METAMORPHISM OF FELSIC AND PELITIC ROCKS LOCATED IN THE VIRGIN MOUNTAINS OF NORTHWESTERN ARIZONA

The Virgin Mountains located in northwestern Arizona host a variety of different types of geologic features. Many workers have focused on Miocene extension within the mountain range, but little work has been done on the Paleoproterozoic basement rocks. Mesozoic extension has exposed 1.73 – 1.80 Ga basement material that exhibits intense shear deformation and evidence of high temperature/ high pressure and possibly ultra-high pressure metamorphism. These rocks are well exposed throughout Elbow and Lime Kiln canyons which are located east and south of Mesquite, Nevada. Some exposures enclose ultra-mafic lenses containing pyroxene/spinel pseudomorphs after garnet and relict orthopyroxene containing exsolution lamella of spinel, garnet, and clinopyroxene. These features suggest decompression through the garnet-spinel transition zone, possibly from ultra-high pressure conditions. This study focuses on the orthogneiss, paragneiss and meta-pelites that host these ultramafic lenses. These rocks occur in a broad shear zone exposed over an 80-100 km length. Some textures observed throughout the samples are mylonitic which contain polygonized quartz grains and show sutured boundaries between crystals and mineral foliations. These rocks also have a variety of shear sense indicators which include; sigma and delta structures, mica-fish, and S-C textures. Sillimanite and biotite within the S-C shear fabric suggest deformation and equilibration under upper amphibolite facies conditions (650^o-750^oC and 6-10 Kilobars). Incorporation of the high pressure/ ultra-high pressure ultramafic lenses suggests decompression from at least 22 Kilobars.