Metamorphic patterns and SHRIMP zircon ages of medium-to-high grade rocks from the Tongbai orogen, central China: Implications for multiple accretion/collision processes prior to terminal continental collision

摘要

The Qinling-Tongbai-Dabie-Sulu orogenic belt comprises a Palaeozoic accretion-dominated system in the north and a Mesozoic collision-dominated system in the south. A combined petrological and geochronological study of the medium-to-high grade metamorphic rocks from the diverse Palaeozoic tectonic units in the Tongbai orogen was undertaken to help elucidate the origins of Triassic ultrahigh-pressure metamorphism and collision dynamics between the Sino-Korean and Yangtze cratons. Peak metamorphic conditions are 570-610°C and 9.3-11.2kbar for the lower unit of the Kuanping Group, 630-650°C and 6.6-8.9kbar for the upper unit of the Kuanping Group, 550-600°C and 6.3-7.7kbar for the Erlangping Group, 770-830°C and 6.9-8.5kbar for the Qinling Group and 660-720°C and 9.1-11.5kbar for the Guishan complex. Reaction textures and garnet compositions indicate clockwise P-T paths for the amphibolite facies rocks of the Kuanping Group and Guishan complex, and an anticlockwise P-T path for the granulite facies rocks of the Qinling Group. Sensitive high-resolution ion microprobe U-Pb zircon dating on metamorphic rocks and deformed granite/pegmatites revealed two major Palaeozoic tectonometamorphic events. (i) During the Silurian-Devonian (c. 440-400Ma), the Qinling continental arc and Erlangping intra-oceanic arc collided with the Sino-Korean craton. The emplacement of the Huanggang diorite complex resulted in an inverted thermal gradient in the underlying Kuanping Group and subsequent thermal relaxation during the exhumation. Meanwhile, the oceanic subduction beneath the Qinling continental arc produced magmatic underplating and intrusion, leading to granulite facies metamorphism followed by a near-isobaric cooling path. (ii) During the Carboniferous (c. 340-310Ma), the northward subduction of the Palaeo-Tethyan ocean generated a medium P/T Guishan complex in the hangingwall and a high P/T Xiongdian eclogite belt in the footwall. The Guishan complex and Xiongdian eclogite belt are therefore considered to be paired metamorphic belts. Subsequent separation of the paired belts is inferred to be related to the juxtaposition of the Carboniferous eclogites with the Triassic HP metamorphic complex during continental subduction and exhumation.