In the central erogenic belt of China, at least two UHP metamorphic belts are identified based on detailed structural analysis of the Kanfenggou and Xiangfanggou metamorphic microlithons exposed in the eastern Qinling orogen, together with previous regional structural, petrological and geochronological data at the scale of orogenic domain. The first is the South Altun—North Qaidam—North Qinling UHP metamorphic belt and the second is the Dabie—Sulu UHP and HP metamorphic belts. These two belts are separated by a series of fault-bounded tectonic slices composed of the Qinling Group, Danfeng Group and Liuling or Fuziling Group, and are different in peak age of UHP metamorphism and geodynamic implications for deep subduction and collision of the continent. Regional field and petrological relationships suggest that the Kanfenggou UHP metamorphic microlithon, which contains a large volume of coesite- and microdiamond- bearing eclogite lenses, is compatible with the rocks and structures recognized in the South Altun and North Qaidam UHP metamorphic microlithons exposed in the western part of China, and formed a large UHP metamorphic belt, up to 1000 km long along the erogenic belt. This UHP metamorphic belt represents an intercontinental deep subduction and collision belt between the Yangtze and Sino-Korean cratons, occurring during the Early Palaeozoic (500~400 Ma). On the other hand, the well-constrained Dabie—Sulu UHP and HP metamorphic belts occurred mainly during the Triassic (250~220), and were probably produced by intracontinental deep subduction and collision within the Yangtze craton. The Kanfenggou UHP metamorphic microlithon, situated in the northern Qinling erogenic belt and extending eastwards, does not appear to link with the Dabie—Sulu UHP and HP metamorphic belts, including the northwestern Dabie Mountains and the southern Dabie Mountains, along the orogen. The common occurrence of rutile and representative mineral assemblages of the Xiangfanggou microlithon exposed in the Xixia area, southern Qinling suggest that the Xiangfanggou microlithon underwent high-pressure amphibolite facies metamorphism at pressures close to those of the eclogite facies, and that the Dabie—Sulu UHP and HP metamorphic belts extend from the Tongbai Mountains across the Nanxiang basin to the eastern Qinling Mountains. Consequently, there is no reason to assume the two UHP metamorphic belts as a single giant deep subduction and collision belt in the central orogenic belt between the Yangtze and Sino-Korean cratons. Moreover, any dynamic model for the orogen must account for the development of the UHP metamorphic rocks belonging to two separate tectonic belts of different ages.
Abstract The present‐day observable tectonic framework of the ultrahigh‐pressure (UHP) and high‐pressure (HP) metamorphic belts in the Dabie‐Sulu region was dominantly formed by an extensional process, mostly between 200 and 170 Ma, following the Triassic collision between the Sino‐Korean and Yangtze cratons. The framework that controls the present spatial distribution of UHP and HP metamorphic rocks in particular displays the typical features of a Cordilleran‐type metamorphic core complex, in which at least four regional‐scale, shallow‐dipping detachment zones are recognized. Each of these detachment zones corresponds to a pressure gap of 0.5 to 2.0 GPa. The detachment zones separate the rocks exposed in the region into several petrotectonic units with different P‐T conditions. The geometry and kinematics of both the detachment zones and the petrotectonic units show that the exhumation of UHP and HP metamorphic rocks in the Dabie‐Sulu region was achieved, at least in part, by non‐coaxial ductile flow in the multi‐layered detachment zones, and by coaxial vertical shortening and horizontal stretching in the metamorphic units, under amphibolite‐ to greenschist‐facies conditions, and in an extensional regime. All ductile extensional deformations occurred at depths below 10 to 15 km, i.e. below the brittle/ductile deformation transition.
Abstract A series of ductile shear zones of the overthrust and strike‐slip types and related ductile shear metamorphic rocks, including tectonic melange and mylonites, were formed in the core of the Qinling orogenic belt in the course of the Caledonian‐Indosinian ductile and brittle‐ductile reworking. The study on their petrography, variations in composition and conditions of formation is conducive to revealing the metamorphism‐deformation history of the core of the Qinling orogenic belt and further to understanding the dynamic mechanism of its evolution.
Abstract The metamorphic history and tectonic evolution of the Qinling Complex is divided into formation and modification stages. During the Proterozoic formation stage, three deformational sequences are recognized. Andalusite–muscovite, sillimanite–muscovite and sillimanite–K‐feldspar zones of amphibolite facies regional metamorphism are earlier than, or synchronous with the first or second phase of folding. Ductile shear zones were formed and Caledonian granites were emplaced during the modification stage. The granites superimposed contact aureoles (garnet–K‐feldspar zone) on the regional metamorphic fabric. Metamorphic reactions, P–T conditions of metamorphism and P–T–t paths were estimated by analysis of mineral textures and standard thermobarometric techniques. The P–T–t path of the Proterozoic tectonometamorphic cycle shows prominent clockwise decompression. The P–T–t path of the Caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature, followed by isothermal decompression (rapid uplift) and finally with isobaric cooling. The P–T–t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the Qinling orogenic belt during the Proterozoic and Caledonian–Hercynian periods, respectively.
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