U‐Pb Geochronology and Isotope Geochemistry of the Archean and Proterozoic Rocks of North‐Central Madagascar

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Abstract

New U‐Pb zircon ages and Sm‐Nd and Rb‐Sr isotopic data are presented for orthogneisses from north‐central Madagascar, including Île Sainte Marie, Alaotra‐Beforona, Maevatanana, and Ambatolampy‐Ambatomarina. A migmatite tonalite gneiss from Île Sainte Marie is dated precisely at $$3187\pm 2$$ Ma and has a Sm‐Nd model age (TDM) of 3204 Ma, thereby establishing a Middle Archean age for the oldest, juvenile gneisses in northeast Madagascar. Dated orthogneisses, intrusive into the schist/paragneiss sequences, range in age between 2522 and 2494 Ma and have Sm‐Nd model ages (TDM) between 3207 Ma and 2541 Ma. These data establish a Late Archean or older age for two of the schist/paragneiss sequences of Madagascar and suggest that the cratonal regions of north‐central Madagascar and south India were once contiguous. Strontium and neodymium isotopic data from the Late Archean rocks are interpreted to reflect mixing between depleted mantle magmas and evolving Middle Archean crust. U‐Pb geochronology of other plutonic igneous rocks demonstrates that the Middle Neoproterozoic (800–640 Ma) represents an important period of igneous activity throughout north‐central Madagascar. In addition, a latest Neoproterozoic–Early Cambrian (580–520 Ma) period of high‐grade metamorphism and intrusive igneous activity is recorded in western and central parts of north Madagascar. We attribute this later activity to the effects of continental collision between East and West Gondwana.

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