ARC ORIGIN OF FRANCISCAN HIGH-GRADE METAMOPRPHIC ROCKS CONSISTENT WITH TECTONIC MODEL OF MOORES (1970)

Major elements, compatible, incompatible trace element concentrations and ratios, and Sr-Nd-Pb isotopic data suggest that the protoliths of high-grade Franciscan tectonic blocks were arc lavas with no continental crust-derived components. Although the protoliths underwent high-grade metamorphism at the inception of subduction, the tectonic affinity of the protoliths can be distinguished by their geochemical fingerprints. Our geochemical data indicate that high-grade blocks have a different origin than lower-grade Franciscan basalts of OIB and MORB-derivation that were incorporated in the subduction complex later. Because the high-grade blocks apparently were metamorphosed at the inception of Franciscan subduction, their protoliths must have been formed over a pre-Franciscan subduction zone. We suggest that the pre-Franciscan subduction zone dipped west because there is no evidence of pre-Franciscan subduction west of the Francisan and there is evidence for a subduction suture or sutures of potentially appropriate age and/or polarity in the Sierra Nevada and/or beneath the Great Valley. Based on the geochemical evidence for an arc origin and the available geochronological data, we suggest that protoliths of the high-grade blocks of the Franciscan Complex and the Coast Range Ophiolite formed in the same infant arc setting over a west dipping subduction zone. The collision of the continental margin stopped this subduction and resulted in a subduction polarity flip with east-dipping Franciscan subduction beginning metamorphism beneath the young Coast Range ophiolite. A similar model was first proposed by Moores in 1970. As Franciscan subduction initiated, high-grade metamorphism of the high-grade blocks took place. Continued subduction resulted in refrigeration of the subduction zone overprinting of the high-grade blocks blueschist facies assemblages, and eventually led to the subduction of older MORB and OIB crust, some of which were offscraped to form the younger and lower grade Franciscan volcanic rocks. The evidence of an arc origin for sub-ophiolitic high-grade rocks of the Franciscan may be important for interpreting the initiation of subduction and ophiolite formation in other orogenic belts.