Petrological-tectonic units in the coesite-bearing metamorphic terrain of the Dabie Mountains, central China and their geotectonic implications

Four main faults outline the metamorphic terrain in the Dabie Mountains, central China. The difference of metamorphic grade in the northern, central and southern parts is well-established. This study suggests that these metamorphic rocks belong to three individual geological-tectonic units, which are the northern Andean-type magmatic arc, the central UHPM rock-bearing melange zone and the mobilized sedimentary cover and basement of Yangtze southern continental margin. An old ductile sheared zone through Wuhe and the south of Yuexi-Shuihou divides the Dabie metamorphic complex into northern and central units. The northern unit is predominantly occupied by tonalitic-trondhjemitic grey gneisses and amphibolites facies metasediments. Several approximately E-W-trending ductile-sheared mylonitic belts cut through the grey gneisses, and are especially developed in the northern and southern margins of the unit (high-strain zones). There is a series of lenses of basic granulites, serpentinites and Grt-pyroxenites in the high-strain zones. They do not have the petrological and geochemical characteristics of ophiolitic rocks. However, they may represent the root of an arc combined with grey gneisses. Our study also demonstrates that all rocks from the northern block did not undergo eclogite facies metamorphism. Mesozoic post-collisional granites occur extensively in the Dabie terrain, especially in the northern unit. Coesite and diamond-bearing eclogites in the central unit indicate a regional ultra-high-pressure metamorphism of pressures of > 30 kbar and temperature of 600–800°C. Except for eclogites, various ultra-high-pressure metamorphic (UHPM) rocks include ultramafic rocks, marbles and other metamorphosed sedimentary rocks. They, as large-scale blocks, are mixed with lower P-T encompassing host rock orthogneisses. The central block is a UHPM rock-bearing melange zone. The south block is a metasedimentary sequence of continental margin of the Yangtze plate. The boundary between the central and southern blocks is located in the north from Huangzhen, extending eastwards through the dam of Huangliangting reservoir. In addition, the Foziling Group, north of the Dabieshan terrain was possibly deposited in an extensional basin on an extensional shear zone, comparable to the basin in the Variscan of France and the Caledonides of S. Norway (Dewey, Ryan and Anderson, Geological Society Special Publication No. 76, pp. 325–343, 1993). The authors hope that this information will provide more valid evidence of regional tectonic evolution.