Geochemistry of volcanic rocks from the Geysers geothermal area, California Coast Ranges

The Geysers geothermal reservoir in the California Coast Ranges is the worldʹs largest economically used geothermal system. Surface exposure of volcanic rocks and extensive drill penetration of a > 300 km3 subsurface plutonic body (Geysers Plutonic Complex or GPC) that underlies the reservoir allow unique insights into the evolution of a volcanic–plutonic magma system. We present compositional data for major elements, trace elements, and Nd, Sr and Pb isotopes of late Pliocene to early Pleistocene lavas that are spatially related to the geothermal reservoir and cores from coeval subsurface plutonic rocks. Isotopic ratios of dacitic to rhyolitic lavas from Cobb Mountain, Pine Mountain and Tyler Valley, and core samples from the shallow microgranite phase of the GPC range between 0.70343–0.70596 (87Sr / 86Sr), 0.512813–0.512626 (143Nd / 144Nd), and 19.084–19.189 (206Pb / 204Pb). These compositions fall between values for two major isotopic end-members: (1) coeval basalt of Caldwell Pines that has compositional affinities to magmas extracted from a subcontinental mantle-wedge, and (2) regional crustal rocks equivalent to those of the Franciscan and Great Valley sequence. Model calculations suggest that lower crustal intrusion of basalt triggered crustal assimilation in mass proportions of ∼5 : 1 to produce magmas of intermediate composition. Because rhyolites are isotopically indistinguishable to dacites, we interpret them as partial melts derived from precursor intrusions of granodioritic compositions. Our data suggest that two major mantle and crustal reservoirs continually interacted over the > 1 Ma duration of volcanism and pluton emplacement at the Geysers. Individual magma pulses, however, evolved independently, arguing against the presence of a single long-lived magma body.