THE SPIRIT MOUNTAIN BATHOLITH AND SECRET PASS CANYON VOLCANIC CENTER: PERSPECTIVES ON SILICIC MAGMATISM FROM THE MID TO UPPER CRUST

The Miocene Spirit Mountain Batholith (SMB) and Secret Pass Canyon Volcanic Center (SPCVC), in southern Nevada and western Arizona, document the shallow batholithic and hypabyssal portions of two large silicic magma systems. Crustal thinning in the Colorado River Extensional Corridor during the mid-Miocene resulted in near-cross-sectional exposures of these two systems. The two areas currently lie ~40 km apart on opposite sides of the Colorado River, but inferred large-scale movement on the regional detachment system (Faulds et al., 2000, 2001) permits proximity of the two at the time when they were magmatically active.

Magmatic activity commenced at the SPCVC around 18.5 Ma, when voluminous trachydacite was emplaced as a shallow, flat-topped sill-laccolith complex into a sequence of tuffs and volcanogenic sediments. The laccolith appears to have been emplaced incrementally, comprising many sheets which coalesced to form a fairly homogeneous mass. Subsequently, between ~17.7 and 17.3 Ma, dome-derived rhyolite flows and breccias and at least one silicic ignimbrite were deposited. Immediately afterward, this sequence was intruded by andesitic to dacitic dikes, sills, and volcanic necks.

The SMB represents gradual accumulation of magma to form a patchwork batholith over 2 m.y. (from ~17.4 to 15.3 Ma) by repeated intrusion. Field evidence suggests that magma ponded as sill-like structures within the pluton. Repeated in situ fractionation and melt segregation of magma batches resulted in the formation of a gradationally stratified granite pluton with high silica granite at the roof grading downward through granite into a magmatically foliated quartz monzonite at the base of exposure. For later magma batches which intruded a cooler, waning SMB system, intrusion geometries are preserved as a plexus of dikes and sills and suggest a similar construction mechanism for the SMB as a whole.

Despite their proximity and the overlap in lifetimes, no evidence has surfaced to definitively connect the two systems. The onset of SPCVC activity appears to have predated that of SMB by a million years, though sparse antecrystic zircons from the SMB coincide with the earliest eruptions. Also, profuse rhyolites of the SPCVC are coeval with the early dated stages of the SMB, permitting a possible linkage which we intend to explore further.