ULTRAHIGH-PRESSURE PALEOPROTEROZOIC METAMORPHISM IN THE BEAVER DAM MOUNTAINS, SOUTHWESTERN UTAH

The currently mapped portion of the extraordinary Paleoproterozoic (1.7-1.8 Ga) basement terrain exposed in the Beaver Dam Mountains is comprised of two lithotectonic blocks separated by a wide zone of intensely sheared rock. The two blocks appear to record different metamorphic histories with the western block containing features indicative of decompression from very high pressures. Lenses of retrograded eclogite within the shear zone provide evidence for extreme metamorphic conditions. Mafic lenses display significant retrogression to amphibolite but preserve eclogite facies assemblages comprised of garnet (up to 30% by mode) + clinopyroxene + epidote/clinozoesite + phengitic muscovite + quartz + rutile. Possible kyanite has been observed in thin section. A more felsic assemblage consists of garnet (up to 60% by mode), quartz, and symplectitic intergrowths of albite and muscovite that are thought to pseudomorph former jadite. The relict high-pressure assemblage is overprinted by retrograde biotite and tremolite. Grossular-rich garnet porphyroblasts in both mafic and felsic compositions contain unusual inclusion assemblages. Quartz occurs as rational inclusions and as quartz rods, both indicative of very high-pressure conditions. Quartz inclusions are often surrounded by radial fractures. Importantly, these unusual inclusions often contain small rutile grains. Rutile also occurs as free inclusions, unassociated with other minerals. Rounded, polycrystalline quartz inclusions are interpreted as quartz pseudomorphs after coesite. Mineral assemblages, inclusion characteristics and the presence of former coesite are consistent with metamorphism under ultrahigh-pressure (UHP) conditions. PT conditions are estimated to range from 650-750°C and 2.8-3.5 GPa using the graphite-diamond transition to bracket upper pressure limits.