Abstract
The Central Belt of the Sierra Nevada, California, is a belt of ophiolitic, arc, and subduction complex rocks assembled during Triassic-Jurassic subduction along the Cordilleran margin. One of the best exposures of the ophiolitic section is at the Jarbo Gap Ophiolite, located north of Lake Oroville, California. There, previous studies of Central Belt rocks in the Jarbo Gap area documented a 10-km2 section of mafic, ultramafic, and minor pelagic sedimentary rocks and mafic amphibolite that have been interpreted as an ophiolitic mélange. However, the presence of rutile in some of the amphibolite-facies mafic rocks indicate the presence of an upper-plate ophiolite underlain by a high-temperature lower-plate subduction complex.
Typical subduction complexes are metamorphosed in the low-temperature blueschist- and eclogite-facies, making the high-temperature subducted rock at Jarbo Gap unusual. Field observations rule out high-temperature metamorphism due to plutonic intrusions and regional amphibolite-facies metamorphism. Two commonly proposed tectonic mechanisms that can explain the high-temperature subduction complex rocks structurally underlying an ophiolite are subduction initiation or the subduction of a spreading ridge. To determine which mechanism is responsible, this thesis utilizes whole rock and trace element geochemistry, petrography, U-Pb zircon geochronology, and field observations to identify and characterize the upper plate and lower plate and quantitative and semi-quantitative thermobarometry to identify the peak pressure and temperature of the amphibolite.
The upper-plate ophiolite consists of primarily ultramafic rock and metasedimentary cover with limited exposure of metavolcanics of supra-subduction zone origin and cross-cutting dikes with adakite geochemistry. The ophiolite underwent peak metamorphism in the greenschist-facies after the intrusion of the dikes, though a higher grade of metamorphism has affected all units in the northern part of the study area. Thermobarometry in the lower-plate mafic amphibolite constrains pressure-temperature conditions to 1.2-2.2 GPa and a minimum of 570 °C. Ilmenite overgrowing rutile and partial-melt textures indicate an increase in temperature suggesting a clockwise P-T-t path. U-Pb zircon geochronology, combined with petrography and geochemistry, indicate limited arc magmatism on the ophiolite at approximately 204-197 Ma. The subduction of a spreading ridge is the preferred hypothesis to explain the genesis of adakitic magmatism and the clockwise P-T-t path in the amphibolite at Jarbo Gap. The Jarbo Gap Ophiolite is coeval with other ophiolites in the Central Belt where high-pressure amphibolite has been found, indicating the presence of a > 250-km long east-dipping subduction zone in the late Triassic to early Jurassic which may have terminated with the subduction of a spreading ridge.