A FRESH LOOK AT CORE COMPLEX DEVELOPMENT IN THE NORTHERN SNAKE RANGE, NEVADA: INSIGHTS FROM METAMORPHISM

The Southern Snake Range, Northern Snake Range (NSR), and the Deep Creek Range in eastern Nevada and Western Utah constitute one of the longest extensional axes within the Basin and Range province. This series of mountain ranges is comprised of a group of north-south trending metamorphic core complexes displaying up to 300% extension. In the NSR, lower plate metamorphic rocks consist of strongly sheared quartzite and pelitic schists. These rocks preserve a complex metamorphic and deformational history recording Cretaceous (?) crustal thickening (D1) and Tertiary extension and uplift (D2). Early compressional deformation is indicated by discordant S1 foliation evidenced by aligned inclusions in garnet and staurolite. Thermobarometry conducted during this study and by previous workers, indicate peak metamorphic conditions of 600°C and 8.0 to 9.5 kbar, conditions consistent with 30 to 34 km paleodepths. D1/M1 textures and mineral assemblages are overprinted by Tertiary extensional fabrics (D2). These rocks display well developed C-S or C'-S fabrics that resulted from east directed stretching and core complex development. The lowermost exposed metapelite units preserve M1 amphibolite facies mineral assemblages. D2 deformation modified M1 minerals resulting in biotite fish and deformed staurolite porphyroblasts. Textural relationships indicate a second generation of biotite and staurolite growth (M2) related to initiation of extensional deformation. Biotite growing in strain shadows and in D2 cleavage domains indicate syn-extensional growth. Additionally, small, subhedral to euhedral staurolite grains growing parallel to D2 fabrics indicates syn-extensional growth of this mineral. These textural relationships could only develop if extension occurred under amphibolite facies conditions. Application of the Ti-in-Biotite thermometer to biotites in the extensional fabric yielded temperatures in the range of 500 to 560°C, lower than peak metamorphic conditions but consistent with growth under staurolite-grade conditions. Thus, M2 mineral assemblages are interpreted to have grown in the upper most part of a retrograde P-T path, just below peak metamorphic conditions. Importantly, this implies that extension affected rocks to at least 30 km depths.