U-Pb, Single Zircon Pb-Evaporation, and Sm-Nd Isotopic Study of a Granulite Domain in SE Madagascar

Abstract

Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite lenses containing giant monazites. Finally, the emplacement of zircon-bearing calcite veins at 520 Ma represents the latest Pan-African event in this SE Madagascar domain. This precise determination of the Pan-African overprint in this region facilitates comparative discussion of the geodynamic evolution of Madagascar and surrounding areas at the end of the Proterozoic. There is good agreement between the single zircon Pb-evaporation results and conventional isotopie dating; nevertheless slightly younger apparent ages are obtained by the former method. This discrepancy, which may have some important implications, could be related to the discordancy of the analyzed zircon populations.