Rhyolite intrusions in the intracaldera Bishop Tuff, Long Valley Caldera, California

Drilling of the Long Valley Exploratory Well on the resurgent dome in the Long Valley Caldera revealed > 300 m cumulative thickness of granophyric intrusions within the 1180-m-thick, 760 ka intracaldera Bishop Tuff. The intrusions are aphyric to sparsely plagioclase-phyric, high-silica, high-barium and low-strontium rhyolites. They resemble the lavas of the Early Rhyolite, the first phase of post-caldera volcanism. A mean 40Ar/39Ar age of 590 ± 17 ka from a part of a shallow intrusion is coeval with Early Rhyolite volcanism. A second mean age of 454 ± 17 ka from the same intrusion may reflect either younger Early Rhyolite activity with no external equivalent or hydrothermal resetting of the argon system. Hydrothermal alteration of the intrusions is characterized by introduction of quartz, calcite and pyrite and formation of illite/smectite. High CO2 content of fluids apparently inhibited zeolite formation. Alteration varies locally within intrusions and intrusive groups and does not vary systematically with depth. Oxygen shows consistent depletion of the 18O isotope from an initial magmatic composition of + 6.0 to + 8.5‰ to values ranging from +1.4 to −0.4‰. The constant oxygen isotope depletion most likely reflects alteration of intrusions due to local emplacement-induced hydrothermal circulation rather than a caldera-scale hydrothermal system. In contrast, 18O depletion of the host Bishop Tuff increases regularly with depth (except at an intrusive contact). A pre-Early Rhyolite geothermal gradient of approximately 70 °C/km was inferred. This is substantially higher than the current gradient but substantially lower than expected for the case of a conductive regime over a shallow residual magma chamber. Either the intrusions were fed from a deep chamber, or a cool hydrologic recharge regime was established early in caldera history. e, thickness and suspected lateral extent of these shallow intrusions are such that emplacement of the intrusions, rather than inflation of a shallow chamber, is responsible for resurgence of the central Long Valley Caldera. Similar intrusions occur in another well on the resurgent dome ( LV13-21 ) but not in wells located off the dome.