MAPPING MIDDLE MIOCENE VOLCANIC ROCKS IN THE WEISER EMBAYMENT, SOUTHWEST IDAHO: NEW U-PB TIMS ZIRCON AGE DATA AND ITS IMPLICATIONS

New STATEMAP-supported mapping, XRF chemistry, paleomagnetism, and U-Pb zircon dating have helped subdivide and constrain timing of middle Miocene volcanism in the Weiser embayment in southwest Idaho. Embayment volcanism includes Steens Basalt, Imnaha Basalt, and Grande Ronde Basalt of the Columbia River Basalt Group (~16.9-16.4 Ma; CRBG), newly identified rhyolitic domes and ignimbrites, dacitic and andesitic lavas, (≥16.4 Ma), Weiser volcanics (~15.1-12.4 Ma), and other localized flows. New high-precision U-Pb TIMS dates on zircons from rhyolite ignimbrites interbedded or capping volcanic flows provides timing controls in this complex regional volcanic system. A laterally discontinuous rhyolite ignimbrite marks the end of major volcanism in the southern part of the embayment at 16.394 ± 0.008 Ma. This 16.4 Ma ignimbrite is the capping member on a thin package of mixed volcanics and sediments unconformable to the underlying Grande Ronde Basalt. This date indicates the Grande Ronde members of southwestern Idaho are significantly older than the more well documented Grande Ronde members in Oregon, Washington, and northern Idaho (16.0-15.6 Ma; Barry and others, 2010). Detailed mapping and whole rock chemistry data have also led to the identification and correlation of several andesite flows that appear syneruptive with Grande Ronde Basalt yet don’t fit chemically into traditional CRBG formations. The andesite flows are volumetrically significant in this part of the embayment and their appearance in the stratigraphy mark the end of CRBG volcanism. The identification of CRBG volcanic rocks in close juxtaposition to those of the Weiser embayment, which span the entire silica range of the IUGS chemical classification, invites direct comparison with both the middle Miocene Strawberry volcanic rocks and the Powder River volcanic field both of eastern Oregon.