluted solutions were analyzed by an ICP-OES at

the Department of Geological Sciences of the University of Missouri. Synthetic standards

were made by addition of analyte elements to blank solutions. USGS standards SDC-1,

DNC-1 and GSP-1 were analyzed for quality control.

For trace elements (and P), samples were diluted in 3% HNO3 also by a factor of 1

to 10. The resulting solutions were analyzed in a high resolution ICP-MS using a VG

28

Axiom at the Research Reactor of the University of Missouri. The same quality control

standards were used.

29

5. RESULTS

Appendix 1 shows location and major and trace element compositions of granitoids,

and some metabasalts and schists (tables A1 to A5).

Mafic Rocks

Analyses of mafic rocks reported by Sims and Day (1999) are presented here

together with analyses done in this study (e.g. Figure 9). In Sims and Day (1999), the

geographic coordinates of some metabasalts do not correspond to those mapped in the field.

Thus, for this work, the basalts are grouped following the map of Sims and Day (1999)

instead of their tables.

Major elements are very similar in all basaltic suites (Sims and Day, 1999). Figure

9 shows major oxides (wt. %) plotted against Mg# [molar MgO/(MgO+FeO)]. Most oxides

present nearly identical trends for the three suites. The Muskrat Canyon metabasalt can be

distinguished because the average of the samples is higher in TiO2 and Na2O. The higher

K2O distinguishes the metadiabase dikes from the Mother Featherlegs metabasalt.

Differentiation trends are only visible for TiO2 and FeO, negatively correlated with Mg#,

and CaO, positively correlated with Mg#.

The main differences between the suites come from trace elements (tables A1 and

A2). Figure 10 shows REE patterns for the three mafic suites (all normalized to chondrites

of McDonough and Sun, 1995). Mother Featherlegs metabasalt presents mainly flat

patterns (Figure 10a). Few of analyses from Sims and Day (1999) presented higher La/Yb

ratios, but in those cases the shape of the trend is rather concave upward and is rare with

respect to the flat trends. The Muskrat Canyon metabasalt (Figure 10b) presents

30

systematically straight patterns with enrichment in LREE’s. Metadiabase dikes (Figure

10c) show flat REE patterns similar to those of the Mother Featherlegs metabasalt. An

exception to this is sample 15-HU-77-01B that is enriched in LREE’s and presents a strong

negative Eu anomaly.

Figure 11 depicts chondrite-normalized spider diagrams for the mafic rocks. They

underscores that the differences between the suites are subtle. Positive Th and U anomalies

can be recognized in some samples.

31

Figure 9. Major elements vs. Mg# diagrams for the three basaltic suites.

32

Figure 10. REE patterns for the three basaltic suites. Shaded areas represent data form

Sims and Day (1999).

Figure 11. Chondrite-normalized ‘Spider’ diagrams for the three mafic suites.

15-HU-77-01B

15-HU-77-01B

33

Granitoids

Major elements for granitoids are reported in tables A3, A4 and A5 (Appendix 1).

Figure 12 shows ‘Harker’ diagrams for the major oxides of the granitoids. The first feature

of these diagrams is that the Twin Hills diorite is compositionally very different from rest

of the granitoids. The tonalitic pods in the metadiabase dikes are easily distinguished by

their higher Na2O and CaO contents, and extremely low K2O. The Haystack Range granite

and the Flattop Butte granite are very similar, showing inverse correlations with SiO2 for

most elements. The sillimanite-baring phase of the Rawhide Buttes granite is effectively

indistinguishable from rest of the Rawhide Buttes granite. Anomalous compositions are

those of the grey gneiss phase of the Rawhide Buttes granite, which plots systematically

low with respect to the main phase of the Rawhide Buttes for all oxides except K2O.

Figure 13 shows REE patterns for the granitoids of the Hartville Uplift. The

Rawhide Buttes granite (Figure 13a) presents very steep slopes and moderate anomalies in

Eu. 15-HU-28-01 is a sample of the sillimanite-bearing phase of the Rawhide Buttes

granite. It has more elevated heavy REE’s. 15-HU-33-02 is a sample from the grey gneiss

of the Rawhide Buttes, its shape is very similar to the rest of the Rawhide Buttes samples,

with a pattern similar to 15-HU-28-01. The tonalitic pods of the metadiabases (Figure 13b)

have the lowest abundances in REE of all the granitoids and present high positive Eu

anomalies. The Twin Hills diorite samples (Figure 13c) range for nearly flat REE patterns

to moderately steep patterns with enrichment in LREE’s and moderate negative Eu

anomalies. The Flattop Butte granite (Figure 13d) and the Haystack Range granite (Figure

13e) are very similar, with moderate slopes and high negative Eu anomalies.

34

The REE patterns of most granitoids resemble very strikingly the patterns of the

metapelites (Figure 13f). Exceptions to this are one Twin Hills diorite sample, dikes of the

Haystack Range granite within the Twin Hills diorite, and the tonalitic pods within the

Rawhid Buttes metadiabases, which present positive Eu anomalies.

Spider diagrams (Figure 14) confirm the similarities between high-K granites

(Rawhide Buttes granite, Flattop Butte granite and Haystack Range granite) and the

metapelites of the Whalen group. They also show how different these rocks are from the

mafic portions of the Twin Hills diorite and the tonalitic pods.

35

Figure 12. Harker Diagrams for the granitoids of the Hartville Uplift.

36

Figure 13. REE patterns for the granitoids and metapelites of the Hartville Uplift.

15-HU-28-01

15-HU-33-02

37

Figure 14. Chondrite-normalized ‘Spider’ diagrams for the granitoids and metapelites of

the Hartville Uplift. Elements ordered by Ionic Radius.

38

6. DISCUSION

Mafic Rocks

Despite similarities, differences are evident between the series. In the TAS diagram

(Figure 15), it can be seen that the Muskrat Canyon metabasalt is more alkaline, which is

consistent with the higher contents in TiO2 (Figure 9). The flatter REE patterns of the

Mother Featherlegs metabasalt (Figure 10) is also an important difference. It was used by

Sims and Day (1999) to distinguish between the two suites.

Figure 15. Total Alkalis vs. Silica (TAS) diagram. After LeBas et al (1986). Line separating

Alkaline from Subalkaline fields after Irvine and Baragar (1971).

39

The metadiabases are a singular case: in all major and trace elements they plot close

to the Mother Featherlegs metabasalt; however, they show a clear enrichment in K2O

(Figure 15), where they plot with the Muskrat Canyon metabasalt. This K2O enrichment

could be secondary due to crustal contamination. As stated above, the metadiabase dikes

have tonalitic pods in gash fillings (Figure 5c). Around these pods, concentrations of biotite

are visible (Figure 5d). I suggest that the tonalitic pods are melts extracted from the

metadiabase leaving biotite crystals behind. Some metadiabase dike samples (e.g. 15-HU-

32-01B) don’t show these pods. When they do, they present abundant chlorite and

poikilioblastic textures (Figure 5a), suggesting recrystallization. I suggest that when they

were emplaced, the metadiabases were geochemically indistinguishable from the Mother

Featherlegs metabasalt, and that the current differences are due to assimilation of K from

the crust or the host granite during ascent and emplacement.. Sample 15-HU-77-01B is

probably an exception. The negative Eu anomaly, the steep REE pattern, and elevated Ba

and Rb concentrations suggest that this rock either (a) came from a different source, (b)

has undergone a differentiation process that other diabases did not, or (c) assimilated

different crustal rocks during transport.

Discrimination diagrams are used here to infer the tectonic setting of the mafic

suites. Only diagrams with elements considered immobile in fluids are used due to the

metamorphic processes that these rocks have suffered. This is clearly evidenced in the Th

and U anomalies of the spider diagrams (Figure 11) and signature of the crustal

assimilation in Figure 16.

40

Figure 16. Th/Yb vs. Nb/Yb diagram. After Pearce (2008). Notice that despite the apparent

crustal contamination in both basaltic suites they remain clearly distinct. The N-MORB

characteristic of the Mother Featherlegs metabasalt and the E-MORB characteristic of the

Muskrat Canyon metabasalt are consistent with discrimination diagrams in Figure 17.

Figure 17 shows two different discrimination diagrams. Mother Featherlegs metabasalt

falls within the fields of N-MORB / Ocean Floor Basalt. This is consistent with the flat

REE patterns (Figure 10a) and low alkali concentrations (Figure 15) that characterize N-

MORB’s (Sun and McDonough, 1989). Muskrat Canyon metabasalts coincide roughly

with the fields of Within Plate Tholeiite and Within Plate Basalt, with is consistent with

more fractionated REE’s, more elevated alkalis (Figure 15), and higher Nb/Y and Th/Y

41

rations (Figure 16). The metadiabases tend to fall in the same field that the Mother

Featherlegs metabasalt.

Figure 18. Tectonic discrimination diagrams for the mafic suites. Symbols as in figure 2

(no Nb data for the metadiabases was available from Sims and Day, 1999). a) After

Meschede (1986). WPA = Within Plate Alkali basalts; WPT = Within Plate Tholeiitic

basalt; VAB = Volcanic Arc Basalt. b) After Pearce and Cann (1973). LKT = Low K

Tholeiites; OFB = Ocean Floor Basalt; CAB = Calc Alkali Basalt; WPB = Within Plate

basalt.

The following tectonic explanation is proposed for these basalts. Whether they are

related or not, the Muskrat Canyon metabasalt represents an older basalt, related to the

melting of a plume source (or at least a more fertile mantle; Sun and McDonough, 1989),

whereas the Mother Featherlegs metabasalt is related to a rifting process where the basalts

were sourced from an asthenospheric mantle. Because more similarities than differences

are found between the metadiabases and the Mother Featherlegs metabasalt, I propose that

the metadiabases are plutonic equivalents of these metabasalts.

The relationship of these basalts to those in nearby Laramide uplifts is complicated.

The lack of modern geochemical studies for basalt suites in these ranges makes a direct

42

comparison difficult. However, some similarities occur. The Muskrat Canyon metabasalt

is relatively undeformed and presents very well preserved basaltic flows and pillow lavas

(Figure 3a, b). Well preserved pillow lavas have been found as well in the Medicine Bow

and Sierra Madre mountains (Houston et al., 1992) and the Laramie Range (Graff et al.,

1982), and Allard (2003) found amphibolites with calc-silicate pods (as in the Muskrat

Canyon metabasalt, Figure 3c) in the Laramie range. A common feature of these rocks is

that they have been classified as Archean, although there is discussion on whether they are

older (Houston et al., 1992; Allard, 2003) or younger (Graff et al., 1982) than the 2.6

granites (see below). Van Boening and Nabelek (2008) have found rocks with a very

similar REE geochemistry to the Mother Featherlegs metabasalt in the Black Hills

(Pactola-Rushmore amphibolite and/or Bear Mountain amphibolite). If these rocks are

related with the Mother Featherlegs metabasalt, it would mean that the age of this suite is

Proterozoic, as opposed to the probable Archean age of the Muskrat Canyon metabasalt.

Granitoids.

All granites of the Hartville Uplift have similar major and trace element

compositions (Figure 12). Flattop Butte and Haystack Range granites are nearly identical

and share a Nd model age. Although the Rawhide Buttes granite presents steeper REE

patterns, it also includes the sillimanite-bearing phase (15-HU-28-01) that has identical

REE patterns to the Flattop Butte and Haystack Range granites and a more peraluminous

composition (Figure 12 through 14).

The reason for the occurrence of sillimanite in these rocks is their high ASI

(Aluminum Saturation Index; Appendix 1, Table A4) with respect to other Rawhide Buttes

samples (15-HU-73-02A, 15-HU-77-01 and 15-HU-85-02). The grey gneiss of the

43

Rawhide Buttes, having a very high K2O/Na2O ratio, is an enigmatic phase and may have

a different origin from rest of the Rawhide Buttes granite (e.g. Krugh, 1997).

Because of intense alteration, shearing and metamorphism (e. g. Figure 6), the use

of trace element discrimination diagrams for the granitic rocks of the Hartville Uplift is not

prudent, but tectonic inferences can still be done on a geochemical basis. The Rawhide

Buttes, Flattop Buttes and Haystack Range granites are nearly identical in major elements

and most trace elements. Their REE and spider diagrams are strikingly similar to the

metapelites. This characteristic, together with high K2O and ASI, point toward melting of

metapelites as the source for the granites of the Hartville Uplift. Steep REE patterns are

consistent with crustal melting of metapelites that occurs during continental collisions

(Nabelek and Bartlett, 1998). In fact, Nd model ages are nearly identical for the granites

and the metapelites (3.0 Ga; Day et al., 1999; Frost and Frost, 1993; Snyder and Peterman,

1982), indicating that they have a common crustal history.

The Rawhide Buttes granite has a slightly younger Nd model age (2.6 Ga), but it is

based on a single sample (Day et al. 1999). The Sm/Nd ratio in the Rawhide Buttes granite

is variable, so a range of model ages can be obtained from homogeneous 143Nd/144Nd ratios.

In fact, more peraluminous compositions (e. g. the sillimanite-bearing phase of the

Rawhide Buttes granite) crop out mainly in the western part of the Rawhide Buttes (Snyder

and Peterman, 1982), which indicates a possible compositional zoning in the Rawhide

Buttes granite. I suggest that the western part of the Rawhide Buttes granite represents the

top of a magmatic chamber (pluton) where later melts (more peraluminous, higher

abundances of incompatible elements) crystallized. These rock are equivalent to the

composition of the Flattop Butte granite (which has been described as being more

44

peraluminous than the Rawhide Buttes granite). In fact, both granites share identical ages

within error, which suggests that they can be considered a result of a single collisional

event.

The Haystack Range granite presents two dike samples within the Twin Hills

diorite that have positive Eu anomalies. These samples are interpreted here as plagioclase-

K-feldspar cumulates (in feeder dikes?), which is consistent with their REE patterns.

Major and trace elements of the Twin Hills diorite are completely different from

granites in the uplift. Major elements, including low Na2O/K2O ratios (Appendix 1, table

A3), reflect a mafic igneous source. REE patterns range from nearly flat to moderately

steep (Figure 13c). All these characteristics point toward a subduction-related origin.

Because no other subduction-related pluton of ca. 1.74 Ga is found to the north of the

Cheyenne Belt, the most likely associated process is a westward subduction beneath the

uplift related to either the Black Hills-Dakotan orogeny, in accord with Geist et al. (1989)

who have suggested the possibility of underplating of Proterozoic crust to explain the

isotopic composition of the Twin Hills diorite.

The last felsic plutonic rock to appear are the tonalitic pods of the metadiabases of

the Rawhide Buttes. Bear and Lofgren (1991) proved that tonalitic melts can be produced

by melting of amphibolites and that the Na2O/K2O ratio was a function of both, the original

composition of the rock and the pressure. In fact, the REE patterns are very similar to those

found in leucosomes of migmatites from the Black Hills (Nabelek, 1999) which are

supposed to have attained solid-state equilibrium with the melanosome. This, together with

the ‘neoformed’ appearance of the biotite (Figure 5b) and the dark haloes around the pods

45

(Figure 5d), points toward an in-situ melting of the metadiabases during shearing at ca.

1.72 Ga.

As opposed to the mafic rocks, the comparison with nearby uplift is very

straightforward. The K2O rich Archean Rawhide Buttes and Flat Top granites of the

Hartville Uplift show a 87Rb-87Sr age between 2.5 and 2.6 Ga. Only one U-Pb zircon age

of ca. 2.66 has been obtained for the Rawhide Buttes granite on the west side of the

Hartville Fault. Condie (1969) described a K2O rich granite in the Laramie Range (Figure

1), that has a 87Rb-87Sr age of 2.55 Ga (Johnson and Hills, 1976) and an U-Pb age of 2.61

Ga (Snyder et al., 1998). Additionally, a granitic phase with a 87Rb-87Sr age of ca. 2.55 Ga

(Peterman and Hildreth, 1978) and zircon U-Pb age older than 2.6 Ga (Ludwig and

Stuckless, 1978; Langstaff, 1995) also occurs in the Granite Mountains (Figure 1). These

granites crop out along the Oregon Trail Structure (Chamberlain et al., 2003) that has been

interpreted as a ca. 2.65 Ga structural belt by Grace et al (2006). The Hartville Uplift is

located only slightly to the south of the projection of the Oregon Trail Structure (Figure 1).

I propose that the Rawhide Buttes and Flattop Butte granites are part of this system of

granites related to a ca. 2.65 Ga collision. This precludes the conclusion that the Hartville

Uplift was part of the Wyoming province at that time.

46

7. CONCLUSIONS AND TECTONIC HISTORY

The following conclusions can be inferred from the petrographic and geochemical

data presented in this work:

The Hartville Uplift became part of the Archean Wyoming Province at least as late

as ca. 2.6 Ga. This is evidenced not only by the Rawhide and Flattop Buttes

granites, which may be related to the Oregon Trail Structure, but perhaps also by

the Archean (?) Muskrat Canyon metabasalt.

During the Paleoproterozoic, extension and rifting took place, as evidenced by the

metadiabase dikes, the Mother Featherlegs metabasalt and (probably) the Muskrat

Canyon metabasalt. Pillow structures in this basalt suggest eruption on an ocean

floor during rifting that preceded subsequent tectonic events. Since exposure of this

metabasalt is limited by faults and has not been dated, establishing its relationship

to tectonic events in the region is complicated.

In the Paleoproterozoic, east-west subduction followed by collision (Twin Hills

diorite and Haystack Range granite) took place ca. 1.74 Ga, which places them

between the ca. 1.76 Black Hills orogeny and the ca. 1.72 Dakotan orogeny. The

apparent continuity of ages suggests that the Black Hills and Dakota orogenies were

parts of one continuous collisional event.

A late product of deformation is the tonalitic fracture-fillings within Rawhide

Buttes diabase dikes whose chemical compositions suggests that they were

produced from the dikes during tectonic shearing or decompression.

47

APPENDIX 1: COMPLETE TABLES OF THE ANALYTICAL RESULTS AND

LOCATIONS

Sample

15-HU -32-

01B

15-HU -73-

01

15-HU -73-

03

15-HU -77-

01B

15-HU -80-

01

15- HU-85-

01

Latitude 42o33’15”N 42o34’56”N 42o34’56”N 42o34’43”N 42o35’37”N 42o35’46”N

Longitude 104o30’26”W 104o29’43”W 104o29’43”W 104o30’08”W 104o28’29”W 104o28’50”W

Unit Metadiabase Metadiabase Metadiabase Metadiabase Metadiabase Metadiabase

SiO2 49.35 49.31 48.16 52.06 50.89 47.66

TiO2 1.61 1.36 1.35 1.29 0.74 1.97

Al2O3 11.66 13.35 13.99 13.23 14.39 13.86

FeO* 13.38 14.13 13.77 12.94 9.49 14.31

MnO 0.22 0.25 0.28 0.25 0.18 0.22

MgO 9.92 7.47 7.99 7.21 6.21 5.89

CaO 8.63 9.13 9.24 8.60 9.89 9.63

Na2O 0.76 2.87 3.00 2.81 2.63 2.60

K2O 1.33 1.16 1.30 1.26 0.87 1.01

P2O5 0.11 0.09 - 0.05 0.04 -

LOI 2.45 0.85 0.88 0.67 2.77 1.69

Total 99.42 99.97 99.96 100.37 98.10 98.83

Mg# 0.57 0.49 0.51 0.50 0.54 0.42

V 310 339 - 5.38 249 -

Cr 451.4 129.4 - 20.8 222.4 -

Co 58.1 50 - 1.99 43.5 -

Ni 343 70 - 5.2 57.7 -

Cu 96.8 67.2 - 4.69 65.3 -

Zn 49.9 88.8 - 11.8 60.2 -

Rb 31.2 26.3 - 120 13.8 -

Sr 30.7 98.6 - 87.5 151.5 -

Y 34.9 25.6 - 34.8 16.6 -

Zr 96 79.5 - 165 36 -

Nb 4.03 6.65 - 17.6 1.4 -

Sn 1.62 0.99 - 0.96 0.5 -

Ba 1500 212 - 857 98.5 -

La 10.4 7.4 - 36.6 2.58 -

Ce 25.7 19.3 - 80.4 6.23 -

Nd 19 13.9 - 34.2 6.38 -

Sm 5.61 4.17 - 8.19 1.46 -

Eu 2.27 1.3 - 0.659 0.633 -

Gd 7.65 4.35 - 7.51 2.65 -

48

Tb 1.31 0.734 - 1.21 0.418 -

Dy 8.08 5.16 - 7.58 2.97 -

Yb 2.79 3.1 - 3.12 1.87 -

Lu 0.365 0.392 - 0.325 0.363 -

Ta 0.273 0.528 - 0.56 0.128 -

Pb 3.42 17.4 - 31.9 7.3 -

Th 1.02 0.606 - 49.2 0.31 -

U 0.544 0.306 - 3.93 0.315 -

Table A1.

Sample

15- HU-51-

01

15-HU -39-

01

15-HU -47-

01

15-HU -70-

01

15- HU-89-

02

15- HU-93-

01



Unit Meatadiabase

Muskrat

Canyon

Mother

Featherlegs

Muskrat

Canyon

Mother

Featherlegs

Muskrat

Canyon

SiO2 50.24 51.93 56.56 46.67 45.22 45.69

TiO2 2.39 0.93 1.16 1.47 0.80 1.32

Al2O3 12.17 11.72 13.78 15.17 12.35 14.86

FeO* 15.11 8.15 11.56 13.24 12.15 11.88

MnO 0.26 0.17 0.22 0.17 0.22 0.21

MgO 4.19 6.97 5.86 6.54 12.83 7.13

CaO 7.65 9.40 9.71 5.31 8.34 11.35

Na2O 3.16 3.99 1.46 4.28 1.34 2.37

K2O 0.28 0.24 0.09 0.12 0.15 0.17

P2O5 0.20 0.14 0.07 0.18 0.06 0.09

LOI 2.56 4.67 0.29 5.12 4.48 3.08

Total 98.20 98.31 100.77 98.26 97.95 98.16

Mg# 0.33 0.60 0.47 0.47 0.65 0.52

V 448 203 310 263 221 294

Cr 13.7 602.4 136.4 167.4 148.4 90.4

Co 41.3 35 42.2 53 60.7 54.6

Ni 9.1 154 67.1 111 237.4 82.4

Cu 13.2 24 28 12.6 62.9 83

Zn 120 62.6 74.2 116 88.4 92.8

Rb 4.57 3.09 2.85 1.71 3.42 1.37

Sr 282.5 165.5 67.1 128.5 75.7 182.5

Y 37.2 15.5 19.5 27.4 14.5 15.2

49

Zr 115 75 56 150 40.5 69.3

Nb 3.49 5.18 2.84 7.73 1.92 8.06

Sn 1.66 1.03 0.69 1.37 0.514 0.914

Ba 44 133 168 55.8 35.4 166

La 6.43 7.54 3.16 16.8 2.06 7.53

Ce 19.2 16.3 8.13 40.6 6.15 19.5

Nd 16.3 9.4 7.39 24.4 5.01 12.3

Sm 4.51 2.79 2.39 5.46 1.37 3.15

Eu 1.6 0.937 1.16 1.38 0.664 1.14

Gd 6.54 3.29 2.94 6.13 1.98 2.62

Tb 1.02 0.453 0.594 0.852 0.414 0.509

Dy 7.3 3.21 3.91 5.56 2.57 3.52

Yb 4.47 1.58 2.32 2.58 1.75 1.73

Lu 0.663 0.254 0.319 0.423 0.265 0.213

Ta 0.264 0.35 0.203 0.526 0.142 0.506

Pb 9.2 5.59 1.54 3.51 2.68 7.98

Th 1.29 2.7 0.28 2.12 0.198 0.759

U 0.863 0.452 0.171 0.351 0.106 0.132

Table A2.

Sample

15-HU -96-

02

15- HU-96-

01C

15-HU-22-

02

15-HU-24-

01

15-HU-25-

02

15-HU-25-

04



Unit

Muskrat

Canyon

Muskrat

Canyon Twin Hills Twin Hills Twin Hills

Haystack

Range?

SiO2 44.10 46.94 52.90 51.03 - 65.97

TiO2 2.14 0.52 1.44 0.91 - 0.43

Al2O3 12.51 11.38 15.04 12.69 - 12.76

FeO* 13.94 8.12 8.91 10.57 - 2.83

MnO 0.25 0.84 0.13 0.19 - 0.04

MgO 5.83 7.03 3.12 6.07 - 0.64

CaO 6.83 10.37 6.44 8.35 - 1.13

Na2O 3.65 2.10 2.93 2.67 - 2.61

K2O 0.17 0.86 2.02 0.34 - 4.80

P2O5 0.06 0.47 0.07 - 0.09

LOI 9.32 9.99 3.93 4.67 0.50 4.95

Total 98.75 98.21 97.33 97.56 - 96.26

ASI 0.43 0.61 0.80 0.64 - 1.11

50

V - 95.4 162 282 224 34.2

Cr - 168.4 36.1 177.4 17.4 14.1

Co - 21.5 22.8 47.1 23.9 5.32

Ni - 65.9 12.3 109.4 6.9 8.7

Cu - 6.72 18.5 108 16.3 4.18

Zn - 84.1 94.3 70.5 112 29.9

Rb - 33.4 62.5 8.27 71.1 107

Sr - 100.5 529.5 151.5 403.5 308.5

Y - 20.1 36.5 16.2 50 5.16

Zr - 98.8 429 53.9 353 363

Nb - 8.44 27.1 3.07 29.4 7.74

Sn - 2 1.51 0.65 1.27 0.45

Ba - 615 1120 115 1600 1790

La - 28.2 75.8 4.24 77 48.3

Ce - 57 163 10.6 188 72

Nd - 23 72.8 7.72 88.1 17.7

Sm - 4.14 11.5 2.26 16.4 1.83

Eu - 0.9 2.76 0.91 3.21 1.74

Gd - 3.73 9.3 3.06 13.8 1.47

Tb - 0.53 1.33 0.527 1.78 0.163

Dy - 3.51 8.16 3.62 10.5 1.11

Yb - 2.24 3.64 1.84 4.89 0.482

Lu - 0.31 0.629 0.27 0.776 0.124

Ta - 0.742 1.49 0.228 1.58 0.396

Pb - 10.3 24.9 1.91 14.6 19.6

Th - 11.1 11 0.407 9.2 38.9

U - 1.69 1.99 0.101 2.02 2.41

Table A3.

Sample

15-HU-25-

03

KC-HU-92-

17

15-HU-09-

01

15-HU-19-

01

15-HU -88-

02 FTB

Latitude 42o23’58”N * 42o25’57”N 42o25’38”N 42o24’14”N *

Longitude 104o34’09”W * 104o37’43”W 104o37’57”W 104o38’22”W *

Unit

Haystack

Range?

Haystack

Range

Haystack

Range

Haystack

Range

Haystack

Range Flattop Butte

SiO2 - 74.24 73.46 78.23 72.81 73.74

TiO2 - 0.25 0.27 0.01 0.25 0.21

Al2O3 - 13.44 13.33 12.56 11.35 13.62

51

FeO* - 2.71 2.48 0.56 1.74 1.79

MnO - 0.04 0.04 0.00 0.02 0.02

MgO - 0.42 0.44 0.04 0.12 0.55

CaO - 1.13 0.90 0.03 0.34 0.42

Na2O - 3.08 2.81 3.57 2.45 2.95

K2O - 5.14 5.24 4.85 4.52 5.24

P2O5 - 0.07 0.06 0.01 0.08

LOI 0.07 0.06 1.07 0.37 3.82 1.12

Total - 100.59 100.10 100.23 97.42 99.73

ASI - 1.06 1.12 1.12 1.19 1.21

V 31.7 16.1 17.5 3.53 - 44.2

Cr 15.8 16.8 19.9 15.9 - 24.5

Co 4.231 3.28 2.21 0.2 - 1.56

Ni 4.2 11.7 6.4 3.6 - 7.7

Cu 3.39 3.25 5.48 3.37 - 4.68

Zn 23.6 34 36.2 19.6 - 24.9

Rb 91.7 248 198 354 - 234

Sr 236.5 160.5 111.5 48.6 - 94.5

Y 10.3 30.5 39.8 66.6 - 28.7

Zr 234 238 292 142 - 208

Nb 9.87 29.4 22.1 44.8 - 24.5

Sn 0.522 3.15 16.5 2.31 - 8.76

Ba 1280 911 826 81.7 - 823

La 22.5 78.9 117 12.8 - 76.3

Ce 45.6 150 230 52.3 - 153

Nd 19.3 51.4 80.9 15.4 - 57.5

Sm 2.95 7.07 13.1 4 - 10.4

Eu 1.32 1.05 1.23 <LOD - 0.869

Gd 2.61 5.74 9.46 5.87 - 7.34

Tb 0.426 0.839 1.38 1.51 - 1.13

52

Dy 1.93 5.35 7.49 11.5 - 6.79

Yb 1.12 3.88 4.4 11.8 - 2.99

Lu 0.217 0.57 0.597 1.87 - 0.443

Ta 0.43 2.24 2.1 7.52 - 3.11

Pb 15.4 26 23.2 17.1 - 42.6

Th 5.77 44.8 56 67.8 - 56.9

U 1.39 5.75 6.07 6.59 - 3.28

Table A4. * = sample provided by Kevin Chamberlain (University of Wyoming, 2015).

Sample

KC-HU-94-

01

15-HU-33-

02

15-HU-28-

01

15-HU -67-

01

15-HU -73-

02A

15-HU -77-

01

Latitude * 42o33’15”N 42o33’25”N 42o34’52”N 42o34’56”N 42o34’43”N

Longitude * 104o30’27”W 104o30’24”W 104o34’42”W 104o29’43”W 104o30’08”W

Unit

Flattop

Butte

Rawhide

Buttes

Rawhide

Buttes

Rawhide

Buttes

Rawhide

Buttes

Rawhide

Buttes

SiO2 75.18 74.76 71.41 71.18 72.89 72.53

TiO2 0.14 0.09 0.21 0.33 0.24 0.17

Al2O3 13.58 9.67 13.04 12.39 12.74 12.22

FeO* 1.09 0.37 2.06 2.83 1.66 0.94

MnO 0.02 0.01 0.01 0.01 0.02 0.01

MgO 0.43 0.37 0.69 0.34 0.84 0.69

CaO 0.39 0.61 0.21 0.04 0.65 0.32

Na2O 3.24 1.22 2.81 0.16 3.87 3.45

K2O 6.16 5.51 5.86 7.29 4.15 4.74

P2O5 0.04 0.07 0.09 0.02 0.05

LOI 0.72 3.87 2.13 3.47 1.75 2.72

Total 101.00 96.55 98.53 98.06 98.87 97.79

ASI 1.07 1.06 1.15 1.51 1.06 1.07

V 6.43 20 15.2 22.9 14.1 -

Cr 12.3 20.6 18 25 25.1 -

Co 1.52 2.52 2.24 3.75 3.55 -

Ni 3.6 4.1 4.1 8.8 4.4 -

Cu 3.36 15 19.5 62.6 57.7 -

Zn 17.2 4.6 18.5 19.9 20.6 -

Rb 243 185 194 219 80.3 -

Sr 65.2 60.4 93.5 60.4 64.9 -

53

Y 44.1 22.9 55.9 14.4 7.45 -

Zr 112 146 228 206 186 -

Nb 17.6 15.6 19 8.74 8.19 -

Sn 5.8 1.9 3.91 2.13 0.975 -

Ba 590 862 862 1540 1220 -

La 38.3 59.6 80.9 61.9 27 -

Ce 74.6 109 164 115 54.8 -

Nd 28.1 43.2 62.3 42 21.8 -

Sm 6.23 8.28 10.5 6.28 3.98 -

Eu 0.511 1.45 0.93 0.858 0.718 -

Gd 5.78 5.87 8.38 4.36 2.61 -

Tb 1.11 0.862 1.25 0.616 0.327 -

Dy 7.25 4.35 8.93 2.9 1.72 -

Yb 4.5 2.46 5.36 1.14 0.569 -

Lu 0.664 0.413 0.838 0.181 0.099 -

Ta 1.63 1.67 1.03 0.553 0.162 -

Pb 38.8 21.7 33.4 23.6 135.5 -

Th 33.8 35.5 53.3 60.3 35.2 -

U 2.77 4.16 7.08 3 2.51 -

Table A6. * = Sample provided by Kevin Chamberlain (University of Wyoming, 2015).

Sample

15-HU -85-

02

15-HU -73-

01 POD

15-HU -73-

03 POD

15-HU-65-

02

KC-HU -92-

15

15-HU -34-

01


Longitud

e 104o28’50”

W 104o29’43”W 104o29’43”W 104o34’46”W 104o34’46”W 104o31’24”W

Unit

Rawhide

Buttes Tonalitic Pod Tonalitic Pod Metapelite Metapelite Metapelite

SiO2 73.68 74.64 70.14 63.62 61.50 62.21

TiO2 0.20 0.16 0.25 0.44 0.69 0.80

Al2O3 12.79 12.25 13.43 13.66 16.61 18.26

FeO* 0.75 0.88 1.79 4.81 7.00 7.60

MnO 0.01 0.02 0.03 0.01 0.09 0.13

MgO 0.57 0.52 0.95 0.98 2.31 2.39

CaO 0.28 3.01 3.71 0.13 0.60 0.73

Na2O 3.65 3.92 4.11 1.54 1.30 0.78

K2O 5.24 0.49 0.56 5.51 6.06 5.56

P2O5 0.01 0.03 0.10 0.11 0.24

LOI 1.67 2.29 2.75 - - -

Total 98.84 98.20 97.75 5.37 2.88 1.96

54

ASI 1.05 0.98 0.95 1.57 1.70 2.12

V - 22.9 43.2 80 97.8 107

Cr - 26.6 45.2 13.1 105.4 108.4

Co - 7.07 8.84 11.1 14.8 18.2

Ni - 11.1 19.7 9.7 41 31.2

Cu - 47.2 36.6 5.67 33 50.2

Zn - 13 20.2 39.5 118 108

Rb - 11.7 16.5 154 217 216

Sr - 197.5 203.5 76.6 116.5 71.4

Y - 1.56 3.58 23.3 23.6 25.1

Zr - 28.5 44.9 70.5 151 177

Nb - 0.604 1.39 13.2 13.9 14.8

Sn - 0.299 0.245 11.9 3.1 4.7

Ba - 158 156 913 1200 1120

La - 1.34 1.89 27.4 39.9 42.8

Ce - 2.13 3.32 49.9 81.8 90.7

Nd - 0.922 1.82 20.7 36.2 41.1

Sm - 0.289 0.364 3.94 6.47 7.62

Eu - 0.27 0.357 0.661 1.89 1.02

Gd - 0.238 0.786 3.38 5.19 5.84

Tb - 0.039 0.085 0.559 0.695 0.964

Dy - 0.391 0.617 3.98 4.46 4.8

Yb - 0.183 0.456 2.5 2.45 2.58

Lu - <LOD 0.053 0.408 0.36 0.395

Ta - 0.037 0.109 1.92 0.955 1.03

Pb - 23.8 25.4 12.9 33.2 16.5

Th - 0.142 0.131 13.5 14.8 13.9

U - 0.082 0.092 1.39 3.59 3

Table A7.

55

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