CONSTRAINTS ON THE LIFETIME OF THE MAGMATIC SYSTEM AT THE GEYSERS, SONOMA COUNTY, CALIFORNIA

The Geysers thermal anomaly is the northernmost and youngest in a series of volcanic centers developed as the Mendocino triple junction was progressively supplanted by Pacific-North American plate interaction. Magmatism is believed linked to upwelling of asthenospheric mantle along the trailing edge of subducted and abandoned oceanic lithosphere. Associated with this thermal anomaly is a huge geothermal system. In spite of over 40 years of exploration and development, the Geysers steam field remains the subject of ongoing controversy regarding the role of continued juvenile heat input and the existence of a magma system that has persisted over the past 1 m.y. Our U-Pb zircon and ⁴⁰Ar/³⁹Ar K-feldspar thermochronologic investigations of the Geysers Plutonic complex (GPC) and related volcanics are directed towards understanding the thermal and magmatic histories with a view to resolving these questions. In order to minimize uncertainties related to initial isotopic disequlibria (e.g., ²³⁰Th), we determined Th and U partition coefficients from measured abundances in zircons, whole-rocks and melt inclusion glasses. After correcting for Th-deficit using these values, U-Pb zircon ages were obtained for three volcanic units apparently coeval with the GPC: the 1.39±0.04 Ma Alder Creek rhyolite, 1.34±0.04 Ma Cobb Mountain rhyodacite, and the1.28±0.05 Cobb Valley rhyodacite. Within each unit, statistically significant age differences imply a span of zircon crystallization of ~100 ka. The zircon crystallization age for the Alder Creek Rhyolite is ~200 k.y. older than eruption ages inferred from published ⁴⁰Ar/³⁹Ar data (1.19±0.01 Ma) suggesting that the Cobb Mountain magma reached zircon saturation considerably prior to eruption. U-Pb zircon ages from the Cobb Mountain samples also overlap with those obtained from the granite phase of the GPC suggesting zircon crystallization in a common magma reservoir. We are currently undertaking a similar investigation of intrusive units within the >100 km³ GPC with a view to correlating the age of these units with the overlying volcanic rocks.