T343 Cenozoic Magmatism and Volcanism in the North American Cordillera

Wednesday, 22 October 2014
1:00 PM - 5:00 PM
306

40AR/39AR AND U-PB AGES OF SILICIC IGNIMBRITES, CALDERAS, AND GRANITIC PLUTONS, SOUTHERN STILLWATER RANGE AND CLAN ALPINE MOUNTAINS, NEVADA: LINKAGES BETWEEN VOLCANIC AND PLUTONIC ROCKS

Paper number: T343-12
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

Faulted calderas in the southern Stillwater Range (SSR) and Clan Alpine Mountains (CAM) reflect intense ~30 to 25 Ma magmatism related to the middle Cenozoic ignimbrite flareup. New geologic mapping, geochemistry, and 40Ar/39Ar and SHRIMP U-Pb zircon dating document 4 nested calderas and related granitoid plutons in sections up to 10 km thick that have been tilted 40->90°.

Cenozoic magmatism in SSR began with eruption of 30(?) Ma rhyolite lava domes. These are overlain by up to 1.5 km of altered intermediate lavas and breccias (29.4±0.4 Ma, U-Pb) that underlie and are inferred to be related to the Job Canyon caldera (JCC). JCC is filled by 2 km of altered rhyolite tuff of Job Canyon (29.4±0.5 Ma, U-Pb), which is overlain by up to 2.5 km of intermediate lavas (59-69% SiO2; 28.5±0.4 Ma, U-Pb; 28.79±0.03 Ma, biotite Ar-Ar). JCC is intruded by the geochemically similar IXL pluton (59-69% SiO2; 28.9±0.4 Ma, U-Pb), which is >4-5 km thick.

Magmatism resumed with eruption of rhyolite lava domes and dikes (25.6±0.3 and 25.5±0.4 Ma, U-Pb), followed by 3 thick ignimbrite units in 3 calderas: tuff of the Louderback Mountains (Tlm; low silica rhyolite; ≥600 m thick; 25.23±0.02 Ma, sanidine Ar-Ar; 25.4±0.2 Ma, U-Pb); tuff of Poco Canyon (Tpc; high-silica rhyolite; up to 4.3 km thick; 25.27±0.05 Ma, 8 sanidine Ar-Ar ages; 26.0±0.2 to 25.7±0.2 Ma, 3 U-Pb ages); and tuff of Elevenmile Canyon (trachydacite to rhyolite; up to 4.5 km thick; 25.12±0.01 Ma, 20 sanidine Ar-Ar ages; 25.7±0.4 to 25.1±0.3 Ma, 4 U-Pb ages). Ages and distribution of calderas indicate nearly simultaneous eruption of Tpc from SSR and Tlm from CAM. The tuff of Elevenmile Canyon (≥2000 km3) fills the much larger Elevenmile Canyon caldera (ECC; now ~15-40 by 45 km) that encompasses most of the SSR and CAM. In the SSR, the composite Freeman Creek pluton (granodiorite, 25.0±0.2 Ma, U-Pb; granite, 24.8±0.4 Ma, U-Pb) intrudes ECC and the Poco Canyon caldera, and at Chalk Mountain, rhyolite porphyry (25.2±0.2 Ma, U-Pb) and granite (24.8±0.3 Ma, U-Pb) plutons intrude the SW margin of ECC.

Field, geochronologic, and geochemical data show that thick plutons (>2-5 km) underlie compositionally and temporally related caldera-filling ignimbrites and indicate that focused silicic magmatism in SSR-CAM from ~30 to 25 Ma entirely replaced the Mesozoic upper crust with Oligocene rocks to depths of >5 to 10 km.