The ultrahigh temperature granulites of southern Madagascar in a polymetamorphic context: implications for the amalgamation of the Gondwana supercontinent

Abstract

We examined the petrological characteristics of the Graphite group and Androyan group in Southern Madagascar, south of the prominent Ranotsara shear zone, and we performed U-Pb SHRIMP dating on zircon and U-Th-total Pb dating on monazite. Widespread high-temperature metamorphism is evidenced by Spl–Qtz assemblages occurring over ca . 75,000 km 2 in the whole Androyan group. The occurrence of symplectites consisting of Crd + Kfs + Qtz + Opx or Crd + Kfs + Q333tz + Bt, which are interpreted as pseudomorphs after osumilite, is restricted to a smaller area of about 250 km 2 . Furthermore, in some pelites Spr + Qtz + Sil or Opx + Sil + Qtz formed the peak-metamorphic assemblage, which broke down to Crd ± Spl. Orthopyroxene in metapelites is aluminous with Al 2 O 3 = 9–10 wt%. Peak-metamorphic conditions of T = 950–1000 °C and P = 8–11 kbar are followed by decompression at high temperatures, as shown by the formation of Crd + Opx 2 (Opx with 6–8 wt% Al 2 O 3 ) symplectites from Grt–Qtz–Opx 1 (8–9 wt% Al 2 O 3 ). The pressure decrease is furthermore constrained by Spr–Crd symplectites in SiO 2 -undersaturated metapelites, and extensive formation of late-stage cordierite in the whole Androyan group. During subsequent cooling, cordierite broke down to form And + Qtz + Carbonate/Chl. Throughout the Androyan group, ages of 560–530 Ma have been obtained from monazite (M 2 metamorphism). Samples which do not contain ultrahigh-temperature assemblages provide evidence for an earlier metamorphic event at 650–600 Ma (M 1 ) in monazite cores. Zircon generally shows both metamorphic ages. Therefore, the deduced clockwise P-T evolution of the UH T metamorphism is interpreted to correspond to the M 2 stage, which affected the whole Androyan group. P-T conditions of the older M 1 metamorphism are generally unrecognisable. High temperature metamorphic conditions during M 2 are likely caused by intense charnockite emplacement. The near-isothermal decompression points to subsequent rapid exhumation of the formerly overthickened crust during the M 2 metamorphism at 560–530 Ma. We interpret this metamorphic stage to reflect the assembly of the Gondwana supercontinent, most likely related to the collision of the Tanzania Craton with the Azania microcontinent subsequent to closure of the Mozambique Ocean.