Timing and PT Evolution of Whiteschist Metamorphism in the Lufilian Arc-Zambezi Belt Orogen(Zambia):Implications for the Assembly of Gondwana

摘要

One of the world's most extensive occurrences of whiteschists(talc-kyanite schists)is located in south central Africa with several exposures along a ca.700 km northwest-southeast striking zone.The metamorphic evolution and age relations of whiteschists and associated rocks from four localities,three in the Lufilian Arc and one in the Zambezi Belt,were investigated.In the Lufilian Arc whiteschists,associated garnet-amphibolites and biotite-kyanite-garnet gneisses record peak metamorphic conditions of about 750 deg+-25 deg C at 13+-1 kbar.Whiteschists,associated antho-phyllite-cordierite-kyanite gneisses,and garnet-staurolite-kyanite schists from the Zambezi Belt indicate peak metamorphic conditions of about 700 deg+-25 deg C at 10+-1 kbar.Mineral reaction textures imply a metamorphic evolution along clockwise PT paths with peak metamorphism culminating at high-pressure amphibolite facies conditions.The shapes of the PT paths are interpreted to result from crustal thickening.On the basis of these interpretations and a concordant monazite age of 529+-2 Ma for a biotite-kyanite-garnet gneiss combined with ~207pb/~235U ages of 531 to 532+-2 Ma from monazites from whiteschists,we concluded that the crustal thickening event affected the entire Lufilian Arc-Zambezi Belt orogen almost simultaneously.Whiteschist formation was related to the final continental collision between the Congo and the Kalahari cratons at ca.530 Ma during the assembly of Gondwana,which followed the subduction of oceanic lithosphere and eclogite formation at about 600 Ma.