MID-MIOCENE VOLCANISM IN NORTHEAST NEVADA: SPATIAL, CHEMICAL, AND CHRONOLOGIC SIGNIFICANCE OF THE JARBIDGE RHYOLITE

At approximately 16.7 Ma, voluminous volcanism occurred across the Pacific Northwest, marking the inception of the Yellowstone Hotspot (YH). This ~2.5 Ma event was characterized by widespread mafic and silicic magmatism, lithospheric extension, and epithermal Ag-Au mineralization. Here, we report new field, petrographic, and chemical data from the Jarbidge rhyolite (JR), in the vicinity of its Jarbidge, NV type section with the goal of providing temporal, physical, chemical and petrologic constraints on the JR. The JR was named and mapped by Coats (1964) as a mid-Miocene porphyritic rhyolite throughout Elko County, NV; however, it is best exposed around its type section. Prior geochronologic work suggests that these exposures represent the easternmost silicic activity associated with the initial YH event. Furthermore, the region around the JR type section displays other salient characteristics that identify regional mid-Miocene magmatic centers (e.g. Santa Rosa-Calico, Owyhee Mts., McDermitt, Lake Owyhee, etc.): a dominantly bimodal basalt-rhyolite assemblage (e.g. the Steens basalt equivalant, Seventy-Six basalt and JR), local epithermal Ag-Au mineralization, and coeval faulting. Field and physical relationships indicate that JR volcanism was primarily effusive. JR lava flows typically contain 25-35% phenocrysts and include (in decreasing abundance) quartz, potassium feldspar, plagioclase, oxides, ± zircon, ± sphene, and ± fayalite. Garnet, hornblende and oxyhornblende are also present in JR samples from flow-dome complexes in the vicinity of Wildhorse Reservoir, NV, west of the thickest JR exposures near the type section. Quartz, sanidine, and plagioclase phenocrysts are commonly embayed and skeletal, suggesting open-system processes affected JR magmas. JR lava flows are peraluminous to slightly metaluminous and are characterized by wt % SiO₂ = 73-79, K₂O = 4.5-7.3, CaO = 0.25-1.50, FeO* = 0.50-3.00, TiO₂= 0.25-0.40, ppm Ba = 500-2,000, Sr = 60-160, Zr = 220-390, Nb = 14-32, and Y =21-58. ⁴⁰Ar/³⁹Ar geochronology is underway to better constrain the duration of volcanism around the type section, provide temporal comparison between regional JR outcrops, and better define how JR volcanism relates to the initial YH event.