Emplacement and Thermal History of a Rhyolite Lava Flow near Fortymile Canyon, Southern Nevada

Abstract

The rhyolite of Comb Peak is one of a series of related rhyolitic lava flows and domes that postdate formation of the Timber Mountain caldera in southern Nevada. The Comb Peak body consists of bedded rhyolitic tuffs overlain by a rhyolitic lava flow that has a foliated interior with fluidal struc- ture, enveloped by breccia. The vent for the lava flow and the pre-eruption topography have been exposed by erosion and indicate the general condi- tions and direction of flow. These indications have been corroborated by study of the foliation pattern and by a semiquantitative statistical analysis of flow folds and related features. Geologic relationships in- dicate that brecciation of the lava flow occurred mainly during periods when the flow was spreading, whereas the body eroded its floor and walls much as does a glacier during periods when the flow was con- fined. Tuffs adjacent to parts of the lava flow have been conspicuously modified. Local compaction and in- duration (fusion) by the hot lava caused the bed- ded tuffs to weld so that they resemble welded ash- flow tuffs except for characteristic bedding and sorting indicative of their ash-fall origin. The zone of fusion is everywhere parallel to the contacts of the lava flow and intersects bedding at high angles. Locally fusion affected the tuffs for a thickness of 75 m or more from the contact of the flow. Approxi- mate calculation of the heat flow necessary to ac- complish the observed alteration indicates that sim- ple conduction from the margins of the flow would not have been adequate. Heat transfer was greatly increased, probably by convection of superheated steam from the edges of the flow through porous tuffs. Simultaneous cooling and crystallization of the fused tuffs and the lava flow produced a series of zones that encompass the entire Comb Peak body and cross contacts between emplacement units. The basal vitrophyre zone, for example, is in the lowest part of the lava flow in many places, but elsewhere it is entirely within fused tuffs and has been mapped as much as 300 m beyond the present posi- tion of the flow contact.