Author Keywords

Virgin Mountains, Elbow Canyon, Lime Kiln Canyon, Beaver Dam Mountains, orthogneiss, metapelites, metamorphism, amphibolites facies, granulite facies, Mojave crustal province, Yavapai crustal province, Laurentia


The Virgin Mountains, located in northwestern Arizona, host a variety of different geologic features. Many workers have focused on Tertiary extension within the mountain range, but little work has been done on the Paleo-Proterozoic basement rocks. Tertiary 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. These features suggest decompression through the garnet-spinel transition. These rocks occur in a broad shear zone exposed over 80-100 km in the Virgin Mountains and the Beaver Dam Mountains to the north. Most samples are mylonitic, but contain polygonal quartz grains consistent with shearing under high-temperature conditions. Other shear indicators 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 to lower granulite facies conditions (650o-800oC and 0.6-1.1 Kilobars). Also, sillimanite pseudomorphs after kyanite found within metapelites suggests decompression from high pressure conditions. Decompression of ultra-mafic lenses through the garnet-spinel transition documents pressures in excess of 2.0 GPa and depths of at least 70 Km. Structural considerations as well as the presence of high-pressure metamorphism are consistent with a collisional suture. The Virgin Mountains appear to host the Paleoproterozoic collisional boundary between Mojave and Yavapai crustal provinces.

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