REVISITING VOLCANOLOGY AND COMPOSITION OF RHYOLITES AND ASSOCIATED REE RICH MAFIC CLASTS OF THE THREE FINGERS CALDERA, SE OREGON

Two adjacent caldera systems, the Mahogany Mountain and the Three Fingers caldera constitute voluminous rhyolitic volcanism on the eastern margin of the Oregon-Idaho graben during the mid-Miocene. Both calderas are part of the Lake Owyhee Volcanic Field, Oregon that in turn is part of widespread rhyolites associated with the Columbia River Basalt province. We focus on establishing relationships between intracaldera units of Three Fingers caldera and caldera-forming Spring Creek outflow tuff and assessing the distribution of entrained mafic clasts and their often anomalously high, ore-grade concentrations of rare earth elements (REE).

Previous mapping identified two intra-caldera rhyolite units: 1) intra-caldera Spring Creek Tuff and 2) younger rhyolite lavas (Trp). Our main reinterpretation is that devitrified Trp is equivalent to surrounding often glassy, pumiceous to dense or brecciated rhyolite mapped before as intra-caldera Spring Creek Tuff. In addition to field evidence, reinterpreted rhyolites lack vitroclastic textures and are geochemically distinct from outflow Spring Creek Tuff . Outflow tuff units are Fe-rich, low silica rhyolites (~74 wt.% SiO₂, and 3 wt.% FeO) as compared to less Fe rich, high-silica rhyolites (~77 wt.% SiO₂, 2 wt.% FeO) of intra-caldera units. We interpret the investigated area as a rhyolite dome field, erupting subsequent to caldera collapse. The proximity of vents resulted in a complex stratigraphic overlap of rhyolite flows and clastic debris issued from adjacent domes. The predominance of high-standing dome interiors reflects the more resistant nature of dense devitrified rhyolite as compared to pumiceous, glassy, or brecciated rhyolite. New ⁴⁰Ar/³⁹Ar data reveal intra-caldera rhyolites and outflow tuff to be indistinguishable at 15.64 ± 0.08 Ma yet field evidence indicates eruption of post-caldera rhyolites occurred after sedimentation within the caldera. Mafic clasts present in dense glassy or porous intra-caldera rhyolites are reworked fragments of preexisting lava flows that were entrained by subsequent eruptions. Ore-grade REE enrichment of over 2400 ppm Nd in these clasts is likely facilitated by mobilization of REE from earlier rhyolites during renewed rhyolite magmatism and subsequent deposition.