USING BIOTITE TEXTURES AND GROWTH TEMPERATURES TO OBTAIN METAMORPHIC P-T PATHS: APPLICATION OF THE TI-IN-BIOTITE THERMOMETER

The Southern Snake Range, Northern Snake Range (NSR), and the Deep Creek Range in eastern Nevada and Western Utah constitute on 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 exposed in the footwall consist of quartzite, quartzite mylonites, and pelitic schists. All of these rocks display well developed C-S or C'-S fabrics that resulted from east-west directed stretching. Biotite textures suggest continuous growth accompanying extension, crustal thinning and uplift. The amount of Ti in biotite has recently been calibrated for use as empirical geothermometer. This offers a potentially powerful tool in tracing the conditions under which biotite growth occurred since the temperatures of biotite grains in different textural relationships can be determined. This study compares Garnet-Biotite temperatures with temperatures obtained using a Ti-in-biotite thermometer for rocks exposed in the Hampton Creek drainage of the NSR. Previous work performed on these exposures yielded temperatures of 610˚C (garnet-biotite thermometry) and pressures of 810 MPa (GASP). In this study, garnet-biotite pairs yielded temperatures of 609˚C, in very good agreement with temperatures obtained by other workers. The Ti-in-biotite thermometer yielded temperatures that can be categorized in two groups; a high temperature group averaging 585˚C, and a low temperature group averaging 505˚C. The high temperature group agrees well with temperatures obtained from garnet-biotite pairs. These were obtained from biotite grains located in garnet strain shadows, and grains that parallel the extensional S foliation direction. The low temperature group was obtained from biotite grains that embay garnet or cross cut the primary foliation, indicating late growth. Thus, careful consideration of textures and biotite growth temperatures can help in deciphering metamorphic P-T paths.