Pyroxene-phyric basalt 15075 - Petrography and petrogenesis

Abstract

Sample 15075 is a holocrystalline gabbroic rock that can be classified as a coarse-grained variant of the Apollo 15 pyroxene-phyric basalt group. The phenocrysts of pyroxene show spectacular compositional zoning, from hypersthene-pigeonite, through subcalcic augite, to 'ferropyroxenes.' The predominant trend of differentiation is one of Fe enrichment, which is also reflected in the progressive increase of FeO content in the normally zoned plagioclase. As in other samples of pyroxene-phyric basalts, the plagioclase in 15075 did not crystallize until the pyroxenes had reached a subcalcic augite composition; the onset of plagioclase and Fe-rich pyroxene coprecipitation is marked by sharp discontinuity in the Ca/(Fe + Mg) and Ti/Al trends of the pyroxene.