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 grano phyric 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 lav as of the Early Rhyolite, the first phase of post-caldera volcanism. A mean Ar-40/Ar-39 age of 590 +/- 17 ka from a part of a shallow intrus ion is coeval with Early Rhyolite volcanism. A second mean age of 454 +/- 17 ka from the same intrusion may reflect either younger Early Rhy olite activity with no external equivalent or hydrothermal resetting o f the argon system. Hydrothermal alteration of the intrusions is chara cterized by introduction of quartz, calcite and pyrite and formation o f illite/smectite. High CO2 content of fluids apparently inhibited zeo lite formation. Alteration varies locally within intrusions and intrus ive groups and does not vary systematically with depth. Oxygen shows c onsistent depletion of the O-18 isotope from an initial magmatic compo sition of +6.0 to +8.5 parts per thousand to values ranging from +1.4 to -0.4 parts per thousand. The constant oxygen isotope depletion most likely reflects alteration of intrusions due to local emplacement-ind uced hydrothermal circulation rather than a caldera-scale hydrothermal system. In contrast, O-18 depletion of the host Bishop Tuff increases regularly with depth (except at an intrusive contact). A pre-Early Rh yolite geothermal gradient of approximately 70 degrees C/km was inferr ed. This is substantially higher than the current gradient but substan tially 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 ear ly in caldera history. The age, thickness and suspected lateral extent of these shallow intrusions are such that emplacement of the intrusio ns, rather than inflation of a shallow chamber, is responsible for res urgence of the central Long Valley Caldera. Similar intrusions occur i n another well on the resurgent dome (LV13-21) but not in wells locate d off the dome.