ELEMENTAL MOBILITY IN SUBDUCTION METAMORPHISM: INSIGHT FROM METAMORPHIC ROCKS OF THE FRANCISCAN COMPLEX AND FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA

The degree of element mobility in subduction metamorphism is a subject of debate with some advocating considerable mobility during metamorphism and others advocating near immobility. We assess chemical mobility during subduction metamorphism by examination of the major, trace element concentrations and Pb-, and Nd-isotopic data of thirty-six rocks from the Franciscan subduction complex and related units of coastal California, and three rocks from the Feather River ultramafic belt of the northern Sierra Nevada, California, spanning a wide range of metamorphic grade. We conclude that the metavolcanic rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios with the exception of the few mobile large ion lithophile elements (LILE), such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of the metabasalt we examined occurred in a largely closed system. The lack of LREE enrichment in the rocks demonstrates the lack of chemical exchange with subducted metasediments even though some of our samples were subducted synchronous with underplating of significant amount of clastic sediments in the Franciscan. The relatively low SiO₂ content (< 48 wt %) of many of the metamorphic rocks examined in this study appears to be due to sea-floor hydrothermal alteration prior to subduction. In contrast to the results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert-derived fluids and the overlying mantle. Accordingly our data show that the fluids that interact with the mantle wedge up dip from the region of arc magma genesis may be preferentially derived from metasediments on the slab. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba and Pb rich fluids interacted preferentially with metabasalt closest to the region of depletion.