CONTRASTING THE WILDCAT MOUNTAIN AND CROOKED RIVER CALDERAS FROM THE PRE-COLUMBIA RIVER BASALT OREGON PLATEAU USING HIGH-PRECISION ZIRCON PETROCHRONOLOGY

Between the accretion of Siletzia at ~50 Ma and the eruption of the Columbia River Basalts at 17-16 Ma there was more limited but still voluminous intraplate volcanism in the Oregon Plateau. We focus on two dominantly rhyolitic systems which are approximately 70 km east of the Cascades Arc, the ~40 Ma Wildcat Mountain Caldera and the immediately adjacent ~30 Ma Crooked River Caldera. The Wildcat Mountain system is calc-alkaline and appears to be typical of large volume tuffs in arcs worldwide. The Crooked River Caldera comprises a much larger tholeiitic system, with the ~500 km³ Tuff of Smith Rock accompanying several other smaller high-silica rhyolite tuffs and lavas. Many of the Crooked River magmas also have distinctly low δ¹⁸O values, which suggest they formed via melting of shallow, hydrothermally altered rocks, a phenomenon also observed in the nearby Yellowstone hotspot track.

We present new high-precision zircon CA-ID-TIMS U-Pb ages and trace element compositions obtained by LA-ICP-MS from both the major tuffs and associated lavas from Wildcat Mountain and Crooked River in order to contrast the development their magmas. Nearly all zircon from the major tuffs crystallized within age uncertainty of each other, but calculated Ti-in-zircon temperatures for Crooked River suggest crystallization temperatures as much as 300° higher than at Wildcat Mountain, and Crooked River zircon also show both milder Eu/Eu* anomalies and significantly lower concentrations of most other trace elements, especially U, Th, and REEs. As these magmas were erupted through essentially the same crust, we use thermal modeling to suggest that a greater magmatic flux from the mantle allowed the hotter magmas of the Crooked River system to melt shallow crust prior to eruption, which the cooler and more crystal-rich Wildcat Mountain magmas could not do. The resulting low-δ¹⁸O values in the Crooked River and Yellowstone magmas may therefore be indicative of high flux, possibly plume or delamination-related magmatism. Applied broadly, similar isotopic signatures may also be indicative of high mantle melting rates and fast rate of intrusion in igneous systems that are too old for precise dating or which are more poorly preserved than the Oregon and Yellowstone rhyolites.