Aurora volcanic field
Aurora volcanic field
late Pleistocene to Pliocene
The Aurora volcanic field consists of compositionally variable, high-K to shoshonitic lavas, dominantly basaltic andesite to andesite plus high-silica rhyolite, forming monogenetic lava and cinder cones, lava flows, shield volcanoes, and rhyolitic domes, erupted across ~325 km2 in the uplands northeast of Mono Lake, mostly beyond map extent (e.g., Lange and Carmichael, 1996; John and others, 2015). The basaltic andesite of Locomotive Point (~3.9 Ma) and the hornblende trachydacite of Cedar Hill (~3.5 to ~2.4 Ma) of John and others (2015) crop out in the map area. Unconformably overlies the more porphyritic volcanic rocks of the Miocene Bodie Hills volcanic field (part of the ancestral Cascade arc; map unit aca).
Compositions vary continuously from trachybasalt to trachydacite (66% SiO<sub>2), plus high-silica rhyolite (75% SiO<sub>2; John and others, 2015). In order of decreasing volume, rock types include basaltic andesite, hornblende andesite, two-pyroxene andesite, high-silica rhyolite, dacite, and basalt (Lange and Carmichael, 1996). Generally weakly to moderately porphyritic (1-30%) with phenocrysts variably consisting of olivine, plagioclase, augite, hornblende, orthopyroxene, and sparse biotite (Lange and Carmichael, 1996).
John and others (2012, 2015), Kingdon and others, (2013), and Kingdon (2016) interpret the field to be bimodal on the basis of the gap between ~66 and 75 wt% SiO<sub>2, but Lange and Carmichael (1966) do not due to the presence of intermediate compositions.
Youngest ages reported are 0.04 ± 0.04 Ma (Reheis and others, 2002) or 0.11 ± 0.08 Ma (Lange and others 1993); oldest ages reported are ~3.9 Ma (Lange and others, 1993; John and others, 2015).