EVALUATING A POTENTIAL CONNECTION BETWEEN THE LATE JURASSIC-EARLY CRETACEOUS OSA CREEK RING COMPLEX AND THE BLACKROCK ANDESITE, SOUTHERN SIERRA NEVADA, CA

Recent exploration along the western edge of the Kern Plateau (Tulare Co.; 36.17 degrees N lat.) revealed a hornblende andesite buttressed against plutonic rocks south of Blackrock Mountain. Neogene lavas and cones decorate this part of the Kern Plateau, but the presence of biotite granite dikes crosscutting the Blackrock andesite supports the intriguing possibility that this non-metamorphosed volcanic rock is instead Mesozoic in age. We first hypothesize that the Osa Creek ring complex (OCRC), located 2.5 km to the west, reveals the shallow footprint of a stratovolcano and evaluate this magmatic center as a possible source for the Blackrock volcanic deposit. Interfingering arcuate dikes of biotite leucogranite and biotite-hornblende diorite, with rare pyroxene-hornblende gabbro, comprise the ring complex. Mineralogy and whole-rock geochemistry are broadly consistent with a comagmatic origin for the ring complex and the andesite, which contains comingled basalt pods. U-Pb LA-ICPMS dating of zircon separated from a biotite granite dike cross-cutting the andesite yields a latest Jurassic to earliest Cretaceous minimum age for the hornblende andesite (mean U-Pb date = 161.1 ± 3.4 Ma; youngest population = 146.1 ± 2.3 Ma). In places, cuspate contacts separate the granite dikes from the andesite, suggesting the granite dikes are coeval with the surrounding andesite. Zircon crystals separated from a hypabyssal-textured OCRC granite yield Early Cretaceous dates (mean U-Pb date = 147.6 ± 0.7 Ma; youngest population = 137.7 ± 2.0 Ma). Core-rim zircon dates document growth of individual, zoned zircon crystals over at least 4 m.y. and support our preferred interpretation that the youngest U-Pb zircon age population provides the best constraint of emplacement age. If correct, the Osa Creek ring complex is a rare exposed product of Early Cretaceous arc magmatism, and is ca. 10 m.y. younger than the Blackrock andesite. Similar reasoning would indicate that the Blackrock andesite is coeval with the hornblende-rich Summit Gabbro, which crops out across the Kern Plateau, and with the regionally extensive Independence dike swarm. Future geochemical and geochronologic studies of plutonic rocks on the Kern Plateau are aimed at revealing details of Sierra Nevada arc evolution across the Jurassic-Cretaceous boundary.