GRANULITE METAMORPHISM IN THE NORTHEASTERN WIND RIVER RANGE AT 2705 MA: EVIDENCE FOR CRUSTAL UNDERPLATING DURING THE STILLWATER THERMAL EVENT AND PLUME-STYLE CRUSTAL GROWTH

Southwest-directed thrusting along the ca. 2680 Ma Mount Helen structural belt (MHSB) exposed granulite facies rocks in the Washakie block, which lies on the hanging wall of the thrust. The granulites have been extensively retrograded by deformation along the MHSB and by fluids evolved from the late syntectonic Bridger batholith. Metamorphic conditions at the time of the MHSB were 4-5 kbars and 650-700°C. Conditions for the granulite facies were P = 6 – 8 kbar and T > 800°C, but these are poorly constrained because of extensive ion exchange during partial retrogression. These conditions require significant heating of the crust as they are outside any reasonable static geothermal gradient. U-Pb zircon ages of metamorphic zircon from throughout the northwestern Wind River Range are consistently around 2705 Ma. We have yet to find evidence for extensive felsic magmatism at 2705 Ma, thus we have no evidence that the granulites were formed due to the movement of felsic melts through the crust. This age, however, is within error of the age of the Stillwater Complex, 250 km to the north in the Beartooth Mts., Montana.

From these relations we conclude that the metamorphism was produced by mafic underplating of the north-central Wyoming province at 2705 Ma. This underplating event is manifested elsewhere in the Wyoming province by the Stillwater intrusion and by mafic dikes in the Beartooth and Bighorn Mountains. The high-grade metamorphism associate with this event is only seen in the Washakie block of the Wind River Range, because deep levels of the crust have been exposed along the MHSB. This mafic underplating event could have formed part or all of the 10-15 km thick, fast, lower crust imaged by Deep Probe beneath the north-central Wyoming province. If this model is correct, this event may represent the last plume-style, non-plate tectonic modification of the Wyoming province, prior to the onset of lateral accretion and linear subduction zones that dominate the tectonic history of the southwestern and southern margins of the province from 2.68 to 2.62 Ga. In this case, the Wind River Range records the transition from vertical tectonics that seems to dominate Early to Middle Archean crustal growth, to lateral, plate tectonics processes that dominate the Proterozoic and Phanerozoic.