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Geological Society of America Bulletin

doi: 10.1130/0016-7606(2000)112<564:LASAMH>2.0.CO;2 2000;112;564-578Geological Society of America Bulletin

B. Ronald Frost, Kevin R. Chamberlain, Susan Swapp, Carol D. Frost and Thomas P. Hulsebosch Evidence for a long-lived active margin on the Archean Wyoming cratonLate Archean structural and metamorphic history of the Wind River Range:

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ABSTRACT

The Archean rocks of the Wind River Rangein western Wyoming record a Late Archeanhistory of plutonism that extends for more than250 m.y. The range is dominated by graniticplutons, including the 2.8 Ga Native Lakegneiss, the 2.67 Ga Bridger batholith, the2.63 Ga Louis Lake batholith, and late 2.54 Gagranites. These plutons provide a means of dis-tinguishing the complex metamorphism anddeformation that affected the range in the LateArchean. Five deformation events are re-corded. D1 is a penetrative deformation that oc-curred during the earliest granulite-faciesmetamorphism; D2 is a folding event, probablyin amphibolite facies, that deforms porphyriticdikes that cut the D1 fabrics. Both D1 and D2predate the intrusion of the ca. 2.8 Ga NativeLake gneiss. D3 is a folding event, accompaniedby upper amphibolite to granulite metamor-phism, that deformed the Medina Mountain se-quence, a sequence of rocks that was either de-posited or thrust upon the Native Lake gneiss.D4 is a fabric-forming event associated with theMount Helen structural belt (MHSB). It is rep-resented by mylonites in the MHSB, a penetra-tive fabric in the Bridger batholith, and foldingof the D3 structures in the Medina Mountainsequence. We consider D3 and D4 to be coevalwith the emplacement of the Bridger batholith,and hence to date at ca. 2.67 Ga. The lateststructures (D5) are fabrics associated with thefolding and thrusting of the 2.65 Ga South Passsequence.

We recognize at least four metamorphicevents. M1 is associated with the D1 fabrics andoccurred at high T (>750 °C) and high P(~7–8 kilobars). M2(650–750 °C and 4–5.5 kilo-

bars) is associated with the intrusion of theBridger batholith and formation of the D3 andD4 structures. The D5 structures of the SouthPass sequence record M3, which is lowP (~2–3kilobars) and low T (~500 °C). The final meta-morphism, M4, is a contact metamorphismaround the Louis Lake batholith. In the southagainst the South Pass sequence, the metamor-phism occurred at ~3 kilobars and at tempera-tures <700 °C. In contrast, in the north wherethe Louis Lake batholith is charnockitic, themetamorphism occurred at 6 kilobars and800 °C. This pressure gradient is probably a re-flection of tilting of the Wind River block dur-ing the Laramide orogeny.

The composition of the plutons and thestructural and metamorphic history of theWind River Range indicate that during theLate Archean this area occupied the activemargin of the Wyoming province. This tec-tonic environment is similar to the long-livedPhanerozoic margins of North America. TheWind River Range represents the best-docu-mented active margin of Archean age.

Keywords: active margin, Archean, metamor-phic petrology, structure, tectonics, Wyomingprovince.

INTRODUCTION

The Wyoming province is the most southwest-ern of the Archean provinces of North America.It has a geologic history that is distinctive fromthat of the Superior province, the largest Precam-brian craton within North America. The majorityof the Superior province formed rapidly between2.7 to 2.8 Ga (Card, 1990; Percival et al., 1994),whereas isotopic evidence indicates that theWyoming province was cratonized before 3.2 Ga(Wooden and Mueller, 1988; Frost, 1993) and

that this early crust was repeatedly reworked bylater Archean events (Frost et al., 1998). Its his-tory of early cratonization is shared by the Slave,Nain, and Minnesota River Valley provinces,small cratons that ring the Superior and Churchillprovinces (Hoffman, 1988).

SUMMARY OF THE GEOLOGY OF THEWIND RIVER UPLIFT

The Wind River Range, a northwest-trendinguplift in western Wyoming, exposes Archeanrocks over an area of more than 10 000 km2. Therange is composed almost entirely of high-gradegneisses and granites that were thrust to the westover Paleozoic and Mesozoic sedimentary rocksduring the Laramide Orogeny 40–80 Ma. Thethrust, which is sparsely exposed, has beenshown seismically to extend to depths of morethan 15 km (Smithson et al., 1978). On the east,the Precambrian rocks are covered by Paleozoicand Mesozoic sedimentary rocks, dipping on av-erage 15° NE into the Wind River Basin. The up-lift plunges gently to the north in the north and tothe south in the south, which suggests that thedeepest levels are exposed in the central-westportion of the range (Mitra and Frost, 1981).

Most of the basement in the range consists ofgranites and granite gneisses (Fig. 1). Frost et al.(1998) recognized four distinct ages of plutonismin the range. In this paper, we use the relation be-tween these plutons and various metamorphicand structural elements in the surroundinggneisses to unravel Late Archean plutonic, sedi-mentary, metamorphic, and deformation eventsthat occurred in the Wind River Range. Theseplutons include:

1. The Native Lake gneiss, a locally deformedcalc-alkalic pluton in the Washakie terrane with apreliminary zircon U-Pb age of ca. 2.8 Ga (Frostand Frost, 1993).

564

Late Archean structural and metamorphic history of the Wind River Range:Evidence for a long-lived active margin on the Archean Wyoming craton

B. Ronald Frost*Kevin R. ChamberlainSusan SwappCarol D. FrostThomas P. Hulsebosch†

Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

GSA Bulletin;April 2000; v. 112; no. 4; p. 564–578; 12 figures; 4 tables.

}

*E-mail: [email protected].†Deceased.



2. The foliated Bridger batholith, which hasbeen dated at 2670 ± 13 Ma (Aleinikoff et al.,1989), and which makes up a major portion ofthe northern part of the range.

3. The undeformed Louis Lake batholith, whichwas intruded at 2630 ± 2 Ma (Frost et al., 1998).

4. A series of latest Archean plutons that havenot been dated precisely, but for which Stucklesset al. (1985) suggested an age of 2545 ± 30 Ma.Included in this group are the Middle Mountainbatholith and the granite of New Fork Lakes inthe northern part of the range, the Bears Ears plu-ton in the center of the range, and isolated gran-ites at South Pass (Frost et al., 1998).

Three areas within the Wind River uplift areparticularly critical for unraveling the geologichistory of the range. They include:

1. The Washakie block, a sequence of gneissesthat occupies the northeastern portion of theWind River Range (Fig. 1, area 1) (Frost et al.,1998). This coherent package of tonalitic andcalc-alkalic gneisses lacks the intense migmati-zation associated with the Bridger batholith andprovides an important window into the earlierhistory of the range.

2. A complex migmatite that surrounds theBridger batholith in which isolated fragments ofolder gneisses survive within an area of felsicneosomes (Koesterer et al., 1987) (Marshall,1987). Some of the neosomes in the migmatiteare of Bridger age, but some of them are clearly

older and are presumed to be associated with Na-tive Lake–aged plutonism (Frost et al., 1998). Westudied two areas where these migmatites con-tained infolded belts of supracrustal rocks, one inthe Medina Mountain (Fig. 1, area 2), and an-other in the Crescent Lake area (Fig. 1, area 3).

3. The South Pass area, on the southern marginof the uplift (Fig. 1), which is dominated by a se-quence of weakly metamorphosed supracrustalrocks. These rocks, which we call the South Passsequence, have been the subject of many studies(Bayley et al., 1973; Harper, 1985; Hull, 1988;Hausel, 1991).

DETAILED GEOLOGIC HISTORY

We have identified a record of geologic eventsin the Wind River Range that spans more than1 billion years (Table 1). To characterize the rela-tive age of events within the range, we have de-termined the relation between deformations dis-played in the rocks and the plutonic eventsoutlined in Frost et al. (1998). Because moststructural elements are present in some areas andabsent in others, we use the following conventionfor naming structural elements. We assign a sym-bol to a given deformation event (i.e., D1, D2,

LATE ARCHEAN HISTORY OF THE WIND RIVER RANGE

Geological Society of America Bulletin, April 2000 565

Louis Lake batholith, 2.63 Ga batholith, locallycharnockitic

Mt. Helen structural belt

110 108 W45 N

42

Wyoming

Wind River Range

Late Archean plutons, 2.55 Ga granites

Bridger batholith, 2.67 Ga batholith, weaklymetamorphosed

Migmatitic gneiss

South Pass sequence, 2.65 Ga volcanic rocksand greywackes

Washakie block, 2.8 Ga orolder gneisses

109 45’

42 45’

Pinedale

Kilometers

0 25

2

1

3

Trace of buried Wind River thrust

Figure 1. Geologic map of theWind River Range compiledfrom Granger et al. (1971),Worl et al., (1986), Koestereret al. (1987), Marshall (1987),Hulsebosch (1993), and our un-published data. Boxes outline ar-eas discussed in detail in the text.These are: (1) Dry Creek–BobLakes–North Fork Bull LakeCreek region, (2) Medina Moun-tain area, (3) Crescent Lakearea. Dots locate samples used inthermobarometry.

TABLE 1. GEOLOGIC EVENTS IN THE WIND RIVER RANGE

Event Age Where found(Ga)

Deposition of early supracrustal rocks, granulite metamorphism, D1 >2.8 CL, MHSB, MM, WB

Intrusion of porphyritic dikes >2.8 CL, MM, WBD2: folding of D1 fabric and porphyritic dikes >2.8 WBIntrusion of Native Lake gneiss ca. 2.8 CL, MM, WBDeposition of Medina Mountain sequence, metamorphism,

and D3: folding of these rocks. 2.7–2.8 MMEmplacement of the Bridger batholith, and D4: formation

of Mount Helen structural belt and folding of D3 structures 2.67 BB,CL,MHSB, MM, WBDeposition of South Pass sequence, metamorphism, D5:

folding of SPS and thrusting onto Wyoming craton 2.65 SPIntrusion of Louis Lake batholith, contact metamorphism 2.63 LL, SPIntrusion of Late Archean plutons 2.54 All of the rangeIntrusion of Proterozoic diabase dikes 1.47 All of the range

Abbreviations: BB—Bridger batholith, BE—Bears Ears batholith, CL—Crescent Lake area, LL—Louis Lakebatholith, MHSB—Mount Helen structural belt, MM—Medina Mountain area, SP—South Pass, WB—Washakieblock.



etc.) with an older event having a lower numberin the subscript. Each fabric element is labeledwith twin subscripts—the first refers to the defor-mation event and the second to the generation ofthat particular feature within the event.

We divide the Wind River Range into fivestructural domains (Fig. 2). The oldest is in theWashakie block, in which younger fabrics are ab-sent or are present only as sparsely occurringcross-cutting mylonite zones. The dominant fab-rics in this block were produced by the D1 and D2events. This block is bounded on the south by amajor deformation zone, the Mount Helen struc-tural belt (Granger et al., 1971). This D4 zone hasthrust displacement and separates the Washakieblock from a domain where the earlier fabricshave been largely obliterated by the Bridgerbatholith. A weak S4 fabric is found throughoutthe Bridger batholith and older fabrics occur lo-cally in this domain. For example, D3 fabrics arerecognized in the Medina Mountain area (Fig. 1,area 2), and D1 and possibly D2 fabric elementsmay occur in the migmatites on the southern andnorthern margin of the Bridger batholith. Theyoungest structural domain in the range is de-fined by D5 deformation of the South Pass se-quence. Most of the central portion of the rangeis underlain by directionless fabrics of the LouisLake and Late Archean batholiths, which wereintruded after D5.

Early Supracrustal Rocks and the D1 Event

The oldest deformation event identified in theWind River Range, a penetrative deformation as-sociated with granulite metamorphism, is bestdisplayed in supracrustal rocks from theWashakie block. Rock types in this associationinclude metaperidotite, metabasalt, sulfidicmetaquartzite, semipelitic gneiss (with lesseramounts of calc-silicate gneiss), pelitic gneiss,and iron-formation. Most of the rocks carry up-per amphibolite-facies assemblages; however,they locally preserve granulite assemblages. Thisrock association, along with its local granulitemetamorphism, is widespread throughout therange and occurs in the Medina Mountain area(Koesterer et al., 1987), Crescent Lake area(Worl, 1968; Marshall, 1987; Sharp and Essene,1991), and Mount Helen structural belt (Hulse-bosch, 1993) (Fig. 1). In these areas, however,later migmatization has disrupted the supracrustalsequence to the extent that early fabric relationsare obliterated.

In the Washakie block, supracrustal rocks oc-cur as lenses that may extend for a kilometer ormore (Fig. 3) interfolded with gray gneisses. Inseveral places they clearly occur in the keels oftight, isoclinal F1,2synforms. Locally within thesesynforms, refolded isoclinal F1,1 folds occur. Themajor schistosity in this area lies parallel to the

limbs of the F1,2 folds, and we designate it as S1,2(Fig. 4). Where the rocks are in granulite facies,the preferred orientation of granulite minerals liesparallel to S1,2. Thus, we conclude that at least thelater stages of D1 occurred during granulite meta-morphism.

It is unclear whether the supracrustal rockswere deposited on the gray gneisses, are in tec-tonic contact with them or are intruded by thegray gneisses. Locally, we have recognized D1folds and granulite assemblages in the graygneisses. This fact means that at least some of thegray gneisses were present during the earlieststructural and metamorphic events recognized inthe range.

Porphyritic Dikes

Porphyritic, weakly foliated amphibolitizeddikes cut the D1 fabrics in the Washakie block.These rocks are invariably boudinaged so that in-dividual dikes typically can be traced for less thana hundred meters. Trains of boudinaged dikeshowever outline the D2 folds that also deform theD1 fabrics. This is clear evidence that the dikeswere intruded between the D1 and D2 events. Por-phyritic dikes that cut D1 fabrics are also presentboth in the Medina Mountain area, where they arecalled the Victor dikes (Koesterer et al., 1987),and in Crescent Lake area (Marshall, 1987). Be-cause the porphyritic amphibolite dikes showsimilar textures in all three areas, and because thecross-cutting relations are the same, we considerthem to represent the same magmatic event, andthe term Victor dikes is applied in all three areas.

D2 Event

The S1,2schistosities in the Washakie block arefolded into broad folds that have a weak axial-planar schistosity that can be recognized only inthe fold hinges. On the basis of the shape of thesefolds, we have divided the Washakie block intotwo domains, which are separated by the DryCreek Ridge Structural Zone (Fig. 3). In the DryCreek domain, these folds are relatively open andplunge at ~45°E (Fig. 4A), whereas in the BobLakes domain the folds are nearly isoclinal andplunge ~30°E (Fig. 4B). Because the Victor dikes,which cut the S1,2 fabric, are folded by these laterfolds, we conclude that this folding event is a dis-tinct deformation event, which we call D2.

Foliations and lineations from the Dry Creekand Bob Lakes domains are plotted on stereodiagrams on Figure 4. The foliations measuredare dominantly S1,2. D2 produced weak mineral-preferred orientations in the fold hinges that areeasily distinguished from the compositional lay-ering that is distinctive of S1,2. Distinction be-tween lineations is ambiguous. The F2,1 event

FROST ET AL.

566 Geological Society of America Bulletin, April 2000

D4 fabric of Mt. Helen Structural Belt, earlier

fabric elements may be present in low strain zones.

D1 and D2 fabrics dominate, late cross-

cutting D4 shear zones may be present.

Age of the fabric is unknown

Base of the Mt. Helen Structural Belt,dashed where inferred

Weak D4 fabric dominates, earlier fabric elements may be

present in migmatitic and supracrustal rocks

D5 fabric of South Pass

No penetrative fabric

109 45’

42 45’

Pinedale

Kilometers

0 25

Figure 2. Structural domains in the Wind River Range.



produced linear features (L2,1) that are seenmainly as parasitic fold axes and crenulations.There are also L1,2 lineations associated with theF1,2 folds. The L1,2 lineations are easily recog-nized in the supracrustal rocks, where they formas crenulations and minor folds parallel to theF1,2 fold axes, but they are not so easily distin-guished in the surrounding gray gneisses. Thoselineations that we could clearly identify as L1,2were not plotted on Figure 4. Nonetheless, thereis a rather large scatter of lineations from theDry Creek domain, and we conclude that a fairnumber of the lineations in Figure 4 formed be-fore D2. This is indicated by a larger scatter oflineations in the Dry Creek domain, where theD2 folds are relatively open, than in the Bob

Lakes domain, where the folds are nearly isocli-nal. We interpret this difference to result fromthe transposition of the L1,2 into parallelismwith the L2,1 within the Bob Lakes domain(Fig. 4).

Early Migmatization

The early migmatites in the Washakie block arerepresented by the Native Lake gneiss, a weaklyfoliated to unfoliated calc-alkalic pluton that wasemplaced late during or post-D2. This is the earli-est body for which we have a direct age; it givesU-Pb zircon dates of ca. 2.8 Ga (Frost and Frost,1993). There are a few localities where the NativeLake gneiss is inclusion-free. Over most of its oc-

currence, it is migmatitic, hosting inclusions of allthe older rock types discussed above. In a fewareas, such as north of Crater Lake (Fig. 3), mas-sive enderbites grade into typical amphibole-bear-ing Native Lake gneiss. In most of the rest of itsexposures, the Native Lake gneiss has a texturesimilar to retrograded charnockites—the horn-blende and biotite form clots rather than distinctgrains. The hornblende and biotite in the clots arepoikilitic in thin section, with numerous inclu-sions of quartz. These textures are similar to thoseproduced by secondary biotite and hornblende incharnockites in which the pyroxene has been par-tially hydrated. We therefore infer that at least lo-cally the Native Lake gneiss was originally py-roxene-bearing.



u n m a p p e d

unmapped

Geology of the Dry Creek Region,Wind River Mountains, Wyoming

Dry Creek gneiss

undifferentiated gneiss,Dry Creek domain

undifferentiated gneiss,Bob Lakes domain

gray gneiss

supracrustal-rich gneiss

Native Lake gneiss

Native Lake migmatite

Bridger batholith

Bridger migmatite

Mt. Helen structuralzone

Dry Creek Ridgestructural zone

Proterozoic diabase dike

Dry Creek Ridge

Quaternary alluvium

Archean

axial trace of f2,1 antiform

axial trace of f2,1 synform

Explanationscale in kilometers

0 5

NIndian Pass

Win

d R

iver

Ind

ian

Res

erva

tion

North Fork Bull Lake Creek

Crater Lake

Bob Lakes

Dry Creek

NativeLake

43 15’

109 30’

unmappedFigure 3. Geologic map of the

Dry Fork–Bob Creek–Bull LakeCreek region (area 1, Fig. 1). Mapcovers portions of U.S. GeologicalSurvey Bob Lakes, Fremont PeakNorth, Hays Park, and Ink Wells7.5′′ Quadrangles. Geology byB. R. Frost, T. Hulsebosch, andK. Chamberlain, 1982–1995.



Weakly foliated felsic orthogneisses that weconsider to be correlative with the Native Lakegneiss are found in strain-free zones of the MountHelen structural belt and in the southwest portionof the Medina Mountain area mapped byKoesterer (1986) (Fig. 5). Like the Native Lakegneiss, the felsic gneiss in both areas containsclotted amphiboles, suggesting an earlier pyrox-ene-bearing history. Both the Medina Mountainand Crescent Lake areas expose a migmatite thatpredates the Bridger batholith and that containsinclusions of granulite-grade supracrustal rocks,which are cut by Victor dikes (Koesterer et al.,

1987; Marshall, 1987). Our interpretation is thatthese early migmatites formed at the same timeas the Native Lake gneiss. It is possible that thesemigmatites represent an intrusive event that isdistinct from that Native Lake gneiss. With ourpresent knowledge, however, we have no evi-dence that it is different.

Medina Mountain Sequence and D3

A belt of supracrustal rocks (the MedinaMountain sequence), is in-folded with the earlymigmatites in the Medina Mountain area

(Fig. 5) (Koesterer et al., 1987). The metasedi-mentary rocks within this sequence includepelitic and semipelitic gneisses and minoramounts of fine-grained amphibolite (probablymetabasalt),quartzite, calc-silicate granofels,and iron formation. In places, such as on thenorthern limb of the synform that runs throughMedina Mountain, a distinct stratigraphy is de-veloped that (from base to top) consists of am-phibolite, metapelitic gneiss, amphibolite, andpsammitic gneiss. The migmatite at the base ofthe Medina Mountain sequence is not stronglysheared, which leads us to conclude that theMedina Mountain sequence probably rests indepositional contact on the migmatites. It is pos-sible that the Medina Mountain sequence wasthrust onto themigmatites, but, if that were true,then the thrusting must have happened at low tem-peratures where the strain would have been con-centrated in narrow faults rather than in a wide my-lonite zone (Sibson, 1977).

The Medina Mountain sequence has beenfolded into tight synforms during a deformationevent (D3) that clearly postdates the injection ofthe neosomes in the migmatite. The synformslack an axial planar schistosity and clearly fold anearlier schistosity that is parallel to the composi-tional layering. We call the schistosity S3,1 andthe major synforms F3,1 because these folds arecaused by the earliest deformation event that wecan identify. The S3,1 is parallel to the northwest-southeast–trending schistosity in the earliermigmatites and the Bridger batholith. The rocksare poorly lineated; the major linear features areaxes of minor folds and crenulations. The L3,1lineations have been folded into a broad fold,which is expressed in the field by the way that theF3,1 folds form closed ovoid structures (Fig. 6).We consider this later fold to be a D4 structurebecause it has the same trend as the F4,2 folds inthe Mount Helen structural belt.

Mount Helen Structural Belt and D4

The Mount Helen structural belt is a majorhigh-temperature shear zone that forms thesouthwestern margin of the Washakie Block(Figs. 1 and 3). It was named by Granger et al.(1971) who correlated it with the Wilson Creekorthogneiss of Perry (1965) and Barrus (1970).Our mapping shows that the zone is lithologi-cally heterogeneous and measures up to 3 kmacross strike, much wider than is shown in themap of Granger et al. (1971). The shear zone hasa gentle northeast dip and a top-to-the-southwestsense of shear (Hulsebosch, 1993). Where ex-posed, the footwall contact of the Mount Helenstructural belt is sharp, with the footwall rock be-ing a weakly foliated gneiss that resembles theNative Lake gneiss. We have not located the

FROST ET AL.


f2,1

f1,2

Dry Creek domain

f1,2

Bob Lakes domain

f2,1

Bob Lakes lineationsN = 68

Bob Lakes foliationsN = 78

s1,2

s1,2

Dry Creek lineationsN = 100

Dry Creek foliationsN = 189

A

B

A

B

Figure 4. Sketch comparing the structural framework of the Dry Creek and Bob Lakesdomains. In both areas, the supracrustal rocks contain rootless isoclinal folds not shown on thefigure that we label as f1,1.



hanging-wall contact because it occurs in thesteep terrane bordering the North Fork Bull LakeCreek. The extent of the Mount Helen shearzone shown on Figure 3 is the minimum widthpossible for the zone; it could be considerablywider.

The Mount Helen structural belt consists of in-tensely foliated, mylonitic, granitic, and tonaliticgneiss. The mylonitic foliation has been foldedinto tight isoclinal folds that locally preserverootless hinges of an earlier folding event(Fig. 7). We refer to the earliest folds as F4,1 andthe isoclinal folds as F4,2. Much of the composi-tional layering in the Mount Helen structural beltwas produced by shear early in the deformationand may even predate the formation of F4,1. Theaxial planes of F4,2 folds have been folded intobroad open F4,3 folds (not shown on Fig. 7). Thefolding of F4,2 around these open F4,3 folds ac-counts for the dispersion of lineations from theMount Helen structural belt (Fig. 7). The majorschistosity in the Mount Helen structural belt isaxial planar to F4,2, hence is designated S4,2. The

S4,2 schistosity anastomoses around giantboudins of weakly sheared rock. It is the anasto-mosing nature of this foliation that produces thedispersion of the foliations evident on the stereoplot (Fig. 7).

The boudins within the Mount Helen struc-tural belt include weakly deformed graniticgneiss as well as isolated blocks of supracrustalrocks that may be as large as 30 m in any dimen-sion. These supracrustal rocks include metaperi-dotites, fine-grained metabasites that may havebeen metabasalts, sulfidic quartzites, and raremetapelitic gneiss. The supracrustal inclusionscommonly contain granulite assemblages andhave a fabric that is oblique to that of the foliationof the Mount Helen structural belt. We considerthese to be equivalent to the S1,2foliations seen inthe Washakie block. On the margins of the inclu-sions are amphibolite-facies assemblages withfoliations that are parallel to S4,2(Fig. 7).

About 30% of the area of the Mount Helenstructural belt in the North Fork Bull Lake Creekarea (Fig. 3) consists of weakly foliated leuco-

granite typical of that of the Bridger batholith(Hulsebosch, 1993). Some leucogranite dikeshave been folded by F4,2, whereas others com-pletely cross-cut the fabric of the shear zone.From this situation, as well from as the map pat-tern which shows the Mount Helen structural beltto be truncated by the Bridger batholith near thecenter of the range (Fig. 1), we conclude that theMount Helen structural belt was active during theemplacement of the earliest stages of the Bridgerbatholith.

The northeast-trending fabric of the MountHelen structural belt is common throughout theBridger batholith and the migmatites that lie tothe south. As noted above, the S3,2 foliations lieparallel to the S4,2 foliations, and the D3 synformin the Medina Mountain area has been foldedaround an axis that is coincident with the generalorientation of the F4,2 folds. These relations sug-gest that D3 and D4 deformations may have beencaused by the same event. Because the deforma-tion style in the Medina Mountain sequence isdistinctly different from that of the Mount Helen



Louis Lake batholith, 2.64 Ga granite and granodiorite

Bridger batholith, weakly foliated 2.67 Ga granite

Medina mountain supracrustals, pelitic gneiss, metabasite, semi-pelitic gneiss

Felsic Orthogneiss

Paragneiss-rich migmatite

HallsLake

Lake Victor

NorthFork Lake

Lake Prue

Alluvium

Faults

axis of f3,2 synform, showing direction of plunge

EXPLANATION

N0 1

kilometers

42 55’109 30’

Figure 5. Geologic map of theMedina Mountain area (area 2,Fig. 1), modified from Koesterer(1986). Map covers areas of theU.S. Geological Survey HallMountain 7.5′′ Quadrangle map.



structural belt, and because no evidence indicatesthat the deformations in the two areas are indeedcoeval, we have designated the deformation ineach area as a distinct event.

A series of sheared gneisses is found in theCrescent Lake area (Fig. 8). Although thegneisses are locally highly migmatized by theBridger batholith, enough areas of coherentsheared gneiss survive for structural analysis. Theshear zone here has a northeast trend with a top-to- the-northwest sense of shear (Marshall, 1987).Despite the difference in trend ,we suggest that itrepresents the same D4 event as that in the MountHelen structural belt, because in both areas theshearing was nearly coeval with the intrusion ofthe Bridger batholith. Furthermore, the countryrock affected by the shearing in both areas showsa similar history (cf. Marshall, 1987; Hulsebosch,1993). In the Crescent lake area the country rockwas the early migmatite, whereas for the MountHelen structural belt it was the Native Lakegneiss. In both areas, the gneiss hosted large in-clusions of older supracrustal rocks. By correlat-ing the sheared gneiss in the Crescent Lake areawith the Mount Helen structural belt, we inferthat the Mount Helen structural belt originallycurved from northwest trending in the centralportion of the range to northeast trending in thenorth (Fig. 2).

Bridger Batholith

The Bridger batholith, a weakly foliated or-thogneiss that crops out over an area of approxi-mately 650 km2 in the north-central portion of theWind River Range, is key to establishing the agesof structural events in the northern portion of therange. The batholith was emplaced at 2.67 Ga(Aleinikoff et al., 1989) and places the youngerlimit on the age of the D3 and D4 fabrics. Thebatholith ranges in composition from diorite togranite, with granodiorite being the most volumi-nous phase (Frost et al., 1998), and forms grada-tional contacts on all margins, which are shownas migmatite on Figure 1. As noted above, thebatholith was emplaced late in the D4 event andhas a weak foliation. In the southern margin, thefabric has a northwest trend (Hulsebosch, 1993),whereas, in the north, it trends toward the north-east (Marshall, 1987). The foliation is presentthroughout the body but increases in intensity tothe east, toward the Mount Helen structural belt.

South Pass Sequence

Unlike other supracrustal rocks in the WindRiver Range, the supracrustal rocks at South Passare weakly metamorphosed, preserve primarysedimentary structures, and present a coherent

stratigraphy. The structurally lowest unit in theSouth Pass sequence (SPS) is the DiamondSprings formation, which consists of serpen-tinites and talc-chlorite schists, considered to bekomatiitic flows and subvolcanic intrusions(Hausel, 1991). Resting above the DiamondSprings formation is the Goldman Meadowsformation, a sequence of mature sediments andiron formation (Bayley et al., 1973) and theRoundtop Mountain greenstone, a series ofmetavolcanic rocks that range from tholeiitic tokomatiitic in composition (Hausel, 1991). Mostof the rocks exposed at South Pass are part of theMiners Delight formation and are thrust againstthe lower units of the SPS (Hull, 1988). The Min-ers Delight formation consists of two packages.The lower portion is a proximal graywacke thatcontains minor interbedded calc-alkalic lavaflows. Thrust against this is an upper distal se-quence that contains only deep-water turbidites(Hull, 1988). Preliminary U-Pb ages of zirconsfrom dacites within the Miners Delight formationindicate that these rocks were deposited at2.64 Ga (K. R. Chamberlain, unpublished data),only a few million years before the emplacementof the 2.63 Ga Louis Lake batholith.

The relationship between the SPS and theolder rocks in the Wind River Range is not clearbecause the lower contact of the SPS has been

FROST ET AL.


s1,2

s3,1

migmatitic gneiss

supracrustal rocks in migmatite, granulite grade

supracrustal rocks in migmatite, amphibolite grade

f4,2

Medina Mountain supracrustals

Medina fold axesN = 76

Medina foliationsN = 98 f3,1

A’

A

A’A

Figure 6. Sketch of the struc-tural relations in the MedinaMountain area looking north.Earliest fabric elements are foundin isolated blocks of supracrustalrocks floating in migmatite. Wedesignate the schistosity in theserocks as S1,2 on the basis of simi-larity to fabric elements in theWashakie block. Included withinthe migmatites are supracrustalrocks that are recorded in thekeels of f3,1synforms. The axes ofthese folds have been folded asshown in cross section A–A′′around open folds that we suggestformed in the D4 event.



obliterated by the later emplacement of the LouisLake batholith. Because the SPS contains severalnorth-directed thrust faults, we believe that theSPS was thrust northward upon the craton. A pri-mary sedimentary contact, however, cannot bediscounted.

Louis Lake Batholith

The Louis Lake batholith, the largest post-tec-tonic batholith in the Wind River Range, was em-placed at 2630 Ma (Frost et al., 1998). Althoughminor enclaves of gabbroic and dioritic rocks arepresent in the pluton, the major rock types are gra-nodiorite and porphyritic granite. On the north-eastern limits of outcrop, in the structurally deep-est level of exposure of the Wind River uplift, bothrock types contain pyroxene (Frost et al., 1998).East and south from this area, the pyroxene-bear-ing assemblages are replaced by hornblende- andbiotite-bearing assemblages. Along the southernmargin, the Louis Lake batholith intrudes andmetamorphoses the South Pass sequence, whereasin the north, the charnockitic portions of thebatholith produce granulite-grade metamorphismin the earlier gneisses.

Late Archean Batholiths

The last Archean event in the Wind RiverMountains was the emplacement of four plutonsof porphyritic granite. These are the Bears Earsbatholith in the east central, the Middle Mountainbatholith in the north central, the Granite of NewFork Lakes in the northeast, and small plutons inthe South Pass area. Whole-rock Pb-isotopic datafrom these rocks indicate that they are roughlycoeval and were emplaced around 2.54 Ga(Stuckless et al., 1985).

Post-Archean Events

The Archean rocks of the range are cut by Prot-erozoic diabase dikes that locally preserve primaryigneous textures. These dikes may be up to 50 mwide and can be followed for up to 30 km. Paleo-magnetic and preliminary U-Pb baddeleyite agesof these dikes indicate that there may be two agesof dikes, one at around 2.0 Ga (Harlan, personalcommun.) and another at 1.47 Ga (Chamberlainand Frost, 1995).

The crystalline rocks of the range are also cutby numerous shear zones. Some of the zones aremylonitic, but most are brittle. These zones arecommonly associated with chlorite- epidote- orhematite-rich alteration halos that may be asmuch as tens of meters wide. Although thesezones locally have Laramide motion, the green-schist-grade assemblages that they contain aretoo high-temperature to be Laramide in age.

They are considered, therefore, to have formed inthe Proterozoic, although their exact age is stillunknown. Some large fault zones can be tracedfrom the overlying Phanerozoic rocks into theArchean basement. These are clearly Laramidein age, but they differ from the Proterozoic zonesin that they are associated with clay alterationrather than greenschist-grade minerals (Mitra andFrost, 1981).

METAMORPHIC HISTORY

We recognize four distinct metamorphicevents in the Wind River Range (Table 2), al-though we are aware that there were other ther-mal events that we cannot characterize becausethey were not associated with distinctive struc-tural events or mineral assemblages. The earliestmetamorphism, M1, is a granulite-facies eventthat is associated with D1 deformation. A thermalevent certainly was associated with the emplace-ment of the 2.8 Ga Native Lake gneiss; however,we cannot distinguish assemblages formed thenfrom those formed during later metamorphism.The M2 is a regional event that accompanied theemplacement of the 2.67 Ga Bridger batholith.The M3 is a regional greenschist-amphibo-lite–grade event that accompanied deformationof the 2.65 Ga South Pass sequence. This meta-morphism is not seen elsewhere in the range, ei-

ther because the granitic gneisses of the range arerelatively unreactive or because the metamor-phism occurred while the South Pass rocks werebeing tectonically thrust onto the Wyoming cra-ton. The M4 contact metamorphism is associatedwith the intrusion of the 2.63 Ga Louis Lakebatholith and is probably limited to a zone withinseveral kilometers of the batholith.

In addition to using published results of Bayleyet al. (1973), Marshall (1987), and Sharp andEssene (1991), we determined the P-Tconditionsfor the various metamorphic events using analy-ses of minerals from the samples listed in Table 3.Minerals from samples 95Bob25 and 85Fl-2A(see Fig. 1) were analyzed on the JEOL SUPER-PROBE 8900 at the University of Wyoming,whereas H-2 and H-150A (Table 4) come fromunpublished analyses of Koesterer (1986) andwere performed on a CAMECATM CAMEBEXmicroprobe. In the thermobarometric calculationsbelow we have used the TWQ program ofBerman (1991) along with the data set of Berman(1988) and the solution models of Berman andAranovich (1996).

Throughout the range, ion-exchange thermo-meters were extensively reset to low tempera-tures (Koesterer, 1986; Sharp and Essene, 1991;Hulsebosch, 1993). During cooling, garnet be-came more iron-rich whereas cordierite and or-thopyroxene become more magnesian-rich.



f4,2

Mt. Helen lineationsN = 96

Mt. Helen foliationsN = 194

f4,1

s1,2

s4,2

s4,1

gray gneiss, lines show foliation trends

supracrustal rocks, granulite grade

supracrustal rocks, amphibolite grade

Figure 7. Sketch of structural relations in the Mount Helen structural belt looking roughlynorth. The earliest structural elements are foliations found in granulite-grade inclusions withinthe mylonitic gneiss. These we label as s1,2. The earliest fabric element in the mylonitic gneiss isan intense mylonitic foliation that has been folded into at least two isoclinal folds, which wedesignate f4,1and f4,2. Not shown are the broad, open f4,3folds that fold the axial planes of the f4,2folds and bodies of the Bridger granite, which may be involved in the f4,2and f4,3folds, but whichmay also cut the shear zone entirely.



Thus, in an effort to “see through” the effects ofcooling, we have chosen the core compositions,which provide the most magnesian-rich garnetand the most iron-rich cordierite and orthopyrox-ene compositions (see Table 4).

M4 Event

Before we discuss the metamorphic history ofthe Wind River Range, we must characterize the

effect of Laramide uplift, because different levelsof crust are exposed across the range and this willaffect how one interprets the results of the thermo-barometry. To do this we use the thermobarometryof the M4 event, which is the contact metamor-phism related to the intrusion of the Louis Lakebatholith. In the South Pass region it is character-ized by the presence of Kfs in rocks containingAnd (Bayley et al., 1973) (mineral abbreviationsafter Kretz, 1983). The And-Sil reaction and the

reaction Ms + Qtz = And + Kfs + H2O intersect atH2O pressures of 2.5 kilobars. At lower H2O pres-sure this intersection moves to higherP.We there-fore show the contact aureole to have formed atpressures around 2.5–3.0 kilobars and tempera-tures around 670–700 °C (Fig. 9).

The charnockites along the north contact lo-cally contain pelitic inclusions. One of these in-clusions (FL85-2A) contains the assemblageGrt-Opx-Crd-Qtz. Using the displaced reactionswritten for the Fe and Mg end members fromthis assemblage, the TWQ program generates11 equilibria that intersect between 740 °C and840 °C and 5 400 and 6 800 bars. The dispersionin T and P is probably caused by Fe-Mg ex-change that continued after the peak of meta-morphism, as indicated by the discordancy ofthe cordierite-opx, cordierite-garnet, and opx-garnet Fe-Mg exchange thermometers (Fig. 9).Despite the scatter, it is clear that FL85-2Aequilibrated at conditions that were consider-

FROST ET AL.


Bear

Lake

LakeDaphne

Faler Lake

Crescent Lake

Roaring Fork

kilometers

0 1

NMiddle Mountain batholith, 2.54 Ga granite

Layered migmatite

Migmatized sheared gneiss

Sheared felsic gneiss

Supracrustal-rich gneiss

EXPLANATION

Taconite

Synformal axis, showing plunge direction

Pink granite and granite gneiss, probablycorrelative with 2.7 Ga Bridger batholith.

Cross Section on A-Ano vertical exaggeration

A A′

A

A

Crescent Lake lineationsN = 102

Crescent Lake foliationsN = 252

109 45′

43 20′

Proterozoicdike

Figure 8. Geology of the Crescent Lake area (area 3, Fig. 1) after Mar-shall (1987). Map covers portions of the U.S. Geological Survey 7.5′′Downs Mountain Quadrangle.

TABLE 2. METAMORPHIC EVENTS IN THE WIND RIVER RANGE

Event Age Description(Ga)

M1 >2.8 Granulite metamorphism associated with D1M2 2.67 Amphibolite-granulite metamorphism associated with the emplacement of the

Bridger batholithM3 2.65–2.63 Regional metamorphism of the South Pass sequenceM4S 2.63 Contact metamorphism of South Pass sequence on south side of Louis Lake

batholithM4N 2.63 Granulite-grade contact metamorphism on north side of Louis Lake batholith



ably hotter and at higher pressure than the rocksalong the southern contact.

The barometry indicates that, at the time theLouis Lake batholith was intruded, the presentnorthern contact was about 10 km deeper than thesouthern contact (Fig. 9). These localities are65 km apart, and this uplift can be accommo-dated by a southward plunge in the Laramidestructures of less than 10°. The attitude ofPhanerozoic sedimentary rocks in the northernportion of the Wind River Range indicate that theLaramide structure is a broad fold that plunges15°N (Mitra and Frost, 1981). If the southernportion of the structure plunges south at a similarangle, then the pressure difference recorded inthe contact aureole of the Louis Lake batholithcan easily be accommodated by Laramie defor-mation, although tilting by Archean or Protero-zoic deformation events cannot be disproved.

M1 Event

Although granulite assemblages are not un-common in the older supracrustal rocks, it is un-usual to find assemblages in which ion-exchangethermometers preserve the conditions of thismetamorphism (Koesterer et al., 1987; Sharp andEssene, 1991). Because the ion-exchange thermo-meters in these rocks are strongly reset, distinctivemineral assemblages must be relied upon to con-strain these metamorphic conditions. Sharp andEssene (1991) used the assemblage Spl-Sil-Rt-Qtz that occurs as inclusions within garnet to con-

strain the earliest metamorphic conditions in theCrescent Lake area. We find similar assemblagesin the Medina Mountain area and in the Washakieterrane. The assemblage occurs as part of the ma-trix of the rock in these areas, however, with Siloriented parallel to S1,2, indicating that it formedas part of the M1 event.

The best-preserved sample with this assem-blage is 95Bob25, which comes from theWashakie terrane (Fig. 3). In this rock, Spl is as-sociated with Sil and both are included in Crd orGrt. The Grt contains inclusions of Rt but notIlm, whereas Ilm occurs in the matrix instead ofRt. As with Spl and Sil, Gar is rimmed by Crd.We infer that the primary assemblage was Qtz-Bt-Ksp-Pl-Grt-Sil-Spl-Rt, whereas the matrixassemblage, which we conclude was M2, wasQtz-Bt-Ksp-Pl-Grt-Crd-Ilm. Garnet is weaklyzoned with moderate enrichment (0.05 mole %)in alm on the margins; likewise XFe in Crdvaries by less than 2 mole %. Plagioclase aver-ages an68 and varies by only 0.02 mole % an,with the rims being slightly more sodic than thecores.

Because M2 was much hotter than the com-mon closure temperature for diffusion in garnet,we consider that the compositions listed inTable 2 formed during M2. It is likely that Crd,the most magnesian mineral in the rock, grew en-tirely during M2, and we believe therefore thatthe Grt was probably more magnesian during M1than is shown by the analyses. It is impossible toback-calculate Grt composition because all Fe-Mg bearing phases (Grt, Bt, and Spl) changedcomposition as Crd grew. Thus, we would haveto know the P-T path followed by the rock toback-calculate. We can use the M2 compositions,however, to limit the conditions of M1. Figure 10shows the location of four limiting curves calcu-lated from the given composition of Grt, Crd, andSpl. The assemblage Spl + Qtz is limited by tworeactions:

2 MgAl2O4 + 5 SiO2 = Mg2Al4Si5O18 (1)

5 SiO2 + 3 MgAl2O4 = 2 Al2SiO5 + Mg3Al2Si3O12 (2)



TABLE 3. MINERAL ASSEMBLAGES OF ROCKS FROM THE WIND RIVER RANGE

Sample Grt Crd Opx Sil an Spl Rt85FL-2A X X X N.P. 0.34 N.P. N.P.95Bob25 X X N.P. X 0.33 X XH-2 X X N.P. * 0.29 N.P. N.P.H-150 X X N.P. * 0.33 N.P. N.P.

Notes: an—anorthite content of plagioclase; X—present in main assemblage, N.P.—absent;*—retrogressive. All rocks contain quartz, K-feldspar, and biotite.

TABLE 4. MINERAL ANALYSES FROM THE WIND RIVER RANGE1

Garnet Cordierite Opx SpinelSample 85Fl2A 95Bob25 H-2 H-150A 85Fl2A 95Bob25 H-2 H-150A 85Fl2A 95Bob25SiO2 38.68 38.59 37.01 36.75 49.77 50.19 48.46 48.93 47.93 0.00TiO2 0.06 N.A. 0.00 0.01 0.00 0.00 0.03 0.00 0.09 0.07Cr2O3 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 4.02Al2O3 21.72 20.81 21.63 21.67 34.29 33.65 33.93 33.95 6.89 56.28FeO 27.19 31.52 32.09 32.76 5.16 5.75 6.64 6.91 25.23 30.70ZnO N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 4.07MnO 0.50 0.63 0.97 0.99 0.01 0.00 0.04 0.07 0.04 0.06MgO 9.69 6.96 6.65 6.00 10.43 10.09 9.79 9.75 19.40 4.24CaO 1.21 2.28 1.27 1.51 0.01 0.00 0.03 0.00 0.07 N.A.Na2O 0.00 N.A. N.A. N.A. 0.03 0.28 0.05 0.11 0.00 N.A.Total 99.05 100.80 99.62 99.69 99.72* 99.95 99.15 99.72 99.65 100.34†

Note: Cation proportions on a basis of 12 oxygens for garnet, 18 for cordierite, 6 for Opx, and 12 for spinel.Si 2.999 3.013 2.938 2.928 4.975 5.019 4.919 4.938 1.817 0.000Ti 0.004 0.000 0.000 0.001 0.000 0.000 0.032 0.000 0.003 0.003Cr N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 0.136Al 1.985 1.916 2.023 2.035 4.040 3.967 4.059 4.038 0.308 2.833Fe 1.763 2.058 2.130 2.183 0.432 0.481 0.564 0.583 0.800 0.731Mn 0.033 0.042 0.065 0.067 0.001 0.000 0.004 0.006 0.001 0.002Mg 1.120 0.810 0.787 0.713 1.554 1.504 1.482 1.467 1.096 0.180Ca 0.100 0.191 0.108 0.129 0.001 0.000 0.003 0.000 0.003 N.A.Na 0.000 N.A. N.A. N.A. 0.005 0.054 0.011 0.026 0.000 N.A.Almd 0.585 0.664 0.689 0.706Py 0.371 0.261 0.255 0.231Gros 0.033 0.062 0.035 0.042Spes 0.011 0.014 0.021 0.022XFe 0.612 0.718 0.730 0.754 0.218 0.242 0.276 0.284 0.422 0.802

Notes: Analyses of minerals from H-2 and H-150A come from Koesterer (1986); opx—orthopyroxene; N.A.—not analyzed.*Includes 0.02 wt% K2O.†Includes 0.90 V2O3.



The reactions shown on Figure 10 are calcu-lated for the displaced Mg-end members becausethese are the extreme limits of the assemblage.The locations displacing the Fe-end-member re-actions lie within the field outlined by these reac-tions; if the minerals in the rock had retained theirhigh-T, Fe-Mg compositions, the locations of theFe end member and Mg end member for reac-tions (1) and (2) would have coincided.

The assemblage Rt + Grt is limited by the re-action:

3 TiO2 + Fe3Al2Si3O12 = 3 FeTiO3 + Al2SiO5 + 2 SiO2 (GRAIL)

Our sample has Rt but not Ilm in the core ofthe Grt. Therefore, to estimate the location ofGRAIL in Figure 10, we used the most mag-nesian Grt, pure Rt (the Rt in the rock shows onlyTi peaks on EDS) and pure Ilm. This establishesthe minimum pressure for this reaction, becausesolution of Mg and Fe2O3 into Ilm will stabilizeit to higherP. The fourth reaction on Figure 10 is

the Grt-Crd thermometer, which, as noted above,was calculated using the most Mg-rich Grt andthe most Fe-rich Crd.

We consider that the Grt-Rt-Spl-Sil-Qtz as-semblage in sample 95Bob25 formed at condi-tions indicated by the vertically ruled area in Fig-ure 10. This field is limited at high pressures byreaction (2), at low temperatures by reaction (1),and at low pressures by GRAIL. As noted abovewe contend that Crd growth during M2 caused allminerals to become more Fe-rich as Mg was se-questered in Crd. If the original Grt and Spl weremore Mg–rich, reactions (1) and (2) would bothhave moved to higherT, restricting the stabilityfield for Spl + Qtz.

Although imprecise, the conditions deter-mined for M1 in 95Bob25 are consistent withthose estimated for the assemblage Grt-Spl-Sil-Qtz-Rt-Ilm from the Crescent Lake area by Sharpand Essene (1991). It must be noted, however,that the rocks studied by Sharp and Essene(1991) also had late Crd. Before the Crd grew inthese rocks, the Grt and Spl were likely to have

been richer in Mg and this change in bulk com-position would cause the parallelogram in Fig-ure 10 to move to lowerP and higherT. A smallchange in Grt bulk composition would havecaused the Sharp and Essene (1991) estimate tooverlap the P-T estimate by Marshall (1987).Marshall (1987) estimated conditions from theoccurrence of sapphirine in metaperidotiticblackwall rocks with assemblages that bracketedthe reaction Crd + Spl = Opx + Spr. Because ofthe Laramide tilting, there is no reason to expectthat the metamorphic conditions for M1 were thesame in 95Bob25 as at Crescent Lake, which islocated 25 km to the northwest. Our results andthose of Marshall (1987) and Sharp and Essene(1991) indicate, however, that M1 occurred athigh pressure (6–8 kilobars) and was very hot(T > 800 °C). This is consistent with textural re-lations suggestive of pigeonite in iron formationfrom the northernmost margin of the range(Sykes, 1985) and with locally preserved (butstrongly reequilibrated) garnet in mafic gran-ulites elsewhere in the range.

M2 Event

The M2 event is recorded in amphibolite-facieshalos around granulite boudins in the Mount He-len structural belt and in assemblages in theMedina Mountain sequence. In most areas, M2was in upper amphibolite facies, however, locallyin the Medina Mountain area, where metabasitesabut partially melted pelite gneiss, granulite-gradeassemblages are formed. This local granulitemetamorphism probably reflects areas of low wa-ter activity around granite melts (Koesterer, 1986).

We calculated the conditions for the M2 eventfrom the cordierite-bearing assemblage in95Bob25. We assumed that the Crd and Grt equi-librated during M2 and used the compositions ofthe Grt core and the most Fe-rich Crd. Because thematrix contains no Rt, the Crd must have grown atconditions below those indicated by GRAIL, or atpressures below around 5 kilobars (Fig. 10).

We also calculated M2 from two samples fromthe Medina Mountain area (H-2 and H-150A).Both of these samples have the assemblage Qtz-Bt-Ksp-Pl-Grt-Crd. These rocks differ from95Bob25 in that they have not undergone M1and that the whole assemblage was producedduring M2 (although the mineral compositionswere subsequently modified during cooling). Inboth rocks, the Grt is moderately zoned, withXalm increasing by as much as 8 mol% from coreto rim. Crd is not zoned and has XFe that variesby only 3 mol%. Pl is irregularly zoned andvaries by about 2% around an29 in H2 and an32 inH-150A.

In addition to the Grt-Crd thermometer, thereare two equilibria involving Grt, Pl, Crd, and Qtz:

500

2000

4000

6000

8000

10000P

(bar

s)

600 700 800 900 1000

T ( C)

conditions of contactaureole at South Pass

conditions of contact aureoleat Burnt Lake

(sample FL85-2A)

cord

-opx

cord

-gar

n

opx-

garn

Figure 9. P-T conditions for metamorphism along the northern and southern margins of theLouis Lake batholith. Sample M5-2A with the assemblage garnet-cordierite-orthopyroxene-quartz comes from the northern contact, where the pluton is charnockitic. Eleven equilibriagenerated from the Fe and Mg end-members of these phases using the TWQ program ofBerman (1991) intersect in an area around 6 kilobars and temperatures around 750 °C. Thesouthern contact with an assemblage K-feldspar-andalusite-sillimanite-muscovite-quartz (Bai-ley et al., 1973) records temperatures below 700 °C and pressures of 2.5–3.0 kilobars. We inter-pret the differences in these conditions to record different degrees of crustal exhumation in re-sponse to Laramide deformation.

FROST ET AL.




2 Fe3Al2Si3O12 + 6 CaAl2Si2O8 + 3 SiO2 = 2 Ca3Al2Si3O12 + 3 Fe2Al4Si5O18 (3)

2 Mg3Al2Si3O12 + 6 CaAl2Si2O8 + 3 SiO2 = 2 Ca3Al2Si3O12 + 3 Mg2Al4Si5O18 (4)

By using the most Mg-rich Grt and the mostFe-rich Crd in these rocks, we obtained an inter-section of the Grt-Crd thermometer with reac-tions (3) and (4) at temperatures of ~700 °C andpressures of 4.5–5.5 kilobars (Fig. 10). Theseconditions are at slightly higher temperature thanthat obtained from 95Bob25. The differences in Tand possibly P may reflect Laramide tilting,since the Medina Mountain rocks, which arestructurally deeper in the Laramide uplift, recordhigher grade conditions.

We conclude that M2 formed at moderate pres-sures (4–5 kilobars) and at temperatures aroundor slightly below 700 °C. This conclusion is con-sistent with the estimate of metamorphism in theMount Helen structural belt (Hulsebosch, 1993)and with the core and rim compositions of “ma-trix” mineral assemblages described by Sharpand Essene (1991), although the temperatures inthe Crescent Lake area may have been somewhathigher (~750 °C) than those in the core of therange.

M3 Event

Evidence of the M3 event is seen in the regionalmetamorphism of the South Pass sequence. Al-though some of the units at South Pass recordgreenschist conditions, this metamorphism waswithin the low amphibolite facies over most of thearea. We have no quantitative thermobarometryfrom the area,. The coexistence of Crd and Andreported by Bayley et al. (1973), however, allowus to limit the conditions to 450–550 °C and2–3 kilobars (Fig. 11).

Summary

A summary of the metamorphic conditions inthe Wind River range is shown in Figure 11. Ofparticular note is the wide swath in P-Tspace oc-cupied by the M4 event. As noted above, thisprobably reflects Laramide tilting of the rangeand suggests that, if the other metamorphicevents could have been sampled over a suitablywide area, they would record a similar range ofconditions. The rocks recording the M1 eventcome from locations that are structurally high inthe range. Were we to find rocks that equilibratedduring this event on the western side of the range,

we might expect that they would record evenhigherT and P.Likewise, had we more locationswhere M2 could be estimated and had we betterprecision, we might be able to see the effect ofLaramide tilting on M2 as well. Finally, becausewe recognize a nearly 3 kilobar difference be-tween the northern contact of the Louis Lakebatholith and the southern contact, and becauseM4 followed shortly after M3, it is likely that therocks farther north were much deeper and pre-sumably hotter at the time M3 was being devel-oped on the southern portion of the range.

P-TTIME PATH FOR THE WIND RIVERRANGE

We can use our estimation of P-Tconditions,the age constraints, and the geological relationsoutlined above to constrain the temperature pathfollowed by the rocks in the Wind River Rangeduring the Late Archean (Fig. 12). The path ispoorly constrained because of the vast amount oftime involved, because of the few points (both inspace and time) where we could constrain condi-tions, and by the certainty that each of the meta-morphic conditions that occurred over the hugearea of the Wind River Range probably was asso-ciated with a range of temperature conditions.Even so, it is clear that during this 300 m.y. pe-riod of time the rocks in the Wind River Rangeexperienced periodic burial and exhumation. Weshow the path beginning at surface conditions toaccount for the deposition of the earliestsupracrustal rocks, although the path from thispoint to the peak conditions at M1 may have beenvery long and probably involved thermal events,the evidence for which has long since been lost.The first thermal pulse that we can identify is M1at ~800 °C and 6 kilobars (see Fig. 12). As notedabove, this event predates emplacement of the ca.2.83 Ga Native Lake gneiss and the Victor dikes,the age of which is unknown. The X-ordinate inFigure 12 is dashed in this range to emphasizethis age uncertainty. The metamorphism could beas old as 3.2 or 3.3 Ga, as indicated by U-PbSHRIMP (super high-resolution ion microprobe)ages from zircons in migmatite (Aleinikoff et al.,1989), or it may have predated the Native Lakegneiss by only a few tens of millions of years.

The Victor dikes are porphyritic and locallychilled, and, therefore, they must have been em-placed into country rocks that were at tempera-tures lower than M1. This temperature is uncon-strained; although we show it to have been~200 °C, it could have been much lower orhigher. We also have no direct constraints on theemplacement temperature of the Native Lakegneiss. We infer that it must have been emplacedat temperatures >700 °C because it was at leastlocally charnockitic.



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95Bob25M2

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1

2

H2M2 in Medina

Mtn. areaM2 in Washakie

block

T ( C)

P (b

ars)

Figure 10. P-T diagram showing conditions of the M1 and M2 metamorphic events in theWind River Range. Reaction curves listed are calculated for the average spinel and the compo-sition of core garnet and cordierite from sample 95Bob25. Ruled area is the stability field for theM1 assemblage garnet-sillimanite-spinel-quartz-rutile. Area indicated as M2 for this sample isthe stability field for garnet-cordierite-quartz-ilmenite. Samples H2 and H150A come from theMedina Mountain area. M, 87—Marshall (1987); S&E, 91—Sharp and Essene (1991). See textfor discussion.



The Medina Mountain Sequence was eitherdeposited directly on the Native Lake gneiss orwas thrust onto it at low temperatures. Thus, wecontend that some portions of the Wind RiverRange were exposed or near the surface after thecooling of the Native Lake gneiss. After deposi-tion of the Medina Mountain sequence, the rangewas buried again. Some of this may have beenaccompanied by tectonic thickening along theMount Helen structural belt and some may havebeen produced by surface accumulation of erup-tive components of the Bridger batholith. By thetime the Bridger batholith crystallized, the condi-tions were around 700 °C with pressures of4.0–5.5 kilobars.

After the cooling of the Bridger batholith atleast the southern portion of the range was ex-humed to low pressure because the South Pass Se-quence was either deposited upon the basement orthrust upon it under low temperature conditions.Shortly thereafter, the Louis Lake batholith wasemplaced, bringing temperature again into the800 °C range. We do not know the thermal historyafter the cooling of the Louis Lake plutons; weshow the area to have cooled slightly before thelatest Archean plutons were emplaced at 2.55 Gaand to have cooled slowly thereafter. We postulatethis slow cooling to account for the extensive re-setting of the ion-exchange thermometers fromthe M4 metamorphism.

CONCLUSIONS

Metamorphic and Isotopic Resetting

The Wind River Range was periodically in-truded by magmas from ca. 2.8 Ga to 2.55 Ga;this fact accounts for many of the complexities indetermining ages and P-Tconditions for variousrocks from the area. In most places, the originalM1 granulite assemblages were retrograded toamphibolite facies. Distinct hydration halos areseen surrounding dikes of both Bridger and LouisLake granitic rocks, indicating that during theLate Archean these rocks were periodicallyflooded with fluids as well as magmas. In thosefew areas where the original granulite assem-blages are preserved, the ion-exchange thermo-meters have been extensively reset to tempera-tures that are far too low for granulite assemblages(below 500 °C for two-pyroxene and olivine-spinel thermometers; Koesterer et al., 1987). Eventhe granulites formed during the latest metamor-phism show evidence of significant ion-exchangeon cooling (Fig. 10), suggesting that the rocksstayed hot after the intrusion of the Louis Lakebatholith.

The prolonged magmatic history and its asso-ciated influx of melts and fluids into the crust mayexplain the complex U-Pb systematics in the older

rocks of the range. For example, Aleinikoff et al.(1989) found that a migmatite yielded 207Pb/206PbSHRIMP ages from zircon that clustered at 2.65,2.72, 2.85, 3.2, 3.3, and 3.8 Ga. Whereas the olderages reported by Aleinikoff et al. (1989) may beages of individual detrital grains, the youngerthree ages probably reflect recrystallization of zir-con during the Late Archean magmatism that af-fected the area. Similarly, DeWolf et al. (1993)

found a complex age distribution in monazitefrom the Crescent Lake area, wherein a singlegrain yielded 207Pb/206Pb ages that clusteredaround 2.78, 2.66, and 2.54 Ga. They interpretedthese ages to represent distinct periods of mon-azite growth. It is likely that the two younger agesrepresent monazite growth due to the influx ofmagmatic fluids, because the samples studied byDeWolf et al. (1993) occur as a complex roof pen-

FROST ET AL.


Figure 11. P-T diagram comparing conditions of the M1, M2, M3, and M4 metamorphicevents.

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nulit

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e G

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Vic

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es

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holit

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2

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ake

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olith

, M4

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tons

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ositi

on o

f ea

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Figure 12. Archean thermal history of the Wind River Range incorporating the thermo-metry observed from the metamorphic rocks. Asterisks denote age constraints discussed byFrost et al. (1998).



dant that lies on the contact between the 2.7 GaBridger batholith and the 2.54 Ga Middle Moun-tain batholith (Fig. 8).

Tectonic Implications

Although we cannot determine the polarity ofthe 2.8 Ga magmatic event in the Wyomingprovince, distribution of the Late Archean gran-itoids indicates that subduction forming the 2.7,2.63, and 2.54 Ga plutons was directed from thewest and south toward the craton (Frost et al.,1998). During the Phanerozoic, three hundredmillion years is not an exceptionally long periodof time for an active continental margin to re-main stationary. The locus of calc-alkalic mag-matism on the western margin of North andSouth America has occupied essentially thesame position for nearly that long. For example,the Canadian Cordillera has been the site formagmatism periodically for the past 200 millionyears (Armstrong and Ward, 1991, 1993). Like-wise, the Sierra Nevada was magmatically ac-tive from 180 to 70 Ma and is still active in theNorth today (Armstrong and Ward, 1991). Likethe Wind River Range, both the CanadianCordillera and the Sierra Nevada areas containfragments of sedimentary sequences that havefrom time to time been deposited or thrust ontothe magmatic arc.

The existence of active continental margins, inwhich calc-alkalic plutons have been emplacedinto an older continental crust, has been postu-lated in many Archean terranes. In some locali-ties, such as Lac des Iles area of the SuperiorProvince (Brügmann et al., 1997), the EasternGoldfields superterrane of the Yilgarn province(Swager, 1997), and the northern magmatic beltof Zimbabwe (Kusky, 1998), the arcs were short-lived and were built upon crust that was onlyslightly older than the plutons themselves. Otherareas, such as the Limpopo belt and the Slave andthe western Yilgarn provinces, are similar to theWyoming province, in which calc-alkalic plutonswere emplaced into a basement that was consid-erably older than the plutons.

In the central zone of the Limpopo belt, calc-alkalic plutons dated from 2.6 to 2.55 Ga wereemplaced into a basement as old as 3.2 Ga.(Holzer et al., 1998). The older rocks of theSlave province are invaded by voluminous2.70–2.58 Ga granitoids (van Breemen et al.,1992). The western Yilgarn province, includingthe Murchinson, Lake Grace, Boddington, andBalingup terranes, consists of fragments of oldgreenstones and gneisses, including some as oldas 3.5 Ga, included in large volumes of LateArchean granites. Granitic magmatism lastedfrom 2.70 to 2.60 Ga in the Murchison province(Wiedenbeck and Watkins, 1993); in the other

terranes in the western Yilgarn, granitoids datefrom 2.67 to 2.53 Ga. (Wilde et al., 1996).

No consensus exists on the origin of the LateArchean calc-alkalic plutons in the Slave and Yil-garn cratons. The 2625 to 2695 Ma plutons in theSlave craton have been ascribed to subduction,but it is unclear whether the subduction was west-dipping (Kusky, 1989) or east-dipping (Fyson andHelmstaed, 1988). Likewise, although Archeangranites in each of the blocks in the Yilgarnprovince are of different ages, ages do not pro-gress across the province (Wilde et al., 1996).This lack of progression, along with a widespreaddistribution of 2.64 Ga gold mineralization, ledQiu and Groves (1999) to conclude that the wide-spread postorogenic plutons in the Yilgarn are notproducts of subduction but rather were producedby mantle delamination. Thus, the composition,age, and distribution of granites and gneisses inthe Wind River Range represent the best docu-mented example of a long-lived active continentalmargin during the Late Archean.

ACKNOWLEDGMENTS

Much of the mapping upon which this paperwas based was conducted by Tom Hulsebosch asa seasonal employee of the U.S. Geological Sur-vey and later as part of his Ph.D. dissertation.Tom was killed in a traffic accident in September1996, and we dedicate this paper to his memory.This manuscript benefited greatly by reviews byPeter Dahl, Dave Mogk, and John Percival.

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