P-T Constraints of Orthogneiss, Metapelites, and Ultra-Mafic Lenses Located in the Virgin Mountains of Northwestern Arizona

Abstract

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.