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

The Pleistocene Coso volcanic field (CVF) is one of a series of eastern Californian Quaternary volcanic centers. The silicic portion of the CVF is composed of 38 crystal-poor rhyolite (~ 77% SiO₂) domes, and although sporadically emplaced over the past ~ 1 Myr, ~99% of the total ~ 2 km³ of rhyolite magma was emplaced since ~ 300 ka. The Pleistocene volcanic system intrudes a seismically active area hosting a productive geothermal field driven by heat from an underlying magma body that may be partially molten. As such, the potential exists for future volcanism from the CVF, making a detailed understanding of the eruptive history of the system important.

Potassium-Argon geochronology published in the late 1970’s, and subsequent ⁴⁰Ar-³⁹Ar and U-Th-Pb geochronology currently define 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 over the lifetime of the system, only 8 were directly dated, with just a handful of these 8 being revisited after initial efforts in the 1970’s. The lack of a published comprehensive eruption history of the most voluminous portion of the volcanic field precludes an accurate assessment of CVF hazard potential.

Zircon and allanite-bearing CVF domes are amenable to modern isotopic measurement techniques such as uranium-series ion-microprobe (SHRIMP-RG) dating, which enables improved accuracy and precision on dome emplacement ages. We present zircon ± allanite in-situ surface dates on the 17 supposed youngest CVF rhyolite domes. Surface dating permits sampling and dating of the last zircon and allanite to crystallize within the magma, and thus constrains eruption timing as closely as is possible with minerals that crystallize pre- or syn-eruption. Surface dating also circumvents issues of biasing eruption age by inclusion of inherited/pre-eruption crystal age domains. Dating co-crystallizing zircon and allanite, which have very different U and Th isotopic compositions, increases accuracy on isochron dates relative to single-phase dating. Our dataset suggests an emplacement interval of ~25 kyr for the 17 youngest Coso domes, all of which are younger than ~100 ka, and four shorter-duration emplacement pulses within this age range.