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The Tobacco Root Mountains are located near the northwestern margin of the Archean Wyoming province. This position places the Archean rocks of the Tobacco Root Mountains approximately midway between the Great Falls tectonic zone, which many believe marks the collisional boundary between the Hearne and Wyoming provinces, and the Madison mylonite zone, 3-km-wide shear zone that is believed to represent major displacement during collision of these two Archean provinces. The Archean of the Tobacco Root Mountains is dominated by quartzofeldspathic orthogneisses but contains substantial and unequivocal metasedimentary rocks that facilitate detailed structural mapping, an allochthonous sequence of metamorphosed mafic and ultramafic volcanic rocks, and mafic dikes and sills that crosscut early fabrics but also are metamorphosed and deformed. The unique location and compositional diversity of these Archean rocks supports new insights into the timing and tectono-thermal effect of the collision of the Wyoming province with the Hearne province. Geochronologic and geothermobarometric results support a collision between 1.78 and 1.72 Ga that produced a metamorphic episode at 5–6 kbar and 650–750 °C and involved partial melting. A sequence dominated by tholeiitic and komatiitic basalts and possible komatiites was brought into tectonic contact with the relatively autochthonous orthogneisses and metasedimentary rocks. The intensity of this tectonothermal event strongly reworked all Archean rocks, so that the dominant folds, fabrics, and mineralogy now observed in Tobacco Root rocks date from this time.

Quartzofeldspathic gneisses and metamorphic mafic rocks are the dominant lithologies in the Indian Creek and Pony–Middle Mountain Metamorphic Suites of the Tobacco Root Mountains. Field relations, geochemical discriminant analysis, and isotopic systematics indicate that these rocks derive from a bimodal volcanic suite ca. 3.3 Ga. The quartzofeldspathic gneisses contain sodic rocks of the tonalite-trond-hjemite-granodiorite suite as well as potassic varieties. This suite of rocks most likely contains some lithologies derived from sedimentary or volcaniclastic sources, and there is evidence that alkali metasomatism occurred prior to or during subsequent major tectonothermal events. The entire suite of gneisses and metamorphic mafic rocks has geochemical characteristics that are indicative of an active continental arc setting, with deposition most likely in an extensional, backarc setting similar to the Mesozoic through Tertiary rocks of the eastern Sierra Nevada Mountains or Mojave Desert. The formation of these rocks represents an early, distinct stage of crustal evolution that preceded the (unconformable?) deposition of one or more platform-type sedimentary sequences (e.g., marbles, pelitic schists, quartzites, banded iron formations). All primary lithologic contacts and textures or structures indicative of possible protoliths have been largely obliterated due to transposition during Archean and Paleoproterozoic (ca. 2.4 and ca. 1.8 Ga) deformation and metamorphism.

The Spuhler Peak Metamorphic Suite consists of a thick sequence of Archean metamorphic rocks, dominated by amphibolite, orthoamphibole-garnet gneiss, and hornblende gneiss and containing minor quantities of quartzite, aluminous schist, quartzofeldspathic gneiss, and meta-ultramafic rocks. Although the Spuhler Peak Metamorphic Suite represents less than 3% of Precambrian exposures in the Tobacco Root Mountains, it has been recognized as a unique suite in the region, due in large part to the unusually abundant amphibolite and the spectacular appearance of the orthoamphibole-garnet gneisses. An analysis of mafic rocks within the Spuhler Peak Metamorphic Suite, using classification diagrams, tectonic discrimination diagrams, and rare earth element plots, provides results consistent with primary igneous processes and pre-metamorphic hydrothermal alteration. Spuhler Peak Metamorphic Suite amphibolites compare well to typical Archean tholeiitic basalts with arc-related affinities. The major element geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is within the range of the suite's amphibolites, excepting CaO and MgO contents. This supports the hypothesis that the amphibolites represent metamorphosed equivalents to the unaltered orthoamphibole-garnet gneiss protolith. When the effects of hydrothermal alteration are taken into account, the geochemistry of the Spuhler Peak Metamorphic Suite orthoamphibole-garnet gneisses is consistent with a volcanic protolith, genetically related to the protolith of its amphibolites. Outcropping within the suite are a number of small bodies of meta-ultramafic rocks, but these rocks are chemically distinct from the suite. We believe these bodies possibly are tectonically emplaced and not directly related to the Spuhler Peak Metamorphic Suite protolith. This protolith, therefore, is interpreted as largely mafic volcanic rocks (90% of the total package) deposited in a marine environment and intercalated with minor amounts of sedimentary material. The Spuhler Peak Metamorphic Suite is distinct from the other Archean suites in the Tobacco Root Mountains—the Indian Creek Metamorphic Suite, which consists of quartzofeldspathic gneisses and metasedimentary rocks including marble, and the Pony–Middle Mountain Metamorphic Suite, which is composed primarily of quartzofeldspathic gneisses with subsidiary hornblende gneiss. Field relationships and radiometric dating demonstrate that the Spuhler Peak Metamorphic Suite was brought into fault contact with the Indian Creek and Pony–Middle Mountain Metamorphic Suites during the Big Sky orogeny, a major tectonothermal event at 1780–1720 Ma.

Meta-ultramafic rocks occur as small (2 to 100 m long), podiform bodies in all three major Precambrian rock suites of the Tobacco Root Mountains of southwest Montana. Most samples consist of a randomly oriented, coarse-grained assemblage of orthopyroxene, olivine, and magnesiohornblende ± spinel, partially replaced by a fine-grained assemblage that may include anthophyllite, talc, cummingtonite, magnesiohornblende, chlorite, serpentine, and/or magnetite. Blackwall reaction zones of anthophyllite, actinolite, chlorite, and/or biotite surround several of the meta-ultramafic bodies. The absence of clinopyroxene with orthopyroxene limits the metamorphic pressure-temperature history of these rocks to temperatures below ∼800 °C. The presence of anthophyllite with olivine indicates that these rocks passed through 650–700 °C at pressures below 0.6 GPa. These constraints are consistent with the detailed pressure-temperature path determined for the surrounding upper-amphibolite to granulite facies metamorphic rocks. Whole-rock chemical analyses of the meta-ultramafic rocks show them to be rich in SiO 2 (44–54 wt%) and poor in MgO (21–34 wt%) relative to mantle peridotites and typical Alpine-type ultramafic rocks. Rare earth element concentrations are all enriched relative to chondritic values and the light rare earth elements are especially enriched (10–30 times), inconsistent with either an upper-mantle or a komatiitic origin. All samples have similar TiO 2 /Zr ratios, suggesting that they have a common or related origin, and that TiO 2 and Zr were conserved in the processes that led to the observed chemical variations. Together, the chemical data point to a protolith that was an ultramafic cumulate rich in orthopyroxene, and therefore probably formed in a continental setting from a basaltic magma enriched in silica. One possible time of origin is a magmatic event during the continental rifting at 2.06 Ga that led to the intrusion of a suite of mafic dikes.

Just over two billion years ago, basaltic magma intruded rocks of the Pony–Middle Mountain Metamorphic Suite and the Indian Creek Metamorphic Suite that now crop out in the Tobacco Root Mountains of Montana near the northwestern margin of the Wyoming province. Numerous examples can be found of mafic dikes that crosscut layering and gneissic banding, demonstrating that the host rocks were metamorphosed to form the gneissic texture prior to intrusion of the dikes. Although many of the intrusions appear to be sills that followed compositional layering, close inspection reveals low-angle discordance in nearly every case, consistent with rotation of dikes by shearing into nearly layer-parallel orientations. The mafic dikes do not intrude the adjacent Spuhler Peak Metamorphic Suite, which we interpret to mean that the Spuhler Peak Metamorphic Suite was not present at the time of intrusion. These dikes and sills were metamorphosed along with their host rocks during the Big Sky orogeny, a major orogenic event at 1.77 Ga that is documented in this volume. The fine-grained, garnet-bearing, rusty-weathering metamorphosed mafic dikes and sills (MMDS) generally have a distinctive appearance in the field. Some MMDS were clearly folded or boudinaged during metamorphism, but many show only weak foliation and still have sharp contacts with their host rocks. A decrease in grain size commonly occurs on the margins of the MMDS and is believed to be a metamorphic texture that developed from the chilled margins of intrusions into cold rocks. Chemically, the MMDS are subalkaline tholeiites that have been modified by significant fractional crystallization, as evidenced by molar Mg/(Mg + Fe) values that vary from 0.6 to 0.3 and TiO 2 values that vary from 0.6 to 3.0 wt%. The ratio of TiO 2 to P 2 O 5 is virtually constant, indicating a common origin for the MMDS and that these two components were similarly incompatible with the fractionating minerals. The weight percentages of both change by a factor of five, which would require crystallization and removal of 80% of the liquid if there were a single magma source. Based on the major element chemistry, plagioclase, orthopyroxene (or pigeonite), and clinopyroxene were the major fractionating phases, with little involvement of olivine. Trace element variations are consistent with this interpretation. For example, there is no clear correlation between Ni and MgO, which would be expected if there were significant olivine fractionation. Measured rare earth element (REE) data for the MMDS are all at least ten times chondrite REE values. In most cases, light rare earth elements (LREEs) are enriched relative to heavy rare earth elements (HREEs). These data are consistent with a model of intrusion of the MMDS into cool, metamorphic rocks at 2.06 Ga during continental rifting of the Wyoming province. The resulting ocean basin closed at 1.77 Ga during the Big Sky orogeny, emplacing rocks of the Spuhler Peak Metamorphic Suite and slivers of an ultramafic, orthopyroxene cumulate, deforming all units together, and metamorphosing all rocks to at least upper amphibolite facies conditions.

Textures and mineral assemblages of metamorphic rocks of the Tobacco Root Mountains are consistent with metamorphism of all rocks during the Big Sky orogeny (1.77 Ga) at relatively high pressure ( P >1.0 GPa) followed by differential reequilibration on a clockwise P-T path at lower pressures (0.6–0.8 GPa). The highest pressures are documented by coarse-grained kyanite and orthopyroxene in aluminous orthoamphibolites, which require P ≥ 1.0 GPa. Other higher-pressure mineral assemblages of note include kyanite + orthoamphibole and kyanite + K-feldspar. Abundant textural evidence for partial melting in pelitic and basaltic rocks includes leucosomes, very large (several cm across) porphyroblasts of garnet, and an absence of primary (foliation-defining) muscovite. Partial to complete overprinting of the coarse-textured, high-pressure assemblages by lower-pressure assemblages and textures occurred across the Tobacco Root Mountains, especially where assisted by deformation and the availability of water. In aluminous rocks, sillimanite bundles typically replace kyanite, and garnet may be rimmed by cordierite + orthopyroxene symplectite or, in quartz-absent rocks, sapphirine + spinel + cordierite symplectite. Orthoamphibolites with partial pseudomorphs of garnet by cordierite are common. Garnet necklaces surround orthopyroxene in orthopyroxene-plagioclase gneisses, whereas orthopyroxene + plagioclase pseudomorphs of garnet occur in nearby hornblende amphibolites. These features appear to require nearly isobaric cooling at pressures near 0.8 GPa, followed by nearly isothermal decompression at temperatures near 700 °C. The resulting P-T path is believed to be the result of tectonic denudation late in the orogenic cycle. Quartz-plagioclase-garnet-hornblende amphibolites occur throughout the Tobacco Root Mountains. Near-rim mineral compositions from these rocks have been used to calculate T s of 650–750 °C at P s of 0.7–0.9 GPa across the terrane. There is no systematic variation in calculated P and T between units nor geographically within units; differences appear to reflect variations in thermometer closure possibly due to the availability of water during cooling. Field relations involving metamorphosed mafic dikes, as well as geochronological data from monazite and zircon, demonstrate that some rocks were first metamorphosed at high temperatures and pressures at 2.45 Ga. However, we have not identified mineral assemblages that can be assigned unequivocally to this earlier event.

Measurements of 60 single-grain, UV laser microprobe 40 Ar/ 39 Ar total gas ages for hornblende from metamorphic rocks of the Tobacco Root Mountains in southwest Montana yield a mean age of 1.71 ± 0.02 Ga. Measurements of 40 Ar/ 39 Ar step-heating plateau ages of three bulk hornblende samples from the Tobacco Root Mountains metamorphic rocks average 1.70 ± 0.02 Ga. We believe that these and the K/Ar or 40 Ar/ 39 Ar ages reported by previous workers are cooling ages from a 1.78 to 1.72 Ga, upper-amphibolite to granulite facies, regional metamorphism (Big Sky orogeny) that affected the northwestern portion of the Wyoming province, including the Tobacco Root Mountains and adjacent ranges. Based on the 40 Ar/ 39 Ar data, this 1.78–1.72 Ga metamorphism must have achieved temperatures greater than ∼500 °C to reset the hornblende 40 Ar/ 39 Ar ages of samples from the Indian Creek Metamorphic Suite, which was previously metamorphosed at 2.45 Ga, and of the crosscutting metamorphosed mafic dikes and sills (MMDS), which were intruded at 2.06 Ga. Biotite and hornblende from the Tobacco Root Mountains appear to give the same 40 Ar/ 39 Ar or K/Ar age (within uncertainty), indicating that the rocks cooled rapidly through the interval from 500 to 300 °C. This is consistent with a model of the Big Sky orogeny that includes late-stage tectonic denudation that leads to decompression and rapid cooling. A similar cooling history is suggested by our data for the Ruby Range. Three biotite samples from the Ruby Range yield 40 Ar/ 39 Ar step-heating plateau ages with a mean of 1.73 ± 0.02 Ga, identical to the best-estimate (near-plateau) age for a hornblende from the same rocks. Two samples of the orthoamphibole, gedrite, from the Tobacco Root Mountains were studied, but did not have enough K to yield a reliable 40 Ar/ 39 Ar age. Several biotite and three hornblende samples from the region yield 40 Ar/ 39 Ar dates significantly younger than 1.7 Ga. We believe these samples were partially reset during contact metamorphism by Cretaceous (75 Ma) intrusive rocks. Hydrothermal alteration associated with ca. 1.4 Ga rifting led to growth of muscovite with that age in the Ruby Range, but this alteration was apparently not hot enough to reset biotite and hornblende ages there.

The Precambrian rocks of the Tobacco Root Mountains have been separated into three suites: the Indian Creek Metamorphic Suite, the Pony–Middle Mountain Metamorphic Suite, and the Spuhler Peak Metamorphic Suite. The Indian Creek and Pony–Middle Mountain Metamorphic Suites are quartzofeldspathic gneiss suites that contain variable amounts of metasupracrustal rocks. The Spuhler Peak Metamorphic Suite contains primarily mafic rocks and is possibly ocean crust. Metamorphosed mafic dikes and sills that intruded the Indian Creek and Pony–Middle Mountain Metamorphic Suites, but not the Spuhler Peak Metamorphic Suite, indicate juxtaposition of the Spuhler Peak Metamorphic Suite with the other two suites after intrusion of the dikes at ca. 2060 Ma. All rocks have been deformed and metamorphosed together, initially at pressures greater than 1.0 GPa and temperatures greater than 750 °C, followed by differential reequilibration at ∼0.6 GPa on a clockwise pressure-temperature path. Two-hundred-seventy-two 207 Pb/ 206 Pb spot ages of monazite grains from seventeen Spuhler Peak Metamorphic Suite, five Pony–Middle Mountain Metamorphic Suite, and eight Indian Creek Metamorphic Suite rocks have been obtained from the University of California at Los Angeles ion microprobe. Based on the distribution of the ages, the samples can be divided into three groups. (1) All seventeen Spuhler Peak Metamorphic Suite, one Pony–Middle Mountain Metamorphic Suite, and two Indian Creek Metamorphic Suite samples have relatively homogeneous spot age populations that vary from ca. 1720 to ca. 1780 Ma. (2) A group of seven Indian Creek and Pony–Middle Mountain Metamorphic Suite samples has spot ages from monazite grains that form an array near 2450 Ma. (3) A group of four Indian Creek and Pony–Middle Mountain Metamorphic Suite samples are bimodal in that they contain spot ages from both the 1720–1780 Ma group and the 2450 Ma array. There are younger and a few older spot ages in these samples that likely represent mixed age domains, the former between the 1720–1780 Ma and the 2450 Ma age domains and the latter between older detrital grain cores and the 2450 Ma array. Monazite grains in the matrix have similar ages to those that occur as inclusions in garnet and kyanite. Thus, the monazite in these rocks, as well as the peak metamorphic minerals, either grew or re-equilibrated during the higher-pressure (>1.0 GPa) metamorphism. The near absence of 207 Pb/ 206 Pb ages older than 1780 Ma in the Spuhler Peak Metamorphic Suite and the common occurrence of older ages in monazite from the Indian Creek and Pony–Middle Mountain Metamorphic Suites are consistent with assembly of the Tobacco Root terrane during a prolonged (60 m.y. long) collision event, the Big Sky orogeny, beginning at ca. 1780 Ma and culminating at ca. 1720 Ma. The Early Proterozoic Big Sky orogeny significantly overprinted the effects of an earlier ca. 2450 Ma orogeny in both the Pony–Middle Mountain and Indian Creek Metamorphic Suites. This older event modified pre-existing Archean rocks. However, of the 272 spot ages on monazite grains reported here, only six are significantly older than 2450 Ma and only one of these is older than 2600 Ma—a 2988 Ma spot age from a monazite inclusion in a garnet from a Pony–Middle Mountain Metamorphic Suite sample. There is no evidence of widespread Archean events recorded in the monazite grains of the Tobacco Root Mountains.

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region. The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.

A Proterozoic map-scale sheath fold is present in the crystalline core of the Tobacco Root Mountains. The fold has distinctive metamafic and associated metasedimentary rocks of the Spuhler Peak Metamorphic Suite in its core and quartzofeldspathic gneisses of the Indian Creek and Pony–Middle Mountain Metamorphic Suites on its outer flank. Fabric and structures throughout the range relate geometrically to this sheath fold and together demonstrate the character of ductile deformation during the 1.72–1.78 Ga Big Sky orogeny in the Tobacco Root Mountains. Intense, progressive, noncoaxial strain either transposed or formed compositional layering, mineral-alignment foliation, and lineation and generated outcrop-scale sheath folds, cylindrical folds with axes dispersed in the plane of foliation, and larger curtain folds. While it can be demonstrated that some gneissic fabrics and folds in the Indian Creek and Pony–Middle Mountain Metamorphic Suites formed in a much earlier phase of deformation that predates the intrusion of a suite of metamorphosed mafic dikes and sills, the Big Sky orogeny effectively reoriented preexisting structures and fabrics into a single Proterozoic pattern. It is most likely that tectonic juxtaposition of the Spuhler Peak Metamorphic Suite with the quartzofeldspathic gneiss suites was an integral part of this shear strain. Mesoscopic asymmetric folds record the displacement of the Spuhler Peak Metamorphic Suite down and to the north relative to the Indian Creek and Pony–Middle Mountain Metamorphic Suites during the formation of the map-scale sheath fold. “Unfolding” this sheath fold demonstrates that the Indian Creek and Pony–Middle Mountain Metamorphic Suites are lateral equivalents but leaves undetermined whether the Spuhler Peak Metamorphic Suite was juxtaposed above or below the quartzofeldspathic gneisses during Big Sky deformation.

Integrated studies by Keck Geology Consortium participants have generated many new insights into the Precambrian geology of the Tobacco Root Mountains. We have clarified the tectonic setting and origin of two suites of metamorphic rocks: (1) a quartzofeldspathic gneiss complex with associated metasupracrustal rocks (the combined Indian Creek and Pony–Middle Mountain Metamorphic Suites) that originated in a continental arc setting between 3.35 and 3.2 Ga with subsequent sedimentation and (2) mafic metavolcanic rocks with intercalated metasedimentary rocks (the Spuhler Peak Metamorphic Suite) from a suprasubduction zone ophiolite or backarc basin possibly of Proterozoic age. A poorly preserved metamorphic event at 2.45 Ga affected the former but not the latter, as did the intrusion of rift-related mafic dikes and sills at 2.06 Ga. Both suites were amalgamated, metamorphosed to at least upper amphibolite facies, subjected to simple shear strain and folded into map- and outcrop-scale sheath folds, and tectonically unroofed during the period 1.78 to 1.71 Ga. We name this event the Big Sky orogeny. The Proterozoic geology of the Tobacco Root Mountains can be integrated with coeval features of the geology of the northern Wyoming province to outline a northeast-trending, southeast-vergent belt as the Big Sky orogen. The Big Sky orogen consists of a metamorphic hinterland flanked to the southeast by a foreland of discrete ductile shear zones cutting older basement, and to the northwest by arc-related meta-plutonic bodies and the trace of a fossil subduction zone in the upper mantle. Archean blocks to the north of the Big Sky orogen may have been accreted as allochthonous terranes during collision and convergence. The remarkable synchroneity of collision along the Big Sky orogen with tectonism in the Trans-Hudson orogen along the eastern margin of the Wyoming province and in the Cheyenne belt to the south of the province raise profound but unanswered questions about the process by which the Wyoming province was added to the rest of the ancestral North American craton.

The core of the Tobacco Root Mountains, a domal uplift of southwestern Montana, is composed of regionally metamorphosed Archean rocks that have been intruded by diabase dikes of late Precambrian age and by igneous rocks of late Mesozoic and Cenozoic age. The Archean rocks, originally a stratiform sequence of epiclastic, volcaniclastic, and carbonate sedimentary rocks, have been metamorphosed to amphibolite-granulite rank with attendant migmatization. The distribution of the metamorphic rocks and the later intrusions is shown on the map. In this report to accompany the map, the nature of the crystalline rocks is described briefly. Two thermal events affected the area in Precambrian time, one about 2.7 b.y. B.P. and the second at about 1.6 b.y. B.P. A third event may have begun in very late Precambrian time, but is most strongly represented by changes of late Mesozoic age. Because of the structural complexities, correlation of the metamorphic rocks in the Tobacco Root Mountains with Precambrian sequences elsewhere in southwestern Montana is extremely difficult.