MID-MIOCENE SILICIC VOLCANISM IN THE OWYHEE MOUNTAINS (ID): LOCAL PHYSICAL AND GEOCHEMICAL CHARACTERISTICS AND IMPLICATIONS FOR REGIONAL MAGMATISM LINKED TO THE INCEPTION OF THE YELLOWSTONE HOTSPOT

Similar to many other areas in and adjacent to the northern Great Basin (NGB) and Oregon Plateau (OP), the Silver City mining district (SCD), Owyhee Mountains, ID, experienced a period of extensive magmatism in the mid-Miocene that is often associated with the inception of the Yellowstone hotspot. This study focuses on local SCD silicic volcanism and its relationship to contemporary mid-Miocene silicic magmatic systems (e.g. Santa Rosa-Calico volcanic field, SC; Jarbidge Rhyolite, JR) that are located in the NGB and OP. The mid-Miocene SCD magmatic suite spans a complete compositional spectrum from locally erupted Steens Basalt to high-Si rhyolite. SCD rhyolites have been mapped as the upper-most stratigraphic member of the mid-Miocene volcanic package (e.g. Silver City rhyolite; previous workers and this study). New geochemical data allow us to evaluate the major and trace element characteristics of the Silver City rhyolite and compare it to other local and regional, coeval, silicic magmatism. Silver City rhyolite lava flows and shallow intrusive bodies are characterized by <200 ppm Zr, <12 ppm Nb, <2000 ppm Ba, 10-25 ppm Pb, 20-45 ppm La and <32 ppm Nd. Conversely, the other most extensive SCD silicic unit (Tql of Panze, 1975) has >400 ppm Zr, >26 ppm Nb, >2000 ppm Ba, 15-35 ppm Pb, 25-80 ppm La, 30-90 ppm Nd. Other major and trace elements (e.g. TiO₂, Sc, Sr, etc.) often show complete overlap between these two groups and a third, much less areally extensive silicic unit that is characterized by geochemistry transitional between the two other groups. Overall, the within- and between-group chemical variations, coupled with field relationships and physical characteristics, indicate that silicic magmatism in the SCD was primarily effusive and sourced from at least two chemically and spatially distinct magma reservoirs. Sr-Nd-Pb-O isotope data currently being obtained will provide constraints on the sources of these silicic magmas. However, the geochemical characteristics of SCD rhyolites closely resemble those erupted from the SC and JR which appear to have formed via middle/upper crustal melting of quartzofeldspathic rocks. This is in contrast with the younger rhyolites of the central and eastern Snake River Plain that are characterized by a much greater mantle component.