BIMODAL MAGMATISM IN THE LATE JURASSIC SIERRA NEVADA CONTRACTIONAL ARC, CALIFORNIA

Late Jurassic volcanic and plutonic rocks of the Sierra Nevada continental arc are exposed along the East Sierran thrust system from north of the Garlock fault into the Mojave Desert. The composition of the Late Jurassic Sierran rocks reflects bimodal magmatism from alaskite to gabbro and rhyolite to basalt. Late Jurassic plutonic bodies occur in stock-sized bodies and are commonly cross-cut by widespread and roughly coeval mafic to bimodal Independence dike swarm (IDS). The volcanic rocks are primarily felsic, although some mafic flows are present. The volcanic rocks and intrusive bodies are locally highly deformed in by the Eastern Sierra thrust system (ESTS). Deformed volcanic rocks typically form the footwall of thrust faults and shear zones in the ESTS. Contractional structures are ENE-directed north of the Garlock fault and bend to ESE or SE-directed in the Mojave Desert. The IDS bodies are generally undeformed and locally cross-cut the contractional fabrics. They appear to represent the last intrusive event recorded in the Late Jurassic arc.

To better understand the timing and duration of magmatism and contractional deformation, we have dated many intrusions using CA-TIMS U-Pb zircon geochronology. Previous dates clustered around 150 Ma but lacked the precision now available in the U-Pb method. Quartz monzonite and ignimbrite in the Panamint Range reveal that the coeval felsic volcanism and plutonism were greatest at ca. 151 Ma in this area. Similar rocks in the western and southern parts of the Owlshead Mountains and Slate Range date to ca. 149. In the Mojave Desert south of the Garlock fault, dates from the Cronese Hills, Soda Mountains, Cruecero Hills, and Cave Mountain span the range from 147 to 152 Ma. These ages are similar regardless of the composition of the rocks data. Our field and petrography studies combining with geochronology data support late Jurassic syntectonic bimodal magmatism in a highly contractional setting of the late Jurassic Sierran arc.