ZIRCON U-PB AGES AND TRACE ELEMENT COMPOSITION FOR ERUPTIVE UNITS BETWEEN 193–588±69 KA, LASSEN VOLCANIC CENTER, NORTHERN CALIFORNIA

The Lassen Volcanic Center (LVC) is the southernmost active volcanism of the Cascade Range, which formed as a result of the subduction of the Juan de Fuca Plate under the North American Plate in western North America. Volcanism in the LVC began around 825 ka, forming the Rockland caldera complex (825–610 ka), which culminated with the voluminous eruption (~50 km³) of the Rockland Tephra at ~609 ka, and ranges in composition from dacite to rhyolite. This was followed by the Brokeoff Volcano (590–385 ka), an andesite composite cone, which erupted a compositional range of andesite to dacite. The most recent activity is the Lassen Domefield (300 ka to present), which ranges in composition from basaltic andesite to dacite in the Twin Lakes sequences, from dacite to rhyodacite in the Bumpass sequence, and from dacite to rhyodacite in the Eagle Peak sequence.

Zircon from four samples in the LVC dated between 193–588±69 ka are to be analyzed for U-Pb ages and trace element composition via SHRIMP-RG in August 2014. The eruptive units being analyzed are two units from the Bumpass Sequence, the rhyodacite of Dersch Meadow (193 ka) and the rhyodacite of Mt. Conard (298 ka); and two units from the Rockland Caldera Complex, the rhyolite of Rockland Tephra (609 ka) and the rhyolite of Raker Peak (588±69 ka, K-Ar age). These sequences are separated by the Mill Canyon sequence and the Diller sequence, produced from Brokeoff Volcano (590–398 ka), which are primarily andesite and are not included in this study.

These data will extend the zircon age and compositional record for the LVC from the most recent activity (<90 ka, see Klemetti et al., 2013 and Bertolett et al., this volume) to the later part of the Rockland caldera complex. This will allow for an examination of the evolution of crystal storage and rejuvenation under the LVC over at least 588±69 ka. Additionally, U-Pb ages and trace element compositions in the zircon extracted from the Rockland Tephra and the rhyolite of Raker Peak will help determine if the ~588±69 ka rhyolite of Raker Peak was the early precursor magma to the voluminous (~50 km³) Rockland Tephra eruption (609 ka).