Debating the petrogenesis of Proterozoic anorthosites – Reply to comments by Vander Auwera et al. on “Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho”

Abstract

We welcome the opportunity for discussion on the petrogenesis of Proterozoic anorthosites stimulated by a recent comment by Vander Auwera et al. (2014). In their comment Vander Auwera et al. accept our new geochronologic and isotopic data on comagmatic, high-aluminum orthopyroxene megacrysts (HAOMs) and anorthosites (Bybee et al., 2014), but provide an alternate interpretation of the data, in line with their model of magma genesis through melting of a mafic lower crust. In short, they suggest that the similarity of the ages of the HAOMs and a potential lower crustal source for the Rogaland Anorthosite Province (RAP) is evidence that the HAOMs crystallized at 1.05 Ga at the base of a thickened crust from the parent magma of the gabbronoritic Feda suite. They suggest that this hypothesis is supported by overlapping Nd and Pb isotopic compositions of the HAOMs and Feda suite. Vander Auwera et al.’s argument centers on their suggestion that the HAOMs, observed in most Proterozoic anorthosites, are restitic source material entrained in the melts from their proposed gabbronoritic source. This model relies on a two-stage petrogenesis, wherein an underplating event forms lower crustal gabbronoritic cumulates, followed approximately 100 million years later by a lower crustal melting event that entrains HAOMs and then crystallizes vast amounts of plagioclase to form Proterozoic anorthosites. We strongly disagree with this interpretation of our geochronologic