Age and magmatic history of the Antananarivo Block, central Madagascar, asderived from zircon geochronology and Nd isotopic systematics

We report single zircon (207)pb/(206)pb evaporation and SHRIMP ages, combin ed with whole-rock Nd isotopic systematics for granitoid rocks from the Ant ananarivo Block (terrane), one of five tectono-metamorphic units making up the Precambrian basement of central and northern Madagascar. Our data revea l three distinct age groups at similar to 560 to 530, similar to 820 to 720 , and 2520 to 2500 Ma respectively that reflect major magmatic events and c orrelate with similar events in various parts of East Africa and Sri Lanka but not in southwestern India. A widespread high-grade metamorphic event at similar to 550 Ma transformed many of the earlier granitoid gneisses into enderbite-charnockite assemblages. This granulite-facies event is common to Madagascar, East Africa, and southernmost India/Sri Lanka and reflects the final amalgamation of East and West Gondwana, Contrary to previous interpr etations, there is a distinct lack of Kibaran-Grenvillian magmatism or meta morphism in Madagascar, making it unlikely that the island played a major r ole in the accretionary history and amalgamation of the supercontinent Rodi nia, The widespread and voluminous granitoid magmatism at similar to 824 to 720 Ma remains enigmatic, and the tectonic scenario with which it is associated is difficult to reconstruct due to severe tectonic transposition of most g neisses, The Nd isotopic systematics as well as abundant zircon xenocrysts attest to extensive remelting of Archean and Paleoproterozoic crust. On pre sently available data the similar to 740 to 820 Ma granitoids are either re lated to magmatic underplating following plume generation, subcrustal mantl e delamination during break-up and dispersal of Rodinia, or to continental are magmatism related to subduction of the Mozambique ocean. They were empl aced into the ancient crust of central Madagascar as it lay either attached to East Africa or formed a microcontinent within the Mozambique ocean.