Geochemistry and U-Pb zircon dating of the Toudaoqiao blueschists in the Great Xing'an Range, northeast China, and tectonic implications

The Toudaoqiao Complex is a sequence of high-pressure metamorphic rocks located along the suture zone that separates the Xing'an and Erguna blocks in northeast China. The rocks were metamorphosed up to epidote-blueschist facies at P-T conditions of approximately 0.9-1.1 GPa and 320-450 degrees C. Lithological associations and major and trace element compositions indicate that the blueschists are metabasalts with ocean island basalt (OIB) and normal mid-ocean ridge basalt (N-MORB) affinities, similar to those of the Philippines intraoceanic accretionary complex formed by subduction of oceanic crust. Magmatic zircons extracted from two samples of the epidote-blueschist facies metabasalts from the North Mountain in the Toudaoqiao Complex exhibit Pb-206/U-238 ages of 511 +/- 5 and 516 +/- 17 Ma, whereas a greenschist from the South Mountain has a (206)pb/U-238 age of 511 +/- 2 Ma; These data show that the protoliths of the Toudaoqiao Complex are of early Paleozoic age. In addition, a granitic dike from the South Mountain exhibits a weighted mean Pb-206/U-238 age of 492 +/- 1 Ma. These ages constrain the timing of high-pressure metamorphism in the Toudaoqiao Complex to 490-510 Ma. The new data support the view that the suture zone between the Xing'an and Erguna blocks is the Toudaoqiao-Xinlin Fault, rather than the Derbugan Fault as previously thought. This suture, which extends from Xinlin, through Toudaoqiao, and southern Mongolia, named the 'South Mongolia-Toudaoqiao-Xinlin Suture Zone', may be an important boundary between the Central Mongolia-Erguna and Xing'an blocks. (C) 2014 Elsevier Ltd. All rights reserved.