Early Rhyolite
Early Rhyolite
middle Pleistocene
Rhyolitic pyroclastic deposits and subordinate lava domes and flows located within Long Valley caldera on, and north of, the resurgent uplift. Represents first major post-caldera eruptive phase. Pyroclastic component is volumetrically dominant and consists of varied rhyolitic pyroclastic flow and fall deposits, with most being nonwelded but some welded, and some reworked; typically massive to vaguely bedded, and phenocryst poor; commonly containing accidental lithics of Bishop Tuff, and more rarely of basalt and Mesozoic granitic and metavolcanic rock types (Bailey, 1989; Hildreth and Fierstein, 2016). Domes and flows are aphyric to sparsely porphyritic with 0-3% phenocrysts of plagioclase, orthopyroxene, rare biotite, and other trace minerals (Bailey, 1989; Hildreth, 2014). Rare mafic enclaves and small blebs are reported to occur in one dome and four lava flows (Bailey, 2004; Hildreth and Fierstein, 2016). Mankinen and others (1986) reported K-Ar ages ranging from 751 ± 16 ka to 652 ± 14 ka. Maximum thickness is at least 622 m (Hildreth, 2004); unit occurs much more extensively across caldera floor in the subsurface (Hildreth and others, 2014). Volume ~100 km3, similar to pre-caldera Glass Mountain Rhyolite (map unit lvg; Hildreth, 2014). Notably, includes Lookout Mountain, a topographically prominent rhyolitic shield with unusual summit caldera (Hildreth and Fierstein, 2016), with Ar-Ar glass date of 678 ± 9 ka (Simon and others, 2014). Also includes Tuff of Little Antelope Valley, a relatively voluminous and extensively distributed high-silica rhyolitic tuff cropping out around and north of Antelope Spring and stratigraphically overlying Bishop Tuff in the subsurface (Hildreth and Fierstein, 2016) and probably one of the earliest post-caldera releases.