Paper No. 6
Presentation Time: 9:50 AM
OXYGEN ISOTOPE CONSTRAINTS ON MID-MIOCENE RHYOLITE PRODUCTION IN THE SANTA ROSA-CALICO VOLCANIC FIELD, NV
The origin of Snake River Plain-Yellowstone (SRPY) low-δ18O silicic magmas is controversial, and centers on two disputed models:  magma production via repeated eruption-caldera collapse, hydrothermal alteration, and then melting of intra-caldera fill that is then incorporated into an evolving magma body eventually producing low-δ18O magmas and  melting of previously hydrothermally altered crust to form low-δ18O magmas. To evaluate these models, we have studied the O isotope composition of silicic magmas from the 16.7-14 Ma Santa Rosa-Calico volcanic field (SC), northern NV. Quartz and feldspar phenocrysts from fifteen samples of SC silicic units (e.g. ash-flow tuffs and lavas) were chosen for 18O analyses based on existing field/temporal relations and geochemical/radiogenic isotope compositions. Our results show that low-δ18O values are limited to the youngest erupted silicic unit, the 15.5 Ma Cold Springs tuff. Cold Springs tuff feldspar δ18O values range from 2.36 to 4.05‰; the unit lacks quartz. Older lavas that are not petrogenetically related to the Cold Springs tuff are characterized by normal δ18O feldspar values (in equilibrium with quartz) that range from 7.19 to 10.04‰. Petrologic modeling indicates that the source of the Cold Springs tuff is primarily hydrothermally altered local Cretaceous granitoid upper crust, with a δ18O range of ~2 to 4‰. Mid-Miocene hydrothermal fluids were sourced from the epithermal system centered on Buckskin Mountain (e.g. Buckskin-National), adjacent to the Cold Springs tuff source. Vikre (1987; 2007) calculated fluid δ18O values to fall between -12‰ to 7‰ and these fluids most likely traveled down syn-magmatic extensional faults to depths >5 km. Our results are consistent with the prior alteration model. Given that similar mid-Miocene epithermal systems are found across the northern Great Basin, our results also suggest that mid-Miocene crustal alteration, facilitated by coeval extension, could have been a viable mechanism for setting the stage for <15 Ma low δ18O silicic magma production in the region.