Megacrystic pyroxene basalts sample deep crustal gabbroic cumulates beneath the Mount Taylor volcanic field, New Mexico

Abstract

Distributed over the ~2.3m.y. history of the alkaline and compositionally diverse Mount Taylor Volcanic Field (MTVF), New Mexico is a widespread texturally distinct family of differentiated basalts that contain resorbed megacrysts (up to 3cm) of plagioclase, clinopyroxene, and olivine±Ti–magnetite±ilmenite±orthopyroxene. These lavas have gabbroic cumulate inclusions with mineral compositions similar to the megacrysts, suggesting a common origin. Gabbroic and megacrystic clinopyroxenes form positive linear arrays in TiO2 (0.2–2.3wt.%) with respect to Al2O3 (0.7–9.3wt.%). Plagioclase (An41–80) from representative thin sections analyzed for 87Sr/86Sr by laser ablation ICP-MS range from 0.7036 to 0.7048. The low 87Sr/86Sr plagioclases (0.7036 to 0.7037) are associated with high Ti–Al clinopyroxenes. Likewise, the higher 87Sr/86Sr plagioclases (0.7043 to 0.7047) are associated with the low-Al clinopyroxenes. Taken together, the pyroxene and plagioclase megacrysts appear to track the differentiation of a gabbroic pluton (or related plutons) from alkaline to Si-saturated conditions by fractional crystallization and crustal assimilation. Clinopyroxene-liquid geobarometry calculations suggest that crystallization occurred near the crust–mantle transition at an average of ~1200°C and 12–13kbar. The distribution of the megacrystic pyroxene basalts suggests that a gabbroic intrusive body underlies subregions of the MTVF that have generated silicic magmas. The gabbro is interpreted to be a significant heat and mass input into the lower crust that is capable of driving the petrogenesis of diverse silicic compositions (through fractionation and crustal assimilation), including mugearites, trachytes, trachy-andesites and dacites, high-Si rhyolites, and topaz rhyolites of the MTVF.