METAMORPHISM AND OROGENESIS OF PROTEROZOIC ROCKS IN THE SOUTHWESTERN MOJAVE PROVINCE

We present new U-Pb geochronologic data (conventional ID-TIMS and SHRIMP-RG) for zircon and monazite from Proterozoic rocks of the southwestern Mojave Province. These data confirm earlier conclusions that peak magmatism occurred at 1770 ± 10 Ma, and ~1745Ma. They also reinforce conclusions that metamorphism and deformation occurred during distinct events at ~1740, ~1700 Ma and ~1660 Ma, and had concluded by ~1648 Ma. The Proterozoic rocks (arc intrusive rocks and associated metasedimentary rocks) occur as roof pendants in Mesozoic intrusive rocks in the San Bernardino, Pinto and Lost Horse Mountains. New crystallization ages include dates for orthogneisses in the San Bernardino Mountains (granitic gneiss of Baldwin Lake - 1766 ± 12 Ma; granitic gneiss of Lightning Gulch - 1773 ± 14 Ma; trondhjemite gneiss of Dry Lake ~1780 Ma; augen gneiss of Erwin Lake ~1648 Ma). Orthogneisses from the Pinto Mountains yield crystallization dates of ~1748 and 1760 Ma, whereas those from the Lost Horse Mountains yield less reliable dates of ~1715 and 1700 Ma. Most of the ortho- and paragneisses are also characterized by overgrowth of high U/Th rims on igneous zircon and/or occurrence of monazite. Zircon overgrowths across the entire region are consistently ~1740 or ~1700 Ma and monazite growth is consistently 1665 ± 10 Ma. These data, combined with a 1648 Ma date for the late synkinematic augen gneiss in the San Bernardino Mountains suggest three distinct pulses of metamorphism and deformation that had ended by 1648 Ma. Furthermore, the overlap of intrusive and deformational ages between the Mojave Province and Proterozoic provinces lying to the east, suggests close ties between the Mojave and Laurentia since at least 1780 Ma. There is only limited evidence for metamorphism and deformation at either 1400-1500 Ma or during the Mesozoic, despite the fact that Middle Proterozoic intrusive rocks are abundant immediately north and east of the region and the rocks occur as pendants in Mesozoic batholiths. In the San Bernardino Mountains, the latest metamorphism is associated with penetrative west-vergent deformation. In the Lost Horse and Pinto Mountains, monazite growth accompanies development of a penetrative foliation and upright tight to isoclinal folding in both the authochton and allochthon of the Red Cloud thrust of Powell.