METAMORPHISM AND TECTONIC EVOLUTION OF THE LANCANG PAIRED METAMORPHIC BELTS, SOUTH-WESTERN CHINA

The Lancang metamorphic terrane consists of an eastern low-PIT belt and a western high-P/T belt divided by a N-S-trending fault. Protoliths of both units are mid-late Proterozoic basement and its cover. The low-P/T belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo-Triassic volcanic and volcaniclastic rocks. Most whole-rock Rb-Sr isochron and U-Pb zircon ages of the Lincang batholith are in the range 290-279 and 254-212 Ma, respectively. Metamorphism of the low-P/T belt reaches upper amphibolite with local granulite facies (735-degrees-C at 5 kbar), subsequently retrogressed at 450-500-degrees-C during post-Triassic time. The high-P/T rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P-T conditions follow the high-P intermediate facies series up to about 550-600-degrees-C, at which oligoclase is stable. The 40Ar/39Ar plateau age of sodic amphibole in blueschist is 279 Ma. The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south-east Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P-T conditions, perhaps during collision of the Baoshan block with south-east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in south-east Asia represents a suture zone between two contrasting microcontinents. The Lancang metamorphic terrane consists of an eastern low-PIT belt and a western high-P/T belt divided by a N-S-trending fault. Protoliths of both units are mid-late Proterozoic basement and its cover. The low-P/T belt includes the Permian Lincang batholith, related amphibolite facies rocks of the Damenglong and Chongshan groups, and Permo-Triassic volcanic and volcaniclastic rocks. Most whole-rock Rb-Sr isochron and U-Pb zircon ages of the Lincang batholith are in the range 290-279 and 254-212 Ma, respectively. Metamorphism of the low-P/T belt reaches upper amphibolite with local granulite facies (735-degrees-C at 5 kbar), subsequently retrogressed at 450-500-degrees-C during post-Triassic time. The high-P/T rocks grade from west to east from blueschist through transitional blueschist/greenschist to epidote amphibolite facies. Estimated P-T conditions follow the high-P intermediate facies series up to about 550-600-degrees-C, at which oligoclase is stable. The 40Ar/39Ar plateau age of sodic amphibole in blueschist is 279 Ma. The paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south-east Asia since the latest Precambrian. Tectonic juxtaposition of paired belts with contrasting P-T conditions, perhaps during collision of the Baoshan block with south-east Asia, suggests that an intervening oceanic zone existed that has been removed. The Baoshan block is a microcontinent rifted from the northern periphery of Gondwana. Successive collision and amalgamation of microcontinents from either Gondwana or the Panthalassan ocean resulted in rapid southward continental growth of c. 500 km during the last 200 Ma. Hence, the Lancang region in south-east Asia represents a suture zone between two contrasting microcontinents.