Magma dynamics at the Moho: insights from high-pressure megacrysts and Proterozoic anorthosites

High-aluminium orthopyroxene megacrysts (up to 1 m in length) are common cogenetic features of Proterozoic anorthosite massifs and are amongst the deepest crystallization products in this magmatic system, at upper mantle or lower crustal depths (11-15 kbar). The anorthositic intrusives which host these megacrysts are enigmatic features of the Proterozoic, with ongoing debate surrounding their source (mantle vs. mafic lower crust), tectonic setting, and restricted temporality. Using the geochemistry of megacrysts and their host anorthosites, we assess the petrogenesis of the anorthosites and magmatic processes operating at deep levels in these systems. Isotopic compositions from three populations of megacrysts in classic Proterozoic anorthosite localities (Mealy Mountains Intrusive Suite, Rogaland Anorthosite Province and Nain Plutonic Suite) indicate that the highest-pressure (highest Al₂O₃) megacrysts crystallized in a closed magmatic system, 130-110 m.y. before the host anorthosites (Figure 1). The most tenable means of creating such a closed-system scenario is for the rising mafic magmas to pond and begin crystallizing at the Moho. Most of these mafic cumulates sink into the mantle, but some megacrysts are entrained by plagioclase-rich crystal mushes that rise to upper crustal levels (±4 kbar) to form the anorthosite massifs. These results thus provide direct petrologic and geochemical evidence for magma ponding at the Moho - a commonly proposed, but unsupported phenomenon. The duration of anorthosite crystallisation varies from 12-80 m.y., and in combination with these megacryst ages, indicates that the entire magmatic system was in operation for about 100 m.y. An Andean-arc setting is therefore likely, given its capability of sustaining such prolonged, voluminous, geographically-restricted magmatism. Our observation of formation of mafic cumulates at the Moho may also provide an explanation for the missing crustal silicate reservoir inferred from the discrepancy between predicted and observed average crustal compositions. Using energy-constrained AFC modeling, we also show that the megacrysts and anorthosites formed from a depleted-mantle-derived magma which was contaminated by assimilating lower crustal, and later, upper crustal material. These results provide constraints on the dynamics of mafic magma rise through the lithosphere, illustrate the geochemical effects of ponding and crustal contamination on these magmas at the Moho and refine our current understanding of the source and tectonic setting of Proterozoic anorthosites.igure 1