Volcanism, structure, and geochronology of Long Valley Caldera, Mono County, California

Abstract

Long Valley caldera, a 17‐ by 32‐km elliptical depression on the east front of the Sierra Nevada, formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68‐0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende‐biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende‐biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self‐sealing of near‐surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated north‐west‐trending Sierra Nevada frontal faults that tap hot water at depth.

The unique radar properties of silicic lava domes

“…The finely porphyritic rhyolite shows both chemical and mineralogical zonation [Bailey et al, 1976], suggesting that mingling of two different magma types occurred during emplacement [Vogel et al, 1989]. According to detailed surface mapping by Sampson [1987], Deadman Dome consists primarily of coarsely porphyritic lava, Glass Creek Dome contains roughly equal amounts of coarsely and finely porphyritic varieties, and Obsidian Dome is entirely finely porphyritic rhyolite.…”

“…[5] Two distinct lava types comprise the three most recently emplaced Inyo domes: a finely porphyritic (phenocrysts generally <2 mm) and a coarsely porphyritic rhyolite (phenocrysts generally 3 -10 mm) [Bailey et al, 1976[Bailey et al, , 1983Sampson, 1987;Sampson and Cameron, 1987;Swanson et al, 1989]. The finely porphyritic rhyolite shows both chemical and mineralogical zonation [Bailey et al, 1976], suggesting that mingling of two different magma types occurred during emplacement [Vogel et al, 1989].…”

San Jacinto Intrusive Complex: 2. Geochemistry

“…Linear relationships may also result from the m1x1ng of relatively unfractionated and relatively fractionated liquids near the top of a fractionally crystallizing magma chamber [McBirney, 1980;McBirney et al, 1985;Baker and McBirney, 1985]. There appears no doubt that this mechanism can be quite efficient in developing a range of compositions within a cooling pluton, as shown by both experimental work [McBirney, 1980;McBirney et al, 1985;Turner, 1980] and by the study of exhumed natural examples [e.g., Wilcox, 1954;Bailey et al , 1976] • A variant on this process is where the felsic cap contains a significant component derived from assimilation or melting of country rocks, and mixing is between relatively mafic and felsic materials derived ultimately from quite different sources [e.g., Gerlach and Grove, 1982;Grove et al, 1982].…”

Mineral resources of the Minarets Wilderness and adjacent areas, Madera and Mono counties, California

“…The Pleistocene volcanic and structural evolution of the Long Valley caldera has been described by Bailey, Dalrymple, and Lanphere (1976) and will only be briefly summarized here.…”