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.
Nielson, Bryan R.; Nielson, Brady M.; Schroeder, Andrew; and Colberg, Mark R.
"P-T Constraints of Orthogneiss, Metapelites, and Ultra-Mafic Lenses Located in the Virgin Mountains of Northwestern Arizona,"
The Compass: Earth Science Journal of Sigma Gamma Epsilon:
1, Article 2.
Available at: http://digitalcommons.csbsju.edu/compass/vol85/iss1/2
Appendix 1 Table 1
Nielson Appendix1_Table2.pdf (140 kB)
Appendix 1 Table 2
Nielson Appendix1_Table3.pdf (134 kB)
Appendix 1 Table 3
Nielson Appendix1_Table4.pdf (135 kB)
Appendix 1 Table 4
Nielson Appendix1_Table5.pdf (152 kB)
Appendix 1 Table 5
Nielson Appendix1_Table6.pdf (141 kB)
Appendix 1 Table 6
Nielson Appendix1_Table7.pdf (137 kB)
Appendix 1 Table 7
Nielson Appendix1_Table8.pdf (139 kB)
Appendix 1 Table 8
Nielson Appendix1_Table9.pdf (131 kB)
Appendix 1 Table 9