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JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 PAGES 627649 2000

The PlagioclaseMagma Density ParadoxRe-examined and the Crystallization ofProterozoic Anorthosites

JAMES S. SCOATES*DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES (DSTE), UNIVERSITE LIBRE DE BRUXELLES, CP160/02,

AVENUE F. D. ROOSEVELT 50, B-1050, BRUSSELS, BELGIUM

RECEIVED FEBRUARY 4, 1999; REVISED TYPESCRIPT ACCEPTED OCTOBER 5, 1999

characteristic feature during the crystallization of many ProterozoicIntermediate-composition plagioclase (An4060) is typically less denseanorthosites and layered intrusions.than the relatively evolved basaltic magmas from which it crystallizes

and the crystallization of plagioclase produces a dense residualliquid, thus plagioclase should have a tendency to float in thesemagmatic systems. There is, however, little direct evidence for

KEY WORDS: magma; density; Proterozoic anorthosites; blocks; plagioclaseplagioclase flotation cumulates either in layered intrusions or inProterozoic anorthosite complexes. The layered series of the PoeMountain anorthosite, southeast Wyoming, contains numerous anor-thositeleucogabbro blocks that constrain density relations during

INTRODUCTIONdiVerentiation. All blocks are more mafic than their hosting anor-During the low-pressure crystallization of basaltic magma,thositic cumulates, their plagioclase compositions are more calcic,the densities of the common ferromagnesian silicatesand each block is in strong Sr isotopic disequilibrium with its hostthat crystallizeolivine, orthopyroxene, clinopyroxene,cumulate. Associated structuresdisrupted and deformed layeringpigeoniteincrease progressively with decreasing mg-indicate that (1) a floor was present during crystallization and thatnumber (Fig. 1), allowing for relatively easy separationplagioclase was accumulating and/or crystallizing on the floor, (2)of crystals and melt. The density of plagioclase, however,compositional layering and plagioclase lamination formed directlydecreases progressively with fractionation, such that pla-

at the magmacrystal pile interface, and (3) the upper portions of gioclase of intermediate composition (An4060) is less densethe crystal pile contained significant amounts of interstitial melt. than the relatively evolved basaltic magma from whichLiquid densities are calculated for proposed high-Al olivine gabbroic it crystallizes (Fig. 1). The crystallization of plagioclaseparental magmas and Fe-enriched ferrodioritic and monzodioritic alone results in depletion of the relatively light oxideresidual magmas of the anorthosites taking into account pressure, components SiO2, Al2O3, CaO and Na2O, and en-oxygen fugacity, P2O5, estimated volatile contents, and variable richment in the heavier components TiO2, FeO*andtemperatures of crystallization. For all reasonable conditions, cal- MgO, resulting in progressively denser residual magmas.culated block densities are greater than those of the associated melt. The combination of these processes implies that inter-The liquid densities, however, are greater than those for An4060 mediate-composition plagioclase should have a strongplagioclase, which cannot have settled to the floor. Plagioclase must tendency to float in evolved basaltic magmas. However,either have been carried to the floor in relatively dense packets of there is abundant evidence for the crystallization ofcooled liquid plus crystals or have crystallized in situ. A sloping floor, plagioclase on the floor of magma chamberslayering,possibly produced by diapiric ascent of relatively light plagioclase-rich lamination, scoursand very little demonstrable evi-cumulates, is required to allow for draining and removal of the dence for plagioclase flotation as an eVective mechanism

of diVerentiation. This contradiction is known as thedense interstitial liquid produced in the crystal pile and may be a

*Telephone: +322-650-4714. Fax: +322-650-3748; e-mail:[email protected] Oxford University Press 2000

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 5 MAY 2000

worst-case scenario for the plagioclasemagma densityparadox as they consist of enormous volumes (1000sto 10 000s of km3) of cumulate anorthositeleuconoriteleucotroctolite (7595% plagioclase of inter-mediate composition, An4060), associated with minorintrusions of troctolite and Fe-enriched dioritic rocks,and large granitic batholiths (Morse, 1982; Emslie, 1985;Wiebe, 1992; Ashwal, 1993). Each complex contains anumber of diVerent plagioclase-rich plutons that span astructural range from massive to layered to diapiric(Wiebe, 1992), the last being characterized by stronglydeformed marginal zones. Individual plutons typicallyshow little variation in plagioclase compositions, althoughextensive FeMg fractionation in the ferromagnesiansilicates may be present (Wiebe, 1992; Scoates, 1994).Individual plagioclase grains commonly show only minorzoning, although prominent oscillatory zoning may befound locally (Wiebe, 1992; see also the cover of theJournal of Petrology for 1998).

If plagioclase were to float in evolved basaltic magmas,then Proterozoic anorthosites should be the ideal type-example of this process. This presumption has beenexploited by numerous workers and forms the basisfor the general model for the formation of Proterozoic

Fig. 1. Mineral density vs diVerentiation index (DI) for plagioclase, anorthosites involving polybaric crystallization of mantle-where DI = An/(An + Ab), and for olivine, orthopyroxene, clino-derived basaltic magma (Morse, 1968; Emslie, 1985;pyroxene and pigeonite, where DI = Mg/(Mg + Fe2+). Mineral

densities for the FeTi oxides are given with respect to Fe3+/(Fe2+ + Longhi & Ashwal, 1985; Wiebe, 1992; Ashwal, 1993;Fe3+) for reference only. All mineral densities are at 1 atm from Deer Longhi et al., 1993). Fractional crystallization in chamberset al. (1992). The general range of densities for basaltic magmas is

at the base of the crust produces ultramafic cumulatesshown by the shaded field. The densities of the ferromagnesian silicatesincrease with decreasing mg-number, whereas those of plagioclase on the floor and relatively evolved resident magma.decrease with decreasing An. Plagioclase with compositions less than Plagioclase eventually saturates and, because of the en->An60 are less dense than the relatively evolved basaltic magmas from hanced density contrast between basaltic magma andwhich they crystallize.

plagioclase at higher pressures (Kushiro, 1980), floats tothe top of the chambers, where it may be locally remeltedas a result of periodic replenishment of hotter, less evolvedplagioclasemagma density paradox (Morse, 1973) andmagma, thus enriching the interstitial liquid in plagioclasehas resulted in a major re-evaluation of how layeredcomponents (Wiebe, 1992). Buoyancy-driven ascent ofintrusions crystallize (Campbell, 1978; Campbell et al.,plagioclase-rich diapirs (5070 vol. %) into the crust is1978; McBirney & Noyes, 1979; Chen & Turner, 1980;then proposed for the formation of individual plutonsIrvine et al., 1983; Huppert & Sparks, 1984), and is the(Emslie, 1985; Longhi & Ashwal, 1985; Longhi et al.,source of continued discussion (McBirney, 1985; Morse,1993) that consolidate essentially as flotation cumulates1986a; Irvine, 1987). The plagioclasemagma densitywith progressive crystallization of the interstitial meltparadox has major implications for the formation ofand removal of the dense FeTiP-rich components.anorthosite horizons in layered intrusions (e.g. StillwaterUnambiguous evidence of flotation cumulates, however,intrusion, Montana: Raedeke & McCallum, 1980; Salpashas been diYcult to identify in the field. Some layeredet al., 1983; Czamanske & Bohlen, 1990; Haskin &leucotroctolites grade upwards into massive, undeformedSalpas, 1992) and especially for Proterozoic anorthositeleuconorite to anorthosite that could be interpreted ascomplexes.stagnant accumulations of suspended plagioclase beneathMuch of the debate over where plagioclase crystallizesthe roofs of magma chambers (Emslie, 1970; Wiebe,in basaltic magma chambers has focused on layered1992). In contrast, numerous layered anorthosites, leuco-mafic intrusions, in particular the Skaergaard intrusionnorites and troctolites from many Proterozoic anor-(e.g. McBirney & Noyes, 1979; Irvine, 1987; McBirney,thosite complexes show abundant evidence for bottom1995; Irvine et al., 1998), which are typically multi-accumulation of plagioclase (Kiglapait, Nain: Morse,saturated in ferromagnesian silicates and plagioclase

FeTi oxides. Proterozoic anorthosites represent the 1969, 1979; Harp Lake: Emslie, 1980; Paul Island, Nain:

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SCOATES PLAGIOCLASEMAGMA DENSITY PARADOX

Wiebe, 1990; BjerkreimSokndal, Rogaland: Wilson et The LAC consists of a central mass of anorthositic rocks(550 km2) intruded by dioritic and troctolitic rocks ofal., 1996).

Resolution of the plagioclasemagma density paradox the Strong Creek complex (Mitchell et al., 1995, 1996),monzonitic rocks of the Maloin Ranch pluton (Kolkeris clearly critical to better understanding the magmatic

evolution of Proterozoic anorthosites, and will certainly & Lindsley, 1989; Kolker et al., 1990, 1991), the Sybilleintrusion (Fuhrman et al., 1988; Frost & Touret, 1989;have applications to many layered mafic intrusions where

plagioclase is an important cumulus mineral. In this Scoates et al., 1996) and the Red Mountain pluton(Anderson, 1995), and granitic rocks of the Shermanpaper, I present new field observations and geochemistry

from leucogabbroic to anorthositic blocks in the layered batholith (Geist et al., 1989; Edwards, 1993) (Fig. 2).series of the Poe Mountain anorthosite, southeastern Emplacement pressures for the monzonitic rocks wereWyoming. The presence of deformational structures be- >3 kbar in the north (Anderson et al., 1987; Fuhrmanneath these blocks demonstrates the existence of a floor to et al., 1988) and>4 kbar in the south (Kolker & Lindsley,the magma chamber where plagioclase was accumulating 1989) (Fig. 2).and crystallizing. Layering and plagioclase lamination The Poe Mountain anorthosite, exposed over 200 km2

formed directly at the magmacrystal pile interface. Dens- in the northern LAC (Fig. 3), is structurally a north-ity calculations involving appropriate parental and re- plunging antiform. The core of the antiform is occupiedsidual magma compositions are consistent with observed by pervasively recrystallized anorthosites containing fewblock impact structures, but clearly indicate that inter- relict magmatic features. Along the western and northernmediate-composition plagioclase was not capable of sink- margins of the intrusion, the core anorthosites gradeing in any of the associated magmas; thus alternative outward into a 57 km thick marginal layered seriesmechanisms must be sought to explain bottom ac- (Fig. 3) that displays abundant igneous layering and acumulation. The density contrast between plagioclase prominent, pervasive plagioclase lamination. The layeredand dense residual magma, which becomes even greater series is composed of two distinct zones, the lower oras crystallization of intermediate-composition plagioclase inner anorthositic layered zone (ALZ) with shallow toprogresses, poses a major problem for the formation of moderate dips of 3060 and the upper or outer le-pure anorthositic cumulatessloping floors are required ucogabbroic layered zone (LLZ) with moderate to steepfor drainage and removal of the interstitial liquid (Morse, dips of 6090. Both layered zones contain distinct,1986a, 1988). Diapiric ascent of hot, relatively light, laterally continuous layered sections (lower, middle andconsolidated anorthosite deeper in the crystal pile is upper in the ALZ, and lower and upper in the LLZ)responsible for progressive rotation of the magma cham- that are defined by changes in mineral assemblages,ber floor. layer characteristics, mineral compositional variation,

plagioclase abundance and Sr isotopic variation (Scoates,1994; Scoates & Frost, 1996) (Fig. 4). The uppermostlevels of the LLZ are not preserved, because of subsequent

GEOLOGIC SETTING OF THE POE emplacement of monzonitic rocks of the 143 Ga SybilleMOUNTAIN ANORTHOSITE intrusion (Scoates et al., 1996). Anorthosite xenoliths,

including both strongly laminated and pervasively re-The Poe Mountain anorthosite is one of three compositecrystallized types, occur within the Sybille intrusion. Theintrusions dominated by anorthositic cumulates in thesouthern limit of the Poe Mountain anorthosite is moreunmetamorphosed 725 km2 Laramie anorthosite complexdiYcult to define as the majority of anorthosites in the(LAC), southeast Wyoming (Fig. 2). The LAC is thecentral part of the LAC are pervasively recrystallized andlarger of two Proterozoic anorthosite complexes exposedprimary magmatic structures are rare. The southernmostin the core of one of the Late Cretaceous to early Jurassicextension of the LLZ is clearly truncated by troctoliticLaramide uplifts, the Laramie Mountains, that form therocks that may be related to the larger Strong Creekpresent-day Rocky Mountains in the western USA. Thetroctolite to the south (Mitchell, 1993). Recent mapping143 Ga LAC (Scoates & Chamberlain, 1995), and thehas shown that the upper and middle ALZ in the sameearlier 176 Ga Horse Creek anorthosite complex to thearea are cut by coarse-grained megacrystic anorthosite,south (Scoates & Chamberlain, 1997), intruded alongsimilar to that of the Snow Creek anorthosite (D. H.the Cheyenne belt, the >178 Ga collisional zone thatLindsley & B. R. Frost, personal communication, 1998).separates Archaean rocks of the Wyoming Province toWhere recrystallization is less pronounced, the centralthe north from accreted Proterozoic island arc terranescore area of the Poe Mountain anorthosite is char-to the south (Karlstrom & Houston, 1984) (Fig. 2). Onlyacterized by megacrystic anorthosite (iridescent anda portion of the LAC is exposed at present; west-dippingstrongly zoned) of the Snow Creek anorthosite, moreLaramide thrust and high-angle reverse faults truncate thecalcic (>An50) than plagioclase from the core of the Poeeastern margins, and shallowly dipping early Palaeozoic

sediments unconformably overlie the western margins. Mountain anorthosite (>An46).

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Fig. 2. Geologic map of the southern Laramie Mountains of southeastern Wyoming showing the major anorthositic (Poe Mountain, Chugwaterand Snow Creek) and monzonitic (Sybille, Red Mountain and Maloin Ranch) intrusions of the 143 Ga Laramie anorthosite complex and graniticintrusions of the contemporaneous Sherman batholith. The inferred trace of the Cheyenne belt is noted and average crustal ages north and south ofthe suture zone are shown. Pressure estimates from the surrounding monzonitic intrusions are indicated and increase slightly to the south.

extensive weathering limit the number of occurrencesBLOCKS IN THE POE MOUNTAIN that can confidently be identified. A number of blocksANORTHOSITE have been identified in outcrop along Wyoming State

Highway 34 (WY34) and correspond to nearly the sameBlocks of igneous origin occur throughout the layeredrelative stratigraphic position in the Poe Mountainseries of the Poe Mountain anorthositethe major loc-anorthosite (-2400 to-2240 m) near the upper contactalities are noted in Fig. 3and range in compositionof the lower ALZ with the middle ALZ. A single blockfrom anorthosite to olivine leucogabbronorite (Table 1).of 3 m width located in the middle ALZ to the south ofIt is likely that a significant number of additional blocksthe Sybille FeTi oxide deposit contains very coarse-occur, but unfavourable exposures (sections per-

pendicular to layering show the best relations) and locally grained plagioclase (50100 cm) and high-Al

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Fig. 3. Simplified geologic map of the Poe Mountain anorthosite showing stratigraphic relations and block locations (C). An arrow pointsupsection where stratigraphic tops could be determined in outcrop. ALZ, anorthositic layered zone; LLZ, leucogabbroic layered zone. Thecontact between the ALZ and LLZ is shown as a continuous line and the contacts between layered sections within the two layered zones (lower,middle, upper) are shown as dashed lines. Layering dips 3060 to the west and north in the inner parts of the complex and 6090 along theupper and outer portions.

clinopyroxene megacrysts, and will be discussed sep- envelopes each block (Figs 6 and 7). Stratigraphicallyarately in more detail below. below the blocks, anorthosite typically shows strongly

disrupted or deformed igneous layering. The prominentplagioclase lamination that parallels compositional lay-

Outcrop relations between blocks and ering in the undisturbed sections of the Poe Mountainlayered anorthosites anorthosite wraps around individual blocks (Figs 6 and 7).

In addition, several localities contain abundant irregularlyThe blocks are typically elongate and sub-rounded withdistributed mafic pegmatoids within the zone of de-the longest dimension ranging from 1 to 50 m (Fig. 5).formationdisruption. Stratigraphically above each block,Where the exposures are perpendicular to compositionalcompositional layering and plagioclase lamination arelayering in the anorthosites, an asymmetric set of struc-

tures in the hosting anorthositic cumulates consistently coplanar or drape slightly over individual blocks (Fig. 6).

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Fig. 4. Stratigraphic subdivisions of the Poe Mountain anorthosite. (a) Schematic stratigraphic column showing major subdivisions of the layeredseries and general characteristics. It should be noted that monzonitic rocks of the Sybille intrusion truncate the upper portions of the leucogabbroiclayered zone. The Sybille intrusion shows no evidence of recrystallization and contains xenoliths of both recrystallized and layered anorthosite.The relative stratigraphic position (RSP) reflects the relative horizontal distance from the ALZLLZ contact. The RSP is not a true stratigraphicthickness as no attempt has been made to correct for the variable dips of layering. (b) RSP vs plagioclase abundance in the anorthosites andblocks, where the percent plagioclase equals the sum of the cation normative feldspar components (An + Ab + Or). The majority of blocksare less rich in plagioclase than their hosting cumulates. Data from Scoates (1994).

There is no structural evidence of disruption above blocks contain a structural fabric defined by the preferredorientation of ferromagnesian silicates and this foliationthe blocks. There is also no evidence that these blocks

represent the products of late-stage metasomatism as has is in each case oblique to the general foliation (com-positional layering plus plagioclase lamination) of thebeen proposed for many of the anorthosite inclusions in

the Skaergaard intrusion (McBirney, 1996; Sonnenthal enveloping anorthositic cumulates. After impact, pro-gressive accumulationcrystallization of plagioclase on& McBirney, 1998)contacts are sharp, structures can-

not be traced through the blocks, and the bulk com- the chamber floor continued, resulting in the coherent,planar compositional layering and lamination that drapesposition of the blocks is more mafic than that of their

host cumulates (Figs 6 and 7). over the tops of the blocks.The outcrop relations have important implications forI interpret the structural and textural features to record

the impact of settled blocks onto a pre-existing floor of the crystallization of plagioclase-rich cumulates in thePoe Mountain anorthosite:plagioclase cumulates containing interstitial melt, similar

to those described in the Duke Island and Skaergaard (1) a floor to the Poe Mountain anorthosite magmachamber was present during crystallization. The presenceintrusions (Irvine, 1987; Irvine et al., 1998). The disrupted

layering and curvilinear plagioclase lamination may be of this interface between the anorthositic parent magmaand the crystal pile indicates that intermediate-com-attributed to the force of impact. The presence of mafic

pegmatoids in the zones of disruption probably reflects position plagioclase (An4555) was capable of accumulatingon the floor of the chamber and forming thick piles ofthe migration of interstitial liquid produced during com-

paction of the plagioclase-rich crystal pile by the blocks. plagioclase cumulates.(2) Compositional layering and plagioclase laminationSmall block fragments associated with larger blocks sug-

gest that some may have broken apart on impact. Many formed directly at the magmacrystal pile interface, as

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Table 1: Geochemistry of blocks in the Poe Mountain anorthosite (PMa)

Blocks Megacryst Layered Series

Sample: GR289 PM466 PM555 SR248 GR258 SR246 PM623 ALZ LLZ

RSP (m): -2400 -2300 -2240 -1500 -560 870 -1660 av. av.

Type: lg lg lgn an olgn olgn cpx

SiO2 5200 5260 5220 5360 4700 5080 4790 5415 5076

TiO2 071 060 062 055 216 163 181 059 125

Al2O3 2580 2400 2330 2620 1930 2210 620 2558 2250

Fe2O3t 282 473 538 276 1280 693 1140 229 693

(FeOt) 254 425 484 248 1151 623 206 623

MnO 006 006 007 003 015 009 021 003 008

MgO 155 256 273 085 511 178 1240 069 207

CaO 1090 960 883 970 736 961 1870 933 944

Na2O 415 422 426 477 332 404 038 507 406

K2O 064 084 087 092 076 102 004 109 099

P2O5 004 006 008 005 009 029 005 015 025

LOI 123 062 185 093 154 208 040 115 157

Total 9990 9989 10019 10036 9959 10037 9949 10012 9989

mg-no. 052 052 050 038 044 034 068 038 036

An* 548 507 491 495 508 480 805 460 489

Cation norm

Qz 00 00 00 00 00 00 01 00

Or 38 49 52 54 47 62 64 59

Ab 373 377 386 424 309 371 444 371

An 499 439 422 469 368 400 432 410

Ne 00 00 00 00 00 00 05 00

Di 32 25 12 04 04 54 14 40

Hy 31 63 79 18 96 59 14 48

Ol 13 29 32 19 123 14 11 37

Mt 04 07 09 04 21 11 04 11

Il 10 08 09 08 31 23 08 18

Ap 01 01 02 01 02 06 03 05

Plag (%) 91 87 86 95 72 83 94 84

ppm

Sc 845 114 102 462 717 122 127 666 128

V 628 425 320 284 792 132 656 265 103

Cr 877 350 497 999 196 477 510 465 244

Co 231 352 320 211 505 238 55 207 342

Cu 602 111 295 525 125 127 4 139 216

Zn 363 412 537 301 116 730 71 268 717

Ga 229 219 253 273 217 286 14 299 285

Rb 407 341 556 366 368 625 13 649 868

Sr 678 838 842 874 676 735 71 902 788

Y 248 265 267 142 245 114 266 382 104

Zr 195 364 333 341 337 693 141 332 659

Nb 276 213 245 215 568 841 15 295 733

Ba 338 472 513 558 460 610 26 651 624

La 416 553 457 459 457 139 528 656 133

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Table 1: continued

Blocks Megacryst Layered Series

Sample: GR289 PM466 PM555 SR248 GR258 SR246 PM623 ALZ LLZ

RSP (m): -2400 -2300 -2240 -1500 -560 870 -1660 av. av.

Type: lg lg lgn an olgn olgn cpx

Ce 840 100 868 900 920 309 1904 140 306

Nd 386 456 408 372 444 173 2018 695 165

Sm 0746 0920 0823 0695 0841 416 624 135 332

Eu 144 145 175 209 134 227 204 226 217

Gd 0650 0851 0709 0468 0668 361 666 121 309

Dy 0430 0607 0563 0311 0430 220 615 0734 210

Er 0206 0292 0225 0115 0201 103 264 0304 0911

Yb 0209 0260 0219 0100 0207 0922 213 0268 0804

Lu 0039 0044 0034 0020 0043 0166 0288 0040 0121

Total 201 246 217 211 219 765 706 337 729

Eu/Eu* 64 51 71 113 55 18 10 89 25

Cen/Ybn 103 99 101 230 114 86 23 164 102

Initial ratios

ISr(1434) 070431 070436 070461 070456 070410

eNd(1434) -26 -23 -18 -15 -15 -02

Average major, trace and rare earth element chemistry for the Poe Mountain anorthosite (PMa) from Scoates (1994). Rocktypes: lg, leucogabbro; lgn, leucogabbronorite; an, anorthosite; olgn, olivine leucogabbronorite. Initial isotopic ratios fromScoates & Frost (1994).

both planar features are extensively disrupted or de- 1986; Frost & Simons, 1991). Large clinopyroxene me-gacrysts (20 cm across) with visible plagioclase exsolutionformed by the impact of the blocks.lamellae are associated with very coarse-grained pla-(3) The upper portions (>5 m) of the crystal pile mustgioclase (individual crystals exceed 1 m in diameter),have contained, at least locally, significant proportionsolivine and FeTi oxides. The contacts with the sur-of interstitial liquid whereby compaction of the plagioclaserounding layered and laminated anorthositic cumulatesnetwork occurred and interstitial liquid was mobilized tocannot be observed, but given the obvious diVerence inform mafic pegmatoids.grain-size, this occurrence is interpreted as a block.(4) The consistent orientation of the asymmetric de-Coarse-grained mafic pegmatoids do occur in the Poeformation structures beneath the blocks throughout theMountain anorthosite, although they are readily iden-Poe Mountain anorthosite indicates that the floortifiable by the presence of fayalitic olivine and abundant(magmacrystal pile interface) also maintained a con-apatite (Scoates, 1994). Much smaller clinopyroxene me-sistent orientation and that crystallization proceeded fromgacrysts, 12 cm diameter, with visible plagioclase ex-the inner portions of the ALZ towards the margins.solution lamellae also occur in the large central block inFig. 6.

Blocks containing high-Al clinopyroxenemegacrysts Geochemistry of the blocksHigh-Al clinopyroxene megacrysts, characterized by fine Six blocks in the Poe Mountain anorthosite were analysedexsolution lamellae of calcic plagioclase, have been found for major, trace and rare earth element (REE) con-at two localities in the Poe Mountain anorthosite, and centrations to compare with the observed compositionalin each case, the megacrysts occur within blocks. The variation in the composite stratigraphic section of thefirst locality is in the middle ALZ (Fig. 3), on the road Poe Mountain anorthosite (Scoates, 1994) and for use in

density calculations (Table 1; see also sample locationsleading up to the Sybille FeTi oxide deposit (Bolsover,

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Fig. 5. Photographs of blocks and associated deformational structures. (a) Anorthositic block in the middle ALZ (SR248). The xenolith is sub-rounded in shape. The deformation of layered anorthosite beneath the block should be notedalternating anorthositic and olivine leucogabbroiclayers are bent and thin considerably immediately beneath the block. Photograph by R. F. J. Scoates. (b) Olivine leucogabbroic block in themiddle ALZ (PM555). The xenolith is sub-rounded in shape and deformation of underlying anorthositic cumulates is extensive (see Fig. 6 fordetailed map of this xenolith occurrence). Both the block and the anorthositic cumulates are cut by numerous thin sub-vertical monzodioriticdykes and veins. Photograph by O. R. Eckstrand.

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Fig. 6. Detailed map of asymmetric deformational structures associated with the impact of a group of leucogabbroic blocks in the lower ALZ(RSP = -2400). The map is of an inclined cliV-face, parallel to layering (eastwest) and oriented approximately perpendicular to dip; layeringand lamination dip away into the page. Beneath the blocks, mineral lamination in the cumulates is extensively deformed and wraps aroundindividual blocks, whereas above the blocks lamination is planar. Numerous mafic-rich pegmatoids are found beneath the blocks, and nopegmatoids are found above them. The preserved structures indicate that the blocks were deposited onto the chamber floor where they disruptedunconsolidated cumulates. The stratigraphic younging direction is towards the top of the figure. Locations for samples relevant to this study arenoted (GR289, xenolith; GR209, hosting laminated anorthositic cumulate) and compositional characteristics are shown for comparison (datafrom this study; Scoates, 1994; Scoates & Frost, 1996). The large central block contains several high-Al clinopyroxenes with visible plagioclaseexsolution lamellae.

in Fig. 3). Four of the samples were collected using a described by Scoates, (1994) and Scoates & Frost (1996).The analysed blocks range in cation normative pla-geological hammer (PM466, PM555, SR248 and SR246)

and the remaining two samples (GR289 and GR258) gioclase content (An + Ab + Or) from 72 to 91%(Table 1; Fig. 4b). The majority of the blocks containswere retrieved with a diamond drill coring device to

ensure fresh material (510 individual cores, 25 cm less total normative plagioclase than the anorthositiccumulates that they are contained within. Major and20 cm long). In addition, a single 4 cm 10 cm high-Al

clinopyroxene megacryst from the coarse-grained block trace element compositions correspond to the range ofcompositions observed in the Poe Mountain anorthosite:south of the Sybille FeTi oxide deposit was analysed.

Tungsten carbide (WC) sandpaper was used to remove An*= 5549, mg-number = 052038, Sr = 676874and Ba = 338610, where An*= [An/(An +Ab +potential contaminants from the core tubing on each core.

Coarse-crushing was done using a WC-plated hydraulic Or)] 100 and mg-number=Mg/(Mg+ Fe2+) (Table1; Scoates, 1994). There is no apparent correlation ofpress, and a single homogenized aliquot (75100 g) of

the finely crushed material from each sample was composition with stratigraphic position. However, eachblock contains higher An*than the hosting cumulate (Fig.powdered in a WC shatterbox for 3 min. Major element

compositions were determined using X-ray fluorescence 8a) suggesting that they are not locally derived. The high-Al clinopyroxene megacryst is compositionally distinct. Itspectrometry (XRF) at XRAL Laboratories, Toronto,

Canada, and standardized using University of Wyoming contains bleb-like exsolutions of calcic plagioclase andolivine, 62 wt % Al2O3 and 510 ppm Cr with mg-numberstandards and replicates (Scoates, 1994). Trace element

and REE concentrations were determined by inductively of 068 (Table 1). In contrast, oikocrystic clinopyroxenein the ALZ typically contains 12 wt % Al2O3 with mg-coupled plasma mass spectrometry at the University of

Nebraska (analysts A. Kolker, M. Ghazi and J. S. Scoates). number in the range 064038 ( J. S. Scoates, un-published data, 1994).Most of the blocks were also analysed for Sr and Nd

isotopic compositions (Table 1). Dissolution procedures, The blocks show a restricted range of REE con-centrations with total REE = 2025 ppm (Fig. 8b),analytical techniques and associated errors have been

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Fig. 7. Detailed map of a block impact structure and extensive layer disruption in the lower ALZ (RSP = -2300). The map is from an 80 mlong section of roadcut along Wyoming State Highway 34, parallel to undeformed layering and lamination in the area (eastwest) that dips>50 to the north (into the page). A single large, sub-rounded block is exposed in the westernmost part of the section (see also Fig. 5b).Plagioclase lamination wraps around the block. Locations for samples relevant to this study are noted (PM466, block; PM465, hosting anorthositiccumulate) and compositional characteristics are shown for comparison (data from this study; Scoates, 1994; Scoates & Frost, 1996). The entireoutcrop area is extensively disrupted, suggesting either the presence of additional blocks not exposed in the section or large-scale disruptionrelated to slumping in the crystal pile. Areas shown in light grey represent both coarse-grained olivine leucogabbro typical of the layeredcumulates and mafic pegmatoids that may represent pockets of interstitial liquid remobilized by block impacts. Areas shown in white areanorthosite that contains a chaotic, swirly lamination. All rocks are cut by sub-vertical monzodiorite dykes (mz), which are in turn cut by easterly-dipping granitic dykes (gr).

and uniformly high positive Eu anomalies (Eu/Eu*= anorthosite (Scoates & Frost, 1996). Their isotopic com-positions are relatively restricted compared with those of51113) similar to samples from the lower ALZ and

the recrystallized core of the Poe Mountain anorthosite the layered cumulates, initial 87Sr/86Sr= 0704307046and initial eNd =-15 to -26, but each block displays(Table 1). The one exception is SR246, a very large

(>20 m diameter as exposed in a roadcut) block in the marked Sr isotopic contrasts with its host cumulate orequivalent relative stratigraphic position (RSP); DISr =upper LLZ, which is strongly enriched in total REE

(76 ppm) and has a correspondingly lower positive Eu 0000200007 (Fig. 8c). The three blocks from theALZ where Sr isotopic compositions are available areanomaly (18) (Fig. 8b). With 029 wt % P2O5, SR246

probably contains a much greater proportion of com- significantly less radiogenic than the surrounding cu-mulates. The range in initial eNd for the blocks is limitedponents crystallized from the interstitial melt than do the

other blocks (004009 wt % P2O5), thus accounting for (-15 to -26) and within the range for the majorityof the surrounding cumulates, although GR289 doesthe elevated REE abundances. The high-Al clino-

pyroxene megacryst also has unique REE characteristics. show Nd isotopic disequilibrium outside analytical error(Fig. 8d). The isotopic composition of the high-Al clino-It has relatively high concentrations (total REE = 71

ppm), prominent La and Ce depletion relative to the rest pyroxene megacryst, ISr = 07041 and eNd =-02, alsodemonstrates that it is not in isotopic equilibrium withof the light REE (LREE), overall heavy REE (HREE)

depletion and no Eu anomaly (Fig. 8b). the equivalent RSP host cumulate (ISr > 07051 andeNd > -20) or any other rock in the ALZ (Fig. 8cThe initial Sr and Nd isotopic compositions (calculated

at 1434 Ma) of the analysed blocks indicate that they and d).The combined An*and ISr disequilibrium shown bycrystallized from magmas distinctive in composition from

those that formed the layered series of the Poe Mountain the blocks and the high-Al clinopyroxene megacryst

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Fig. 8. Geochemical characteristics and isotopic geochemistry of the blocks compared with the layered cumulates of the Poe Mountainanorthosite. (a) Relative stratigraphic position (RSP) vs normative An, where normative An = An/(An + Ab) in cations. It should be notedthat in the ALZ, all blocks are characterized by normative plagioclase compositions more calcic than those in the hosting cumulates (Scoates,1994). (b) Chondrite-normalized REE patterns for all blocks and a high-Al clinopyroxene megacryst [normalizing values from Hanson (1980)].The patterns for all the ALZ blocks show prominent LREE enrichment and are similar in form with nearly identical LREE contents and positiveEu anomalies. Sample SR246 from the LLZ is distinctive by its higher total REE contents, but its pattern is similar in shape to the others andthe Eu content is nearly identical. (c) RSP vs ISr(1434) for representative samples from the layered series, four blocks and a high-Al clinopyroxenemegacryst (Scoates & Frost, 1996). The range in ISr is relatively small, 0704307046, and all samples show strong Sr isotopic disequilibrium.(d) RSP vs eNd(1434) for representative samples from the layered series, five blocks and a high-Al clinopyroxene megacryst (Scoates & Frost, 1996).The variation is relatively small (initial eNd =-15 to-26), but one sample (GR289) does show Nd isotopic disequilibrium outside of analyticalerror.

indicate that they cannot represent fragments of the recorded only in the block compositions. The observedisotopic disequilibrium between the high-Allayered series that were transported to the chamber floor,

unless diVerent parts of the chamber were crystallizing clinopyroxene megacryst and the surrounding ALZ cu-mulates is also consistent with a high-pressure origin forfrom magmas of diVerent isotopic compositions that are

638


this megacryst and, by inference, for the block within crystals. Calculated magma densities are also stronglydependent on relative oxygen fugacity conditions duringwhich it resides (Scoates & Frost, 1996).crystallization, the P2O5 content of the magmas, as wellas their predicted volatile contents (H2O and CO2), asdiscussed below.Age relations between blocks and layered

anorthositesBlock compositions (major, trace and rare earth element,

Parental and residual magma compositionsand isotopic) are similar to those of the Poe Mountainto the Poe Mountain anorthositesanorthosite in general, thus they either represent frag-

ments of a now-eroded roof zone to the same chamber, One of the major goals in the study of Proterozoicpossibly formed by disruption during magma re- anorthosites is the determination of appropriate parentalplenishment, or perhaps a distinct earlier phase of anor- and residual magma compositions. This matter is com-thositic magmatism. This can be evaluated, as the ages plicated by the fact that (1) the anorthosites themselvesof the Poe Mountain anorthositic cumulates and one of represent cumulate rocks, typically extreme adcumulatesthe blocks are known precisely (Scoates & Chamberlain, with little remaining evidence of interstitial melt, and this1995). Concordant to very slightly discordant (


is a function of the oxygen fugacity conditions of thesystem of interest. The relative oxygen fugacities duringthe crystallization of the dykes can be determined byconsidering the QUILF relations between ferro-magnesian silicates and FeTi oxides (Frost & Lindsley,1992; Lindsley & Frost, 1992; Andersen et al., 1993). Afterdetermining the relative oxygen fugacity at a specifiedtemperature and pressure, the calculation of Fe2O3 andFeO abundances in the whole-rock analyses, assumingthat they represent melt compositions, is model de-pendent. In this case, the thermodynamic model forchemical mass transfer in magmatic systems, MELTS(Ghiorso & Sack, 1995), which incorporates the redoxrelation of Kress & Carmichael (1991), has been used Fig. 9. Polythermal Dlog f O2 vs XFe (olivine) for anorthositic, gabbroic,by inputting the major element composition of the ferrodioritic and monzonitic rocks in the LAC. The diagram is adapted

from Frost et al. (1996) and was calculated with QUILF (Andersen etrock of interest, the pressure (3 kbar), and the tem-al., 1993) assuming a pressure of crystallization of 3 kbar. Oxygenperature and oxygen fugacity determined from the fugacity is normalized to that of the fayalitemagnetitequartz (FMQ)

QUILF relations. buVer, where Dlog f O2 = log f O2 - log f O2 (FMQ). The XFe ofolivine, where XFe = Fe2+/(Fe2+ +Mg), is the actual or fictive olivineThe majority of the gabbroic and dioritic dykes in thecomposition and monitors Fe enrichment during crystallization. TheLAC are characterized by the coexistence of olivine and majority of the anorthositic cumulates, high-Al gabbroic dykes and

pyroxenes with both ilmenite and magnetite, which allows monzodioritic dykes in the LAC contain olivine, two pyroxenes, mag-netite and ilmenite, and are constrained to lie on the olivine-saturatedfor the oxygen fugacity to be closely constrained. FigureQUILF surface. Many ferrodiorites do not contain olivine and thus9, adapted from Frost et al. (1996), shows the variation plot above the olivine-saturated surface, and some olivine ferrodiorites

in relative oxygen fugacity, where Dlog f O2 = log f O2 do not contain orthopyroxene or inverted pigeonite and must plotslightly below the olivine-saturated surface. Redox calculations were(actual) - log f O2 (FMQ; fayalitemagnetitequartz),performed assuming oxygen fugacities equivalent to that of the FMQwith respect to the Fe/Mg of the system, as expressed buVer for anorthositic rocks and high-Al gabbroic dykes, one log unit

by XFe in olivine. The diagram is polythermal [see Frost below FMQ for the ferrodiorites, and two log units below FMQ forthe monzodiorites.& Lindsley (1992) for similar usage], with a linear dis-

tribution of temperature from T = 1200C at XFe = 0to T= 1000C at XFe = 1, and calculated for a pressureof 3 kbar. The temperature range is consistent with the Significance of P2O5 contentsresults of thermometry studies in rocks of the LAC The addition of phosphorus to a ferrobasaltic melt re-(Fuhrman et al., 1988; Kolker & Lindsley, 1989). Nearly duces magma densities because of the very large partialall the rocks under discussion crystallized along the molar volume of P2O5 (645 cm3/mol at 1300C) (Toplisolivine-saturated QUILF surface [reaction (13) of et al., 1994). Phosphorus enrichment will tend to coun-Lindsley & Frost (1992)OpAUIlO: opx/pig + ul- teract to a certain extent the increase in density causedvospinel= olivine+ augite+ ilmenite), and underscore by iron enrichment, which implies that during progressivethe proposed petrogenetic relationship between the mafic diVerentiation the actual densities of the residual liquidsdykes in the LAC. Some ferrodiorite dykes lack olivine may be lower than those calculated without consideringand thus crystallized at conditions slightly more oxidized the role of P2O5. The reduction in density is about 002than the olivine-saturated QUILF surface and higher g/cm3 per 2 wt % added P2O5 (Toplis et al., 1994),silica activity, whereas some of the olivine ferrodiorites a significant decrease when considering silicate meltlack orthopyroxene or inverted pigeonite, and thus crys- densities.tallized at slightly more reduced conditions and lower The LAC dykes are ferrobasaltic in composition, andsilica activity. The total range of oxygen fugacity con- those with near-liquid compositions can show significantditions during crystallization of the LAC gabbroic and enrichment in P2O5: 163249 wt % in all ferrodioritesdioritic dykes is from conditions at or slightly above the and 069145 wt % in monzodiorites (Mitchell et al.,FMQ buVer in the high-Al olivine gabbros to one log 1996). The calculated reduction in melt density for allunit below FMQ for the majority of the ferrodiorites to dykes in the study is 00003 g/cm3 for the high-Alabout two log units below FMQ for the monzodiorites. olivine gabbros, 001002 g/cm3 for all ferrodioritesAverage calculated Fe2O3/FeO is 016 for the high-Al and 0003001 g/cm3 for the monzodiorites. Althoughgabbros, 010 for the ferrodiorites and 010 for the important in the variation of melt density shown by the

LAC dykes, the eVect of P2O5 on melt density is smallermonzodiorites.

640


than that produced when assuming the presence of very Fe-enriched ferrodiorites and have >050 wt % H2O;after 75% crystallization, the predicted melt compositionssmall amounts of volatile constituents as discussed next.share some compositional similarities with the mon-zodiorites and have >075 wt % H2O. Thus, for thisstudy, density calculations were carried out at 3 kbar for

Realistic volatile contents the high-Al olivine gabbros at 1200C assuming 025 wtAddition of even small amounts of H2O can significantly % H2O, for the ferrodiorites at 1150C assuming 050reduce the densities of multicomponent silicate melts wt % H2O, and for the monzodiorites at 1100C assumingbecause of the low molecular weight (18016 g/mol) and 075 wt % H2O. The LAC magmas may have beenrelatively small partial molar volume (17 cm3/mol) of relatively CO2 enriched as discussed above. AlthoughH2O (Lange, 1994). The eVect of adding CO2 is less somewhat controversial, the CO2 content in submarinepronounced because of its higher molecular weight MORB glasses is typically


magma densities of group 1 (HAG1) decrease pro-gressively to a minimum of >268 g/cm3 at mg-num-ber= 057. The other two high-Al olivine gabbro groups,HAG2 and HAG3, depart from the HAG1 trend withincreasing density as mg-number decreases and trendtowards the ferrodiorites. Calculated densities for theferrodiorites show a maximum in the region of 285290g/cm3 at mg-number >03. The monzodiorites show astriking trend of rapidly decreasing magma density witha small decrease in mg-number, down to 265 g/cm3 at mg-number = 013 (Fig. 10). The regular density variationshown by this diverse group of dykes in the LAC furthersupports the proposition that they may be related throughfractionation, albeit open system (Mitchell et al., 1995,1996; Scoates et al., 1996).

The density relations for the blocks clearly show thatFig. 10. Calculated density vs whole-rock mg-number showing therelative density variations of blocks, intermediate-composition pla- they are nearly all capable of sinking through the majoritygioclase and mafic dykes from the LAC. All densities are calculated at of the proposed high-Al olivine gabbroic parental3 kbar using the formulation of Lange (1994) and considering both the

magmas, consistent with previously described field re-eVect of volatiles and the molar volume of P2O5 (Toplis et al., 1994) onthe densities of the proposed melt compositions. The diagram is lations. Blocks GR258 and SR246 are denser than allpolythermal with both temperature and relative oxygen fugacity de- high-Al olivine gabbros, whereas blocks PM466, PM555creasing with decreasing mg-number. The solid densities of the blocks

and GR289 fall within the calculated range (Fig. 10). Theare noted in histogram form for comparison along the left side of thediagram. The dykes, although clearly not related by a single liquid- capability of these blocks to sink is evidently controlled byline-of-descent (Mitchell et al., 1995, 1996), display coherent variations the relative degree of fractionation of the high-Al olivinein density with diVerentiation: HAG1 densities decrease with decreasing

gabbros. The one possible exception is SR248, which ismg-number, HAG2 and HAG3 densities increase with decreasing mg-number, and ferrodiorite and monzodiorite densities decrease sig- less dense than all calculated magma densities, except thenificantly with very small decreases in mg-number from a maximum of most fractionated monzodiorite (Fig. 10). The physicalabout 285290 g/cm3 at mg-number = 03. The typical crystallizing

evidence for impact on the chamber floor appears in-mineral assemblages in the dykes are noted. All of the blocks, with theexception of SR248, are denser than the majority of the high-Al controvertible (Fig. 5a). It is possible that the samplegabbros. All plagioclase of intermediate-composition, An4060, is less collected is not representative of the bulk composition ofdense that the proposed candidates for parental and residual magma

the block, but instead represents that of a more pla-compositions. Abbreviations: HAG1, high-Al gabbro group 1 dykes;HAG2, high-Al gabbro group 2 dykes; HAG3, high-Al gabbro group gioclase-rich, and thus less dense, portion.3 dykes; FDI, ferrodiorite dykes; OLFDI, olivine ferrodiorite dykes; In sharp contrast to the blocks, all of the plagioclaseMZDI, monzodiorite dykes.

compositions associated with the Poe Mountain anor-thosite (An4555) are less dense than those of the calculatedmelt densities (Fig. 10), ranging from the relatively MgO-fugacity constraints, P2O5 contents and evolving volatilerich high-Al olivine gabbros to the evolved ferrodioritescontents, can be eVectively shown in a diagram of densityand monzodiorites. Similar conclusions with respect toas a function of the whole-rock mg-number, where mg-relative plagioclasemagma densities have been reachednumber = Mg/(Mg + Fe2+) (Fig. 10). Figure 10 isin experimental studies of ferrodioritic or jotunitic meltsisobaric (3 kbar) and polythermal, and the relative oxygenfrom other Proterozoic anorthosite complexes. In bothfugacity decreases from that equivalent to the FMQthe Newark Island layered intrusion within the NainbuVer at mg-number = 0607 to 2 log units belowPlutonic Suite, Labrador (Snyder et al., 1993) and theFMQ at mg-number = 015020. The solid densities ofBjerkreimSokndal layered intrusion within the Ro-the blocks calculated at 1000C are shown in histogramgaland anorthosite complex (Vander Auwera & Longhi,form on the left side of the diagram for comparison; their1994), melt densities from a range of fractionated com-whole-rock mg-number is not relevant to this discussion.positions (63 wt % MgO) remain persistently well aboveAlso shown is the field for An4060 plagioclase (shadedthat of the equilibrium plagioclase. Thus, evidence fromfield at bottom of diagram independent of mg-number).several diVerent Proterozoic anorthosite complexes con-The density variation shown by the mafic dykes of thesistently shows that plagioclase cannot settle in the as-LAC in Fig. 10 is remarkably coherent and similar tosociated melts, and in situ or other methods ofthat shown for the progressive diVerentiation of naturalcrystallizationaccumulation must be invoked (see dis-anhydrous basalts (Sparks et al., 1980; Stolper & Walker,

1980; Sparks & Huppert, 1984). High-Al olivine gabbro cussion below).

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Crystallizing mineral assemblages andfractionation densitiesThe mineral assemblage that crystallizes from a magmacontrols the resultant density variations in the melt. At3 kbar, the LAC high-Al olivine gabbros are co-saturatedin plagioclase, olivine and high-Ca pyroxene low-Capyroxene, the ferrodiorites are co-saturated in plagioclase,high-Ca pyroxene, pigeonite olivine and FeTi oxides,and the monzodiorites are co-saturated in sodic pla-gioclase (nearing ternary feldspar), high-Ca pyroxene,ilmenite, magnetite, olivine and apatite (Mitchell et al.,1995, 1996). The density maximum shown in the fer-rodiorites and the subsequent dramatic decrease inmagma density clearly marks the onset of FeTi oxide

Fig. 11. Fractionation densities of the major crystallizing phases incrystallization (Fig. 10). The existence of a density mini-basaltic magmasplagioclase, clinopyroxene, pigeonite, orthopyroxene

mum in basaltic systems is commonly inferred to mark the and olivinecompared with the general field of basalts and the overalltendency shown by the Laramie mafic dykes. Fractionation densities,onset of plagioclase ( high-Ca pyroxene) crystallization,the density of the components of the fluid being selectively removedand the initial decrease in density at relatively highby fractional crystallization, are calculated after the method of Sparks

mg-number is generally considered to result from the & Huppert (1984) at a pressure at 3 kbar using the partial molarvolumes of Lange (1994). The diagram is polythermal and reflects thecrystallization of olivine pyroxene (Stolper & Walker,decrease of temperature with diVerentiation. It should be noted that1980). However, all the LAC high-Al olivine gabbroicthe decreasing density of the high-Al gabbroic dykes with decreasing

dykes are saturated in plagioclase (Mitchell et al., 1995). mg-number is consistent with the combined crystallization of bothferromagnesian silicates and plagioclase.The influence of the fractionating assemblage on melt

density can be examined considering fractionation dens-ities, the ratio of the gram formula weight to molar

is probably replaced by pigeonite as the low-Ca fer-volume of the chemical components in the liquid phaseromagnesian phase, which is reflected in the relativethat are being removed by fractional crystallizationdensity relations of the evolved magmas.(Sparks & Huppert, 1984). Fractionation densities of the

major minerals that crystallize from basaltic magmasplagioclase, clinopyroxene, pigeonite, orthopyroxene andolivinehave been calculated after the method of Sparks

Plagioclase crystallization and the& Huppert (1984) at 3 kbar incorporating the partialformation of plagioclase-rich layered rocksmolar volumes and associated compressibilities of LangeDensities for plagioclase (An4060) in the anorthositic rocks(1994) (Fig. 11). Because diVerentiation is associated withrange from 261 to 265 g/cm3, significantly below thosedecreasing temperatures, the eVect of thermal expansionfor calculated parental magma compositions (Fig. 10)on the fractionation densities has been considered. Figureand yet plagioclase was clearly accumulating on the11 is polythermal with linearly decreasing temperature,chamber floor before the arrival of the blocks (Figs 5, 6like Fig. 9, from 1200C at mg-number or An*= 1 toand 7). Considering the simple end-member case where1000C at mg-number or An*= 0. For the high-Althe resident magma contained few suspended crystals,

olivine gabbro densities to systematically decrease with for plagioclase to have ended up on the intrusion floordecreasing mg-number, a fractionating assemblage of beneath the blocks, it must either have crystallized in a>50% plagioclase and subequal amounts of olivine and boundary layer at the crystal pilemagma interfaceinclinopyroxene is initially required (mg-number = 065). situ crystallization ( Jackson, 1961; Campbell, 1978;The relative amount of plagioclase that crystallizes can McBirney & Noyes, 1979; Morse, 1986a; Langmuir,increase during fractionation as the density of plagioclase 1989)or it must have been carried to the floor indecreases and those of the ferromagnesian silicates pro- relatively dense packets of cooled liquid plus crystals fromgressively increase. To account for increasing densities the roof or walls of the intrusiontwo-phase convectionwith decreasing mg-number as observed in the HAG2 (Grout, 1918; Morse, 1986b). Once at the floor, pla-and HAG3 dykes, the crystallizing assemblage must gioclase will probably remain there if there is negligiblebecome more rich in plagioclase and/or much poorer density contrast (>01 g/cm3) and if there is the slightestin olivine (Fig. 11). Pigeonite becomes the stable low-Ca amount of yield strength to the magma (Irvine, 1987).pyroxene at about mg-number = 06 (Lindsley & Frost, Crystallization of interstitial ferromagnesian silicates and

FeTi oxides, much of which is oikocrystic (Scoates,1992). In conjunction with increased silica activity, olivine

643


1994), will consolidate the crystal pile and allow it to formation of anorthosites, which are defined by thepresence of abundant excess plagioclase compared withremain in place.established cotectic proportions with ferromagnesian sil-The above description of plagioclase crystallization isicates, requires that the parental magmas lie well withinconsiderably at odds with the prevailing model for thethe plagioclase-only field and thus are richer in the lighterformation of Proterozoic anorthosites. As discussed atoxide components Al2O3, SiO2, CaO and Na2O, andthe start of this paper, buoyancy-driven ascent of crystal-poorer in the denser oxide components MgO, FeO*andrich diapirs containing 5070 vol. % plagioclase is aTiO2. Could the addition of plagioclasemelt componentsmechanism favoured by many workers to account forsuYciently reduce magma densities such that inter-the characteristic large masses of anorthositemediate-composition plagioclase would actually sink?leucotroctoliteleuconorite (Emslie, 1985; Longhi & Ash-

The polybaric model for anorthosite formation dis-wal, 1985; Ashwal, 1993; Longhi et al., 1993). Thiscussed in previous sections involves extensive frac-mechanism leaves little room for the existence of dynamictionation at depth of basaltic magmas that are multiplymagma chambers at the final level of emplacement, assaturated at high pressures, perhaps corresponding to thethe high proportion of crystals to liquid approaches (orbase of the crust. Plagioclase that has accumulated byexceeds) the limit of critical crystallinity, where viscositiesflotation, as a result of the increased compressibility ofincrease so dramatically that the rheology of the magmasilicate melts at high pressure, at the roof of these stagingbecomes essentially that of a solid (Marsh, 1981). Thechambers may be partially remelted and resorbed duringfield evidence from the Poe Mountain anorthosite inperiodic replenishments of higher temperature, lessthe LAC, and from layered anorthosites from otherevolved magma (Wiebe, 1992). Diapirs containing sus-Proterozoic anorthosite complexes (Wiebe, 1992), sug-pended plagioclase are then assumed to rise through thegests that a significant component of melt was presentcrust from these deeper staging chambers. Because of thein the mid- to upper-crustal magma chambers afterexpansion of the plagioclase stability field with decreasingemplacement. Assuming that the resident magmas con-pressure (Morse, 1982), cotectic magmas at depth lietained some amount of suspended plagioclase crystalsin the plagioclase field on ascent and thus additionalupon emplacement, both two-phase convection and insuspended plagioclase could be remelted, provided thatsitu crystallization may still be viable processes. If two-little heat is lost to melting of the crust, and reduce thephase convection was operating, then the descendingoverall magma density. A simplified test of this hypothesispackets of cooler melt may have entrained some of theis shown in Fig. 12, which shows magma density as asuspended plagioclase and transported it to the floor.function of the percentage of added remelted plagioclase,Conversely, if plagioclase was crystallizing directly on thethe composition of the remelted plagioclase and pressure.floor of the intrusion then the suspended plagioclaseThe initial magma composition (filled squares) is takenwould be progressively incorporated in the advancingas the average of the high-Al olivine gabbroic group 1solidification front from the floor. Both of these processes(HAG1) from the LAC: 273 g/cm3 at 3 kbar (1200C,may explain the rather common occurrence of randomlyFMQ , 025 wt % H2O and 200 ppm CO2) and 283 g/distributed blocky megacrysts of plagioclase within cu-cm3 at 10 kbar (1250C, FMQ , 025 wt % H2O andmulates consisting of tabular, laminated plagioclase of200 ppm CO2). Use of a ferrodioritic composition resultsthe Poe Mountain anorthosite layered series (Scoates,in a much greater initial plagioclasemagma density1994)the megacrysts may represent plagioclase crys-contrast. For both pressures, the eVect of adding remeltedtallized at depth and transported in suspension in aplagioclase components ranging from An40 to An60 andfeldspathic magma.taking into account the eVect of compressibility is shown(equivalent melt densities3 kbar: An4060 = 249254g/cm3; 10 kbar: An4060 = 260265 g/cm3). At 10 kbar,

The eVects of plagioclase remelting on the magma densities remain higher than the crystallizingplagioclasemagma density contrasts plagioclase for all reasonable values of percent remeltedThe plagioclasemagma density paradox obviously ceases plagioclase. At 3 kbar, >4050% remelted plagioclaseto be a problem if the resident magma density is less is required to eliminate the plagioclasemagma densitythan that of the crystallizingaccumulating plagioclase. contrast, and >50% remelted plagioclase would be neces-Is there evidence for parental magmas that are less dense sary to allow the plagioclase to eVectively sink. Thisthan intermediate-composition plagioclase in Proterozoic simplified test does not take into account the heat neces-anorthosite complexes? The mafic dykes in the LAC, sary to remelt plagioclase, but demonstrates only theand those found in other anorthosites, are multiply sat- density requirements needed to allow plagioclase to sink inurated in plagioclase + ferromagnesian silicates (olivine appropriate parental magmas. By incorporating thermal+ pyroxenes) FeTi oxides at their level of em- expansion and heat capacity terms, Longhi et al. (1999)

calculated that pressure release from 13 to 4 kbar of anplacement, typically in the middle to upper crust. The

644


case of a sloping floor for the crystallization of felsicadcumulates. A sloping or inclined floor allows for theprogressive downslope migration of dense, rejected inter-stitial liquid, which may eventually rejoin the main massof magma above the crystal pile, ensuring the magmaresident in the chamber will always be denser than thecumulates. An important consequence of this process isthat compositionally evolved liquids can be redistributeddownslope, thus influencing the bulk composition of theresultant cumulates as evolved liquids interact with lessevolved liquids. This would occur before significant com-paction, upward expulsion of interstitial liquid and as-sociated compositional modification (Meurer &Boudreau, 1998), and should be incorporated into current

Fig. 12. Magma density as a function of the percentage of added models for the evolution of solidifying crystal piles. Aremelted plagioclase, the composition of the remelted plagioclase (An40sloping floor certainly appears to be required for the60), and pressure (3 and 10 kbar). The initial magma composition used

in the calculations is the average of the high-Al olivine gabbro group crystallization of layered anorthosites, leuconorites and1 (HAG1) from the LAC. The field of plagioclase densities for com- leucotroctolites in Proterozoic anorthosite complexes, andpositions typical of Proterozoic anorthosite complexes is shown by the

may be a general characteristic of all feldspathic cu-shaded field. At 10 kbar, the resultant magma densities are higher thanthe crystallizing plagioclase for all reasonable values of percent remelted mulates in layered intrusions where the rejected interstitialplagioclase. At 3 kbar, >4050% remelted plagioclase is required to liquid is relatively dense.eliminate the plagioclasemagma density contrast. The crystallization regimes for the layered series

cumulates in the Poe Mountain anorthosite involvinga sloping floor can be envisaged as consisting of (1) aascending anorthositic diapir could remelt as much asresident magma composed of high-Al olivine basalt4% of the suspended plagioclase, producing only minorplus suspended plagioclase that crystallized at depthchanges in magma density with respect to the relationsor during ascent through the crust, (2) an upper mushshown in Fig. 12. Thus, if plagioclase remelting is to bezone to the crystal pile that contains cumulus plagioclasean eVective mechanism for decreasing magma density,and dense FeTiP-rich interstitial liquid (ferrodiorite),it appears that extensive remelting must be done at depthand (3) a deeper zone in the crystal pile that is belowbefore ascent. Additionally, anorthositic magmas (meltthe solidus and consists of solidified anorthosite (Fig.+ crystals) emplaced at pressures >3 kbar will be sub-13). Plagioclase accumulates and crystallizes on thejected to progressively larger plagioclasemagma densityfloor through combined in situ crystallization, the arrivalcontrasts as a result of the eVects of silicate melt com-of dense two-phase packets composed mainly of coolerpressibility.liquid and some crystals, and progressive incorporationof suspended megacrysts. Anorthositic to leucogabbroicblocks periodically struck the floor causing extensive

The fate of the residual liquid and the disruption and deformation beneath them, and dem-significance of sloping floors onstrating that most layering and lamination forms

directly at the magmacrystal pile interface. Crys-The accumulation and crystallization of plagioclase onthe floor of the Poe Mountain anorthosite magma cham- tallization of intermediate-composition plagioclase pro-

duces a dense residual liquid enriched in FeTiP andber leads to another major problem encountered whenconsidering the crystallization of anorthosites: the rejected somewhat depleted in silica, essentially ferrodioritic in

composition. The dense liquid is gravitationally unstableinterstitial liquid is denser than the plagioclase in thecrystal pile (Figs 10 and 11) and will remain so until well in the upper part of the inclined crystal pile and

migrates downslope through a permeable network ofafter saturation in FeTi oxides. As a result, this processshould lead to stagnation of the dense residual liquid plagioclase crystals, perhaps similar to the way interstitial

liquid drains from crystal networks in partially meltedamong the cumulus plagioclase on the floor with theadded result that no anorthosite can form. Some dense basalts (Philpotts & Carroll, 1996; Philpotts et al., 1998).

The ferrodioritic liquid may mix with other less evolvedresidual liquid could infiltrate downward eventually form-ing small dykes or pods of ferrodiorite that are so char- and more evolved liquids during percolation and may

react with cumulus plagioclase. The extent to whichacteristic of Proterozoic anorthosite complexes. However,to form an anorthosite the majority of this dense liquid the dense liquid can seep downwards is probably

limited by lithologic variationslayers of nearly puremust be removed from the crystal pile. To overcome thisproblem, Morse (1986a, 1988) proposed the general anorthositic adcumulate will act as impermeable barriers

645


Fig. 13. Proposed crystallization regimes in the magma chamber of the Poe Mountain anorthosite involving the impact of blocks, aninclined floor and downslope removal of dense residual liquid. The resident magma is assumed to be similar in composition to the high-Al gabbroic dykes and contains suspended plagioclase crystallized at depth. The crystal pile contains an upper part where the temperatureis above the solidus that consists of cumulus plagioclase and interstitial ferrodioritic residual liquid, and a lower part of solidified anorthositewhere the temperature is below the solidus. Layering of diVerent types is shown schematically, as are scour structures (Scoates, 1994) andtrapped mafic pegmatoids (Mitchell et al., 1996). The blocks struck the chamber floor, causing extensive disruption and deformation ofplagioclase lamination and layering beneath them. The ferrodioritic residual liquid is denser than the plagioclase cumulus network. Aninclined floor is required to allow the dense liquid to drain downslope through the crystal pile and eventually rejoin the main mass ofmagma.

to both downward and upward migrating liquid CONCLUSIONS(Fig. 13). The much modified dense residual liquid

The age, compositions and structures associated withmust eventually rejoin the main mass of residentanorthositic to leucogabbroic blocks in the layered seriesmagma.of the Poe Mountain anorthosite, combined with cal-The presence of a sloping floor may be a requirementculations of proposed parental and residual magma dens-for the evolution of Proterozoic anorthosites. The overallities, place important constraints on the plagio-density of the anorthositic cumulates on the floor willclasemagma density paradox in Proterozoic anorthositesalways be less than that of the resident magmas aboveand plagioclase-rich layered intrusions. The blocks maythem, producing an inherently unstable situation. Largerepresent fragments of a now-eroded roof zone, a distinctvolumes of relatively light anorthositic cumulates mayearlier phase of anorthositic magmatism or materialrise diapirically because of density contrasts with thecrystallized at high pressures. Disrupted and deformedunderlying material, progressively tilting the floor duringlayered anorthosites beneath the blocks provide consistentcrystallization and allowing for the downward escape ofstratigraphic tops indicators. The blocks fell throughdense residual liquid. Small amounts of deformationresident magma and struck a floor that was present duringrelated to slow, diapiric rise could be responsible for thecrystallization of the layered series, where intermediate-recrystallized texture of many Proterozoic anorthositescomposition plagioclase (An4555) was accumulating and/that formed through high-temperature fast grain bound-or crystallizing, and where compositional layering andary migration (Lafrance et al., 1996). The resultant dom-lamination were forming. The upper parts of the crystalical structure would also be consistent with the forms ofpile contained significant amounts of relatively densemany separate plutons in Proterozoic anorthosite com-

plexes (Emslie, 1980; Frost et al., 1993). interstitial melt that was remobilized by the block impacts.

646


Block densities are greater than, or nearly identical to, this project would not have been possible without thethe calculated magma densities. The density variation co-operation of the numerous landowners in the southernof mafic dykes in the LAC, which have compositions Laramie Mountains, past and present, and the staV ofappropriate for magmas that produced the anorthositic the Wyoming Game and Fish Experimental Station incumulates (high-Al olivine gabbros) or FeTiP-rich re- Sybille Canyon. Many thanks are due also to A. Kolkersidual magmas (ferrodiorites and monzodiorites), is re- and M. Ghazi for providing high-quality trace elementmarkably coherent and similar to that for the progressive and REE analyses. External reviews by D. H. Lindsley,diVerentiation of natural anhydrous basalts. Densities B. R. Frost, R. F. J. Scoates and J. N. Mitchell, andfor intermediate-composition plagioclase are significantly journal reviews and comments by S. A. Morse, R. A.below those for the calculated magma compositions. Wiebe and S. R. Tait have materially improved thePlagioclase cannot have settled to the chamber floor; it presentation of arguments in the manuscript.must either have crystallized in situ in a boundary zoneat the magmacrystal pile interface or arrived at the floorin dense two-phase packets composed mainly of liquidand some crystals from the roof zone of the chamber.

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Deer, W. A., Howie, R. A. & Zussman, J. (1992). An Introduction to theACKNOWLEDGEMENTSRock-Forming Minerals, 2nd edn. Harlow, UK: Longman, 696 pp.

I would like to thank B. R. Frost, D. H. Lindsley, J. N. Edwards, B. R. (1993). A field, geochemical, and isotopic investigationMitchell and W. P. Meurer for invaluable assistance and of the igneous rocks in the Pole Mountain area of the Sherman

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was supported by two Geological Society of America central Labrador: an example of Elsonian magmatism. GeologicalSummer Research Grants, and field and laboratory work Survey of Canada Bulletin 293, 136 pp.were both funded by National Science Foundation (NSF) Emslie, R. F. (1985). Proterozoic anorthosite massifs. In: Tobi, A. C.

& Touret, J. L. R. (eds) The Deep Proterozoic Crust in the North Atlanticgrants EAR8618480 and EAR8816040 to D. H. LindsleyProvinces. Dordrecht: D. Reidel, pp. 3960.and EAR9017465 and EAR9218360 to B. R. Frost and

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