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Late Archean structural and metamorphic history of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton

B. Ronald Frost^*,1, Kevin R. Chamberlain¹, Susan Swapp¹, Carol D. Frost¹ and Thomas P. Hulsebosch^,1

¹ Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA

The Archean rocks of the Wind River Range in western Wyoming record a Late Archean history of plutonism that extends for more than 250 m.y. The range is dominated by granitic plutons, including the 2.8 Ga Native Lake gneiss, the 2.67 Ga Bridger batholith, the 2.63 Ga Louis Lake batholith, and late 2.54 Ga granites. These plutons provide a means of distinguishing the complex metamorphism and deformation that affected the range in the Late Archean. Five deformation events are recorded. D₁ is a penetrative deformation that occurred during the earliest granulite-facies metamorphism; D₂ is a folding event, probably in amphibolite facies, that deforms porphyritic dikes that cut the D₁ fabrics. Both D₁ and D₂ predate the intrusion of the ca. 2.8 Ga Native Lake gneiss. D₃ is a folding event, accompanied by upper amphibolite to granulite metamorphism, that deformed the Medina Mountain sequence, a sequence of rocks that was either deposited or thrust upon the Native Lake gneiss. D₄ is a fabric-forming event associated with the Mount Helen structural belt (MHSB). It is represented by mylonites in the MHSB, a penetrative fabric in the Bridger batholith, and folding of the D₃ structures in the Medina Mountain sequence. We consider D₃ and D₄ to be coeval with the emplacement of the Bridger batholith, and hence to date at ca. 2.67 Ga. The latest structures (D₅) are fabrics associated with the folding and thrusting of the 2.65 Ga South Pass sequence.

We recognize at least four metamorphic events. M₁ is associated with the D₁ fabrics and occurred at high T (>750°C) and high P (7–8 kilobars). M₂ (650–750 °C and 4–5.5 kilobars) is associated with the intrusion of the Bridger batholith and formation of the D₃ and D₄ structures. The D₅ structures of the South Pass sequence record M₃, which is low P (2–3 kilobars) and low T (500°C). The final metamorphism, M₄, is a contact metamorphism around the Louis Lake batholith. In the south against the South Pass sequence, the metamorphism occurred at 3 kilobars and at temperatures <700 °C. In contrast, in the north where the Louis Lake batholith is charnockitic, the metamorphism occurred at 6 kilobars and 800°C. This pressure gradient is probably a reflection of tilting of the Wind River block during the Laramide orogeny.

The composition of the plutons and the structural and metamorphic history of the Wind River Range indicate that during the Late Archean this area occupied the active margin of the Wyoming province. This tectonic environment is similar to the long-lived Phanerozoic margins of North America. The Wind River Range represents the best-documented active margin of Archean age.

Key Words: active margin • Archean • metamorphic petrology • structure • tectonics • Wyoming province

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