Disequilibrium crystal–liquid processes at Hamblin–Cleopatra volcano, Lake Mead area, Nevada

The 60 km3 Hamblin–Cleopatra stratovolcano produced shoshonite, latite, and trachyte lavas throughout its Miocene eruptive history. Low-silica rhyolite and silica-undersaturated hawaiite erupted before and after lavas of the Hamblin–Cleopatra volcano. Shoshonite, latite, and trachyte resulted from contamination of felsic (trachyte to low-silica rhyolite) anatectic liquids with crystals from hawaiite. Most of the entrained crystals were not in equilibrium with liquid represented by groundmass, but were mingled with liquid shortly before eruption. Crystal aggregates are common inclusions in the lavas, and are sources of the contaminating minerals. The resulting bulk compositions of these porphyritic lavas form a continuum that resembles a liquid line of descent, as dictated by mass balance.

► Lava compositions range from 46 to 62 wt.% SiO2.
► Groundmass in lavas is trachyte to low-silica rhyolite.
► Crystals in lavas are mostly out of equilibrium with groundmass.
► Disrupted crystal aggregates, derived from alkali basalt or hawaiite, became entrained in felsic liquids.
► Contamination of felsic magma by xenocrysts results in chemical trends resembling “liquid lines of descent.”