POLYMETAMORPHISM OF ARCHEAN PELITIC SCHIST IN THE GROUSE CREEK MOUNTAINS, NORTHWESTERN UTAH

Pelitic schist from the Archean Green Creek Complex in the central Grouse Creek Mountains, Utah, exhibit textural and mineralogical characteristics suggestive of two episodes of metamorphism, an Archean episode which was overprinted in the Cretaceous by metamorphism associated with the Sevier orogeny. The first metamorphism (M1) was granulite in grade and occurred when a 2.55 Ga monzogranite intruded the pelitic schist, causing a dehydration partial melting reaction that yielded trondjhemitic (mus + albite + quartz) leucosomes that occur as layers up to 1 meter thick, and as stringers, pods, and lenses. Subsequently, M1 garnets in the schist became finely crushed and smeared along the present-day dominant subhorizontal foliation. Subsequent metamorphism, probably of Sevier age (M2), crystallized idioblastic staurolite and kyanite that overgrew the foliation. During M2, the garnet fragments reequilibrated to compositions consistent with amphibolite facies conditions. Thermobarometry on garnet cores indicates equilibration at 6-6.8 kb and 590-645°C; overgrowths show higher pressures at similar temperatures, 9 kb and 610°C, which suggests a thrust-related pressure-temperature path similar to that found at Basin Creek, ~23 km to the north, where the recorded temperatures are ~120°C lower. The sampled rocks appear to be little affected by later Tertiary deformation, which imparted at least one known exhumation-related mylonitic fabric to the area. The rocks also do not display chemical evidence of contact metamorphism associated with the Oligocene Red Butte stocks, from which it may be inferred that the stocks are too small to have had a significant or prolonged thermal effect. The findings of this study provide evidence of thrusting in the hinterland of the Sevier orogenic belt, in a mountain range where thrust relationships appear to have been obscured by later Tertiary extensional faulting.