Flare Up of Hot-Dry-Reduced Ignimbrites Related to Extension in the Cascades Arc: The Deschutes Formation, Central Oregon.

This article focuses on the petrogenesis of the Deschutes Formation ignimbrite flare-up in Central Oregon’s Cascades arc, a rare period of intense silicic volcanism occurring between 6.25 and 5.45 million years ago. The study finds that these ignimbrites are crystal-poor, hot, dry, and reduced rhyolites distinct from typical arc magmas, with geochemical and isotopic evidence indicating their origin primarily from partial melting of low-K tholeiitic (LKT) basaltic crust rather than simple fractional crystallization. Complex magma storage and mingling of distinct magmas occurred in a shallow crustal system influenced by intra-arc extension, which facilitated decompression melting of the mantle and hydrothermal alteration of the crust, producing low δ^18O melts. The flare-up is linked to a high flux of decompression melts focused into the arc by extensional tectonics, paralleling contemporaneous volcanism in the nearby High Lava Plains, and reflects progressive depletion ("basaltification") of the crustal source over time.