Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780°C) pressure up to c. 24kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486±9Ma and 459±9Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.