Petrology of an oxidized blueschist cobble from the San Onofre Breccia, California, USA
Bachelor of Arts
F. Zeb Page
Blueschist, San Onofre, Subduction, California, Franciscan, Epidote, Oxidized
The mid-Miocene San Onofre Breccia (SOB) found along the southern California borderlands contains clasts of several lithologies including high-pressure metamorphic rocks commonly thought to be shed from the Catalina Schist. Sorensen concluded the San Onofre Schist was part of the Franciscan Complex, although at that time the Catalina subduction was considered to be part of the Franciscan Complex. In this study, a ~10 cm cobble collected from the San Onofre type locality was studied to describe its mineralogy and estimate its conditions of metamorphism. The cobble is composed of glaucophane (35%) + epidote (15%) + garnet (13%) + phengite (12%) + omphacite (7%), with minor quartz, sphene, apatite, chlorite, rutile, and zircon components. Garnets (~3 mm) are zoned with Alm56Grs27Pyp3Sps14 cores and Alm66Grs25Pyp9 rims. Blue amphibole in the rock is intermediate between glaucophane and barroisite. Garnet and epidote porphyroblasts occur within a blue amphibole (intermediate between glaucophane and barroisite) matrix with minor pyroxene. Prismatic epidote is aligned with the dominant foliation. Phengite is present as smaller crystals throughout the matrix and surrounding the garnets. Dark green clinopyroxene Ac13Jd34Di39Hd11CaTs1 is notably rich in Fe3+. I estimated metamorphic conditions using Equilibrium Assemblage Modeling in the MnNCKFMASHTO system. T-X and P-X diagrams were used to estimate the amount of additional O2 in the system. Results suggest metamorphism at 520°C and 16kbar with the addition of around 0.3 wt% O2to reproduce the observed Fe3+-rich assemblages. Under these conditions, calculated modes for major minerals match those observed in the sample. A potential source of error includes uncertainty in Fe3+-bearing solution models, which predict more Fe3+ in Amp than in Cpx, the opposite of what is observed. This rock more closely resembles Franciscan garnet blueschist tectonic blocks from Northern California and Catalina in mineralogy and pressure- temperature (P-T) conditions than younger, lower-grade blueschists from Catalina. Geochronology will further help place this sample in multiple episodes of Californian subduction cycles.
Helm, Alaina A., "Petrology of an oxidized blueschist cobble from the San Onofre Breccia, California, USA" (2021). Honors Papers. 834.