Geochemistry of Gneisses from Dabie Complex and Tongbai Complex in Qinling-Tongbai-Dabie Orogenic Belt:Implications for Location of Yangtze-Sino-Korean Suture

The Dabie complex (DC) and the Tongbai complex (TBC) are separately distributed in the middle and eastern parts of the Qinling-Tongbai-Dabie orogenic belt. In this study, the Dabie complex can be divided into two units: one is the complex with no high pressure and ultrahigh pressure metamorphic rocks (DC1), and the other is the complex containing coesite-bearing eclogite lenses or boudins (DC2). Gneisses are predominant in the TBC, DC1 and DC2. Major and trace element data of gneisses in the TBC, DC1 and DC2 show them to be the orthogneisses. The gneisses in the DC1 have higher incompatible element contents and higher ratios of w(K 2O)/w(Na 2O) and w(La) n/w(Yb) n than those in the DC2. However, no obvious differences arise in other element contents and the ratios of w(La)/ w(Nb), w(Nb)/w(Th), w(Nb)/w(Hf), w(Ba)/w(La), w(Sm)/w(Nd) and w(Th)/w(U) between the gneisses in the DC2 and those in the DC1. These observations suggest that the protoliths of the gneisses in the DC2 have affinities to those in the DC1. The difference between the DC1 and DC2 gneisses in incompatible element contents could reflect the difference in their partial melting extent. The TBC gneisses are geochemically similar to the DC1 gneisses, suggesting that the TBC and DC1 gneisses are the same lithologic unit in the Qinling-Tongbai-Dabie orogenic belt and that they have experienced similar formations and evolution histories. In the Qinling-Tongbai area, the TBC is part of the northern blocks of the Yangtze craton. Given the similarity of geochemical characteristics, the rock assemblage and the ages between the TBC and DC1 gneisses, we can infer that the Dabie complex also belongs to the northern blocks of the Yangtze craton. In terms of the distribution of eclogites and metamorphic facies, we propose that the collisional suture in the Dabie area is distributed along the Xiaotian-Mozitan fault, at the contact with the Shang-Dan-Tongbai fault to the west.

Dynamic Mechanism of Migmatization in the Dabie Complex, Northeastern Hubei, China

On the basis of the detailed field work, compositions and contents of plagioclase and K - feldspar,determination of ordering degree, statistical analysis of plagioclase elongation index, mass-balance calculation and mineral spatial distribution and geochemistry, it is concluded that the migmatites in the Dabie complex are characterized by the presence of thermocenters. There are regular changes in mineral character in the migmatites from the centers outwards. The dominant genetic mechanism is anatexis and metasomatism, whose intensities decrease from the centers outwards. Finally, according to the simulated experiment on Liesegang' s rings and non-linear dynamics (dissipative structure theory), the dynamic mechanism of migmatization is profoundly expouded as consisting of the early-stage metasomatism induced by the thermal anomaly, the cardinal-stage anatexis induced by the early-stage matasomatism and finally the last-stage post-anatexis metasomatism.

New U-Pb Geochronological Constraints on Formation and Evolution of the Susong Complex Zone in the Dabie Orogen

Objective The Susong complex zone（SCZ）is a relatively lowgrade metamorphic unit mostly with an epidoteamphibolite facies,located in the southernmost part of the Dabie orogen.However,its rock compositions,ages,

A Uniform Orogen-parallel Extension System of the Shear Zones in the Tongbai-Dabie Orogenic Belt, Central China

Large-scale magmatism affected the Tongbai-Dabie orogenic belt during post-collisional lateral tectonic extension in the Cretaceous, which was suggested to account for the widespread deformation and migmatization in the Tongbai-Dabie complexes. However, it cannot explain the most deformations in the shear zones. The northwest-southeast shear zones are developed around or wrapped the Tongbai-Dabie complexes. They play an important role for the interpretation of the tectonic evolution of the Tongbai-Dabie orogenic belt. By a systematically observation and description of the geometry and kinematics of these shear zones, we found that the shear zones to the north dip NE and have a uniform sinistral shear sense, the shear zone to the south dips SW and has a uniform dextral shear sense, and the shear zones at the core are sub-horizontal and have a uniform top-to-NW sense of shear. Combining with the comparison of previous and our geochronological studies, we interpret these associations as indicating that these shear zones were originally a single, more flat-lying and sub- horizontal shear zone with a uniform top-to-NW shear sense before the folding-doming of the Tongbai- Dabie complexes and suggest that the Tongbai-Dabie orogenic belt experienced a uniform top-to-NW orogen-paraUel extension in the ductile lithosphere before the widespread magmatism in the Cretaceous.