Geology of the Jurassic Basement Rocks, Santa Cruz Island, California, and Correlation with Other Mesozoic Basement Terranes in California

Abstract Jurassic plutonic and metamorphic rocks are exposed on Santa Cruz Island and comprise three mappable units: (1) Santa Cruz Island Schist (2) Willows Plutonic Complex and (3) Alamos Pluton. The Santa Cruz Island Schist consists chiefly of volcanic and metavolcanic rocks (keratophyre, quartz keratophyre, and spilite), and less abundant meta-sedimentary rocks (metachert, tuffaceous metasandstone, metapelite, and gabbroic metasandstone), metamorphosed hypabyssal rocks (meta-microdiorite and meta-microleucotonalite), and fault slivers of metaplutonic rocks including altered gabbro and hornblendite. The schist contains greenschist facies mineral assemblages including chlorite, epidote, artd albite. A variation in the development of metamorphic textures exists ranging from no schistosity with relict volcanic or sedimentary textures to incipient or locally well-developed schistosity that is folded in places. Meta-hypabyssal rocks within the Santa Cruz Island Schist are interpreted as intrusions from the Willows Plutonic Complex. The Willows Plutonic Complex is composed mainly of hornblende diorite and hornblende quartz diorite. It contains less abundant gabbro and minor amounts of leucotonalite and ultramafic rocks. Most ultramafic rocks (clinopyroxenite, hornblendite, and peridotite) occur along the fault contact between the Santa Cruz Island Schist and the Willows Complex. Some gabbro and ultramafic rocks show possible cumulate textures. The Alamos Pluton, a body of leucotonalite, lies within and intrudes the Santa Cruz Island Schist; similar leucotonalite intrudes the Willows Complex. The relatively young radiometric age of the Alamos Pluton suggests that it maybe genetically unrelated to the other two units (Mattinson and Hill, this volume). The Santa Cruz Island Schist formed during volcanism associated with intermittent sedimentation. The volcanic strata were intruded by the Willows pluton(s), and then by the Alamos Pluton. Metamorphism of the basement complex, which may have begun during a period of igneous intrusion, continued after the two periods of intrusion. Radiometric ages indicate that plutonic crystallization and metamorphism occurred during Late Jurassic time, and metamorphism may have continued into Cretaceous time. The Santa Cruz Island basement complex resembles the following three Mesozoic basement terranes exposed in California: (1) the upper part (the diorite and volcanic rocks) of the Point Sal ophiolite (Hopson and others, 1975), (2) the Eagle Rest Peak gabbro and metavolcanic rocks, exposed along the San Andreas Fault in the San Emigdio Mountains (Hammond, 1958; Ross, 1970), and (3) a gabbroic body on Catalina Island (Platt, 1973). It is concluded that the Santa Cruz Island basement complex represents a weakly metamorphosed segment of Late Jurassic oceanic crust. Southern California basement terranes that are dissimilar to the Santa Cruz Island basement complex in petrology and age are the metamorphic rocks and granitic intrusions of the Salinian block, the Santa Monica Mountains, and the Peninsular Ranges; the Franciscan eugeosynclinal complex exposed in the California Coast Ranges, locally in the Transverse Ranges, and on Catalina Island; and the Pelona Schist which occurs in patches along the San Andreas and Garlock faults.