U–Pb zircon geochronology and geochemical constraints on the age and origin of late Neoarchean leucosomes in migmatites from the Maevatanana area, Madagascar

Abstract

Migmatites represent the dominant lithological facies in the Maevatanana area of Madagascar. The migmatites are composite rocks with hybrid metamorphic and magmatic features, comprising 60% melanosome bands that are interlayered and/or tectonically interleaved with 40% of centimeter-to decimeter-scale quartz–feldspar leucosome veins. The leucosome and granite veins are bordered by a series of gently dipping shear zones in the migmatites. U–Pb zircon dating shows that the leucosome veins formed at 2548.3 ± 5.3 Ma and are coeval with intrusion of the granite veins at 2552.2 ± 6.1 Ma. Furthermore, the leucosome and granite veins have uniform chondrite-normalized rare earth element (REE) patterns which are characterized by light REE enrichment relative to heavy REE, and negative or slightly positive Eu anomalies (δEu = 0.61–1.13). Primitive-mantle-normalized trace element patterns of the leucosome and granite veins are enriched in large-ion lithophile elements (Ba and K) and show pronounced depletions in high-field strength elements (Th, U, Ta, Nb, Zr, Hf, and Ti), suggesting that the leucosome and granite veins in the migmatites are syntectonic and synigneous intrusives. Primitive-mantle-normalized trace element patterns of the leucosome veins and melanosome bands in the migmatites are similar and suggest that the granite veins in the migmatites were generated by partial melting of medium-to high-K metabasalt, whereas the melanosome bands originated through metamorphism of calc-alkaline basalts. As such, the leucosome veins were likely generated by partial melting of melanosome bands in the migmatites. Calculated zircon saturation temperatures (TZr) of the leucosome veins are 599–685 °C, which represents the magma crystallization temperatures, and the pressures of magma formation range from 1.2 to 3.2 GPa. These P–T estimates for magma formation and crystallization are consistent with those of 605–658 °C and 1.22–1.43 GPa for metamorphism of the melanosome bands in the migmatites, suggesting that the leucosome veins in the migmatites were formed during peak metamorphism of the melanosome bands.