STRUCTURE AND MAGMATISM INSIDE THE ANTANANARIVO BLOCK (MADAGASCAR): STAGES OF THE EAST AFRICAN OROGEN

During Neoproterozoic time, Madagascar occupied a position between east and west Gondwana, and was strongly involved in the East African Orogeny (EAO). Accordingly, Madagascar's Precambrian rocks are highly deformed and metamorphosed, with a strong north-south structural grain. These rocks have been subdivided into a number of distinct tectonic blocks, of which the Antananarivo block is one. The EAO in the north-central Antananarivo block is characterised by the following stages: emplacement of stratoid granites at 630 Ma (Paquette and Nedelec 1998); an east-west flexure (the Antananarivo virgation zone); the north-south oriented Angavo shear belt (Ralison and Nedelec 1997; Nedelec et al. 2000); intrusion of the porphyritic Carion granites (and other related granites) during the waning shearing stage at 532 Ma (Meert et al. 2001).

The stratoid granites are fine-grained, and occur interlayered with gneisses and migmatites of the country rocks, apparently having intruded along the foliation. They are A-type, and fall into two different suites: (1) a strongly alkali suite of alkaline quartz syenites and hypersolvus alkaline granites, representing differentiates from a mafic parental magma; and (2) a mildly alkaline suite of subsolvus granites, produced by high temperature partial melting of a granodioritic protolith (Nedelec et al. 1995). Their emplacement at midcrustal depth (4-5 Kb) led to formation of a layered crust possibly in a post collisional extensional regime at 630Ma ( D1).

The Antananarivo virgation corresponds to a sinistral transpression in the flexure zone (D2), sealed by the Ambatomiranty granitic dykes at 560 Ma (Paquette and Nedelec 1998). To the east of the virgation zone, and along the eastern margin of the Antananarivo block, the linear north-south Angavo Shear Zone is a major strike-slip shear (D3), formed under low pressure granulitic conditions (3-3,5 Kb; 790°C). The D2 and D3 events are likely related.

The Carion pluton was emplaced at the end of this transcurrent tectonics. The largely undeformed Carion granites represent the latest stage of EAO magmatism in Madagascar. They are metaluminous I-type granitoids belonging to ferriferous and potassic series. They display a tonalitic to monzogranitic composition. A detailed AMS study revealed that the pluton was built by four nested subunits (Madison Razanatseheno et al., GJI, submitted).