PLUME-TRIGGERED DELAMINATION AND THE EARLIEST LOW δ18O CALDERA-FORMING ERUPTIONS OF THE YELLOWSTONE PLUME: IMPLICATIONS FOR LARGE 30–40 MA OREGON CALDERAS

We present new isotopic and trace element data for four newly mapped and dated Paleogene eruptive centers in Oregon: Crooked River (32–28 Ma), Wildcat Mountain (40 Ma), Tower Mountain (ca. 32 Ma), and Mohawk River (32 Ma). The first three calderas are located too far east to be sourced through renewed subduction of the Farallon slab beneath Oregon following accretion of the Siletzia terrane at ~50 Ma, but are located along the suture of the accreted Siletzia terrane and North America. Basalts of all three of these eruptive centers yield high Nb/Yb and Th/Yb ratios, indicating an enriched sublithospheric mantle source, while Mohawk River and other ancestral Cascade calderas have trace element and isotopic (δ¹⁸O and εHf) values that correlate with their location in a subduction zone. The voluminous rhyolitic tuffs and lavas of Crooked River, the largest (41 x 27 km) eruptive center, are hot and dry, densely welded, crystal-poor and zircon-rich, similar to younger Yellowstone-Snake River Plain rhyolites. These multi-cyclic rhyolites have variable δ¹⁸O_zircon values including seven low δ¹⁸O_zircon units (1.8–4.5 ‰), one high δ¹⁸O_zircon unit (7.4–8.8 ‰), and two heterogeneous units (2.0–9.0 ‰). This indicates the need for melting of high δ¹⁸O country rock and previously erupted, hydrothermally altered rhyolites. In order to produce these low δ¹⁸O values, a large heat source, widespread hydrothermal circulation, and repeated remelting are all required to lower the δ¹⁸O value of the high δ¹⁸O country rocks prior to their incorporation. In contrast, the Wildcat Mountain and Tower Mountain eruptive centers have only high δ¹⁸O_zircon values (6.4–7.9 ‰) and normal to low εHf_i values (5.2–12.6), indicating crustal melting of high-δ¹⁸O regionally abundant Phanerozoic supracrustal rocks of the leading edge of North America. We propose that voluminous caldera-forming volcanism in eastern Oregon is produced by the reappearance of the Yellowstone plume east of the Cascadia subduction zone. This is supported by geodynamic plate reconstructions, which put the Yellowstone plume under Crooked River at 32–28 Ma (Wells et al., 2014). Therefore, we suggest that the Yellowstone hotspot is responsible for plume-assisted, piece-meal delamination of portions of the edge of the subduction-influenced leading edge of North America.