NEW ZIRCON U-PB AND (U-TH)/HE AGES OF MIOCENE RHYOLITE FROM THE KERN VOLCANIC FIELD; KERN PLATEAU, SOUTHERN SIERRA NEVADA, CALIFORNIA

Regional correlations, petrology, and geochemistry suggest a newly recognized outcrop of hornblende rhyolite ash flow tuff located in the Domeland region of the Kern Plateau (southern Sierra Nevada, CA) is a member of the Miocene Kern volcanic field (Farmer et al., 2002). New zircon U-Pb and (U-Th)/He data reveal the timing of Miocene melt production and eruption, provide insight regarding the degree and source of Miocene crustal contamination, and constrain the location of the volcanic edifice.

A total of 45 zircon were separated from the leucite-bearing rhyolite and U-Pb ratios were measured via LA-ICP-MS. Of these grains, 38 yield concordant Late Cretaceous ages between ~122 and ~78 Ma, with a weighted mean average of 92.6 ± 2 Ma; the remaining five zircon form a coherent age population and yield a U-Pb age of 11.5 ± 0.5 Ma. A zircon (U-Th)/He cooling age of 11.2 ± 1.2 Ma (2SE) was measured based on the average of four zircon, including both Cretaceous- and Miocene-aged grains. The zircon cooling age reflects the eruption age of the rhyolite, whereas the U-Pb age generally reflects the timing of active Kern volcanic field magmatism. Previously reported ⁴⁰K/⁴⁰Ar ages for basaltic to andesitic samples from the Miocene Kern volcanic field lavas range from ~10.6 to ~12.8 Ma (Bergquist and Diggles, 1986; Marvin and Cole, 1978), consistent with U-Pb and (U-Th)/He data of the associated rhyolite.

The 38 Late Cretaceous zircon present in this Miocene ash flow are xenocrysts recording assimilation of Mesozoic arc plutons within the crustal column. Laser ablation U-Pb analysis of zircons from five plutons in the immediate vicinity of the Miocene rhyolite yield ages ranging from 100.7 ± 1 to 92.3 ± 1 Ma; when combined, all analyzed zircons from the five plutons have a weighted average age of 94.8 ± 1 Ma (n=341 grains). The analyzed zircon from the Domeland plutons are a statistical match for the population of Cretaceous xenocrystic zircon from the Miocene rhyolite. Overlap between the U-Pb ages of xenocrysts and of zircon from Domeland plutons indicate the Miocene rhyolite assimilated the local plutons, and suggest the Kern volcanic field vent was located within the greater Kern Plateau and not in the adjacent Basin and Range province. These findings have implications for detrital zircon studies of Basin and Range and regional Cenozoic basin strata.