Cordierite-bearing granulites from Ihosy,southern Madagascar:Petrology,geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.