Geochronology and geochemistry of Precambrian gneisses, metabasites, and pegmatite from the Tobacco Root Mountains, northwestern Wyoming craton, MontanaThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.T.E. Krogh deceased April 2008.

Abstract

The Middle Mountain Metamorphic Domain of the Montana Metasedimentary Terrane, northwestern Wyoming Craton, within the northwestern Tobacco Root Mountains, mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic (hornblende) gneiss, both of which are cut by metamorphosed mafic rocks. Together, these gneisses are defined as Middle Mountain Gneiss. Archean tonalitic gneiss from west of, and amphibolitic gneiss from east of, the Bismark Fault give, from chemically and air-abraded zircon grains, U–Pb ID–TIMS ages of 3325.5 ± 1.7 and 3340 Ma, respectively. These results reflect primary magmatic ages and show that the Middle Mountain Gneiss extends into the northern area of the Central Fault Block, between the Bismark and Mammoth faults. Older crustal processes in the tonalitic gneiss are evidenced by inherited grains, the oldest of which is >3460 Ma. A metabasite hosted in tonalitic gneiss in the Bismark Fault selvage zone yields a zircon age of 2468 Ma, which is interpreted as the time of metamorphism. This date and other ca. 2470 Ma dates known in the region reflect a series of thermotectonic events designated here as the Beaverhead – Tobacco Root Orogeny. Geochemical evidence in the Central Fault Block metabasites suggests that their >2470 Ma precursors evolved in a back-arc – arc-rift setting, whereas their equivalents west of the Bismark Fault were largely mid-ocean ridge basalt-related tholeiites and east of the Central Fault Block were back-arc tholeiites showing some continental affinity. The metabasite was metamorphosed, deformed, and intruded by pegmatite at 1756 Ma during the Big Sky Orogeny. This orogenic event also produced new zircon growth in Archean tonalitic gneiss. Monazite with an age of 75 Ma, found at one location, reflects nearby intrusion of the Cretaceous Tobacco Root Batholith.

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

“…Support for the latter comes from a ca. 2.45 Ga metamorphic event recognized in the Tobacco Root Mountains and elsewhere in southwest Montana (Roberts et al, 2002;Cheney et al, 2004;Foster et al, 2006;Loehn, 2009;Krogh et al, 2011) and Utah (Mueller et al, 2011). The 2.45 Ga age may be synchronous with the deformation event that produced the foliation locally recorded as inclusion trails in garnet from this dike (also listed in the broad category of D 1 structures here).…”

Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization

“…These consist of Archean to Paleoproterozoic orthogneiss units with several suites of deformed intrusive mafic dikes and interleaved supracrustal metamorphic rocks [21][22][23][24]. The majority of these lithologies have Archean protoliths, and along the northwestern margin of the Wyoming province, they contain evidence for at least two high temperature thermotectonic events: the enigmatic 2500-2450 Ma Tobacco Roots-Tendoy orogeny [20,[25][26][27] and the 1800-1710 Ma Big Sky orogeny [20,22], interpreted as the results of the Wyoming province docking with the rest of the Archean core of Laurentia during the amalgamation of supercontinent Nuna [28,29]. To the southeast of the rocks overprinted by Paleoproterozoic thermotectonism, K-Ar and 40 Ar- 39 Ar thermochronology indicates that the Archean rocks have not been thermally overprinted since the Neoarchean (Figure 1A,B; [30][31][32]).…”