HR: 13:40h
AN: V53F-01    [Abstracts]
TI: Magmatic Evolution of the Coso Geothermal Area, California
AU: * Glazner, A F
EM: afg@unc.edu
AF: Dept. of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, United States
AU: Miller, J S
EM: jsmiller@email.sjsu.edu
AF: Dept. of Geology, San Jose State University, San Jose, CA 95192, United States
AU: Leeman, W P
EM: leeman@nsf.gov
AF: National Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230, United States
AU: Johnson, B R
EM: breckj@email.unc.edu
AF: Dept. of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315, United States
AU: Monastero, F C
EM: francis.monastero@navy.mil
AF: Geothermal Program Office (ESC-25), Naval Air Weapons Station, 429 E. Bowen Road, Mail Stop 4011, China Lake, CA 93555-6108, United States
AB: Geothermal energy in the Coso field owes its origin to basaltic magmatism. Volcanism commenced ~3.5 Ma ago, coincident with a widespread Pliocene outburst in eastern California. Although most basalts associated with this event are highly potassic, those at Coso are not. Pliocene volcanic rocks at Coso (erupted between 3.5-2 Ma) range from basalt to rhyodacite, show abundant petrographic evidence for open-system behavior (e.g., quartz xenocrysts in basalts), and have compositions consistent with mixing. In contrast, Pleistocene rocks, erupted <1 Ma ago, comprise a strongly bimodal suite of mildly alkalic basalt and high-silica rhyolite. Pleistocene basalts differ from their Pliocene counterparts in generally having more depleted ⁸⁷Sr/⁸⁶Sr and ε_Nd values (0.703, +7 vs. 0.704, +4); higher TiO₂ and Nb; lower MgO; greater stalling depths in the crust. Pliocene rocks are distinctly arc-like even though they were erupted ~10 Ma after subduction ceased. In contrast, Pleistocene basalts have a distinctly OIB-like geochemical signature, with undepleted high field strength elements and plume-like radiogenic isotope ratios; these characteristics are shared with late Cenozoic basalts across the western U.S. Rare Pleistocene basalts that were erupted from within the footprint of the rhyolite field have notably high TiO₂ contents (>3 wt%), similar to basalts from the Columbia River and Snake River Plain fields. Unlike Pliocene rocks, which scatter toward isotopic values of local basement with increasing SiO₂, Pleistocene rhyolites generally have high and consistent ε_Nd (+1 - +2.5). Producing this signature by AFC processes involving basalt and basement rocks requires remarkably consistent mixing and fractionation at small-volume volcanic centers separated by several km. Alternatively, high ε_Nd values in the rhyolites could have been produced by partial melting of Pliocene basalts and andesites, which have very similar Nd isotopic compositions. Increasing ε_Nd in silicic rocks as the geothermal production area is approached suggests that the magmatic flux is highest there even though erupted volumes are significantly larger outside the geothermal area. One scenario consistent with the above data is as follows. Post-subduction tectonic events triggered magmatism at 3.5 Ma, tapping fertile, subduction-metasomatized lithospheric mantle. Basalts stalled in and partially melted the mid-crust, generating a mixed-magma series and copious volcanism. Depletion of the mantle source by 2 Ma led to a hiatus in magmatism. A change in basalt chemistry to OIB- affinity in the last 1 Ma suggests a profound change in magma source – likely involving decompression melting of ascending asthenospheric mantle, perhaps related to lithosphere delamination. Injection of such magmas into the lower crust, would have generated rhyolites by remelting of earlier emplaced mafic bodies – imparting a juvenile isotopic signature in the late rhyolites. Precursory Pliocene magmatism is a common feature of other western U.S. geothermal areas, including Twin Peaks, The Geysers, and Long Valley.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3640 Igneous petrology
DE: 3641 Extrusive structures and rocks
DE: 3651 Thermobarometry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting