GEOTHERMOMETRY OF PELITIC GNEISS IN THE LITTLE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA

Quartzofeldspathic and biotite gneisses, locally containing abundant garnet, are exposed between Mesozoic plutonic rock bodies in the Quail Mountain area of the Little San Bernardino Mountains. Garnet-bearing gneisses were collected with the objective to analyze the Fe-Mg distribution between garnets and biotites in metapelitic rocks in order to obtain temperature estimates and determine metamorphic conditions. Mineral phases present in metapelitic rocks include quartz + plagioclase + garnet + biotite + fibrolite + prismatic sillimanite + cordierite + zicon. Quartzofeldspathic rocks have the assemblage amphibole + quartz + biotite + plagioclase. These two mineral assemblages are consistent with the upper amphibolite to granulite facies. Mineral chemistry was determined through electron microprobe (EMP) analysis. For garnets, the almandine end-member composed 79 to 86% of the analyzed garnets. The pyrope end-member made up 8 to 15 % of the overall composition of the garnets. Diffusion zoning is present around the edges of the garnets. In biotite, total Fe and Mg concentrations range from 2.4 to 3.4 % and from 1.6 to 2.3 %, respectively. Several garnet-biotite geothermometers were applied to these compositions to determine the temperature conditions of metamorphism. The maximum temperature determined was 670^oC + 30^oC. Lower edge temperatures, which are 590^oC + 25^oC, are inferred to represent continued diffusion during retrograde cooling and exhumation of these gneisses. Because of the mineral assemblages include the key phases cordierite, plagioclase, garnet, and zircon, it will be possible to determine the age and the pressure during metamorphism. If these gneisses were metamorphosed due to the intrusion of the Mesozoic plutonic rock bodies, this would predict a Jurassic to Cretaceous metamorphic age at pressures equivalent to those determined using hornblende + plagioclase assemblages in the plutons.