GEOLOGY OF THE AMPHITHEATRE CANYON AREA OF THE CALICO MOUNTAINS, CENTRAL MOJAVE DESERT, CALIFORNIA

The Miocene Mojave Extensional Belt is an extensional orogen represented by a discontinuous NW-SE trending outcrop belt extending from the Buttes to the Rodman Mountains. Extension along this belt led to the formation of the central Mojave metamorphic core complex (CMMCC) and coeval magmatism. The Calico Mountains is located on the upper plate of the Waterman Hills detachment fault in the CMMCC. This study presents new geologic mapping, petrographic analyses, and major element geochemical data from an ~1.25 km² area of the NE Calico Mountains to characterize a section of the early Miocene Pickhandle Formation. This section consists of silicic ignimbrites and fluvially reworked tuffs cut by younger rhyodacitic intrusions. Four main lithologic units with several subunits are identified in the study area: 1) hypabyssal intrusions, including a) rhyodacite with subaerial flow banding and jasper, and b) dacite with jasper-filled brecciated margins and mafic inclusions; 2) tan-reddish silicic ignimbrites, including c) weakly welded biotite-hornblende tuff, d) welded polymict lapilli tuff with fiamme and clasts of rhyodacite, dacite, and plutonic basement, and e) welded monomict rhyodacitic lapilli tuff; 3) fluvially reworked tuffs and tuffaceous conglomerates/sandstones, including f) tannish, bedded, polymict tuffaceous conglomeratic sandstone with fluvial sedimentary structures and clasts of dacite, rhyodacite, and plutonic basement; and 4) pre-Tertiary metasedimentary basement that nonconformably underlies the Pickhandle Formation (units 1-3). These rocks are interpreted as early Miocene pyroclastic deposits episodically erupted onto a volcanic debris apron. Steepened slopes shed debris onto the apron via massive avalanches and debris flows. Pyroclastic flows travelled from distal vents and covered the debris apron, with some of these flows deposited within channels and reworked by alluvial systems. Later, silicic domes intruded these deposits as dikes and plugs along preexisting faults. Rapid cooling fractured the silicic domes, which provided conduits for superheated fluids that hydrothermally altered the preexisting silicic ignimbrites and deposited supersaturated silica solution, represented by the large red jasper veins that fill fractures within the hypabyssal intrusion unit.