THE TIMING AND TEMPO OF QUATERNARY RHYOLITE DOME CONSTRUCTION AT THE COSO VOLCANIC FIELD

The Coso volcanic field (CVF) is one of a series of volcanic centers in eastern California active during the Pleistocene. The Pleistocene CVF is compositionally bimodal, with roughly contemporaneous basaltic and rhyolitic magmatism throughout its ~ 1 Ma history. The silicic portion of the CVF is comprised of 38 crystal-poor rhyolite (~ 77% SiO₂) domes, and although sporadically active, the volumetric majority (~99%) of the total ~ 2 km³ of rhyolite magma was emplaced since ~ 300 ky. The Pleistocene volcanic system intrudes a seismically active area which hosts an active geothermal field driven by heat associated with an underlying magma body that may be partially molten. As such, the hazard potential of the CVF is high, making a detailed understanding the eruptive history of the system critically important.

Potassium-Argon geochronology published in the late 1970’s, and subsequent ⁴⁰Ar-³⁹Ar and U-Th-Pb geochronology defined the emplacement history of the rhyolitic portion of the CVF. However, of the 17 youngest rhyolite domes, which represent ~60% of the total silicic volume erupted by the entire system, only 8 have been directly dated, with just a handful of these being revisited after initial efforts in the 1970’s. Existing data precludes construction of a comprehensive eruption history of the most voluminous portion of the volcanic field, hindering an accurate assessment of hazard potential at the CVF. This presented an opportunity to apply modern isotopic measurement techniques to more accurately and precisely date CVF domes.

Toward this goal, we present zircon ± allanite in-situ uranium-series ion-microprobe (SHRIMP-RG) surface dates on the 17 youngest CVF rhyolites. Dating co-crystallizing zircon and allanite, which have very different U and Th isotopic compositions, increases age accuracy and precision relative to single-phase dating. Surface dating permits sampling and dating of the last zircon and allanite to crystallize within the erupted magma, and thus accurately constrains eruption timing, circumventing issues of biasing eruption age by inclusion of inherited/pre-eruption crystal age domains. These data suggest an emplacement interval of ~ 25 ky for the 17 youngest Coso domes, all of which are younger than ~ 100 ka, and shorter-duration emplacement pulses within this age range.