New Mexico Geological Society Annual Spring Meeting — Abstracts

Eruption of alkali-olivine basalt has dominated Quaternary volcanism in the area east of Elephant Butte Reservoir. Cinder cones with 60-70 meters of relief are built on thick sequences of pyroclastic debris, basalt flows, cone sheets, and radial dikes. The base of South Larson Cone's northern flank is underlain by a volcanic vent breccia. Several varieties of lithic and single-crystal inclusions occur hoth within this breccia and in adjacert flows. Lithic inclusions include granulites ( hypersthene + oligioclase + sanidine + quartz) and spinel lherzolites (forsterite + bronzite + augite + spinel). Megacrysts include anhedral vitreous augite, subhedral anorthoclase, and rare olivine.

Petrological and geochemical data are suggestive of a cognate origin for the megacrysts with respect to host basalts. Trace element compositions for both augite and anorthoclase megacrysts relative to host basalt compare favorably to experimental partition coefficients. Similar initial ⁸⁷Sr/⁸⁶Sr values for the anorthoclase megacryst and host basalt (.7038 and .7036, respectively) further supports a cognate origin. Lherzolite xenoliths possessing a metamorphic fabric could represent upper mantle residua related to generation of the basalts. Elevated present ⁸⁷Sr/⁸⁶Sr values of .7239 and .7385 for two granulite inclusions indicate a crustal origin.

Two-pyroxene geothermometry on spinel lherzolite inclusions yield equilibration temperatures in the range of 1170-1240°C. A pressure-dependent two-feldspar geothermometer was used to calculate upper and lower limit temperatures of 880 and 950°C for granulite inclusions.