GEOLOGIC FRAMEWORK AND EVOLUTION OF THE HIGH ROCK CANYON VOLCANIC CENTER, NORTHWESTERN NEVADA: AN EARLY CALDERA-FOCUSED SYSTEM OF THE YELLOWSTONE HOTSPOT TRACK

The High Rock Canyon volcanic center is one of three nearly coeval centers that commenced 16.5 to 16.0 Ma on initiation of the Yellowstone hotspot. The High Rock caldera (HRC), centered at 41°27’N 119°23’W, formed during eruption of the Summit Lake Tuff (SLT; 16.33±0.03 Ma), a zoned ash-flow sheet of mostly low-Si rhyolite containing distinctive complexly-twinned anorthoclase phenocrysts and light and dark pumices. The SLT is preserved over a 100 km diameter area N, E, and S of the caldera and is covered to the W. Although strongly influenced by paleotopography, the sheet thins, and sparse flow indicators point, outward from the caldera. Lithic fragments of rhyolite to basalt are 15+ cm in diameter near the caldera; size and abundance decreases with distance. Neither the topographic margin nor intracaldera tuff are exposed. Vents of voluminous post-collapse silicic to mafic tuffs and lavas mark the 30 – 40 km ring-fracture zone (RFZ). Tuffs, lahars, tuffaceous lacustrine sediments, and diatomites fill the intracaldera basin; tuffs associated with vertebrate-bearing lahars are 15.96±0.10 and 15.62±0.06 Ma. Highly evolved peralkaline rhyolite (comendite), subalkaline rhyolite, and olivine basalt, in part aphyric (superheated), were associated throughout the evolution of the center. Post-collapse volcanic rocks erupted from or near the RFZ dip into the HRC and some units also extend outward; some drape reactivated faults of the RFZ. 500+ m of basalt, overlain by phenocryst-poor to aphyric rhyolite and comendite (16.22±0.03 Ma) erupted along the western caldera margin; the peralkaline tuff of Trough Mountain erupted along the eastern margin. The overlying Soldier Meadow Tuff (SMT; Smith et al., 2009) erupted at 16.12±0.05 Ma; very similar tuffs and lavas above the SMT are 16.04±0.03 – 15.86±0.03 Ma. Vents of the younger, low Fe and zircon-bearing tuffs of Badger Mtn. – Alkali Flat may define the northern margin of the HRC. Rhyolite flows (15.71±0.03 Ma) erupted south of the HRC. Undated, mostly intermediate to mafic magmas then formed thinly layered and commonly welded pyroclastic deposits and lavas within the caldera. Explosions placed 5+ m blocks of SMT within this sequence. Three domes (largest ~5 km wide, 200 m high in the center of the caldera) of uplifted intracaldera deposits appear to mark subjacent intrusions.