UNUSUAL CHEMISTRY AND COMPLEX MAGMATIC HISTORY OF THE ENIGMA STRATOVOLCANO, MCCULLOUGH RANGE, SOUTHERN NEVADA

Single crystal microprobe data, along with whole rock chemistry and isotope data, from the ca. 18.8 – 15.2 Ma Engima Stratovolcano, McCullough Range, reveal a complex magmatic history from depth of melt generation in metasomatized lithospheric mantle (LM) to various crustal reservoirs. The Enigma volcano is composed of two volcanic sequences, but we discuss only the older sequence here. In the older sequence, unusually Mg-, P-, and LREE-enriched compositions prevail and common mineralogy includes olivine (Fo65-76), clinopyroxene (cpx; Di80+), and fluorapatite, with accessory phlogopite and Cr-spinel. Several units contain amphibole and orthopyroxene (opx) as well. Feldspar occurs in all but the most mafic unit, the Hopper basanite, although it is often restricted to the groundmass. Ultramafic inclusions are common regardless of bulk composition (44 – 62% SiO2), and include spinel peridotite and both ortho- and clinopyroxenite. Excellent preservation of mineral phases allowed application of the 2-pyroxene, cpx-liquid, opx-liquid, olivine-liquid, and Si-activity thermobarometers. Because liquid compositions were unavailable on most rocks, bulk compositions were applied to mineral core compositions for several mineral species where there was sufficient evidence of equilibrium. Depth of melt generation was constrained for basalts to 18-22 kbar, well within the LM for the lower Miocene. Olivine-liquid thermometry for two units indicates temperatures (T) >1200 C, consistent with cpx data that indicate T of 1100-1300 C, though these are all anhydrous T in systems containing hydrous phases. Cpx and opx barometric data indicate, regardless of bulk composition, consistent crystallization pressures (P) of 8-9 kbar, 6-7 kbar, and 3-5 kbar, indicating that magmas stalled at these levels in the crust. Pyroxenite not in equilibrium with the host rock show P of 19 – 22 kbar, perhaps indicating a source near the depth of melting. In one andesite, a population of unusually Mg-rich, anhedral opx cores showed equilibrium with a bulk composition similar to the Hopper basanite at P of 11 kbar. The cores had significant overgrowth of opx with identical composition to other grains in the andesite. The equilibration of opx with a more mafic unit may indicate magma mixing at depth prior to the main evolution of the andesitic magma.