GEOCHEMICAL VARIATION WITHIN BLOCKS-IN-MELANGE AS POSSIBLE EVIDENCE FOR SEDIMENTARY FORMATION OF A MELANGE EXPOSED AT RING MOUNTAIN, FRANCISCAN COMPLEX, CALIFORNIA

Mélanges consist of blocks within a finer matrix that may record homogenous pressure-temperature (P-T) conditions or a wide range of P-T conditions. Mélanges with isofacial peak P-T conditions recorded in blocks and matrix appear, on the basis of structural relationships, to have formed by progressive deformation. In contrast, mélanges with blocks that record heterogeneous P-T conditions, including those with blocks of higher grade than the matrix, have spawned divergent explanations of block-matrix mixing 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 rocks into matrix by submarine sliding into the trench prior to progressive subduction-accretion. Geochemistry of blocks in serpentinite mélange at Ring Mountain (RM), of the Franciscan subduction complex, coastal California help test these alternatives. Peak metamorphic mineralogy in blocks at RM range from lawsonite albite to garnet amphibolite facies. Previous geochemical study of RM blocks focused on high-grade blocks, whereas we present preliminary data focused on lower grade rocks and matrix. Metamafic blocks have basaltic protoliths with SiO2 ranging from 46 - 49 wt.%. Harker diagrams show little variation between samples with no systematic variation related to metamorphic grade, suggesting that these variations are related to protolith variation instead of metamorphic modification. Most samples have flat or slighly depleted LREE, with one sample that is slighly enriched in LREE. MORB-normalized multielement diagrams show that all samples are depleted in Nb and one sample is enriched in Ba and Pb. These patterns as well as tectonic discrimination diagrams (e.g. La/Yb-Nb/La, Ti-V, Ti-Zr-Y) suggest that two samples have MORB-like protoliths and two samples have arc-like protoliths. The presence of low-grade metavolcanic rocks of arc origin is difficult reconcile with tectonic incorporation from the subduction zone hanging wall, but compatible with submarine landslide transport to the trench from the upper plate. In addition, we will compare these results with geochemical data from the matrix and metagraywacke blocks, to assess material mixing in the matrix and metamorphic element mobility.