Precambrian geology of Lake Plateau, Beartooth Mountains, Montana by Douglas P Richmond

Transcription

1 Precambrian geology of Lake Plateau, Beartooth Mountains, Montana by Douglas P Richmond A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Earth Sciences Montana State University Copyright by Douglas P Richmond (1987) Abstract: The Lake Plateau area in the central Beartooth Mountains of southern Montana is comprised of voluminous late Archean intrusive rocks ranging from quartz diorite to granite in composition, with a variety of supracrustal inclusions. The inclusions range in size from centimeter to kilometer scale and include biotite hornblende schists (bio-qtz-hbld-epi-plag) and pelitic schists (bio-qtz-cord-plag-gar sill). These inclusions have experienced upper amphibolite grade metamorphism at 6-8 kbar and C, with penetrative deformation creating a north striking foliation. The intrusive rocks vary in modal mineralogy and texture on a meter scale. In some places they have an hypidiomorphic-granular texture, and in others they have weak foliation or foliated augen texture. Assimilation of inclusions is common with foliated granites occurring at gradational contacts with inclusions. Pegmatite and aplite veins associated with the intrusive rocks cut across nearly all Archean rocks and comprise 15-20% of the total rock volume. Structural trends include north-south foliation with associated isoclinal folds, broad open kilometer scale folds, and unfolded shear zones with mylonitic textures and retrograde metamorphism to chlorite and epidote. Younger rocks include amphibolite dikes and a few Tertiary felsic dikes. Lake Plateau and the surrounding Beartooth Mountains evolved by: 1) burial of supracrustal rocks to depths of km; 2) penetrative deformation and upper amphibolite grade metamorphism; 3) generation of high-na intrusives such as the Long Lake granites; and 4) generation of the K-rich granites of Lake Plateau. The Lake Plateau granitoids are interpreted as mid-crustal melts emplaced at approximately 20 km and generated from a slightly deeper crustal source. These large volumes of K-rich granites are different from the Na-rich rocks reported in the eastern Beartooths (Mueller and others, 1985). Large volumes of granite with subordinate quartz diorite and a variety of supracrustal inclusions are consistent with characteristics in younger examples of post-collisional tectonic settings. The Beartooth Mountains represent the remaining mid-crustal evidence of development of thickened continental crust in the late Archean by collisional tectonics and post-collisional uplift, similar in many ways to modern day tectonic processes.

2 PRECAMBRIAN GEOLOGY OF LAKE PLATEAU, BEARTOOTH MOUNTAINS, MONTANA by Douglas P. Richmond A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana June, 1987

3 MMN Li /1/37? «f/4 J (Lof- APPROVAL of a thesis submitted by Douglas P. Richmond This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready for submission to the College of Graduate Studies. ci, IcItf ISlo Chairperson, Graduate Committee Approved for the Major Department Date Head, or Department Approved for the College of Graduate Studies Date, ^ 3 -S ^ Graduate Dean

4 iii STATEMENT OF PERMISSION TQ USE In presenting this thesis in. partial fulfillment of the requirements for a master's degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Permission for extensive quotation from or reproduction of this thesis may be granted by my major professor, or in his/her absence, by the Director of Libraries when, in the opinion of either, the proposed use of the material is for scholarly purposes. Any copying or use of the material in this thesis for financial gain shall not be allowed without my permission. Signature

5 iv ACKNOWLEDGEMENTS. I wish to thank the late Dr. Robert A. Chadwick, Dr. David W. Mogk (Committee Chairman), Dr. David R. Lageson, and Dr. Ken Emerson for their suggestions, guidance, and criticism during the preparation of this thesis. This thesis was partially funded by the NASA Early Crustal Genesis Project through a grant secured by Dr..Mogk. Further thanks is extended to Dr. Mogk for help in field mapping..other field assistance was given by: David Hazen, Ken Salt, Hugh Safford, and Jim Barnaby. Extensive help in the field was given by my wife Linda and our dog Shade, who, along with the other field assistants, helped by carrying supplies and rock samples on the arduous trail to Lake Plateau. Mike Trombetta provided invaluable assistance and advice in the final preparation of this thesis. Finally I would like to thank my loving parents, Howard and Elizabeth, for their moral and financial support. Without their help this study would not have been possible.

6 V TABLE OF CONTENTS Page LIST OF TABLES... LIST OF FIGURES... vi vii LIST OF PLATES......"... viii ABSTRACT ,_... INTRODUCTION... ix I REGIONAL SETTING... 3 ROCK UNITS... 7 General Statement... 7 Inclusions... 8 Biotite Garnet Schist... 8 Hornblende Biotite Schist Intrusive Units Hornblende Quartz Diorite Granite - Granodiorite - Pegmatite Dikes STRUCTURE TECTONIC CONDITIONS CONCLUSIONS REFERENCES CITED 39

7 Vi LIST OF TABLES Table Page I. Granite characteristics of tectonic settings... 35

8 vii LIST OF FIGURES Figure Page 1. Precambrian outcrops of the Beartooth Mountains Archean outcrops of the Wyoming Province Photomicrograph of biotite garnet schist AFM composition diagram for biotite garnet schist Garnet-Cordierite phase relations in biotite garnet schist Modal percent quartz K-spar -plagioclase for the Lake Plateau intrusive units Photomicrograph of rounded zircon with dark reaction rim in Lake Plateau granite Photomicrograph of muscovite in Lake Plateau granite Iron-magnesium-titanium compositional range for muscovite in Lake Plateau granite Pressure-temperature diagram showing water-saturated granite solidus and muscovite+quartz reaction curve... = Photomicrograph of subhedral epidote of probable magmatic origin in Lake Plateau granite Photomicrograph of relict ophitic texture in continuous dikes Generalized structure map of Lake Plateau Photomicrograph of quartz in granite from a shear zone Photomicrograph of mylonitized granite from a shear zone Photomicrograph of bent and fractured plagioclase in sheared granite... '... 29

9 viii LIST OF PLATES Plate Page I. Geologic Map of Lake Plateau, Beartooth Mountains, Montana.... back pocket

10 ABSTRACT The Lake Plateau area in the central Beartooth Mountains of southern Montana is comprised of voluminous late Archean intrusive rocks ranging from quartz diorite to granite in composition, with a variety of supracrustal inclusions. The inclusions range in size from centimeter to kilometer scale and include biotite hornblende schists (bio-qtz-hbld-epi-plag) and pelitic schists (bio-qtz-cord-plaggar sill). These inclusions have experienced upper amphibolite grade metamorphism at 6-8 kbar and C, with penetrative deformation creating a north striking foliation. The intrusive rocks vary in modal mineralogy and texture on a meter scale. In some places they have an hypidiomorphic-granular texture, and in others they have weak foliation or foliated augen texture. Assimilation of inclusions is common with foliated granites occurring at gradational contacts with inclusions. Pegmatite and aplite veins associated with the intrusive rocks cut across nearly all Archean rocks and comprise 15-20% of the total rock volume. Structural trends include north-south foliation with associated isoclinal folds, broad open kilometer scale folds, and unfolded shear zones with mylonitic textures and retrograde metamorphism to chlorite and epidote. Younger rocks include amphibolite dikes and a few Tertiary felsic dikes. Lake Plateau and the surrounding Beartooth Mountains evolved by: I) burial of supracrustal rocks to depths of km; 2) penetrative deformation and upper amphibolite grade metamorphism; 3) generation of high-na intrusives such as the Long Lake granites; and 4) generation of the K-rich granites of Lake Plateau. The Lake Plateau granitoids are interpreted as mid-crustal melts emplaced at approximately 20 km and generated from a slightly deeper crustal source. These large volumes of K-rich granites are different from the Na-rich rocks reported in the eastern Beartooths (Mueller and others, 1985). Large volumes of granite with subordinate quartz diorite and a variety of supracrustal inclusions are consistent with characteristics in younger examples of post-collisional tectonic settings. The Beartooth Mountains represent the remaining mid-crustal evidence of development of thickened continental crust in the late Archean by collisional tectonics and post-collisional uplift, similar in many ways to modern day tectonic processes.

11 I INTRODUCTION The Lake Plateau study area encompasses sixty square kilometers in the central Beartooth Mountains of southern Montana (Fig. I). It provides excellent exposures of Precambrian metamorphic and granitic rocks in a glacial topography of polished knobs and deep cirques. Lake Plateau is a small area in a region of extensive Archean exposures, and has difficult access (five hours on foot). But it is geologically important because it is located near the boundaries of three major tectonic blocks within the Beartooth Mountains. link to other recent studies around the area, It provides a central and it is therefore important to the understanding of the genesis and evolution of this Archean region. The purpose of this study is to characterize the Lake Plateau rock units and to establish the geologic history of this Archean terrane. By doing soi the following questions are addressed: 1) What Archean crustal levels are now exposed at Lake Plateau? 2) What was the source for these rock units? 3) What tectonic conditions created this terrane? 4) What constraints do these data place on theories about Archean crustal development in the Beartooth Mountains and surrounding Southwest Montana?

12 2 LIVINGSTON MONTANA /NORTH 'SNOWY LOCK,STILLWATER -^COMPLEX LAKE SOUTH > SNOWY V blo c k PLATEAU CENTRAL EARTOOTH BLOCK RED LODGE MAMMOTH COOKE CITY MONT WYO Figure I. Precambrian outcrops of the Beartooth Mountains. Lake Plateau is shown in relation to the various blocks and to the Mill Creek - Stillwater Fault Zone.

13 3 REGIONAL SETTING The Beartooth Mountains are near the north end of the Archean outcrops of the Wyoming Province (Fig. 2). This province consists of numerous mountain ranges cored by Archean granites and supracrustal rocks. In the southern part of the Wyoming Province, late Archean to Proterozoic accretion from the south has been demonstrated (Condie, 1982; Karlstrom and Houston, 1984), but tectonic conditions in the north are less clear. In southwest Montana, Archean exposures are truncated by a west to northwest system of faults that have been active from the late Archean to the present (Geissman and Mogk, 1986; ' Schmidt and Garihan, 1986). North of these faults are extensive exposures of Proterozoic Belt rocks and Phanerozoic sediments and volcanics. To the south are Archean-cored Laramide uplifts (Foose and others, 1961; Schmidt and Garihan, 1983) that show changing characteristics from west to east. In the west, these uplifts include the Blacktail, Ruby, and Tobacco Root Ranges (Fig. 2) which are dominated by metasediments that include quartzofeldspathic gneisses, schists and marbles (Heinrich and Rabbit, 1960; Garihan and Okuma, 1974; Clark, 1987). In the Madison Range and western BeartoOths, there is a complex set of te'rranes with varying rock types that include metasediments and a wide range of intrusive units (Spencer and Kozak, 1975; Erslev, 1983; Mogk, 1984; Salt, 1987). To the east, the Beartooth Mountains, including Lake Plateau, are dominated by major granitic intrusions (Mueller and others, 1985).

14 4 LITTLE BELT MOUNTAINS % TOBACCO BOOT / MOUNTAINS D NOBTHEBN MADISON WYOMING PROVINCE I ~ X N BANGS I BLACKTAIL MOUNTAINS : TETON / I BANGS BEABTOOTH M OUNTAINS SOUTHEBN MADISON OW L CBEEK MOUNTAINS MONTANA W y o m i n g ' BIG H O BN MOUNTAINS BLACK HILLS ALBION RANGE RAFT RIVER W IND RIVER RANGE GRANITE MOUNTAINS WASATCH RANGE NORTHEASTERN * '* *. UINTA MADRE f MOUNTAINS - ^TJSCL M EDICINE BOW MOUNTAINS W T O M.IN G ToioiABT-- Figure 2. Archean outcrops of the Wyoming Province (Clark, 1987, after Condie, 1976). This eastward progression of Archean sediments, accreted terranes, and major intrusions in southwest Montana is the product of Archean tectonic conditions. By answering the questions about granite compositions, source rocks, and crustal thickness at Lake Plateau, this study will add to the understanding of these tectonic conditions and of plate tectonics in general during the late Archean. The Beartooth Mountains were subdivided into blocks by J.T. Wilson (1936) as shown in Figure I. The South Snowy block consists mainly of metasediments, including metagraywackes and ironstones with minor

15 5 granitic intrusions (Hallager, 1980; Casella and others, 1982; Thurston, 1986). Paleozoic and Mesozoic. sediments and Tertiary volcanics separate this block from other Archean exposures in the Beartooths, making structural relationships unclear. Part of the North Snowy block has been mapped as a Late Archean mobile belt with large- scale eastward thrusting of supracrustal schists, marbles, and amphibolites over northeast-trending gneisses, schists and amphibolites (Mogk, 1984). Along the north edge of the Beartooths is the Stillwater Complex, a platinum-bearing, layered mafic and ultramafic intrusion with an associated contact aureole. The contact aureole, has been shown to be in fault contact with the adjoining North Snowy and Central Beartooth blocks (Geissman and Mogk, 1986). Lake Plateau is part of the fourth and largest block, the Central Beartooth block. Early studies of the central block with its large volumes of granite and granodiorite were done by Arie Poldervaart and his students (Eckelmann and Poldervaart, 1957, Poldervaart and Bentley, 1958, Larsen and others, 1966, Butler, 1966, and Butler, 1969). The earliest of these studies suggested that the granites formed by static metasomatism of folded sedimentary rocks. Later studies suggested that folding and metamorphism were contemporaneous and that layering may have been produced by flow of rocks rather than by sedimentary processes (Casella, 1969). The current study indicates a magmatic origin for the large volumes of granitic rock at Lake Plateau. This is consistent with recent magmatic interpretations of similar rocks fifty kilometers east of Lake Plateau in the central block (Mueller and others, 1985).

16 6 The intrusions to the east have been age dated at 2.75 Ga old intrusions (Mueller and others, 1985), with 3.4 Ga old granulite inclusions (Henry and others, 1982). Other available age dates from the region include 2.7 Ga for the Stillwater Complex (Lambert and others, 1985). The published age dates nearest to Lake Plateau are from a drill core study near Hawley Mountain, seven kilometers north of Lake Plateau (Lafrenz and others, 1986). This study yielded an age date of 2.75 Ga for a foliated biotite granite and 2.1 Ga for a poorly foliated pink granite. The timing and physical conditions of the magmatism at Lake Plateau is important to understanding the relationship of the Central Beartooth block to the Stillwater Complex and the North Snowy block.

17 7 ROCK UNITS General Statement The outcrops exposed at Lake Plateau are dominated by numerous tabular and irregular bodies of granodiorite to granite.and by pegmatite and aplite veins associated with these late to post-kinematic intrusions. Mineralogical and textural variations are generally gradational within the intrusive units. The pegmatite and aplite veins are ubiquitous and crosscut nearly all other rock units. Within the intrusions are numerous aligned xenoliths of biotite-gamet schist and homblende-biotite schist. These inclusions range from centimeter scale up to a few hundred meters by a few kilometers (Plate I). They were metamorphosed in amphibolite facies with textures ranging from finegrained schist to coarse compositionally banded gneiss. Contacts with granitic units are sharp in some places but are more commonly gradational, showing partial assimilation by the granite. There are mafic intrusive rocks including a continuous 500 meter. wide body of hornblende quartz diorite which predates the granites, and numerous amphibolite dikes which cut the granites. The only rocks younger than Precambrian at Lake Plateau are rare Tertiary felsic dikes associated with the Eocene Absaroka Volcanics which occur south of the study area, covering the older rocks with a layer of andesite and dacite (Chadwick, 1985).

18 8 Inclusions Biotite garnet schist Biotite garnet schist is the dominant rock type in inclusions of the western third of the study area (Plate I). Inclusion size ranges from meter scale in single outcrops, to a north-south trending inclusion just west of Mirror Lake that measures over two kilometers in length. This inclusion is cut by numerous pegmatite veins and by irregular granitic intrusions. Contacts with surrounding units are sharp with pegmatite veins and generally gradational with granitic units, showing varying degrees of partial assimilation. Foliation of inclusions is typically concordant with foliation of intrusives. The biotite garnet schist has the assemblage of biotite, garnet, cordierite, quartz, and plagioclase (An^), plus or minus sillimanite (Fig. 3). Accessory minerals include allanite, zircon, apatite, iron titanium oxide, and secondary chlorite after biotite. Figure 3. Photomicrograph of biotite garnet schist. B: biotite; G: garnet; C; cordierite; Q; quartz.

19 9 Foliation and centimeter scale isoclinal and open folds are defined by weak alignment of biotite grains and locally by compositional layering. The biotite is brown to red-brown. It is not bent or broken, and many grains have random orientations indicating both post- and syn-kinematic crystalization. Garnet is poikiloblastic ranging in size up to two centimeters with inclusions of quartz and biotite. No internal pattern of inclusions was recognized. Quartz occurs as scattered anhedral grains and also as aggregates of grains in irregular blebs to 15 centimeters long. These blebs contain strained grains showing slip bands and mosaics of smaller, strain free, recrystallized grains. Cordierite occurs as irregular grains growing over biotite foliation with characteristic dusting of opaques, pleocroic halos, and pinite alteration. Cordierite locally comprises up to 20% of the rock in the large inclusion at Mirror Lake. Metamorphic temperature and pressure estimates were made for the biotite garnet schist based on microprobe analyses done by D.W. Mogk on biotite, garnet, and cordierite. Analyses were done inside the rims of garnets, away from retrograde effects near the rim, and on biotite grains near, but not in contact with, the garnets (Mogk and Mueller, in review). minerals. Figure 4 shows the measured compositions of these three Peak metamorphic temperature estimates of C were calculated using the biotite-garnet geothermometer of Ferry and Spear (1978). Pressure estimates of 7-8 kilobars were calculated based on the

20 10 garnet-cordierite barometer calibrated by Lonker (1981). Figure 5 shows a graphical solution to the pressure calculation. A GAR CORD J i l l I ' Figure 4. AFM composition diagram for biotite garnet schist. Projected from muscovite (after Best, 1982). Excess silicate and water assumed. Values are calculated from microprobe data. A: Al2O3; F : FeO; M: MgO Pelitic schists similar to the biotite garnet schist of Lake Plateau have been described elsewhere in the region. A large pendant of pelitic schist in granite has been reported on the West Boulder Plateau, kilometers northwest of Lake Plateau (Geissman and Mogk, 1986). Staurolite-bearing schists with greater than 25% biotite have been reported on the West Fork of the Stillwater River, 8 kilometers

21 _I i I I Temperature OC Figure 5. Gamet-Cordierite phase relations in biotite garnet schist. Based on the calibration of Lonker (1981).

22 12 NNE of Mount Douglas (Page and Nokleberg, 1972). In the Cathedral Peak area, 12 kilometers ENE of Mount Douglas, Butler (1966) has described biotite garnet schist with minor amounts of staurolite, cordierite, and anthophyllite, and with chemical.signatures similar ' to those of Precambrian slates. He interpreted these schists as representing.lower to middle amphibolite facies metamorphism. Weeks (1980) has mapped extensive cordierite and staurolite-bearing schist just across the Mill Creek - Stillwater Fault Zone to the northwest of Lake Plateau. These widespread occurrences suggest that at least the western part of the Beartooth Mountains experienced deposition of Archean sediments, which were subsequently buried and metamorphosed prior to granite generation. The estimates of 7-8 kilobars indicate burial of these sediments to kilometers. Hornblende Biotite Schist Hornblende biotite schist comprises most of the inclusions in the eastern two thirds of the Lake Plateau study area (Plate I), with the highest concentrations of inclusions north and east of Rainbow Lakes. The inclusions range from meter scale to tens of meters and, like the biotite garnet schist, have both sharp and gradational contacts with the intrusive units. In contrast to the biotite garnet schist, however, the hornblende biotite schist spans a broad range of mineralogies and textures, suggesting that there may be hornblende-bearing rocks from more than one protolith or inclusions of similar rocks that have undergone different amounts of interaction with the intrusive rocks and their associated fluids.

23 13 The typical mineral assemblage of the hornblende biotite schist includes hornblende, biotite, plagioclase (An^^y), quartz, and epidote with accessory sphene, iron titanium oxide, and secondary chlorite. Color index ranges from 20-60%, and the lighter varieties contain 5-10% K-spar. There is also a wide variation in modal amounts of hornblende and biotite. Hornblende ranges from 0-50%. Biotite ranges from 0-35% and is olive colored in some samples and red-brown in others. Rock textures range from fine-grained, dark schistose rock with strong alignment of hornblende and biotite imparting cleavage, to more gneissic rock with gradational light and dark compositional banding. There is also a lineated variety with scattered elongate clots of hornblende and biotite in a lighter matrix. Homblende-plagioclase phase relations in the hornblende biotite schist (unpublished microprobe data from. D. Henry) indicate metamorphism in the mid- to upper amphibolite facies using the empirical calibration of Spear (1980; 1981). Application of the homblende-plagioclase geothermobarometer of Plyusnina (1982) indicates temperatures and pressures on the order of 580 C and 7 kilobars, which are near the temperature and pressure estimates from the biotite garnet schist inclusions. Rocks with similar modal mineralogy to the Lake Plateau hornblende rocks are found throughout the central Beartooths. Butler (1966) described amphibolites with similar biotite-hornblende variations in the Cathedral Peak area. He later suggested three possible protoliths: I)tuffs or flows of intermediate igneous composition, 2) graywacke, or

24 14 3) metasomatically altered mafic rocks (Butler, 1969). In the eastern Beartoqths, Mueller and others (1983) have described very similar rocks which are an abundant type of inclusion in that area, and which yield a geochemical signature consistent with andesitic magmas with, a mantle origin. Thus, the widespread occurrence of hornblende-biotite rocks throughout the Central Beartooth Block may be due to extensive andesitic magmatism prior to metamorphism and granite generation. Intrusive Units Hornblende Quartz Diorite Hornblende quartz diorite occurs as a continuous, 500 meter wide outcrop that trends north-south across Lake Plateau, with the best exposures near Mirror Lake (Plate I). There are also scattered minor outcrops in the eastern third of the plateau. It appears from crosscutting relationships to be the oldest intrusion in the area. At Mirror Lake, the large body is in contact with biotite garnet schist on the west and granitic rocks on the east. The contact with the schist is sharp and irregular with minor intrusions of hornblende quartz diorite into the schist. The contact with the granites is also sharp with very few intrusions of granitic rock or pegmatite into the dense hornblende rock. Most of these intrusions are thin (less than I cm) leucocratic veins, although in a few places there are large outcrops of granitic rock with numerous inclusions of hornblende quartz diorite. The mineral assemblage of the hornblende quartz diorite includes plagioclase (An^Q_^), hornblende, biotite, and quartz, with accessory epidote, iron-titanium oxide, sphene, and retrograde chlorite and

25 15 sericite. It falls in the quartz diorite field on a quartz plagioclase K-spar ternary diagram (Fig. 6). The hornblende and biotite comprise 25-50% of the total volume. Grain size varies from fine-grained varieties (0.25 mm) to course (up to 3 cm), with 15-20% magnetite occurring locally in the coarsest varieties. LAKE PLATEAU MODAL % GRANITE GRANODIORITE QUARTZ DIORITE K-SPAR (STRECKEISEN, 1976) FLAG Figure 6. Modal percent quartz K-spar plagioclase for the Lake Plateau intrusive units (after Streckeisen, 1976). Based on point counts and visual estimates.

26 16 The rock is typically unfoliated with relict.hypidiomorphic- granular texture, and it is locally recrystallized to equilibrium textures with triple junctions and straight grain boundaries. Foliation is present in some finer grained outcrops and is defined by alignment of hornblende and biotite grains. Most outcrops show scattered one to two centimeter round clots of randomly oriented biotite grains. These clots weather more easily, giving weathered surfaces a pitted appearance. Microscopically, the biotite grains in these clots show fine-grained epidote rims and cleavage fillings. Hornblende is blocky, anhedral, and locally poikilitic with rounded quartz inclusions. Similar rocks are described by Butler (1966) under the heading of "hornblende-bearing rocks", which also includes schistose varieties such as the hornblende biotite schist described above. The larger outcrops with granoblastic textures may be the same early intrusive unit that is exposed at Mirror Lake. Casella (1969) noted similar amphibolites throughout the Central Beartooth Block, and he reported an increase in these rocks from the south and east toward the core of the Beartooths. Granite - Granodiorite - Pegmatite This granite suite comprises a wide variation in mineralogy, texture, and grain size. They fall within the granite and granodiorite fields (Fig. 6), and they comprise 80-90% of the Lake Plateau outcrops (Plate I) % of this volume is pegmatite and aplite veins. Texture and mineralogy vary from outcrop to outcrop, and contacts are sharp to gradational, with gradational changes more common. Except for pegmatite, and aplite veins, large distinct bodies with uniform

27 17 characteristics are rare. Instead, there is. evidence of extensive interaction between intrusive units and inclusions. Textures near contacts with inclusions commonly show relict compositional layering with granitic layers apparently injected into existing foliation of the inclusions. Further from contacts, granitic foliation decreases to wispy biotite layers a few biotite grains thick, and within some larger granitic bodies little foliation is apparent. Increased mafic content and stronger foliation of granites near inclusions suggests extensive assimilation of inclusions or invasion of alkali-rich solutions into the country rock. Pegmatite and aplite veins range from millimeter scale, homogeneous veins, to zoned pegmatite veins a few meters thick with feldspar and quartz borders grading inward to mostly quartz interiors. There are also meter scale homogeneous aplite veins, and a few ptygmatic aplite bodies. Veins are typically planar with sharp contacts and little deformation. They commonly parallel joint patterns, and many large granitic boulders weather out with planar, pegmatitic faces. The granite suite main assemblage is quartz, microcline, 'and plagioclase in the range of ratios shown in Figure 6, with generally less than 5% each of biotite, muscovite, and epidote, although larger amounts of these minerals occur locally, especially near inclusions. Accessory minerals include zircon, apatite, hornblende, garnet and iron titanium oxide. Microscopic textures are typically hypidiomorphic-granular with various reaction textures including: myrmekite, patchy K-spar replacing plagioclase, albite rims on plagioclase, and recrystallized quartz in

28 18 mortar texture between larger grains. In some thin sections, biotite, muscovite, epidote, and minor hornblende occur together in irregular veinlets a few grains wide. Biotite is olive to olive-brown, similar to that in hornblende biotite schist, and it occurs as scattered grains as well as in veinlets. Garnets up to 5 mm occur in both aplite and pegmatite veins, and muscovite up to 5 cm occurs locally in pegmatites. Zircons are rare, but those present are conspicuously rounded. Some have dark reaction rims (Fig. 7), and others show euhedral overgrowths. Similar zircons in granitic rocks of the eastern Beartooths have been described as predominantly detrital (Eckelmann and Poldervaart, 1957) and have been dated at more than 3.1 Ga. (Catanzaro and Kulp, 1964). Figure 7. Photomicrograph of rounded zircon with dark reaction rim in Lake Plateau granite.

29 19 Figure 8. Photomicrograph of muscovite in Lake Plateau granite. Large size and subhedral shape indicate probable magmatic origin. M: muscovite; Q; quartz. Blocky subhedral muscovite is also present (Fig. 8) and may provide another line of evidence for high pressure conditions at the time of granite emplacement. The muscovite in the Lake Plateau granites meets the following criteria used to recognize magmatic muscovite (Speer, 1984); I) grain size is comparable to other magmatic minerals; 2) the muscovite has subhedral to euhedral shape; and 3) the granite is relatively unaltered. Speer also reports that examples of magmatic muscovite have greater titanium content than examples of post- magmatic or hydrothermal muscovite. Figure 9 is a plot of TiOg-Fe^Dg- MgO compositions from microprobe analyses of muscovite in Lake Plateau granites. The elevated titanium values are comparable to those for magmatic muscovite shown by Speer (1984). If this is primary magmatic muscovite that crystallized in equilibrium with the associated quartz and sodic plagioclase, then the pressure and temperature of granite emplacement must be above the two curves shown in Figure 10 (Hyndman,

30 ). This places the pressure minimum for granite emplacement at about A kilobars. Fe. O Figure 9. Iron-magnesium-titanium compositional range for muscovite in Lake Plateau granite. Shown in relation to line representing average measured compositions of magmatic muscovite (after Speer, 1984). Based on microprobe analyses. Microscopic textures suggest that epidote may also be a primary magmatic mineral in the Lake Plateau granites (Fig. 11). Grains are subhedral, ranging up to I mm in size, with straight grain boundaries where they contact biotite or quartz, and irregular to myrmekitic boundaries with plagioclase. Some epidote grains have allanite cores. These textures match those described as magmatic by Zen and Hammarstrom (1984), who interpret such magmatic epidote to be an indication of high pressures (above 7 kilobars) during crystallization.

31 21 GRANITE SOLIDUS T0C Figure 10. Pressure-temperature diagram showing water-saturated granite solidus and muscovite+quartz reaction curve. Patterned region represents probable pressure and temperature conditions of Lake Plateau granite emplacement indicated by the presence of primary muscovite (modified from Hyndman, 1981). Figure 11. Photomicrograph of subhedral epidote of probable magmatic origin in Lake Plateau granite. E: epidote; B: biotite; Q; quartz; P: plagioclase; F; iron-titanium oxide.

32 22 The existence of these epidote and muscovite textures. combined with other evidence, such as abundant pegmatites and assimilation at gradational contacts between the granites and inclusions, provides strong evidence for high water pressures and therefore deep crustal levels during granite crystallization. Dikes There are four Precambrian dikes exposed within the Lake Plateau study area (Plate I). All four are near-vertical tabular units. Three of the dikes are continuous, planar, and uniform in thickness (25-35 m) with sharp contacts. There is no apparent alteration of the older rock units that are cut by the dikes. The fourth dike is discontinuous with irregular contacts, and it is cut extensively by pink granite, pegmatite, and aplite. The three continuous dikes all have the same mineralogy and texture. The main assemblage is plagioclase (An 45-50)1 augite, pigeonite (2V: 20-25), hornblende, and quartz with accessory biotite, and iron titanium oxide. Secondary epidote and white mica replace plagioclase. Relict ophitic texture is recognizable in most thin sections. Plagioclase is subhedral to euhedral ranging up to 4 mm long and showing various stages of alteration from clear to completely replaced. Ophitic pyroxenes are partially or wholly replaced by patchy mats of fine amphibole laths and rims of anhedral green and blue-green hornblende. Augite is the more common remaining pyroxene with pigeonite recognizable only as patchy cores (Fig. 12). Quartz occurs as minor anhedral grains and in micrographic texture with plagioclase.

33 23 Figure 12. Photomicrograph of relict ophitic texture in continuous dikes. P: dark, patchy pigeonite; H: hornblende; PL: plagioclase. The discontinuous dike has the mineral assemblage of plagioclase, hornblende, epidote, clinozoisite, and quartz, with accessory iron titanium oxide, and retrograde sphene and chlorite. Plagioclase phenocrysts up to 3 cm comprise 10-20% of the rock, and they have been almost completely replaced by epidote and clinozoisite. The matrix is a dense, faintly foliated fabric of equigranular hornblende, epidote, and quartz. There are no recognizable pyroxenes. Oxides are rimmed by sphene. Prinz (1964) produced a comprehensive study of dikes in the Beartooth Mountains to the east and south of Lake Plateau, and he described dikes similar to those described here. The discontinuous dike closely resembles his " Archean metadolerites" in mineralogy, texture, and relationship with the granites. Prinz states that these dikes probably intruded rock that was still ductile during late stages of granite generation. The three continuous dikes match his "Late Precambrian dolerites" which are "abundant in all parts of the range

34 24 and are remarkably uniform in composition, indicating a single period of intrusion" (Prinz, 1964, p. 1222). He states that this intrusion happened after uplift had fractured and faulted older units cut by these dikes. All four dikes appear to have recrystallized directly into amphibolite facies in the late stages of granite generation or shortly after when the country rock was still hot. deformation indicates post-kinematic timing, The lack of penetrative and the cross-cutting pegmatites and lack of granite alteration at contacts indicate a close association between the dikes and the granites.

35 25 STRUCTURE The structural geology of Lake Plateau is characterized by broadscale open folding of a regional north-south striking foliation. These folds are cut by undeformed dikes and shear zones. The following structural features were observed and measured in the field: foliation, including schistosity and compositional layering; lineations defined by preferred orientation of biotite and hornblende aggregates, or by hinges of isoclinal to open, centimeter-scale folds; contacts of major inclusions with granitic units; and shear zones. A generalized overview of this data is shown in Figure 13. The 'foliation generally strikes north-south and represents the imposed fabric associated with the main-stage, regional, upper- amphibolite metamorphic event. Foliation is defined by strong alignment of biotite and hornblende in schists, and by compositional layering in gneissic inclusions. Individual grains are rarely bent or broken, but rather have recrystallized into the imposed fabric during metamorphism. The foliation is generally planar within inclusions, but shows some millimeter to centimeter scale intrafolial isoclinal folds and some crenulation cleavage in biotite-rich varieties. The isoclinal folds do not show bent or broken grains and are interpreted to have developed during the main metamorphic event. At contacts with intrusive units, the foliation is locally contorted into discontinuous ptygmatic folds. In other places, granitic material has been injected into foliation creating compositional layering, and at gradational contacts where

36 LAKE PLATEAU / SHEAR ZONE FOLIATION ATTITUDE LAKE PINCHOT HORSESHOE MIRROR RAINBOW WOUNDED I MAN LAKES Figure 13. Generalized structure map of Lake P l a t e a u. Representative foliation attitudes are from granites and inclusions. Stereonet A: Mirror Lake synform with five degree plunge to the south-southwest. Countours at %, 22% point m a x i m u m. Based on 41 points. Stereonet B : Rainbow Lakes antiform with twenty degree plunge to the south-southwest. Countours at %, 17% m a x i m u m. Based on 53 points.

37 27 assimilation has occurred, relict foliation is recognizable in the granite as preferred orientation of scattered biotite grains and as wispy mafic layers. Lineations also show a general north-south trend with shallow to horizontal plunges. Most mineral lineations observed are elongate aggregates of hornblende crystals within hornblende biotite schist outcrops. Other lineations measured are fold hinges of isoclinal and open folds at outcrop scale. These lineations all appear to have occurred during development of the regional imposed fabric. Figure 13 shows a second folding event characterized by kilometer- scale open folds that plunge gently south. These folds are defined by changes in foliation attitudes, and their axes fit into a regional pattern mapped over 200 square kilometers including Lake Plateau, by Butler (1966). He described them as open, cylindrical folds with gentle north or south plunges. Open folds similar to these have been reported in the eastern Beartooths as a post-metamorphic event (Rowan, 1969; and Mueller, 1979). The shear zones are near vertical and do not appear to be deformed by the open folding event. These are linear zones ten to thirty meters wide showing retrograde metamorphism and ductile deformation characteristics. Biotite schist inclusions have been altered to chlorite schist with irregular crenulated chlorite layers and mosaics of fine grained quartz that has recrystallized from larger grains as the result of ductile shearing. Granitic units within the shear zones also show quartz recrystallization ranging from mortar texture around larger strained grains (Fig. 14), to fine grained mylonitic textures

38 28 characterized by smaller, recrystallized, strain-free quartz grains (Fig. 15). Plagioclase grains are commonly fractured (Fig. 16) and locally have been replaced by mats of fine chlorite and epidote. The shear zones are recognizable in outcrop as layers of fine-grained green schist and red and black banded, chert-like siliceous layers of mylonitized granite. They can be traced along strike as straight linear features that commonly produce swales in the topography. Figure 14. Photomicrograph of quartz in granite from a shear zone. Fine-grained recrystallized quartz in mortar texture surrounds large, strained Quartz grains.

39 29 Figure 15. Photomicrograph of mylonitized granite from a shear zone. Quartz is completely recrystallized to smaller, strain-free grains. Figure 16. Photomicrograph of bent and fractured plagioclase in sheared granite.

40 30 Based on the above structural features, history of Lake Plateau can be constructed. a partial deformational The first recognizable event is the upper amphibolite metamorphism with associated imposed fabric, isoclinal folding, and late granite generation. This event destroyed evidence of original sedimentary structures or earlier deformational events. It was followed by broad open folding without accompanying penetrative deformation, and finally by shearing and local retrograde metamorphism.

41 31 TECTONIC CONDITIONS A tectonic model for the Archean rocks of the Beartooth Mountains can be developed by summarizing the available data and by comparing these data to theories of Archean crustal development and to models of tectonic settings developed from younger rocks. From the Lake Plateau data presented in this study, we can answer some of the questions posed in the introduction. First, what Archean crustal levels are now exposed at Lake Plateau? The metamorphic pressure estimate of 7-8 kilobars based on cordierite-garnet geobarometry corresponds to about km crustal depth. The magmatic epidote in the granites suggests pressures of crystallization on the order of 7-8 kilobars. The existence in the granites of magmatic muscovite with quartz and sodic piagioclase indicates greater than 4 kilobars of water pressure (Hyndman, 1981) and places a minimum granite emplacement depth at about 15 km. Gradational contacts of xenoliths and granites with textures suggesting assimilation, indicate emplacement into hot country rock, and support a conclusion that the rocks now exposed at Lake Plateau formed at mid to lower crustal levels. Therefore a minimum depth of 15 km is strongly supported with probable depths of km during the mainstage metamorphic event and subsequent emplacement of magma. This estimate, coupled with the C metamorphic temperature estimate from biotite-gamet geothermometry, yields a metamorphic gradient of C/km which is lower than most reported Archean values (Grambling.

42 ) and is similar to present day values for. gradients in collisional tectonic environments (e.g. Spear and others, 1984). The second question is: what was the source of these rock units? A supracrustal origin has been determined for the schists, with a probable sedimentary origin for the biotite garnet schist based on its pelitic assemblage, and a possible andesitic origin for the hornblende biotite schist based on similarities to metamorphosed andesites in the eastern Beartooths (Mueller and others, 1983). The granitic units are interpreted to have a magmatic origin based on hypidiomorphic-granular textures, idiomorphic' zoned plagioclase, and cross-cutting relationships. The source of these.magmas was probably partial melting within the lower crust, near the crustal levels now exposed. This crustal source is indicated by the granodioritic to granitic magma types, as opposed to tonalitic or more mafic magmas characteristic of direct mantle source derivatives. Other indications of a crustal melting source include: rounded zircons, possible anatectic textures at contacts with inclusions, and high water content producing abundant pegmatite. This high water content was probably generated by dehydration reactions in the supracrustal rocks during melting at pressures and temperatures near the granite solidus.. The third question is: what tectonic conditions created this terrane? The answer to this question must allow for: I) burial of supracrustal rocks to about 20 km; 2) penetrative deformation producing north-south foliation and isoclinal folds; 3) metamorphic gradients similar to present values in thickened continental.crust; and 4) partial melting within the crust to produce granitic magmas. These

43 33 conditions would be met in a collisional tectonic setting involving magmatic arcs and interarc basins. The scale of the involved arcs and basins, and their orientation is difficult to determine based on the limited evidence exposed in the Beartooth Mountains. Such a setting is supported by some of the regional geology. In the North Snowy Block, metasediments are thrust eastward over trondhjemitic gneisses (Mogk, 1984), demonstrating east-west shortening. The wide-spread occurrence in the Beartooth Mountains of high-grade metasupracrustal rocks indicates extensive deep burial of surface rocks. In the eastern Beartooths1 the andesitic geochemical signature of inclusions (Mueller and others, 1983 and 1985) supports the existence of magmatic arcs at 2.8 Ga. Continued accretion of such magmatic arcs and associated sediments may have eventually created the thickened crust, crustal melting, and granitic intrusions observed at Lake Plateau. These ideas are consistent with current theories about Archean plate tectonics. Dewey and Windley (1981) faster-moving plates existed in the Archean. theorize that thinner, The faster motion would allow for dissipation of higher Archean amounts of radiogenic heat without calling on higher geothermal gradients. They also state that arc systems amalgamated into cores of late Archean rigid continental plates,. giving rise to stable continental portions.of plates and to modern-style plate tectonics. Lake Plateau probably represents mid to deep crustal levels of such an early proto-continent. Dickinson (1981) argues that over two thirds of the present continental crust had emerged from the mantle before 2.5 Ga as a product of accelerated plate

44 34 tectonic activity. He states that early continents formed during this time as collages of oceanic island arcs, and he calls on remelting of lower crust in thickened Archean plates to achieve internal fractionation of the crust. This process seems to have occurred at Lake Plateau where andesitic and quartz dioritic chemistries are subordinate to large volumes of granite and granodiorite. Studies of younger magmatism where modern plate tectonic settings are discernible have shown a strong relationship between tectonic environment and granite characteristics (e.g. Pitcher, 1983). If modern style plate tectonics operated by the late Archean as suggested (e.g. Dewey and Windley, 1981), then the characteristics of the Lake Plateau granites may be clues to the tectonic environment in which they developed. Pitcher (1983) has summarized granite characteristics for five tectonic environments. Of these the!-caledonian type most closely describes Lake Plateau. In fact the similarities are striking, as can be seen in Table I. The!-Caledonian setting is one of post-collision uplift with associated granites generated by crustal melting. Pitcher calls on a post-kinematic tensional environment to produce these high.potassium granites that contrast with the higher sodium, syn-tectonic granites of the I-Cordilleran environment. The evidence from the Beartooth Mountains suggests that these granite classifications may be valid for the late Archean. The thrust faulting in the North Snowy Block indicates a collisional setting that may have coincided with crustal thickening and production of high sodium, I-Cordilleran granites in the eastern and central Beartooth

45 S - TYPE A - TYPE M - TYPE I - TYPE CORD ILLERAN I - TYPE CALEDONIAN L A K E PLATEAU Hetcynotype continental oblique collision Post-orogenic or anorogenic setting Oceanic island-arc Andinotype marginal continental arc Caledonian-type post-closure uplift Best fit: Caledonian-type L e u c o. monzogranite some high in biotite Biotite granite Alkalic granite Syenite Plagiogranite subordinate to gabbro Tonalite dominant Diorite to granite Assoc, with gabbro Granodiorite-granite Minor h o m b l e n d e - diorite G r a n o d iorite-granite Minor hornblendequartz diorite White K-spar megacrysts Perthites Interstitial Pink K-spar Pink K-spar Pink-spar micrographic interstitial and interstitial and interstitial and K-spar xenomorphic invasive megacrysts Metasedimentary xenoliths Cognate xenoliths Basic magma blebs Basic igneous xenoliths Dioritic xenoliths may be restitic Mixed xenolith populations Mixed xenolith population Multiple batholiths plutons and sheets Characteristically lacking in voluminous vx. Multiple cauldron complexes Small volume Caldera-centered alkalic lavas Small plutons Quartz dioritegabbro Associated island-arc volcanism Great multiple, linear batholiths Great volumes of andesite and dacite Complexes of multiple plutons and sheets Some have basalt-andesite lava "plateau" Multiple sheets and irregular bodies No surface data Possible andesitic inclusions W Ln Sustained syn and post kinematic plutonism Short-lived plutonism Short, sustained plutonism Very long-duration episodic plutonism Short, sustained plutonism Post-kinematic Late- to postkinematic plutonism Much shortening Low pressure metamorphism Doming and rifting Open folding Burial-type metamorphism Vertical movements Burial-type metamorphism Dip-slip and strike-slip faulting Retrograde metamorph. High grade metamorph. Ductile shearing m i nor retro, metamorph Sn and W-greisen and vein-type mineralization Columbite Cassiterite Fluorite Porphyry copper and gold mineralization Porphyry copper, molybdenum mineralization Rarely strongly mineralized No mineralization recognized Table I. Granite characteristics of tectonic settings (after Pitcher, 1983). Lake Plateau granite characteristics closely match those of the I-type Caledonian granites.

46 36 Mountains (Mueller and others, 1985). This compressional event may then have been followed by uplift and a tensional environment that produced the!-caledonian granites and later dikes observed at Lake Plateau.

47 37 I CONCLUSIONS Lake Plateau represents a view of late Archean exposures of deep crustal levels on the order of kilometers. It shows intrusions with granite to granodiorite compositions that were generated by crustal melting and injected into upper amphibolite grade supracrustal rocks of sedimentary and possibly andesitic origin. The existence of these rocks demonstrates that by late Archean time ( Ga), the Earth had begun to develop thick, differentiated continental crust. Theories on late Archean tectonics in southern Montana need to allow for this thick crust with granite to granodiorite compositions and low metamorphic gradient (25-30 C/km based on pressure and temperature estimates of 7-8 kilobars and C). The theory that modern style plate tectonics were operating by the late Archean (i.e. Dewey and Windley, 1981) is supported by the data reported here. The characteristics of the Lake Plateau rocks match very closely the characteristics attributed to younger post-collisional uplift regions (Pitcher, 1983). The following sequence of events is thus proposed for the tectonic history of the Beartooth Mountains: 1) Generation of magmatic arcs and interarc basins 2) Collisional tectonics resulting in: a) Crustal shortening and thickening b) Deep burial of supracrustal rocks c) Penetrative deformation and upper amphibolite grade metamorphism

48 38 d) Generation of high sodium granites such as the Long Lake rocks 50 km east of Lake Plateau (Mueller and others, 1985) 3) Post-collisional tension and uplift resulting in: a) Generation of Lake Plateau granite to granodiorite by crustal melting b) injection of amphibolite dikes This sequence can only represent a partial picture of the history of this ancient terrane based on the evidence remaining in its exposed roots and based on theories about tectonic processes operating over two billion years ago.

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54 GEOLOGIC MAP OF LAKE PLATEAU PLATE I LEGEND UNI T B O U N D A R I E S LITHOLOGIES G Grenlte-granodlorlte- I HQ I S t r i k e & dip pegmatlte of f o l i a t i o n H ornblende quartz dlorlte Shear > 20% Blotlte garnet -N > 2 0 % Hornblende biotlte i n c l u s i o n s In g r a n i t e t./ -H--- echlet amphibolite LOCATION I ' v >- ' e dike v I 1:24000 V, O M / _r ",y : 'H 1 In M T. r / «/ N) \ ~-X,S Xr~c/ \ % v, \ M \ y / v L n V V"'\ /, V O O U O L AS IS' " % I / \ SxX W lx I' L v. ~ \ Z \ V X i J V\\ \» ' I / / ( X x» r - :,/ " T x T { \ - 6 I '"-X--:--...:'"" A/ / Lr /.A, 4 Z^x /x: ('HBy \ U A r / Z/i I h Z i'x." 5 " / x ^ JD45I 1 Z Z X ' V < -VU (nimvtam=tt / x \ A-.Z-Z- ;» -<.Z x\ Wj.Z-^ : 9J60 I / >Z / o X, \ ' : ; MS - _z - 3, H u S 1 I Nz % 7% X x - v :L n \ x y X X- ^ D V \\ Xv- i x J 'X. r "x x A y ; > G -7066^ Vw 4 hs^x X.. y " - 77? \ As ^.) & ' L ->/ c m \ \ -----\ y ' W X X y Z' / O s Z OvaSrensle, ZT f \ -7. ^ ft 7 / I: // I I I \ Vs <4 COO " ' s i KiLOHETee i i v y / MONTANA SCALE am phi bo li te dike / 1987 dike Discontinuous Tertiary felaic P. R I C H M O N D \ : Continuous DOUGLAS zone echlet I n c l u a l o n a in g r a n i t e A I BEARTOOTH MOUNTAINS MONTANA STRUCTURE - H X B X X :l X.