PETROLOGIC CHARACTERIZATION OF MAFIC SCHIST PROTOLITHS, BAJA CALIFORNIA, MÉXICO: TESTING HYPOTHESIZED RELATIONSHIP TO CAMBRIAN VOLCANISM OF SONORA

Prebatholihtic rocks in the San Felipe region, Baja California include a metasedimentary succession of marble, quartzite, meta-argillite and mafic to pelitic schist. Protoliths for these rocks are hypothesized to be Paleozoic in age. Tertiary extensional processes have imprinted a schistose fabric in most of these rocks, and the schists bear variable amounts of chlorite-plagioclase-biotite-hornblende yielding different rock densities. Previous workers hypothesized that the San Felipe metasedimentary succession correlates with the Wood Canyon Fm and Zabriske Quartzite of the Death Valley region and/or the Cerro Rajon Fm, Puerto Blanco Fm and Proveedora Quartzite of the Caborca region. This study seeks to test this hypothesis through petrographic and geochemical comparison of rocks from these regions. Preliminary results suggest at least two groups of schists are present. The first group is represented by a quartz-feldspathic protolith with continuous to spatial schistosity dominated by biotite. This group could be formed from detrital protoliths. The second group is higher in density and richer in Fe-Mg minerals such as hornblende and biotite. Samples exhibit low to mid metamorphic grades, represented by a chlorite-actinolite-hornblende metamorphic mineral paragenesis. Geochemical results show high TiO2 values and the major oxides correspond with known signatures of alkaline basalts. If the San Felipe succession is coeval with and genetically related to Cambrian volcanic units in the Caborca region, it could imply that the Cerro Rajón volcanic event area was dissected by a Tertiary extension event and that event displaced a piece of the volcanic field along the western margin of the California Gulf.