INVESTIGATING AMPHIBOLE COMPOSITIONS FROM HIGH-GRADE AMPHIBOLITE IN THE BEAR MOUNTAIN HIGH STRAIN ZONE

The Bear Mountain High Strain Zone is part of the larger Bear Mountain Fault Zone that runs 300km along the Sierra Nevada Foothills, trending NNW-SSE. Within a section of the high strain zone near Auburn, CA, a high-grade hornblende amphibolite block with an ⁴⁰Ar/³⁹Ar cooling age of 195.9+/-2.4 Ma is juxtaposed with surrounding greenschist facies serpentinite-chlorite matrix mélange, mafic, and metasedimentary blocks. To assess the possible origins of the block and determine temperature-pressure conditions associated with the high-grade metamorphism, samples were collected along a NE-SW transect through the block. Samples from the block contain a common mineral assemblage of amp+plg+epi+qtz+chl+spn with texture progressing from fine-grained and strongly foliated to locally mylonitic upon approaching the SW margin. A section in the NE is coarse-grained with no foliation. Major element compositions of amphiboles collected with the electron microprobe reveal that the NE portion of the block contains Mg-rich compositions while the SW section of the block is composed of Fe-rich amphiboles. Actinolite is absent in the block, making in distinct from the surrounding greenschists facies blocks. Comparison of cation formula proportions [Al(VI)+Fe³⁺+Ti+Cr vs Al(IV) and Na^M4 vs Na+K] from electron microprobe analyses of amphibole were used to define pressure ranges. Amphiboles correspond to medium pressure, but those in the NE section span a range in Al content. The range of textures and bulk compositions inferred through mineral assemblages suggest that the block is composed of multiple mafic protoliths that experienced the same amphibolite metamorphic facies event. Two sills, which lack internal deformation, intruded the block at midsection and were sampled to assist in constraining the relative timing of deformation. One enriched in cpx shows primary igneous textures with actinolite rims around cpx cores, while the other contains biotite and Mg-rich amphibole. Both sills intruded post high-grade metamorphism. The cpx-actinolite bearing sill intruded before or during the greenschist metamorphic event that affected the surrounding mélange blocks, and the biotite-bearing sill intruded last, following metamorphism.