NEW CONSTRAINTS ON THE PALEOPROTEROZOIC ASSEMBLY OF THE SOUTHWESTERN MARGIN OF LAURENTIA: HIGH (ULTRA-HIGH?) PRESSURE METAMORPHISM IN THE MOJAVE-YAVAPAI SUTURE, UTAH, NEVADA AND ARIZONA

Precambrian lower crustal rocks within the Mojave-Yavapai boundary zone are exposed in the Beaver Dam Mountains (BDM) and Virgin Mountains (VM) in Utah, Nevada, and Arizona. Ongoing metamorphic studies are providing new constraints on the Paleoproterozoic assembly of southern Laurentia. The BDMs contain a sheared high-grade migmatite complex enclosing lenses of partially melted garnet amphibolite and retrograded eclogite. Geothermobarometry, phase equilibria and thermodynamic modeling reveal an isothermal decompression path from eclogite or high-pressure granulite facies conditions (750 - 800°C at 1.5 – 1.7 GPa) to upper amphibolite facies conditions (680 - 750°C at 0.5 - 0.9 GPa). The VMs are characterized by broad, km-scale zones of intense shearing displaying early compressional and latter transpressional shear sense. Ultramafic lenses often contain rounded aggregates of orthopyroxene/spinel symplectites, sodic-pyroxene, magnetite and olivine. These aggregates are contained in a matrix consisting of pargasitic and edenitic amphibole, olivine, and orthopyroxene and clinopyroxene porphyroclasts. Textures and mineral compositions in the VM ultramafic rocks reflect decompression through the garnet-spinel transition under hydrous conditions, and minimum pressures of 2.2 GPa. Higher pressures, possibly ultra-high pressure (UHP) conditions, are sugested by spinel and other exsolution lamella within pyroxene porphyroclasts. Exhumed from minimum depths of 80 km, these rocks represent the deepest levels of Paleoproterozoic crust exposed anywhere in the southwestern United States, and may comprise the oldest known occurrence of UHP rocks on earth. Metamorphic conditions in the BDMs and VMs are consistent with collision between two continental masses; the Mojave Province and 1.85 Ga basement underlying a portion of the Yavapai Province. Isotopic data indicate that HP-UHP(?) rocks are part of the Yavapai Basement, suggesting 1.74-1.70 Ga collision and partial subduction of the Yavapai Basement below Mojave Province crust. A conceptual model involving collision and partial subduction accounts for many of the isotopic and geochemical boundaries noted by previous workers, including the isotopically mixed zone which has often been considered the boundary between the two provinces.

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