Abstract

Published geologic maps, regional geological relations, and new U-Pb ages for intrusive igneous rocks in west-central Madagascar define a 450-km-long belt of rocks emplaced in middle Neoproterozoic time. We report precise U-Pb zircon and baddeleyite ages for 11 coeval gabbro and granitoid plutons from the Itremo region, located in the southern part of this belt. The ages for all gabbroic and granitic plutons and deformed equivalents define an ∼25 m.y. period of magmatic activity between 804 and 779 Ma (at maximum uncertainty). Granitoids intrusive into the Quartzo-Schisto-Calcaire series provide a minimum depositional age of 791 Ma for this Mesoproterozoic platformal sedimentary sequence. Our results, combined with other recent U-Pb age determinations, define a belt of plutonic rocks in west-central Madagascar emplaced between 804 and 776 Ma. We propose that these middle Neoproterozoic rocks constitute the root of a continental magmatic arc emplaced at the time of, or slightly preceding, the breakup of the Rodinian supercontinent. Neoproterozoic plate reconstructions place Madagascar on the putative margin of Rodinia, and therefore the plutonic belt in west-central Madagascar provides important constraints on the timing and extent of middle Neoproterozoic tectonic events in Rodinia and the critical period of Rodinia's transformation into Gondwana.

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