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Ultramafic and mafic inclusions and megacrysts in Pliocene basalt, Black Range, New Mexico

¹ Department of Geosciences, North Carolina State University, Raleigh, North Carolina 27607

Inclusions in Pliocene alkalic basalt of the Black Range, southwestern New Mexico (and on the western margin of the Rio Grande rift), are spinel Iherzolite, a differentiated suite of clinopyroxenite and gabbroic (metagabbroic) rock types (olivine-spinel gabbro; spinel gabbro; gabbro), and olivine and andesine megacrysts. All gabbros have reaction zones of mainly olivine and alkali feldspar surrounding orthopyroxene. Pyroxene and oxide geothermometry indicates equilibrium temperatures for Iherzolite and gabbroic inclusions of 1000 to 1050 °C.

Mineral compositions indicate that Iherzolite nodules are of the Cr-diopside group, representing mantle material that equilibrated at 33- to 45-km depth, whereas the remaining inclusions and megacrysts belong to the Al-augite group and had a high-pressure cumulate origin. The pyroxene compositions in the Al-augite group and the modal mineralogy (no garnet) indicate an origin in the 10- to 20-kb range. The absence of an olivine + plagioclase reaction in one gabbro, however, restricts its origin to a maximum pressure of about 11 to 14 kb. Differentiation among the Al-augite inclusions is displayed by progressive Fe enrichment in pyroxenes attended by decreasing Al in oxides. The reaction zones in gabbro developed from the contact between orthopyroxene and host-basalt magma. A bulk composition of one reaction zone shows that a contribution was probably made by the basalt, to account, in particular, for the formation of alkali feldspar in the reaction zones.