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

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 comple x metamorphism and deformation that affected the range in the Late Archean, Five deformation events are recorded. D-1 is a penetrative deformation tha t occurred during the earliest granulite-facies metamorphism; D-2 is a fold ing event, probably in amphibolite facies, that deforms porphyritic dikes t hat cut the D-1 fabrics. Both D-1 and D-2, predate the intrusion of the ca, 2.8 Ga Native Lake gneiss. D-3 is a folding event, accompanied by upper am phibolite to granulite metamorphism, that deformed the Medina Mountain sequ ence, a sequence of rocks that was either deposited or thrust upon the Nati ve Lake gneiss, D-4 is a fabric-forming event associated with the Mount Hel en structural belt (MHSB), It is represented by mylonites in the MHSB, a pe netrative fabric in the Bridger batholith, and folding of the D-3 structure s in the Medina Mountain sequence. We consider D-3 and D-4 to be coeval wit h the emplacement of the Bridger batholith, and hence to date at ca, 2.67 G a. The latest structures (D-5) are fabrics associated with the folding and thrusting of the 2.65 Ga South Pass sequence. We recognize at least four metamorphic events. M-1 is associated with the D 1 fabrics and occurred at high T (>750 degrees C) and high P (similar to 7- 8 kilobars).M-2 (65-750 degrees C and 4-5.5 kilobars) is associated with th e intrusion of the Bridger batholith and formation of the D-3 and D-4 struc tures, The D-5 Structures of the South Pass sequence record M-3, which is l ow P (similar to 2-3 kilobars) and low T (similar to 500 degrees C), The fi nal metamorphism, M-4, is a contact metamorphism around the Louis Lake bath olith, In the south against the South Pass sequence, the metamorphism occur red at similar to 3 kilobars and at temperatures <700 degrees C, In contras t, in the north where the Louis Lake batholith is charnockitic, the metamor phism occurred at 6 kilobars and 800 degrees C, This pressure gradient is p robably a reflection of tilting of the Wind River block during the Laramide orogeny. The composition of the plutons and the structural and metamorphic history o f the Wind River Range indicate that during the Late Archean this area occu pied the active margin of the Wyoming province. This tectonic environment i s similar to the long-lived Phanerozoic margins of North America. The Wind River Range represents the best-documented active margin of Archean age.