Isotopic disequilibrium and lower crustal contamination in slowly ascending magmas: Insights from Proterozoic anorthosites

Abstract

Many Proterozoic anorthosite massifs show crustal isotopic signatures that have, for decades, fuelled debate regarding the source of these temporally-restricted magmas. Are these signatures indicative of lower crustal melting or of significant assimilation of crustal material into mantle-derived magmas? Traditional whole rock isotopic tracers (Sr, Nd, Pb and Os), like other geochemical, petrological and experimental tools, have failed to identify unambiguously the origins of the crust-like signature and resolve the source controversies for these feldspathic, cumulate intrusives. We make use of high precision Sr, Nd and Pb isotopic compositions of mineral phases (plag, opx, mag) and comagmatic, high-pressure orthopyroxene megacrysts as well as whole rock anorthosites/leuconorites from the Mealy Mountains Intrusive Suite (MMIS) and the Nain Plutonic Suite (NPS) to probe the origin of the crustal isotopic signatures and assess the importance of differentiation at lower crustal depths. This selection of samples represents fragments from various stages of the polybaric ascent of the magmas, while the study of the Mealy Mountains Intrusive Suite and the Nain Plutonic Suite is instructive as each is intruded into crust of significantly different age and isotopic composition. We observe marked differences in the whole-rock isotopic composition of Proterozoic anorthosites and high-pressure megacrysts (e.g. εNd;T=+2 to −10) intruded into crustal terranes of different ages and isotopic compositions. Evidence for varying degrees of internal isotopic disequilibrium (ΔNd, ΔSr, ΔPb) in anorthosites from these different terranes reinforces the notion that crustal contamination, and more importantly, the nature of the crustal assimilant, has a profound influence on the chemical signature of Proterozoic anorthosites. While most samples from the MMIS and NPS show significant and measurable ΔNd and ΔPb disequilibrium, ΔSr compositions cluster around zero. This decoupling in disequilibrium geometries cannot be explained by melting of the lower crust. Assimilation of crust with distinctive Sr, Nd and Pb isotopic compositions does, however, explain the origin of decoupling in internal mineral isotopic compositions. We also find unexpected patterns of internal isotopic disequilibrium, such as isotopically radiogenic orthopyroxene relative to plagioclase and differences in plagioclase isotopic disequilibrium between orthopyroxene- and olivine-bearing samples. These various lines of evidence provide strong support for the generation of crustal isotopic signatures through assimilation, and not anatexis, of the lower crust. These isotopic data show that anorthosite petrogenesis likely involves significant differentiation and solidification at lower crustal depths, followed by ascent of high-crystallinity bodies (⩾50% crystallinity) to mid- or upper crustal levels. We show that protracted lower crustal differentiation imparts a clear chemical and isotopic signature on mantle-derived magmas of Proterozoic anorthosites and that this process is central in the development of such slowly ascending, plagioclase-rich magmas.