SUPRA-SUBDUCTION ZONE PROTOLITH SIGNATURES IN METAMORPHIC SOLES, INITIATION OF SUBDUCTION, AND MODELS OF OPHIOLITE GENERATION AND EMPLACEMENT

Most extensive ophiolites (on-land remnants of oceanic lithosphere) apparently formed in a supra-subduction zone (SSZ) setting. Thin sheets of high-temperature/high-pressure metamorphic rocks (metamorphic soles), crop out structurally beneath many SSZ ophiolites. Such rocks may have formed at the inception of subduction within young, hot oceanic lithosphere. Researchers dispute whether SSZ ophiolites were emplaced over the same subduction zone over which they formed or over a later one. High-grade metamorphic rocks (blocks-in-mélange and coherent sheets) of the Franciscan Complex, coastal California, yield the oldest metamorphic ages in the subduction complex. Their metamorphism, age, field relationships (for coherent rocks), and lithologies, suggest that they represent a dismembered metamorphic sole beneath the SSZ Coast Range ophiolite (CRO). Major, trace element, and isotopic data suggest that these metamorphic rocks formed in an SSZ environment, whereas later-subducted, lower-grade coherent metabasalts are of MORB or OIB affinities. Rocks from the extensive, intact metamorphic sole of the Feather River ultramafic belt of the northern Sierra Nevada, California, show both MORB and SSZ protoliths. SSZ protoliths have been recognized in metamorphic soles beneath some of the Tethyan ophiolites, as well as beneath ophiolites in the southwestern Pacific. SSZ protoliths of metamorphic rocks formed at the initiation of subduction require the existence of an earlier subduction zone over which the SSZ protoliths formed. In the coastal California example, the CRO and Franciscan high-grade rock protoliths formed over a pre-Franciscan subduction zone that may have dipped west, on the basis of (1) permissible suture positions for the older subduction zone to the east in the Sierra Nevada and (2) the lack of evidence for a pre-Franciscan east-dipping subduction zone west of the Franciscan. High-grade Franciscan rocks were metamorphosed at the inception of east-dipping subduction beneath the CRO, and the ophiolite was subsequently emplaced over this later subduction zone. This dual subduction zone model with polarity reversal may be a common mechanism linking SSZ ophiolite formation, subduction initiation, and ophiolite emplacement.