EXTENSIVE MIDDLE-MIOCENE SILICIC MAGMATISM IN NORTHEASTERN NV: THE JARBIDGE RHYOLITE AND ITS CONNECTION TO THE CENOZOIC EVOLUTION OF THE NORTHERN GREAT BASIN

Recent work suggests that widespread extension, coupled with rapid collapse of the Nevadaplano, initiated at ~17-16 Ma across the northern Great Basin (Colgan and Henry, 2009). Coeval with this event was voluminous bimodal basalt-rhyolite volcanism that is often simply related to the inception of the Yellowstone hotspot and temporally/spatially associated with the formation of world-class Au-Ag epithermal ores. As a result, deciphering the origin of these magmas and ores is an important part of understanding the Cenozoic tectonic evolution of the region because their origins directly impact regional tectonic models. In northeastern NV, the most widespread middle Miocene volcanism is that associated with the Jarbidge Rhyolite (JR; Coats, 1964; 1987). JR lava flows and domes crop out across northeastern NV and extend into UT and ID. The thickest and most areally extensive exposures of these lava flows and flow-dome complexes are found between the Owyhee Plateau and just east of the Jarbidge Mountains, NV. In this region, over 500 km³ of calc-alkalic rhyolite erupted effusively from ~16.3 -15.5 Ma (⁴⁰Ar/³⁹Ar geochronology; Brueseke unpublished), with older K-Ar ages suggesting that some activity occurred till ~14 Ma (Bernt, 1998). Prior mapping indicates that JR magmatism was synextensional in the Jarbidge Mountains and generally coeval with local Au-Ag mineralization (e.g. Coats, 1964; 1987; Bernt, 1998). This faulting precluded the local JR magmatic system(s) from becoming volatile-rich, leading to voluminous effusive volcanism. The geochemical and isotopic characteristics of the JR are consistent with an origin via melting of evolved crust, which, like other parts of the northern Great Basin and Oregon Plateau, appears to have been driven by upwelling mafic magma. Mafic magmas ascended into the crust via lithospheric structures, which also acted as conduits for the magmatic volatiles that were deposited as ores. Essentially, the distribution of the regionally extensive rhyolites in northeastern NV appears to reflect an intimate association with temporally and spatially coincident extension; any model proposed to explain the regional volcanism, ore-formation, and/or extension must take into consideration all three geologic phenomena.