The geochronology and geochemistry of the Bearhead Rhyolite, Jemez volcanic field, New Mexico

Leigh Justet, University of Nevada, Las Vegas


Around 82% of mapped Bearhead Rhyolite (Main Cluster) and Peralta Tuff appears to have been derived from a relatively long-lived ({dollar}\sim{dollar}680 ka), large, shallow ({dollar}\le{dollar}10 km below Earth's surface) magma chamber that did not produce a caldera-forming eruption. Although volatile contents were great enough ({dollar}\sim{dollar}3 wt.% H{dollar}\sb2{dollar}O), no large-scale explosive eruptions occurred because magma may have been tectonically vented. The lack of systematic chemical variation within the Main Cluster with time during this {dollar}\sim{dollar}680 ka interval may imply that erupted magmas were physically separated from each other by fault-formed cupolas in the roof of the magma chamber. These results are significant because Bearhead Rhyolite may represent a poorly documented style of silicic volcanism that may be more common than realized; The remaining {dollar}\sim{dollar}18% of mapped Bearhead Rhyolite is chemically and/or temporally distinct from the majority of Bearhead Rhyolite and is located in the southwest periphery of the field area.