Neoproterozoic continental arc magmatism in west-central Madagascar

Published geologic maps, regional geological relations, and new U-Pb ages f or intrusive igneous rocks in west-central Madagascar define a 450-km-long belt of rocks emplaced in middle Neoproterozoic time. We report precise U-P b zircon and baddeleyite ages for 11 coeval gabbro and granitoid plutons fr om the Itremo region, located in the southern part of this bell. The ages f or all gabbroic and granitic plutons and deformed equivalents define an sim ilar to 25 m.y. period of magmatic activity between 804 and 779 Ma (at maxi mum uncertainty). Granitoids intrusive into the Quartzo-Schisto-Calcaire se ries 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 Mada gascar emplaced between 804 and 776 Ma. We propose that these middle Neopro terozoic rocks constitute the root of a continental magmatic are emplaced a t the time of, or slightly preceding, the breakup of the Rodinian supercont inent. Neoproterozoic plate reconstructions place Madagascar on the putativ e margin of Rodinia, and therefore the plutonic belt in west-central Madaga scar provides important constraints on the timing and extent of middle Neop roterozoic tectonic events in Rodinia and the critical period of Rodinia's transformation into Gondwana.