QUANTIFYING THE RANGE OF P-T TRAJECTORIES IN A SINGLE MELANGE: INSIGHTS FROM MINERAL ASSEMBLAGES AND CHEMISTRY

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. Studies of mélange metamorphism of the latter have typically focused on the highest grade blocks (eclogite and amphibolite), leaving the full range of metamorphic P-T trajectories poorly defined. To evaluate the range of P-T trajectories within a single mélange and inform ideas about melange formation, we chose the ~100-m-thick serpentinite mélange of the Franciscan Complex, exposed at Ring Mountain, California, where petrography compared to published P-T estimates on eclogites suggest a wide range of P-T paths. High grade blocks there 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), and metavolcanics (lws + qtz ± gln ± jd ± ab ± pu ± relict igneous cpx or am). Preliminary chlorite and amphibole mineral composition data obtained by SEM suggest a wide range of P-T trajectories. Amphiboles record compositions of 3.4-12% NaO and 5.7-15.15% Al2O. Chlorites record compositions of 7.0-21.97% FeO and 2-24% MgO. We will combine this mineral compositional data with whole-rock XRF data in order to apply classical thermobarometry and pseudosection analyses to quantify the range of P-T trajectories in the blocks. Additionally, we will obtain ICP-MS trace element data to further evaluate the protolith origins of metaigneous blocks.