Middle Cretaceous ash-flow tuff and caldera-collapse deposit in the Minarets Caldera, east-central Sierra Nevada, California

Abstract

A 2.3-km section of ash-flow tuff and associated caldera-collapse deposit, representing the extrusive facies of part of the Sierra Nevada batholith, is totally exposed from its floor to its top in the Minarets Caldera, east-central Sierra Nevada. Rapid burial and subsequent hornblende hornfels facies metamorphism resulted in remarkable preservation of primary textures and structures, despite the development of cleavage domains in parts of the caldera fill; late Tertiary uplift and Quaternary erosion have produced a rugged terrain where every meter of section is available for study. Large-scale caldera-filling eruption of ash-flow tuff was interrupted by emplacement of a wedge-shaped mass of caldera-collapse deposit as much as 2 km thick, whose volume exceeded 70 km 3 . Individual clasts in the caldera-collapse deposit range to as much as 1.8 km across and include a wide variety of andesitic to rhyoliic lavas and related volcaniclastic rocks, remnants of a precaldera volcanic field that was probably much more extensive than the caldera itself. The caldera-fill sequence rests with angular unconformity on a rugged surface eroded into older volcanic rocks; the sequence is capped by bedded volcaniclastic rocks, including delicately laminated tuffs of probable caldera lake origin. The total aerial extent of the Minarets Caldera is not known, but the area studied, plus scattered pendants of ash-flow tuff and associated volcaniclastic rocks to the west, defines a 30- x 22-km elliptical area that may approximate its original shape. The caldera fill is invaded by a body of quartz monzonite porphyry, locally miarolitic,that was probably emplaced during an episode of caldera resurgence.