Ž . Earth and Planetary Science Letters 155 1998 45–56 A new insight into Pan-African tectonics in the East–West Gondwana collision zone by U–Pb zircon dating of granites from central Madagascar Jean-Louis Paquette a,) , Anne Nedelec b,1 ´´ a Magmas et Volcans, UMR 6524 CNRS, Uni Õersite Blaise-Pascal, 5 rue Kessler, F-63038 Clermont-Ferrand Cedex, France ´ b Petrophysique et Tectonique, UMR 5563 CNRS, Uni Õersite Paul-Sabatier, 38 rue des Trente-six Ponts, F-31400 Toulouse, France ´ ´ Received 6 May 1997; revised 24 October 1997; accepted 9 November 1997 Abstract The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and We Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of EastGondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U–Pb zircon metho order to constrain the timing of orogenic events. The central partof 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 not surprising that the crustally-derived suite contains inherited zircons in the 2.2–2.4 Ga range attesting to the existen 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 ce part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplaceme the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. T 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. q 1998 Elsevier Science B.V. Keywords: magma; Gondwana; granite; Madagascar; orogeny; U–Pb 1.Introduction and geological setting wx The East African Orogen 1 is a major Pan-Afri- can belt that resulted from the collision of two ) Corresponding author. E-mail:paquette@opgc.univ-bpcler- mont.fr 1 Ž. Ž. Tel: 33 05.61.55.65.30; Fax: 33 05.61.52.05.44; E-mail: nedelec@lucid.ups-tlse.fr. Ž continental blocks:WestGondwanaSouth Amer- . Ž ica–Africa and EastGondwanaMadagascar–In- . dia–Antarctica–Australia as shown in Fig. 1. How- ever,the Gondwana assembly remains controversial wx in many respects, as discussed by Rogers et al. 2 . The uniqueness of the collision may be an oversim- plification, since tectonometamorphic and magmatic events are recognized covering the time span from ;800 to 500 Ma, i.e., from the break-up of the 0012-821Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S 0 0 1 2 - 8 2 1 X 9 7 0 0 2 0 5 - 7