THE SEARCHLIGHT MAGMATIC SYSTEM, SOUTHERN NEVADA AND NORTHWEST ARIZONA: ANATOMY AND EVOLUTION OF A PRE- TO SYNEXTENSIONAL PLUTONIC-VOLCANIC SYSTEM IN THE HIGHLY EXTENDED COLORADO RIVER EXTENSIONAL CORRIDOR

The Searchlight magmatic system (SMS) is a large Miocene plutonic-volcanic complex stretching across the Colorado River extensional corridor in S. Nevada and NW Arizona. It includes 1) the 10-km-thick 17.7-15.8 Ma Searchlight pluton (SLP), 2) ~18-16 Ma stratovolcanic complex with a core of intermediate domes and flanking intermediate flows and sills, 3) ~16 Ma rhyolite domes and flows, and 4) N-S and E-W dike swarms. Major E-W extension ~16-13 Ma fragmented this system into steep W-tilted fault blocks, exposing its anatomy in a 15-km-thick cross-section. In the southern Eldorado Mts, the SLP occupies the core of the SMS and is exposed across a mid Miocene ductile-brittle transition. The pre-extensional lower SLP has a steep, W-dipping magmatic foliation overprinted by a subsolidus coaxial to mylonitic fabric at deep levels. The fabric restores to subhorizontal and implies intrusion as subhorizontal sheets followed by synextensional vertical attenuation. Upper, synextensional parts of the pluton intruded as both vertical and subhorizontal sheets. A stratovolcano overlies the pluton to the west and consists of intermediate domes flanked by 2-3 km of flows and sills. Au-bearing veins reside in a hydrothermal alteration halo above the pluton. The east half of the SMS was displaced 20 km to the east to the Mt. Perkins area of the Black Mts, where identical volcanic sequences are found, including ~16 Ma rhyolite lavas-domes (fed by a massive N-S dike swarm) and an 18-16 Ma stratovolcanic complex.

The SMS evolved through an abrupt pre- to synextensional transition ~16 Ma. An isotropic early Miocene stress field favored long-lived, highly evolved magma chambers, subhorizontal intrusions, magma mixing, and intermediate volcanism. As extension began, bimodal volcanism became dominant, with felsic volcanism initially peaking then ending ~0.5 Ma later. Vertical attenuation of mushy plutons and proliferating ~N-S fractures-faults induced rapid evacuation of magma in mostly N-S dike swarms. The rapidly extending crust limited residence time of new mafic magmas, which stymied crustal melting and magma mixing, curtailed intermediate to felsic magmatism, and favored mafic volcanism. However, magmatism locally influenced the stress field, as some E-W dikes and subhorizontal sheets slightly postdate the onset of E-W extension.