ZIRCON AGES AND COMPOSITIONS OF THE EAGLE PEAK AND TWIN LAKES SEQUENCES, LASSEN VOLCANIC CENTER, NORTHERN CALIFORNIA

The Lassen Volcanic Center represents the southernmost extent of active volcanism in the Cascades Range caused by the oblique subduction of the Juan de Fuca plate under North America. Five volcanic centers have been active in the region over the past 3.5 M.y., the most recent being the Lassen Domefield that includes the Eagle Peak (66-1.1 ka) and Twin Lakes (90-0.01 ka) Sequences with lava varying from intermediate to silicic (58.0-71.6 SiO₂ wt%). These sequences are separated from the prior Bumpass Sequence (320-190 ka) by a period of eruptive quiescence (190-90 ka).

Zircon from four samples are being analyzed for ²³⁸U-²³⁰Th ages and trace element composition via SHRIMP-RG. The units sampled include the rhyodacite of Eagle Peak (66 ka), andesite of Hat Mt. (45 ka), rhyodacite of Sunflower Flat (41 ka), and rhyodacite of Kings Creek (35 ka). Data for the rhyodacite of Eagle Peak provide zircon U-Th model ages ranging from 105-342 ka with no zircon crystallizing within error of eruption and 3 of 24 zircon were found to be in secular equilibrium (>350 ka). A majority of zircon crystallization occurred between 180 and 230 ka covering the latter Bumpass Sequence and the beginning of the period of eruptive quiescence. Further ages from the remaining three units will be collected in August 2014 along with U-Pb data from older LVC units (Hernandez et al., this volume).

Trace element data from zircon from the rhyodacite of Eagle Peak provides insight into the magmatic evolution of the LVC over the last 350,000 years. Hf (6915-11432 ppm) and Eu/Eu* (0.14-0.61) are similar to zircon from the LVC (Klemetti et al., 2013) although Eu/Eu* values are lower and Hf spans a narrower range. Ti-in-zircon temperatures (~675-801^oC, assuming (Si)=1 and (Ti)=0.6) are similar to other LVC zircon. The period during which the majority of zircon cores formed (180-230 ka) is accompanied by a peak in zircon temperatures with no accompanying change in Eu/Eu*. These data support the model of zircon crystallization in a crystal mush formed during and after the Bumpass Sequence and followed by localized rejuvenation during the Eagle Peak Sequence. The peripheral location of Eagle Peak to the southwest of Lassen Peak and Chaos Crags may control why the zircon ages are slightly older as the eruption tapped a different area of the Bumpass crystal mush.