A new insight into Pan-African tectonics in the East–West Gondwana collision zone by U–Pb zircon dating of granites from central Madagascar

Abstract

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U–Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as “stratoid” granites. These are of both mantle and crustal derivation. Their U–Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2–2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike–slip tectonic regime at ∼570–560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.