Precambrian Geology of North Snowy Block, Beartooth Mountains, Montana

Reid, Rolland R.; McMannis, William J.; Palmquist, John C.

doi:10.1130/spe157-p1

Cited by 9 publications

(9 citation statements)

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“…2.4-2.6 Ga. Later, Erslev andSutter (1990) andO'Neill (1998) went on to suggest that a northeast-trending Proterozoic orogen underlies the northern Wyoming province and that Giletti's K-Ar boundary separates a reset hinterland to the northwest from a cooler, undisturbed foreland to the southeast in that south-vergent orogen. Evidence for 1.8-1.6 Ga thermal disturbance in much older metamorphic rocks has subsequently been documented in the North (Reid et al, 1975) and South (Montgomery and Lytwyn, 1984) Snowy Blocks of the Beartooth Mountains, along Giletti's line, and in the Stillwater Complex (Houston et al, 1993) south of the line. These additional geochronologic data refi ne the position of Giletti's line but do not fundamentally change his, or Erslev and Sutter's and O'Neill's vision.…”

Section: Architecture Of the Big Sky Orogen Across The Northern Wyomimentioning

confidence: 95%

“…The Madison mylonite zone is roughly parallel to a number of northeast-striking shear zones within the Montana metasedimentary terrane that juxtapose rocks of different geologic character and/or metamorphic grade, including a mylonite zone in the North Snowy Block that is typically cited as the suture between the Montana metasedimentary terrane and the Beartooth-Bighorn magmatic terrane in the northern Wyoming province Mogk and Henry, 1988;Mogk et al, 1992b;Reid et al, 1975), and shear zones that cut the northern Madison and Gallatin Ranges (the Mirror Lake and Big Brother shear zones of Mogk et al, 1992b). If proven to be Proterozoic, these shear zones constitute a belt of ductile thrust faults between the Big Sky metamorphic core to the northwest and Giletti's line to the southeast that straddles the transition from the infrastructural hinterland to the supracrustal foreland of the orogen.…”

Section: Forelandmentioning

confidence: 99%

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Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Harms

Brady²,

Burger³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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Integrated studies by Keck Geology Consortium participants have generated many new insights into the Precambrian geology of the Tobacco Root Mountains.We have clarifi ed 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) mafi c 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 mafi c 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 fl anked to the southeast by a foreland of discrete ductile shear zones cutting older basement, and to the northwest by arc-related metaplutonic 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.

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Section: Architecture Of the Big Sky Orogen Across The Northern Wyomimentioning

confidence: 95%

Section: Forelandmentioning

confidence: 99%

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Harms

Brady²,

Burger³

et al. 2004

Precambrian Geology of the Tobacco Root Mountains, Montana

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“…1.8-1.6 Ga northwest of a NE-trending transitional boundary that was originally drawn through the Southern and Northern Madison Ranges ( Fig. 1B; Giletti, 1966Giletti, , 1971Reid et al, 1975;Mueller and Cordua, 1976). This feature, commonly referred to as "Giletti's line," was interpreted to represent the transition from warmer temperatures to the northwest, which induced complete resetting or new mica growth at 1.8-1.6 Ga, to cooler temperatures to the southeast, which were not high enough in Proterozoic time to significantly disturb the Ar system (Giletti, 1966).…”

Section: Introductionmentioning

confidence: 99%

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

Condit¹,

Mahan²,

Ault³

et al. 2015

Lithosphere

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The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8-1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky-related deformation are preserved, both nearly coeval with peak metamorphic conditions of ~0.9-0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2s). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750-1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (~45-40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (~30-25 km) in the central-southeast area.

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“…This event, generally referred to as the Beartooth orogeny, is deeply engraved in the Precambrian rocks throughout the Rocky Mountains of Wyoming and Montana. In its type area, the Beartooth Mountains of Montana, a partial geochronology of earlier events has been established, notably by Reid, McMannis, and Palmquist (1975) for the North Snowy block. The metasedimentary component of that terrane, which has a minimum age of about 3,100 m.y., was metamorphosed and intruded by the Mount Delano Gneiss of Reid, McMannis, and Palmquist (1975) and underwent further metamorphism and igneous intrusion during the Beartooth orogeny.…”

Section: Geologic Historymentioning

confidence: 99%

“…In its type area, the Beartooth Mountains of Montana, a partial geochronology of earlier events has been established, notably by Reid, McMannis, and Palmquist (1975) for the North Snowy block. The metasedimentary component of that terrane, which has a minimum age of about 3,100 m.y., was metamorphosed and intruded by the Mount Delano Gneiss of Reid, McMannis, and Palmquist (1975) and underwent further metamorphism and igneous intrusion during the Beartooth orogeny. Page (1977) concluded that the Stillwater Complex of the Beartooth Mountains intruded a metasedimentary sequence (including ironformation) with a minimum age of 3,140 m.y.…”

Section: Geologic Historymentioning

confidence: 99%

Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana

James¹

1981

Professional Paper

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Bedded deposits of iron-formation are minor components of the thoroughly metamorphosed and deformed Precambrian rocks that make up the core of the Tobacco Root Mountains. The rocks are Archean in age; they predate a major Precambrian orogeny that affected all of southwestern Montana about 2,750 m.y. ago. The principal bed of iron-formation occurs within a metasedimentary sequence that has dolomite marble at the base and rests on quartzofeldspathic gneiss of uncertain origin. The stratigraphic thickness of the preserved part of the metasedimentary group cannot readily be established because of structural complexities, including both thickening and attenuation, but it probably does not exceed 300 m. The true (original) thickness of the iron-formation is even more difficult to determine because of the structural incompetence of the rock, but it ranges from 15 to 30m. All the rocks, with the exception of a few younger Precambrian (Proterozoic Y) diabase dikes, are metamorphosed to amphibolite or hornblende granulite facies. The iron-formation typically consists of quartz and magnetite, with subordinate amounts of iron silicates, mainly hypersthene, garnet, clinopyroxene, and grunerite. The principal deposits of iron-formation are in the Copper Mountain area, an area of about 13 km 2 in the west-central part of the Tobacco Root range that has been mapped in some detail. The structure consists of an early set of tight isoclinal folds, trending north-south and overturned to the east, that are deformed by later crossfolds that trend and plunge northwest. The most prominent belt of iron-formation is on a tight anticlinal buckle within the northsouth-trending Ramshorn syncline, a major structure of the first fold set. This belt of iron-formation is estimated to contain about 63 million t of potential low-grade ore (taconite) to a depth of 100m. The rock contains about 35 weight percent Fe, mostly in the form of magnetite. Iron-formation occurs in many other localities in the region as distinctive but thin and discontinuous units. Some deposits probably are stratigraphically equivalent to the iron-formation of the Copper Mountain area, but others have quite different lithologic associations and probably different ages.

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Precambrian Geology of North Snowy Block, Beartooth Mountains, Montana

Cited by 9 publications

References 0 publications

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Advances in the geology of the Tobacco Root Mountains, Montana, and their implications for the history of the northern Wyoming province

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana

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