RE-EVALUATING THE "OWYHEE-HUMBOLDT" ERUPTIVE CENTER: RELATIONSHIPS BETWEEN VOLUMINOUS MIDDLE MIOCENE SILICIC VOLCANISM AND THE OWYHEE PLATEAU

Associated with the Yellowstone-Snake River Plain volcanic province (YSRP), the Miocene "Owyhee-Humboldt" eruptive center (OHEC) remains poorly constrained. The OHEC is defined on the basis of voluminous rhyolite lava flows/areally extensive lava-like ignimbrites and regionally exposed air fall tuffs that yield eruptive ages of ~14.5 - 12.8 Ma. The thickest exposures of these are found near the Juniper Mountain volcanic center (JM), where fountain-fed silicic magmas erupted between ~14.5 - 13.7 Ma. Aside from JM, silicic eruptive loci (e.g. dikes and domes) are found across the southernmost Owyhee Plateau (OP) and reflect local activity. Our current work in the ~16.4 - 14 Ma Santa Rosa-Calico volcanic field (SC) is helping to better clarify mid-Miocene volcanism of this region. The SC lies near or within the northern Nevada rift and directly west of the southern OP. The SC was characterized by basalt through high-Si rhyolite volcanism and the numerous SC eruptive loci reflect the influence of ongoing extension during emplacement/eruption that likely helped inhibit the formation of large calderas. Three features differentiate the SC from coeval YSRP volcanism: 1) diverse eruptive styles, 2) compositional diversity of magmas, e.g. substantial intermediate volcanism, and 3) lack of peralkaline volcanism. The temporal and spatial relationship of the SC with both ongoing extension and flood basalt volcanism help differentiate the SC from younger YSRP systems. Along the northern and eastern margins of the SC, the JM derived ~14 - 14.5 Ma "Swisher Mtn. Tuff" (ST) onlaps SC and older units and thickens to the north/east. Along the southeastern SC margin, lacustrine and fluvial deposits underlie the ST. These observations coupled with the large areal extent of the southward focused ST suggest that the southern OP region must have existed as a topographic low prior to ST volcanism. The nature of this low requires further investigation, but may have similarities with the Oregon-Idaho Graben. Our observations in the SC, the mapped OP mid-Miocene silicic eruptive loci, the extent of the ST, and prior research all suggest that mid-Miocene volcanism in this region was characterized by multiple eruptive centers (e.g. SC, JM) resulting from localized extension, that in some cases were the source of highly fluid silicic magmas.