CONSTRAINING THE SOURCE OF THE OROCOPIA SCHIST, SE CALIFORNIA: A COMPARATIVE STUDY OF THE MAJOR AND TRACE ELEMENT CHEMISTRY OF CRETACEOUS SANDSTONES DEPOSITED IN FOREARC SETTINGS

The Orocopia Schist, a quartzofeldspathic Late Cretaceous – early Tertiary metagraywacke of oceanic origins crops out along the San Andreas fault system and within the core of the Chocolate Mountains anticlinorium. The marine origin of the Orocopia Schist has led to numerous models for its emplacement, most involving subduction of sediments of either the Franciscan Complex or Great Valley sequence along the low angle trajectory of the Farallon plate. In order to evaluate the source, composition, and protolith of the Orocopia Schist we collected chemical data from the literature and analyzed 29 samples of the Orocopia from Picacho State Recreation Area, SE California, and 6 samples of the Point Loma Fm collected south of La Jolla Beach, California. On a wide variety of major and trace element tectonic discrimination diagrams most samples analyzed from the Orocopia Schist consistently plot in the active continental margin field or spread from this field into the adjacent continental margin island arc field. These results indicate that the protolith of the Orocopia Schist was deposited in an active continental margin setting lying adjacent to a continental margin island arc. The average composition of sandstones from the Franciscan Complex, the Great Valley sequence, and the Point Loma Fm change in a north to south direction with samples from the Great Valley sequence in the Sacramento Valley forming one end member to the north and samples from the Point Loma Formation another to the south. Within this spectrum of compositions, on the pre-Neogene reconstruction of J. Saleeby, samples from the San Joaquin Valley lie just to the north and samples from the Point Loma Fm lie just to the south of the breached portion of the western Cordilleran margin. Most data from the Point Loma Fm fall within the compositional field defined by samples analyzed from the Orocopia Schist on plots of TiO₂ versus Fe₂O₃ + MgO and La versus Th while data from the San Joaquin Valley do so less frequently. This observation suggests that the source of the Orocopia Schist was probably located somewhere within the breached portion of the Cordilleran margin, and probably somewhere along its southern half. Hence, the pre-Neogene reconstruction of J. Saleeby offers a plausible tectonic framework in which to view the otherwise problematic origins of the Orocopia Schist.