Structure and metamorphism of the granitic basement around Antananarivo: Akey to the Pan-African history of central Madagascar and its Gondwana connections

The Precambrian basement of Madagascar acquired a polyphase imprint during the Pan-African orogeny. In northern central Madagascar, emplacement of str atoid alkaline granites at midcrustal depth (4-5 kbars) led to formation of a layered crust in a postcollisional extensional regime at 630 Ma (D1). Su bsequently, the structures of the stratoid granites were rotated by the sin istral and transpressive E-W Antananarivo flexure (or virgation) zone (D2). East of Antananarivo the structures of the D1 layered crust and the D2 vir gation are crosscut by the steeply dipping N-S foliations of the Angavo bel t. Lineations gently plunging to the north attest that the Angavo belt is a major strike-slip shear zone that formed under low-pressure granulitic con ditions (3 kbars, 790 degrees C). The nearby porphyritic Carion granite was emplaced at the end of this period of N-S shearing (D3), which can thus be no younger than 530 Ma. Late-Pan-African (580-550 Ma) strike-slip motion a long broadly N-S shear zones has been recognized elsewhere in Madagascar an d in its Gondwana connections. Continuation of the Angavo belt as one of th e high strain belts of the Arabian-Nubian Shield is discussed in the genera l framework of Gondwana assembly.