The Nature Of The Tongbai Complex And Its Tectonic Implications

The Tongbai complex, one of important geological unites on the northern margin of the Yangtze craton, is situated at the junction between the Qinling and Dabie-Sulu orogens. Although the complex occurs in between two Indosinian high-pressure (HP) metamorphic belts in the Tongbai area, no evidence indicates that it has undergone Indosinian HP metamorphism. Thus, this complex has long been thought to be a Precambrian metamorphic terrane. Owing to the lack of petrological and geochronological data, the nature and metamorphic process of the Tongbai complex remain unclear. Therefore, the issue concerning formation process and special structural relationship between the Tongbai complex and HP metamorphic belts have a strong appeal to geologists. In this paper, we present original results of our field investigation, and geochronological, petrological and structural studies on the Tongbai complx. Coupled with the available regional data, the geodynamic mechanism of the Tongbai complx during the Indosinian to Yanshanian was discussed.The detailed investigation in the field reveals that the Tongbai complex mainly consists of mylonitic granitic bodies, undiformed granitic bodies and metamorphic enclaves. The mylonitic granites are the main part of the Tongbai complex, which forms 75-80% volume of the deformed rocks. U-Pb zircon dating indicates that the protoliths of the most metamorphic enclaves in the mylonitic granites were emplaced during the Neoproterozoic, whereas metamorphism took placed during the Indosinian. Detrital zircons from metasedimentary rocks came from Archean igneous intrusives and Paleoproterozoic metamorphic rocks, respectively. Furthermore, the emplacement and deformation of mylonitic granites and emplacement of pegmatitic dykes related to orogenic events occurred at 139-131 Ma.Garnet-bearing amphibolite and gneiss enclaves arc found at first in the Tongbai complex. The reaction textures have not been observed in such metamorphic enclaves, although some garnets have been affected by the late diffusion. Thermobarometry suggests that the peak metamorphism of the rocks reaches upper amphibolite facies conditions of 560-670℃and 0.57-0.90 GPa. Since no HP/UHP evidence has been found in the metamorphic enclaves, whether these rocks are retrograde products of the HP/UHP rocks, or are only medium-pressure rocks metamorphosed at the mid to lower crustal levels, remains poorly constrained.On the two sides of the Tongbai complex, there are two eclogite-bearing HP metamorphic belts stretching from NWW to SEE. Two ductile shear zones, which formed at 131 Ma, occur alone the southern and northern boundaries of the complex. The strike-slip directions of the shear zones are opposite; the southern boundary is a dextral ductile shear zone, and the northern boundary is a sinistral ductile one. The mylonitic foliations dip to NNE in the northern shear zone, and to SSW in the southern shear zone. In the ductile shear zones as well as the inner part of the Tongbai complex, the stretching lineations invariably dip toward SEE. On the eastern end of the complex, a south-north stretching thrust ductile shear zone occurs, in which low-angle mylonitic foliations dip toward SEE and lineations on the foliations also plunge toward SEE. These dynamic features indicate that the eastern metamorphic terrane thrusted onto the Tongbai complex with a direction of about 295°-310°. Therefore, the Tongbai complex is actually surrounded by a huge ductile shear zone streching alone the orogen. The deformation characteristics in the shear zone and the inner part of the Tongbai complex suggest a low-angle exhumation for the HP metamorphic terrane from the east to the west along the orogen during the early Cretaceous, which led to the HP metamorphic rocks being exhumed to the Earth's surface from the crustal levels.On the basis of tectonic analysis, we proposed that the main components and the present tectonic framework of the Tongbai complex were formed during the Yanshanian intraplate orogeny. The major geodynamic mechanism that caused the intraplate orogeny is the eastward movement of the Yangtze block after the Yanshanian intraplate compressional deformation. The eastward movement of the Yangtze block resulted in the eastward extrution of the orogenic basement along the Xiaotian-Mozitan fault. At the sametime. HP metamorphic terranes that have been exhumed to the middle-upper crustal levels during the Indosinian were uplifted by the eastward movement of the orogenic basement. After 131 Ma, the eastward movement of the Yangtze block mainly occurred alone the Fangxian-Guangji fault and continued to 84 Ma.