Petrology of the San Simeon Coast Range Ophiolite, San Simeon, California

A remnant of the middle Jurassic (165 Ma) Coast Range ophiolite is present near San Simeon, San Luis Obispo County, California. The ophiolite is exposed along 5.5 km of fresh, wave-cut coastline between Ragged Point and Piedras Blancas Point. The San Simeon ophiolite (SSO) has been correlated with a similar body at Point Sal. For this study, 209 samples were examined petrographically.

Ragged Point (~0.5 km length of exposure) has abundant ultramafic rocks including serpentinized dunite +/- chromite with pyroxenite and intrusive gabbroic and dioritic sheets. Breaker Point (~1.5 km of exposure) has the most continuous outcrop ophiolite with serpentinized dunite +/- chromite, wehrlite, olivine clinopyroxenite, clinopyroxenite, websterite, troctolite, gabbro, and gabbronorite. Point Sierra Nevada (~2.0 km of exposure) has cumulus gabbro in fault contact with sheeted dike/sill complex. The cumulus gabbro is cross-cut by swarms of 3-25 cm diabase dikes. Point Piedras Blancas (~1.0 km of exposure) has volcanics including pillow and breccia flow faulted against layered chert.

Ten representative samples were selected for electron probe microanalysis. The primary plutonic sequence of the SSO ranges from: dunite Fo91, troctolite Fo89 An98-90, olivine gabbro Fo77 An85-80, hornblende gabbro An73-55. The crystallization and fractionation sequence is as expected beneath ocean ridges. However, the presence of intrusions of wehrlite, clinopyroxenite, and microdiorite into cumulate ultramafic and mafic rocks indicate significant late-stage or off-axis magmatism. The ophiolite complex at San Simeon is very similar to that at Point Sal.