MESOPROTEROZOIC MADAGASCAR-AFRICA CONNECTION BASED ON SHRIMP U-PB AGES OF DETRITAL ZIRCONS FROM THE ITREMO GROUP AND SAHANTAHA SERIES IN CENTRAL AND NORTHERN MADAGASCAR

The Itremo Group of central Madagascar was deposited sometime between 1500-1700 Ma based on evidence from U-Pb ages of detrital zircons from quartzites, carbon isotope signatures of marbles, and stromatolite morphology. It contains several Paleoproterozoic and late Archean detrital zircon populations with two dominant ones at 1850 ± 3 Ma (30% of 130 detrital ages analysed by mixture modeling) and 2500 ± 2 Ma (35% of grains analysed). Preliminary data from the Sahantaha Series in northern Madagascar indicate strong correspondence with the Itremo Group, with detrital ages concentrated at 1834 ± 4 Ma and 2502 ± 8 Ma. Basement rocks around 2500 Ma are known from within Madagascar, but 1850 Ma rocks are not, so a non-Malagasy source terrane is required. Comparison of detrital ages with U-Pb basement ages in neighbouring continents indicates a predominantly East African provenance, as there is increasing documentation of 1850 Ma ages in the Tanzanian craton and surrounding area but rocks of this age are absent from the Dharwar Craton of India. Of equal importance, the mid-Archean ages that are common in the Dharwar craton and the Antongil block of northeastern Madagascar are not found in the Malagasy metasediments. We conclude that India and the Antongil block were distant from the continental shelf environments where the Itremo Group and Sahantaha Series were deposited. Paleogeographic reconstructions in which Madagascar is located next to India, along the western edge of Rodinia, are therefore probably incorrect. It is likely that the small Antongil block in northeastern Madagascar was part of India, but our data suggest that most of Madagascar was in fact juxtaposed with the Tanzanian craton in the Paleo- and Mesoproterozoic, possibly forming a passive margin along the western side of the Mozambique ocean. This also means that the components of modern Madagascar were not assembled until Neoproterozoic time.