TWO-STAGE METAMORPHIC EVOLUTION OF THE BEMARIVO BELT OF NORTHERN MADAGASCAR: CONSTRAINTS FROM REACTION TEXTURES AND IN SITU MONAZITE DATING

Abstract

New results on the pressure-temperature-time evolution, deduced from conventional geothermobarometry and in situ U-Th-total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low-grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)-alumosilicate-quartz-plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ~900-950 °C and pressures of ~9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet-sillimanite-bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ~970 °C. Peak metamorphism is followed by near-isothermal decompression to pressures of 5-7 kbar and subsequent near-isobaric cooling, which is demonstrated by the extensive late-stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high-grade metamorphism in Madagascar, and are supposed to reflect the Pan-African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of > 25 km. Approximately 25-30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades. © 2006 Blackwell Publishing Ltd.