Some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites, as validated by the geochemical data presented from high-grade-metamorphic tectonic blocks from the Franciscan Complex in central California. Combining major elements, compatible and incompatible trace element concentrations and ratios, and Sr-Nd-Pb isotopic data, we conclude that the protoliths of these high-grade Franciscan 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 our geochemical study. Our geochemical data are significant in that (1) the protoliths of these high-grade blocks have a different mode of origin than the protoliths of the lower-grade Franciscan basalts that were incorporated later in the subduction complex, as the latter generally are considered to be oceanic-island basalts (OIB) and derived from mid-oceanic-ridge basalt (MORB); and (2) the protoliths of the high-grade blocks were formed over a pre-Franciscan subduction zone.
Based on the geochemical evidence for an arc origin and the available geochronological data, we suggest that protoliths of the high-grade tectonic blocks of the Franciscan Complex and the Coast Range Ophiolite formed in the same infant-arc setting. Initiation of Franciscan subduction in this arc crust led to the formation of the high-grade blocks beneath the young Coast Range Ophiolite. Continued subduction resulted in subduction of older oceanic crust of MORB and OIB origin, some of which was scraped off to form lower-grade metabasites within the Franciscan Complex.