GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 261-1
Presentation Time: 9:00 AM-6:30 PM


SHIELDS, Jessie E. and WAKABAYASHI, John, Department of Earth and Environmental Sciences, California State University, Fresno, CA 93740

Mélanges consist of blocks within a finer matrix that may either record homogenous pressure-temperature (P-T) conditions or a wide range of P-T conditions. Few studies of melange metamorphism have attempted to document the full range of P-T conditions recorded in a single melange. Additionally, most studies focus on higher grade blocks (e.g. eclogite), leaving the lower grade blocks relatively unstudied. To evaluate the range of P-T conditions within a single mélange and inform ideas about melange formation, we chose a 100-m thick serpentinite mélange in the Franciscan Complex, exposed at Ring Mountain, California, where rock types range from greenschist to garnet amphibolite facies. High grade blocks include garnet-amphibolite (hb/NaCa am + ab + grt + ep + qtz + rut ± phen), eclogite (omph + grt + ep + phen + grt ± hb/NaCa am + qtz + rut), and grt-free amphibolites, all of which are variably overprinted with blueschist assemblages of gln + lws ± pu ± ep ± phen. Low grade blocks include metachert (qtz + na-amphibole), metagraywacke (qtz + lws ± jd ± ab; act + ep + chl + ab ± gln), and metavolcanics (lws + qtz ± gln ± jd ± ab ± pu ± relict igneous cpx or am). Mineral composition data obtained by SEM and EMPA suggest a wide range of P-T conditions. Amphiboles record compositions of 3.4-12% Na2O, 5.7-15.15% Al2O and .012-0.91% TiO2. Chlorites record compositions of 7.0-21.97% FeO and 2-24% MgO. Combining this data with thermobarometry as well as mineral assemblages suggests a wide range of P-T conditions. Previous studies have shown that eclogite and amphibolite peak P-T conditions are 2.2 GPa and 550 C and 0.8 GPa and 660 C respectively. The garnet amphibolite in this study records pressures of ~2.2 GPa and temperatures of ~650 C. The low grade blocks in this study also have a range of P-T conditions. Metagraywackes have T conditions ranging from 200 C to 350 C. The low grade metaigneous blocks record P-T conditions of 0.5 GPa and 500 C. These preliminary results on the low grade blocks greatly add to the existing P-T data at Ring Mountain, which was previously obtained via studies of high grade blocks. Remaining thermobarometry will focus on phengite barometry and chlorite thermometry. We will combine this data with whole-rock XRF data in order to construct pseudosection analyses to further quantify the range of P-T trajectories in the blocks.