Chilling and Brecciation of a Devonian Rhyolite Sill Intruded into Wet Sediments, Northern Sierra Nevada, California

Abstract

Localized pockets of peperite breccia occur near the base of a well-exposed hypabyssal rhyolite sill within the Upper Devonian volcanic sequence of the northern Sierra Nevada. The peperites resulted from chilling, fragmentation, and commingling of rhyolite with wet, poorly consolidated sediment. Zones of hyaloclastite breccia extend into the sill above the peperites and represent areas where chilled rhyolite underwent fragmentation but was not intermixed with sediment. A dense spherulitic intergrowth developed rapidly within the chilled rhyolite immediately prior to its fragmentation. Formation of this quench texture probably reflects a reduction in viscosity of the magma due to retention of volatiles in the melt under conditions of relatively high confining pressure.