PIERCING THE VEIL: PRE-1.8 GA METAMORPHISM IN THE MOJAVE PROVINCE, BEAVER DAM MOUNTAINS, UTAH

The Paleoproterozoic Mojave Province (MP) in in the southwestern US preserves a long and very complicated metamorphic and tectonic history. The oldest ages generally recorded in the MP are 1.8 Ga metamorphic and deformational ages. However, these events affect previously existing igneous, sedimentary and possibly metamorphic protoliths. Indeed, metavolcanic rocks dating from 2.09 to 2.3 Ga are reported from the Turtle Mountains in southern California, and inherited zircons yielding Archean (2.59 ± .61 Ga) may indicate a late Archean protolith or inherited zircons. The current study attempts to pierce the mask of the intense 1.8 Ga event and look at the earlier tectonic and metamorphic history of the Mojave Provence. Preliminary results from the Beaver Dam Mountains in extreme southwestern Utah are intriguing. 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. 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, and 3) possible relict garnet granulite bodies. 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, and that the 1.8 Ga metamorphic and tectonic event masks a significant decompression event.