| Continental deep subduction and ultrahigh-pressure (UHP) metamorphism has been one of the hotspot topics in the field of Earth science since the innovational findings of coesite and diamond in the supracrustal rocks from 1980’s. Partial melting of metamorphic rocks which is one of the most significant geological process during the continental deep subduction and exhumation, has been more and more widely recognized in continental subduction-collision zones. Partial melting of HP-UHP metamorphic rocks has different influences on the physicochemical properties of deeply subducting crusts, for example, partial melting not only enhances sub-arc magmatic activities, but also changes the rheological characteristics of rocks, Besides, it can also accelerate the reaction between different minerals in the rock. Unlike the oceanic slabs, the deeply subducting continental crust is generally cold and lack hydrous, thus fluid-induced arc magmatism does not develop during subduction. Although the deeply subducting continental crustal rocks relatively lack of fluids, even "dry" UHP rocks can be partially melted under high-temperature (HT) conditions. Morever, the release of structural and molecular water from nominally anhydrous mineral, and decompression dehydration of minor hydrous minerals, can promote the partial melting of the UHP metamorphic slabs in continental subduction zone. Therefore, to precisely identify the age of partial melting plays a very important role in clarifying the exhumation mechanism and rebuilding the metamorphic P-T-t paths of the deeply subducting continental rocks.Located in east-central China, the Dabie-Sulu orogenic belt containing coesite-and diamond-bearing UHP metamorphic units is well known to the world. The south Dabie low-T eclogite zone, the Central Dabie UHP metamorphic zone and the North Dabie complex zone are three eclogite-bearing UHP crustal slices, hereinafter respectively referred to as SDZ, CDZ and NDZ. Informed study indicates that the UHP granitic orthogneisses in CDZ were formed in Neoproterozoic. Many evidences of partial melting have been already recognized in paragneisses, calc-gneisses and eclogites in succession. However, whether partial melting has occurred in granitic gneisses remains unknown. Also, there is no report about the precise timing and melting mechanism yet, nor is the field evidence. Hence, to figure out their precursor nature, petrogenesis, and to find out whether partial melting occurs in UHP granitic gneisses and to ascertain the accurate age, systematical petrology, geochemistry, isotopic geochronology study and field geological investigation have been carried out on granitic gneisses, garnet-bearing amphibolites and eclogites from Hualiangting, Bixiling and Yezhai in the CDZ. The results provide new constraints on the UHP metamorphic ages, also furnishes references for the continental deep subduction and exhumation in the CDZ. In addition, the geochemical research on granitic gneisses, amphibolites and eclogites indicate that the granitic gneisses from the study areas are of different precursor nature and petrogenesis.Petrography observation and zirconology study show that (1) zircons in granitic genisses contain UHP minerals like coesite; (2) the peak metamorphic assemblage of eclogites consists of garnet+omphacite+rutile+coesite+phengite; (3) both eclogites and granitic gneisses contain amphibolite-facies minerals such as amphibolite, plagioclase, titanite. All above prove that the granitic gneisses suffered the UHP metamorphism. Beisides, combined study of field investigation and zircon U-Pb dating suggests that the granitic gneisses suffered at least two stages of partial melting during the continental collision. The felsic veins crystallized from melt at first stage of～23OMa consist of rutile+quartz+K-feldspar+Plagioclase, and which of the later stage at-220Ma contain tourmaline+quartz+K-feldspar+plagioclase.Zircon U-Pb dating conducted among granitic gneisses, garnet-bearing amphibolite and eclogites indicates that (1) their protolith were formed at respectively 780-800Ma and～750Ma; (2) the UHP metamorphic peak stage took place at 225-235Ma, then subsequently suffered the amphibolite-facies retrogression occurred at about 213Ma; (3) Zircon U-Pb dating on the two kinds of felsic veins discussed above implies that zircons crystallized by partial melting are usually with high content of U, of which the CL images are dark. Morever the dark areas of zircons record partial melting at 233±2Ma and 221±2 Ma. Stated thus, the granitic gneisses and eclogites in the CDZ suffered the UHP metamorphism at 225-235 Ma and the partial melting process at-230Ma and-220Ma, respectively, which suggests that the partial melting occurred between the UHP peak metamorphism and the amphibolite-facies retrogressive metamorphism.Granitic gneisses from Bixiling and Hualiangting display similar REE patterns, but the tectonic setting discrimination diagrams and their precursor ages jointly indicate that the protoliths are of different tectonic setting areas. In addition, both of the Bixiling and Hualiangting granitic gneisses show uniform εNd<t) value whenever t=220 Ma or t=750 Ma, however there is a great divergence in εNd(t) and Pb isotopic compositions between them indicating that they were originated from different magma source regions and of different petrogenesis. |
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