RELICT HIGH-PRESSURE METAMORPHISM AND ADIABATIC PT PATHS IN MOJAVE PROVINCE ROCKS, BEAVER DAM MOUNTAINS. UTAH

The Paleoproterozoic Mojave Province in the southwestern US preserves a long and very complicated metamorphic and tectonic history. Most Mojave Province rocks record an intense period of deformation and metamorphism dating to 1.7 Ga; the Ivanpah orogeny. Much of the Mojave Province reflects metamorphism at mid-crustal depths. Preliminary results from the Beaver Dam Mountains in extreme southwestern Utah display intriguing evidence of earlier, high pressure metamorphism. These exposures are very similar to Mojave Province rocks exposed elsewhere, consisting of variably deformed granitoids, amphibolites and metasedimentary units. Metamorphic grade varies, and quartzofeldspathic and pelitic units are migmatitic in the highest grade areas. Stable mineral assemblages in amphibolite and metapelitic units indicate P-T conditions in the range of 700 - 800°C and 0.7 - 0.8 Gpa (25-28 km paleodepths). Numerous features point towards a higher pressure early phase of metamorphism, including 1) plagioclase coronas rimming garnet in garnet amphibolites, 2) sillimanite pseudomorphs after kyanite associated with the migmatites, and 3) possible relict garnet granulite bodies. Textural characteristics indicate a steep, possibly adiabatic PT path. Migmatites formed as these rocks passed through the reaction Ms + Pl + Qtz = Kfs + Als + Gt + L. The lack of plagioclase in the relict garnet granulite indicates pressures in excess of 1.2 GPa, suggesting that rocks in the Beaver Dam Mountains once occupied a lower crustal position (paleodepths in excess of 43 km), and that the 1.8 Ga metamorphic and tectonic event masks a previously undocumented high pressure event.