MULTIPLE UNITS OF THE CAETANO TUFF, NORTHERN NEVADA: CALDERA SOURCE AND PRELIMINARY GEOCHRONOLOGY AND GEOCHEMISTRY

The Caetano Tuff (CT), a widespread late Eocene ash-flow tuff in northern Nevada, was interpreted to be a single unit that erupted from and accumulated in a 20 x >100 km east-west volcano-tectonic trough (Burke & McKee, 1979), although almost all Great Basin ash-flow tuffs are associated with calderas. The CT is petrographically distinctive with abundant smoky quartz and sanidine, and our new field work, ⁴⁰Ar/³⁹Ar dates, and chemical analyses (including published data) demonstrate at least two compositionally and temporally distinct eruptive units. The younger tuff erupted from and accumulated in a 17 (N-S) x <40 km (E-W, present day, including later extension) caldera that coincides with only the eastern part of the trough, whereas much of the classic outflow CT is older and not clearly associated with the caldera. The caldera extends west from the Toiyabe Range to the Shoshone Range but does not continue into the Fish Creek Mtns. The entire caldera boundary is a steep fault against Paleozoic rocks. An approx. 40°, E-tilted section in the Toiyabe Range is at least 2.7 km thick and contains abundant mega- and mesobreccia of Paleozoic sedimentary and Cenozoic volcanic rocks. Intracaldera sections in the Shoshone Range are >700 m thick. This younger CT is mostly high-SiO₂ rhyolite with 74-77% SiO₂. Seven preliminary sanidine, ⁴⁰Ar/³⁹Ar dates on intracaldera and outflow tuff and an intracaldera intrusion range from 33.50±0.07 to 33.72±0.08 Ma. The older CT, whose correlation is based primarily on megascopic composition, forms extensive outflow in the northern Shoshone Range, Battle Mtn, Fish Creek Mtns, and Tobin Range. It is nowhere more than 200 m thick, and the base is generally exposed. The older tuff is low-SiO₂ (71-73%) rhyolite. Two ⁴⁰Ar/³⁹Ar dates on tuffs in the northern Shoshone Range done at different labs are 34.23±0.04 and 34.45±0.08 Ma. This older tuff could be an early eruption from the caldera that produced the younger tuff or from an unrelated, unknown source. A caldera proposed at Mt. Lewis in the northern Shoshone Range (Wrucke & Silberman, 1975) cannot be the source of any CT because their “Caetano intrusions” are ~39 Ma (Kelson et al., 2005). The correlation of other proposed CT exposures in the region remains uncertain because correlation is based only on petrographic characteristics.