SIGNIFICANCE OF BIOTITE TEXTURES IN AMPHIBOLITE FACIES METAPELITES: DEEP EXTENSION IN THE NORTHERN SNAKE RANGE, NEVADA

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, metamorphic rocks in the lower plate consist of quartzite, quartzite mylonites, and pelitic schists. All of these rocks display well developed C'-S fabrics that resulted from east-west directed stretching. In the Hendry's Creek drainage, one of the lowermost exposed metapelite units consists of a staurolite-garnet-biotite schist. Staurolite occurs as large (8 cm) porphyroblasts aligned parallel to the primary foliation. Garnet occurs as smaller (to 0.5 cm) porphyroblasts. Both minerals contain elongated quartz and opaque (ilmenite) inclusions defining an older, f1 foliation discordant to the primary, f2 foliation defined by micas growing parallel to exterior S-surfaces. Staurolite is frequently rimmed by biotite-free zones consistent with staurolite growth at the expense of biotite. Textural relationships suggest syn-kinematic growth of biotite and garnet, and the presence of biotite in C' shear bands show biotite growth and extensional shearing to be contemporaneous. Previous work has shown that similar kyanite bearing rocks from the nearby Hampton Creek drainage record temperatures of 610±50°C, and pressures of 810±70 MPa, corresponding to mid-crustal depths of 30 km. New garnet-biotite and Ti-in-biotite temperatures for rocks from the Hendry’s Creek drainage yield temperatures ranging from 550°C to 630°C. Assuming similar pressures, the textural relationships in the Hendry's Creek exposures suggest that extension affected rocks to 30 km depths.