RECONSTRUCTING THE RECORD OF JURASSIC ARC MAGMATISM, KERN PLATEAU, SOUTHERN SIERRA NEVADA, CA

Though the record of pre-Late Cretaceous Sierra Nevada arc magmatism is fragmented and largely obscured, a relatively complete record of Jurassic arc magmatism is exposed on the Kern Plateau (~36°N latitude). Completeness is gauged by comparison to the detrital zircon record preserved by Early Cretaceous forearc and intra-arc strata deposited at roughly the same latitude. Detrital zircon U-Pb dates define Jurassic age peaks that seemingly record magmatic pulses: Late Jurassic-Early Cretaceous (144-156 Ma; mean 150 ± 0.6 Ma); Middle Jurassic (157-172 Ma; mean 163 ± 1 Ma); and late Early Jurassic (174-191 Ma; mean 180 ± 3 Ma). New and published U-Pb data indicate that the youngest and oldest of these pulses are represented on the Kern Plateau by small pods of the Summit gabbro and by a large, intermediate to felsic intrusion mapped as the Sacatar granodiorite, respectively. A single granite pluton, previously mapped as Cretaceous, yields a 163 ± 4 Ma zircon age, supporting the possibility that even the Middle Jurassic magmatic pulse is recorded on the Kern Plateau.

The Sacatar granodiorite varies from a biotite-hornblende quartz diorite to a potassium-feldspar-porphyritic biotite granodiorite. Whole-rock compositions within the high-K calc-alkaline Sacatar granodiorite suite vary smoothly from 51 to 73 wt. % SiO₂. In contrast, mineral and chemical compositions of the Summit gabbro are fairly uniform; low Mg-numbers (53 to 41) of these hornblende gabbros suggest derivation from a chemically evolved mafic parental magma. Whole-rock Sr_i values distinguish the Early and Late Jurassic suites: values of 0.7073 ± 1 (n=5) characterize the ~180 Ma Sacatar granodiorite, whereas Sr_i values for the ~150 Ma Summit gabbro are uniformly lower (0.7065 ± 2, n=10). Rare olivine-plagioclase-pyroxene cumulate rocks associated with the Summit gabbro have less evolved compositions (Mg-number ~73); these rocks may be products of fractional crystallization of magmas more primitive than the Summit gabbro. Future mapping, geochronologic and geochemical studies on the Kern Plateau will focus on reconstructing the Jurassic arc record, including the character of mantle-derived magmas and possible deep-crustal differentiation.