PETROLOGY OF AN OXIDIZED BLUESCHIST COBBLE FROM THE SAN ONOFRE BRECCIA, CALIFORNIA, USA

The mid-Miocene San Onofre Breccia (SOB) found along the southern California boarderlands contains clasts of several lithologies including high-pressure metamorphic rocks commonly thought to be shed from the Catalina Schist. Sorensen (1985, GSAB, 96, p. 997–106, ) concluded the San Onofre Schist was Fransiscan, although at that time Catalina Subduction was considered to be part of the Fransiscan. 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 Amp(35%) + Ep (15%)+ Grt (13%) + Ph (12%) + Cpx(7%) with minor Qz, Sph, Ap, Chl, Rt, and Zrn. Garnets (~3 mm) are zoned with Alm56Grs27Pyp3Sps14 cores and Alm66Grs25Pyp9 rims. Blue amphibole is intermediate between glaucophane and barroisite. Garnet and epidote porphyroblasts are found within a blue amphibole (intermediate between glaucophane and barroisite) matrix with minor pyroxene. Prismatic epidote is aligned with the foliation. Phengite is present as smaller crystals throughout the matrix and surrounding the garnets. Dark green clinopyroxene Ac20Jd30 CaTs1 is notably rich in Fe3+. Metamorphic conditions were estimated using Equilibrium Assemblage Modeling in the MnNCKFMASHTO system. T-X diagrams were used to estimate the amount of additional O2 in the system. Preliminary results suggest metamorphism at 500°C and 17kbar with the addition of around 0.3 wt% O2to reproduce the observed Fe3+-rich assemblages. Under these conditions, modes for most major minerals are predicted to match those observed in the sample, but with minor chlorite and no rutile. The model predicts the observed assemblage at slightly higher temperatures (~550˚) but with much less Amp and Ep and too much Grt. 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. Ongoing work to retrieve temperatures from Rt, using [Zr], will further inform these models. This rock more closely resembles Franciscan garnet blueschist tectonic blocks from Northern California and Catalina in mineralogy and P-T conditions than younger, lower-grade blueschists from Catalina. Geochronology will further help place this sample in multiple episodes of Californian subduction cycles.