A VOLUMINOUS PULSE OF HOT AND DRY RHYOLITES IN THE CASCADE ARC: THE DESCHUTES FORMATION, CENTRAL OREGON

Although felsic magmas erupted at arc volcanoes typically have cool-wet-oxidized geochemical signatures, some arc rhyolites are hot and relatively dry, more similar to rift settings, and have been attributed to changes in subduction dynamics or tectonic stresses. Characterizing these unusual arc rhyolites and inferring the processes that lead to their formation is critical to our understanding of the subduction system as a whole. The Deschutes Fm. (6.25-5.45 Ma) of Central Oregon records the highest silicic magma production rate in the Cascades arc during the last 17 Ma and is contemporaneous with the onset of intra-arc extension and eruption of low-K tholeiite basalts (LKTs). Deschutes Fm. dacites and rhyolites (60-77 wt. % SiO₂) have drier signatures that are distinct from the arc-typical compositions of the Quaternary Cascades arc. In addition to a crystal-poor (<5%) anhydrous mineral assemblage (plag + cpx + opx + ox), Deschutes Fm. rhyolites have higher FeO/MgO, Y, Eu/Eu*, Dy/Dy*, Dy/Yb and Zr/Sr, indicative of drier evolution. While Fe-Ti oxide f_O2 data for the Quaternary Cascades indicate more oxidizing conditions (NNO to NNO+1), preliminary Deschutes Fm. data indicate more reduced conditions (between NNO-1 and NNO). Instead, the hot-dry-reduced signatures of the Deschutes Fm. rhyolites are more similar to those of the High Lava Plains (HLP), which lie east of the arc and are also accompanied by LKTs (~7.5 Ma) that reach into the rear arc. Trace element and oxygen isotope data suggest that shallow crustal melting was more dominant during Deschutes Fm. time compared to during the Quaternary. More than a third of plagioclase δ¹⁸O values are lower than the closed system FC array. All these low δ¹⁸O plagioclase are from samples with the lowest La/Yb ratios (~5), providing evidence of shallow crustal melting of hydrothermally altered rocks. We propose a petrogenetic model for the Deschutes Fm. in which the same decompression mantle melts responsible for the pulse of LKTs in the HLP were focused beneath the arc by contemporaneous intra-arc extension. Extensional stresses allowed this heightened flux of LKTs into the shallow crust, leading to a period of enhanced shallow melting and fractionation, thereby producing the pulse of hot-dry-reduced rhyolites of the Deschutes Fm.