Metamorphic evolution of Pan-African granulite facies metapelites from Southern Madagascar

Abstract

Granulite-facies, metapelitic gneisses from five localities in Southern Madagascar exhibit very similar mineral assemblages and textures and reveal a very similar P-T evolution. They show an early assemblage of Grt+Sil+Qtz+Bio+Kfsp+Rt+Ilm±Pl±Spl which records P-T conditions of about 880±60°C at 8±1 kbar. In some samples, this assemblage is overprinted by the later assemblage Crd+Grt+Sil+Bio+Kfsp+Qtz+Mt±Crn which equilibrated at about 690±40°C and 4±1 kbar. The main Crd-producing reaction Spl+ Qtz= Crd was aided by the further reactions Sil+ Bio+ Qtz= Crd+ Kfsp+ H 2 O and Grt+ Sil+ Qtz= Crd. Crn formed from relictic Spl, probably during cordierite formation, by the oxidation reaction 3 Hc+0.5 O 2=3 Crn+ Mt. H 2O activities calculated from phase equilibria for both metamorphic events range from 0.6 to 1 which is in agreement with the observation of partial melting during the HT stage. We interpret age data from the literature to suggest that the HT/HP event occurred at about 560–580 Ma, while the MT/MP event could be related to intrusive activity between 520 and 540 Ma, possibly during the same orogenic cycle. The similarities in textures and equilibration conditions among the Malagasy samples strongly imply — in contrast to the opinion of many previous workers — a common tectonometamorphic history of the various parts of Southern Madagascar between 600 and 500 Ma. The P-T-conditions recorded by the samples and the age data correspond closely to P-T-paths deduced for other Gondwana fragments like Southern India, Sri Lanka and parts of East Antarctica. This correspondence may help to constrain the location of the suture for the collision between East and West Gondwana in the late Proterozoic.