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

The Clear Lake Volcanic Field (CLVF) is located approximately 150 km north of San Francisco and is the northernmost and youngest volcanic field in the California Coast Ranges. Episodic volcanic activity from 2.1 Ma to 10 ka generated an eruptive volume of ~100 km³. A shallow magma body known as the Geysers Plutonic Complex (GPC) underlies a portion of the CLVF and supplies heat to the adjacent Geysers steam field. The CLVF is ranked as a high-risk volcanic hazard due to regional seismicity, the presence of the GPC and associated geothermal activity, its long eruptive history, and its proximity to the densely populated Bay Area. Establishing an accurate and comprehensive eruptive history of the CLVF is critical to understanding and better preparing for volcanic hazards associated with the CLVF.

Located along the southernmost edge of the CLVF and directly overlying the GPC, Cobb Mountain is comprised of three volcanic domes: The Alder Creek rhyolite, a commonly used fluence monitor in the ⁴⁰Ar/³⁹Ar community, the Cobb Mountain rhyodacite, and the Cobb Valley dacite. Cobb Mountain domes are dated by both ⁴⁰Ar/³⁹Ar on sanidine and U-Pb on zircon interiors, which constrain the timing and eruption interval to ~1.2 Ma and 0.15 ± 0.06 Ma, respectively. The short-duration, globally recognized Cobb Mountain Normal-Polarity geomagnetic excursion is recorded within this emplacement interval and persists between 1.18–1.20 Ma within the reverse polarity Matuyama Chron.

In this contribution, we present ion-microprobe (SHRIMP-RG) U-Pb zircon surface dates on the three units comprising Cobb Mountain. By sampling the outermost, unpolished zircon surface with the ion microprobe, we date the timing of final zircon crystallization prior to eruption. In multiple published cases from other volcanic systems, this protocol is shown to yield dates within uncertainty of eruption age as defined by ⁴⁰Ar/³⁹Ar analyses. Applying this protocol to zircon from Cobb Mountain units, we (1) further test the accuracy of surface dating by comparing Cobb Mountain U-Pb dates with published ⁴⁰Ar/³⁹Ar eruption ages on the same units, (2) begin building a comprehensive U-Pb zircon-defined eruption history for the CLVF, and (3) add additional geochronologic constraints to the timing and duration of the Cobb Mountain Cryptochron.