Paleoproterozoic Metamorphism in the Northern Wyoming Province: Implications for the Assembly of Laurentia

Abstract

U-Pb ages measured on zircons from the Tobacco Root Mountains and monazite from the Highland Mountains indicate that the northwestern Wyoming province experienced an episode of high-grade metamorphism at ∼1.77 Ga. Leucosome emplaced in Archean gneisses from the Tobacco Root Mountains contains a distinctive population of zircons with an age of 1.77 Ga but also contains zircons to ∼3.5 Ga; it is interpreted to have been derived primarily by anatexis of nearby Archean schist. A granulite facies mafic dike that cuts across Archean gneissic banding in the Tobacco Root Mountains contains two distinct populations of zircons. A group of small (!50 mm) nonprismatic grains is interpreted to be metamorphic and yields an age of 1.76 Ga; a group of slightly larger prismatic grains yields an age of 2.06 Ga, which is interpreted to be the time of crystallization of the dike. Monazite from a leucogranite from the Highland Mountains yields a well-defined age of 1.77 Ga, which is interpreted as the time of partial melting and emplacement of the leucogranite. These results suggest that the northwestern Wyoming province, which largely lies within the western part of the Great Falls tectonic zone, experienced a metamorphic maximum at 1.77 Ga. This age is ∼100 m.yr. younger than the proposed time of Wyoming-Hearne collision in the central Great Falls tectonic zone (1.86 Ga) and suggests that the northwestern Wyoming province may have been involved in a separate, younger collisional event at ∼1.77 Ga. An event at this time is essentially coeval with collisions proposed for the eastern and southeastern margins of the province and suggests a multiepisodic model for the incorporation of the Wyoming craton into Laurentia.

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].…”

Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

“…At least two phases of deformation and metamorphism have affected this region during the late Archean and Palaeoproterozoic, including an enigmatic event at c . 2.55–2.45 Ga (Cheney, Webb, Coath, & McKeegan, ; Condit, Mahan, Ault, & Flowers, ; Krogh et al., ; Mueller et al., ) and the Late Palaeoproterozoic (1.8–1.71 Ga) Big Sky orogeny (Ault, Flowers, & Mahan, ; Brady et al., ; Condit et al., ; Mueller et al., ). The former is most clearly recognized by metamorphic monazite and zircon growth in various lithologies in the Tobacco Root Mountains, where it was referred to by Krogh et al.…”

“…These rocks lie along the northwest margin of the Wyoming Province, the southernmost Archean component of the core of North America (Figure 1a, Mueller & Frost, 2006;Whitmeyer & Karlstrom, 2007). At least two phases of deformation and metamorphism have affected this region during the late Archean and Palaeoproterozoic, including an enigmatic event at c. 2.55-2.45 Ga (Cheney, Webb, Coath, & McKeegan, 2004;Condit, Mahan, Ault, & Flowers, 2015;Krogh et al, 2011;Mueller et al, 2004) and the Late Palaeoproterozoic (1.8-1.71 Ga) Big Sky orogeny (Ault, Flowers, & Mahan, 2012;Brady et al, 2004;Condit et al, 2015;Mueller et al, 2005). The former is most clearly recognized by metamorphic monazite and zircon growth in various lithologies in the Tobacco Root Mountains, where it was referred to by Krogh et al (2011) as the Tobacco Roots-Tendoy orogeny, and by zircon in mafic dykes and leucosome in the central and eastern Northern Madison Range (Condit et al, 2015;Mueller, Wooden, & Mogk, 2011).…”

Structural geology and tectonics of the paleoproterozoic rocks of the Mount Rushmore Quadangle, Black Hills, Souh Dakota

“…The lack of exposure over the trace of the orogen necessitates dependence on geophysical methods to define the orogen and interpret the structural complexity. Other enigmatic structures, such as the Great Falls Tectonic Zone (GFTZ) and the Vulcan Structure (cf., Mueller et al, 2002Mueller et al, , 2005 (Figure 2), have traditionally been interpreted to be separate from the Trans-Hudson. Recent work in the GFTZ (e.g.…”

“…Precambrian rocks of Hudsonian age are, however, exposed within the GFTZ in the Little Belt Mountains, Montana (cf. Mueller et al, 2002Mueller et al, , 2005 and references therein).…”