HR: 16:45h
AN: V32H-04    [PDF]
TI: New Field, Geochronological, and Geochemical Data on the Miocene Lovejoy Basalt and its Source, Northern California
AU: * Garrison, N J
EM: garrison@umail.ucsb.edu
AF: University of California at Santa Barbara Dept. of Geological Sciences, Webb Hall 1006, Santa Barbara, CA 93106 United States
AU: Busby, C B
EM: Busby@geol.ucsb.edu
AF: University of California at Santa Barbara Dept. of Geological Sciences, Webb Hall 1006, Santa Barbara, CA 93106 United States
AU: Gans, P B
EM: gans@geol.ucsb.edu
AF: University of California at Santa Barbara Dept. of Geological Sciences, Webb Hall 1006, Santa Barbara, CA 93106 United States
AU: Wagner, D L
EM: dwagner@consrv.ca.gov
AF: California Geological Survey, 801 K St. MS 12-31, Sacramento, CA 95814 United States
AB: The Lovejoy Basalt is a distinctive, aphanitic olivine basalt that flowed through a paleovalley across the northern end of the Sierra Nevada from the Diamond Mountains near Honey Lake to the Sacramento Valley in northern California. Approximately 150 km3 of basalt was erupted in a tectonic setting transitional between subduction, Basin and Range extension, and hot spot volcanism, but the eruptive source and timing of the Lovejoy Basalt have remained controversial. Recent field mapping and geochemical analyses suggest that Thompson Peak, located south of Susanville, California, is the source vent for the Lovejoy Basalt. At Thompson Peak, Lovejoy Basalt forms an elongate, NNW-SSE trending ridge that is capped in its central section by the Basalt of Thompson Peak. Scoria deposits, volcanic bombs, agglutinate and other near vent deposits indicate Thompson Peak as the source for the Lovejoy Basalt. Previous and new mapping indicate that the basalt was channelized within granitic basement, and flowed south from the vent 30 km to Red Clover Creek before bending to the southwest and flowing 65 km to the Sacramento Valley. The Lovejoy Basalt consists of up to 13 flows, all aphyric except for an uppermost plagioclase-phyric flow present between Thompson Peak and Red Clover Creek that locally contains xenocrysts of olivine and garnet. At Table Mountain, the basalt appears to have ponded or inflated, forming very thick flows with prominent N-S trending pressure ridges. The Lovejoy Basalt is a quartz-normative tholeiite that is geochemically similar to the Columbia River Basalt. While a conclusive statement cannot yet be made about the source magma for the Lovejoy Basalt, initial analyses show that it does not appear to be related to the main phase of Cascade arc volcanism, which is predominantly calc-alkaline. Recently published age data for the Lovejoy Basalt suggests that it is coeval with the main phase of the Columbia River Basalt Group; however, our further attempts to date the basalt have met with difficulty, primarily due to the presence of a high percentage of altered glass in the basalt's matrix. This difficulty may account for the wide range of previously reported K-Ar dates for the Lovejoy Basalt, and call into question the recently published Ar$^{40}$/Ar$^{39}$ ages. We are currently attempting Ar$^{40}$/Ar$^{39}$ analyses on plagioclase crystals from the uppermost flow of the basalt, and on groundmass from more holocrystalline flows present at Table Mountain.
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting