U-PB ION MICROPROBE ZIRCON SURFACE DATING OF COBB MOUNTAIN UNITS WITHIN THE CLEAR LAKE VOLCANIC FIELD

The Quaternary Clear Lake Volcanic Field (CLVF) is located within the San Andreas Transform ~150 km north of San Francisco within the California Coast Ranges. CLVF eruptions between ~2.1 Ma and ~10 ka generated ~100 km³ of lava as flows and domes ranging in composition from basalt to rhyolite. An even larger volume of intrusive rock was emplaced as the ~1.1 – 1.75 Ma Geysers Plutonic Complex (GPC), which remains a persistent shallow heat source adjacent to the CLVF and beneath the Geysers geothermal field. The long eruptive history and close proximity to the densely populated Bay Area result in classification of the CLVF as high threat with respect to volcanic hazard. Establishing an accurate and comprehensive eruptive history of the volcanic field is critical to better preparing for potential future volcanic hazards.

Located near the southern boundary of the mapped CLVF, Cobb Mt. is considered to be an erupted portion of the GPC and records the short-duration, globally recognized Cobb Mountain Normal Polarity geomagnetic excursion within the Reverse Polarity Matuyama Chron. Cobb Mt. is composed of the ~ 1.2 Ma normal polarity Alder Creek rhyolite overlain by the reversed polarity Cobb Mountain rhyodacite and Cobb Valley dacite. These three units have been previously dated by both ⁴⁰Ar/³⁹Ar on sanidine and U-Pb on zircon interiors, the latter chronometer yielding older dates than the former. This chronometer-specific age disparity indicates that the bulk of zircon growth occurred prior to eruption, which precludes zircon interior dates from recording eruption timing.

With this contribution, we present ion-microprobe (SHRIMP-RG) U-Pb zircon surface dates that we interpret as eruption ages for the three Cobb Mt. units. By dating the outermost, unpolished zircon crystal surface with the ion microprobe, we date the timing of final zircon crystallization and thus as close to the actual eruption age as possible with a zircon crystal. Application of the surface dating protocol to the temporally well characterized Cobb Mt. units permits testing accord between zircon surface dates and published ⁴⁰Ar/³⁹Ar eruption ages on the same units, builds a comprehensive U-Pb zircon surface-defined eruption history for this portion of the CLVF, and adds further geochronologic constraints to the timing of the Cobb Mountain Cryptochron.