Paper No. 27-11
Presentation Time: 9:00 AM-1:00 PM
PROTEROZOIC TO EARLY PALEOZOIC METAMORPHIC REMNANTS OF MAFIC VOLCANISM, SAN FELIPE, BAJA CALIFORNIA
Prebatholihtic rocks of Baja California consist of schists, quartzites and carbonates that are preserved in hanging roofs associated with Laramide intrusives. One of these suites is exposed in Sierra Abandonada, near San Felipe, and includes a mafic schist-dominated succession of possible Proterozoic to Paleozoic age that may provide insights on rift-related successions elsewhere in the Cordillera. The focus of this investigation is to characterize the field relations, petrology, and geochemistry of these rocks to test the hypothesis that they are genetically related to mafic volcanic-dominated successions to the east, such as the Cerro Rajon Formation of Sonora. There appear to be three groups of schists/granofels in the succession. The first group is represented by a quartz dominant protolith with continuous to spatial schistosity dominated by biotite. This group could be formed from sedimentary protoliths. The second group is higher in density and richer in Fe-Mg minerals such as hornblende and biotite. The protolith of this group may be basaltic; samples exhibit weak to no foliation, and its texture can be compared to a lava, with some samples preserving pseudomorphs of what could have been pyroxenes. The third group of schists is less abundant, and exhibits features characteristic of intrusive origins. Samples exhibit low to mid-grade metamorphism including a chlorite-actinolite-hornblende metamorphic mineral paragenesis. Preliminary geochemistry show high TiO2 values and exhibit features consistent with an alkaline basalt protolith; they also bear geochemical signatures that could be consistent with formation during an early rifting stage such as OIB spectrums in the Spider diagrams and petrogenetics related to within plane alkaline basalts. If these rocks are correlative with early Cambrian volcanics of Sonora, it is possible that this region was dissected by Tertiary extension and that a piece of the Cerro Rajón volcanic field ended up in the western margin of the California Gulf. This hypothesis would also tie this volcanism with the western Laurentia margin second rifting process during Cambrian-Precambrian age.