OXYGEN ISOTOPE CONSTRAINTS on VOLUMINOUS MID-MIOCENE EFFUSIVE SILICIC MAGMATISM IN NORTH-CENTRAL NEVADA

Mid-Miocene Jarbidge Rhyolite (JR) lava flows crop out across north-central and northeast NV, but are best exposed in the vicinity of the Jarbidge Mountains, NV. Recent work on the JR helps clarify its relationship to other mid-Miocene silicic magmatism further west (e.g. northern NV, southeastern OR, and southwestern, ID). New ⁴⁰Ar/³⁹Ar geochronology coupled with previous work indicates JR magmatism in and near the study area occurred from 16.7 to ~14 Ma, coeval with regional mid-Miocene silicic activity and that this magmatism was accompanied and triggered by local basaltic magmatism (e.g. Coats, 1964, Bernt, 1998; Rahl et al., 2002). The estimated volume of studied JR exposures was calculated via ArcGIS and yields a value of 509 km³, comparable to other large-volume, silicic effusive complexes (e.g. the Central Plateau Member rhyolites of Yellowstone). Based on this volume estimate, conservative intrusive:extrusive ratios of 1:2 and 1:5 yield values of 1527-3054 km³ for the total volume of magma involved in the JR system(s). These calculated volumes are estimates; however, they indicate that the overall JR magmatic system was just as large as some of its mid-Miocene counterparts further west, even though it was primarily characterized by effusive eruptions. To further understand the magmatic processes that affected JR magmatism, we report new oxygen isotope data from JR lava flows. Quartz and feldspar phenocrysts were separated from newly dated lava flows from across the study area; these lava flows span an age range from 16.7 to 15.7 Ma. Analyzed δ¹⁸O values for these phenocryst separates yield values of 6.7-9.0, falling within the range of normal igneous values. These data indicate that hydrothermally altered crust and/or local metasedimentary strata were not substantially involved in the production and evolution of JR magmas. Coupled with other geochemical/petrographic evidence, it appears that JR magmas were likely derived via upper-crustal melting of local Cretaceous granitoid. Further radiogenic isotope work is needed to fully substantiate this hypothesis and also test whether there is any petrogenetic or hydrothermal relationship between the initially low δ¹⁸O outflow associated with the neighboring ~13-8 Ma Bruneau-Jarbidge eruptive center (e.g. the Cougar Point tuff) and the JR.