The Ianapera-Ampanihy Suture, SW Madagascar: A major tectonic boundary on the eastern margin of the Mozambique belt
Résumé
he position of ancient (Paleo) suture zones in the crystalline basement of Madagascar is still the subject of debate. The Ianapera area is part of the Neoproterozoic Vohibory domain, which covers the northern part of the Ampanihy High-Strain Zone (HSZ), in the southwestern part of the basement. By combining Landsat 7 ETM+, airborne magnetic and radiometric data as well as available maps and data, with new field, geochemical and geochronological data in a GIS, we have updated the geological map of this region to investigate a supposed suture in this area. We suggest that the proposed suture in SW Madagascar is a NNE–SSW major lineament, rather than the 20 km-wide Ampanihy HSZ. The lineament separates the Vohibory high-pressure granulites from the older Androyan medium-pressure granulites, and can be outlined from the Ianapera area to the Ampanihy region towards the south; accordingly, we named it the Ianapera-Ampanihy Suture. Based on field, geophysical and geochemical data, we recognised a suite of granites and syenites, which we named the Ianapera suite, and a small sheet-like body of granodiorite (Bevaro granodiorite). The Ianapera suite contains the only known occurrence of corundum-bearing syenite in the Madagascar basement. These bodies were formed by two distinct episodes of felsic magmatism: respectively, high-K calc-alkaline to shoshonitic, and low-K calc-alkaline pulses. The composition, mineralogy, as well as a U–Th–Pb monazite age of 540 ± 22 Ma, is consistent with late to post-collisional emplacement of the Ianapera suite, reinforcing the hypothesis of a suture in this area. Based on the above data, we suggest a northwards continuation of this suture in Kenya, as far as the intrusive-hosted Dusi corundum deposit (Simonet et al., 2004). The Ianapera-Ampanihy Suture could be the major tectonic boundary suggested as the eastern margin of the Mozambique belt, within the central East African Orogen.