ISOTOPIC AND GEOCHEMICAL STUDIES OF HIGH-GRADE BLOCKS AND COHERENT METAMORPHIC ROCKS, FRANCISCAN COMPLEX: NEW RESULTS

Geochemistry of lower grade (prehnite-pumpellyite to lawsonite blueschist) coherent Franciscan volcanic rocks suggests that their protoliths were predominantly ocean island basalts (OIB) and mid-ocean ridge basalts (MORB). In contrast, most volcanics of the Coast Range Ophiolite, a unit of unmetamorphosed oceanic crust structurally overlying the Franciscan subduction complex, exhibit island-arc chemistry and may represent the early stages of arc development. Our earlier work on one Franciscan high-grade block locality (Tiburon Peninsula) showed that the protoliths for those blocks were nascent arc basalts with no evidence of interaction with fluids from continentally-derived sediments. We have analyzed samples from the coherent South Fork Mountain schist (SF) (epidote-blueschist), high-grade blocks from multiple localities, and two high-grade coherent localities: amphibolites from Goat Mountain (GM) and Panoche Pass (PP). Our new trace element data suggests either arc or MORB protoliths, but not OIB. We expect to have isotopic data by the time of presentation to enable us to identify the protolith and evaluate the interaction of the samples with continentally-derived material.

Our new data show consistent major element compositions along a calc-alkaline trend despite their diverse spatial relations. The REE patterns of these rocks are relatively flat. The coherent high-grade rocks (GM, PP) clearly show an arc-signature, whereas SF and the high-grade blocks show a distinctive LREE depletion and may be MORB like. All the samples analyzed fall in the range of the old Tiburon data, suggesting an arc protolith for these rocks. Low Ce/Pb, Nb/U; high Ba/Rb, Ba/Th; and high Pb-enrichments are observed. The overall trace element patterns of these rocks are distinctly different from OIBs.

The presence of arc protoliths among Franciscan high-grade rocks demands the presence of a pre-Franciscan subduction zone, if the high-grade metamorphism resulted from inception of subduction or ridge subduction, two alternatives that have been proposed. The possibility that high-grade blocks were metamorphosed in a pre-Franciscan subduction zone and then shed as olistostrome blocks into the Franciscan trench is precluded by field, geochronologic, and petrologic data.