Multi-stage tectonic exhumation processes of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area, east-central China

Abstract

Exhumation processes of ultrahigh-pressure (UHP) metamorphic rocks, by which these deep-seated rocks were rapidly returned from mantle depths of more than 100 km within deep subduction zones into the upper crust and to the surface, are of paramount importance for the understanding of the kinematics and dynamics of convergent plate margins and continental collision belts. At least four large successive stages of the exhumation process of UHP metamorphic rocks in the Dabie-Sulu area, east-central China, mainly based on structural, petrological, geochemical data and available geochronological data, combined with previously regional polyphased deformation analyses, can be distinguished. Massive eclogites recorded the deep continental subduction/collision between the Sino-Korean and Yangtze cratons during the Triassic (~250-230 Ma). The early exhumation occurred after the peak of UHP metamorphism at conditions of up to >3.1-4.0 GPa and 800±50°C, which may correspond to the D 2 regional deformation phase and took place at mantle depths, displaying the model of vertical ductile extrusion, within the coesite stability field. The subsequent exhumation stages, corresponding with D 3, D 4 and D 5 regional deformation phase, respectively, and two intermediate tectono-thermal events, i.e., the formation of the granulite/amphibolite facies symplectite or corona and the generation of extensive partial melting, occurred in crustal levels. Shear zones of an anastomosing rheological type were developed at diffterent stages and depths, indicating that the partitioning of deformation and metamorphism within the UHP metamorphic belt repeated. These imply that the exhumation of the UHP rocks is a complex tectonic process, in response to subduction/collision or extension of the continental crust, and is strongly constrained by a number of internal and external factors of the belt. In particular, fluids play important roles in the exhumation process of UHP rocks in the Dabie-Sulu area, east-central China.