Fe content in pyroxenes from a calc-alkalic volcanic suite, New Mexico, U.S.A.

Abstract

Electron microprobe analyses of clinopyroxene, both phenocrysts and groundmass, in the calc-alkalic suite (basalt-basaltic andesite-andesite-latite) of the Mogollon Plateau, New Mexico, show little variation in Fe content. The clinopyroxene averages approximately Wo 40En 42Fs 18 in composition and does not vary more than about 7 percent Fs content throughout a suite of rocks that range from 50 to 65 percent in SiO 2 content. Orthopyroxene, on the other hand, is restricted to basaltic andesites and andesites and shows slightly greater variation in Fe content than the clinopyroxenes. It is suggested that the factors limiting concentration of Fe in calc-alkalic magmas, such as high oxygen pressures, are reflected in the clinopyroxenes crystallizing in these magmas by their limited range in Fe content.