LARGE RANGE OF METAMORPHIC CONDITIONS RECORDED IN BLOCKS OF A SINGLE MéLANGE ZONE: AN EXAMPLE FROM THE FRANCISCAN COMPLEX OF CALIFORNIA

Mélanges consist of blocks within a finer matrix. The blocks 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 focused on the highest grade blocks, leaving the range of metamorphic P-T trajectories poorly defined. Mélanges with consistent P-T conditions recorded in blocks and matrix commonly appear, on the basis of structural relationships, to have formed via progressive deformation. However, divergent hypotheses have been presented for melanges with blocks that record heterogeneous P-T conditions and those with blocks of higher grade than the matrix, including (1) mixing of blocks into a matrix by large-scale tectonic movement in the subduction channel and 2) mixing of blocks from previously exhumed terranes into matrix by submarine sliding into the trench, followed by tectonic deformation during subduction-accretion. 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). We will obtain mineral compositional data by EMPA and 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.