STRATOID GRANITES OF MADAGASCAR - STRUCTURE AND POSITION IN THE PANAFRICAN OROGENY

Stratoid granites constitute a major feature of the Precambrian baseme nt of Madagascar. A detailed structural study was carried out NNW of A ntananarivo. New zircon isotopic data on a typical alkaline granite as certain their Panafrican age (585 Ma). The sheets of granites metric t o kilometric of thickness, are interlayered with migmatitic gneisses a nd amphibolites. Their internal structures, determined by anisotropy o f magnetic susceptibility measurements, everywhere yield foliations ge ntly dipping to the west, and lineations striking WSW-ENE. These struc tures were mostly acquired at the magmatic stage in the granites, in t he country-rocks they resulted from high-temperature plastic deformati on. The very constant structural pattern, interpreted in terms of shea r deformation of a section of the crust, as well as the low P (P = 4 - 5 kb) - high T (T # 750-degrees-C) conditions, suggest that the empla cement of the stratoid granites was coeval with a late-orogenic stage in the Panafrican Mozambique belt, and possibly linked to the thinning of the lithosphere.