The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA: Adjacent venting sites of two Dinner Creek Tuff units—the most widespread tuffs associated with Columbia River flood basalt volcanism

Abstract The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts the Columbia River magmatic province. Field mapping along with analytical data tuff samples identify two calderas, Castle Rock and Ironside Mountain as venting sites widespread ignimbrites Tuff. Both calderas lie within area proposed general storage main-phase Basalt magmas. caldera formed during eruption 16.16 Ma 1. northwestern boundary roughly defined by juxtaposition over 300 m densely welded rheomorphic intra-caldera tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale pre-Miocene Ring Butte trachybasalt lavas. Following collapse, fluvial lacustrine volcaniclastic sediments were deposited on floor, outflow tuffs units 2 4 into caldera. Aphyric basaltic andesite icelandite (Fe-rich andesite), which correlate stratigraphically upper Grande Ronde lavas, intrude floor deposits, lavas are interbedded 4. 15.6 2, lies ~15 km north 11 × 6 depression wherein 900 intra-caldera, rheomorphic, partially bound Tureman Ranch granodiorite. Post-caldera dikes sills that also correlative intruded tuff, mostly margins caldera, altered much tuff. Mafic study closely pre- post-date correlated regional Group. Porphyritic aphyric mafic lava flows underlying 1 at Picture Gorge Basalt. overlie westward extensions fractionated tholeiitic magmas seen in late-stage such Hunter Finally, porphyritic basalt 13.5 Tim's Peak At Rock, pre-caldera Group appear lap onto a topographic high stretches northward for tens kilometers based stratigraphic data, it may be related uplift initial impingement mantle upwelling produce exemplify bimodal Eruption rhyolites post-dated local belonging evolved likely concealed but these illustrate close proximity rhyolitic depth centers. Consequently, stratigraphy both somewhat differs from dominated silicic calc-alkaline intermediate post-caldera volcanism.