DIKE EMPLACEMENT AT THE SEARCHLIGHT, NV, VOLCANO-PLUTONIC COMPLEX

Nearly 90º west tilting in the southern Eldorado and northern Newberry Mountains provides a 15 km deep window into the Miocene Searchlight volcano-plutonic system. The 10 km thick pluton is divided into upper, fine-grained quartz monzonite (USL); a middle zone of low to high-silica granite (MSL); and a thick, lower sequence of coarse quartz monzonite (LSL). The 15.8-17 Ma pluton is coeval with a major portion of the overlying volcanic sequence. A swarm of dikes cuts the southern part of the pluton and intrudes Proterozoic gneiss in the pluton roof. Here, we present new field observations and petrologic and SHRIMP U-Pb data for these dikes, referred to as the Searchlight dike swarm.

The Searchlight dike swarm includes two distinct dike generations. Both are large (typically >~5 m across) and characterized by steep dips and near E-W strikes. The first comprises trachydacite porphyry, with prominent plagioclase phenocrysts and lesser biotite, but also includes sparser phaneritic dikes with similar composition, very similar to the dominant USL lithology. Dikes of both types yield ages of 16.6+/-0.3 Ma (2?). Locally, these dikes vary appreciably from E-W strike. Identical porphyries are found as irregularly shaped intrusions into the Proterozoic gneiss and pods within USL. Trachydacites in the volcanic section are also very similar. The second and more voluminous dike generation is marked by low-Si to high-Si rhyolites that cut trachydacite porphyry dikes; one SHRIMP analysis yields an age of 16.0 +/-0.2 Ma (more in progress). These dikes generally strike ~E-W. Individual dikes can be traced from deep in USL, through the Proterozoic roof, and into the overlying volcanic section, a paleodepth range of 5 km, over which dikes show only slight textural variations. The rhyolite dikes commonly have mafic margins and contain local concentrations of mafic magmatic enclaves.

The dikes mark mild N-S extension during emplacement of the Searchlight pluton. The N-S, ~15.5 Ma Eldorado dike swarm, which cuts Searchlight pluton, demonstrates that major E-W extension began after the pluton was largely or entirely emplaced/solidified. The earlier extensional episode tapped early, more intermediate Searchlight magma [dated at 16.5-17 Ma] and probably fed abundant dacite. The younger diking episode tapped later, more MSL granitic magma (dated at about 15.8-16.2 Ma—including highly evolved, high-silica granite.