Study On The Early Precambrain Metamorphic Evolution And Geochronology Of The Badu Group In Southwestern Zhejiang Province

The southwestern Zhejiang region is situated in the middle of the northern Cathaysia Block, which is the boundary of the Cathaysia Block and the Jiangnan orogenic belt, also it the focal area where the Eurasia and the Pacific plate interact with each other. The Paleoproterozoic Badu group outcrops as windows among the phanerozoic cover. By far, it is the oldest rock stratigraphic unit which has experienced the highest level of metamorphism and has the most complicated history of evolution and biggest outcropping area in the Cathaysia Block. Through a combined study of metamorphic evolution, geochemistry and geochronology, we get mainly these following several new findings.In this dissertation, we report pelitic granulite, with mineral assemblage of garnet, perthite and anti-perthite, sillimanite, for the first time that was discovered in the basement of Cathaysia block. This discovery suggests that the area experienced regional metamorphism of up to granulite facies.The protolith of garnet-bearing felsic gneiss is granite generated from the remelting of crustal materials, which is characterized by high content of A1and Si. The protolith of sillimanite garnet biotite gneiss is pelitic to sandy sedimentary rock. The protoliths of garnet amphibole plagiogneiss and garnet two-pyroxene granulite are of the same, which are both andesite-dacite rocks, as are shown by their geochemical characteristics. The slight differences in major and trace elements of the two meta-igneous rocks show that elements migrate differently in metamorphic processes of various metamorphic facies.We present here, for the forst time, the study results of metamorphic evolution of the three different rock types: sillimanite garnet biotite gneiss, garnet amphibole plagiogneiss and garnet two-pyroxene granulite. Three stages of metamorphic evolution can be recognized in all three rocks:early prograde metamorphic stage (M1), peak metamorphic stage (M2) and after peak retrograde metamorphic stage (M3). The PT conditions of the early prograde metamorphic stage are altered in all three metamorphic rocks and they record the PT conditions of only the two later metamorphic stages. For the peraluminous gneiss, the PT condition of the peak metamorphic stage(M2) is800～850℃,0.60～0.70Gpa; the PT condition of the after peak retrograde metamorphic stage (M3) is560～590℃、0.25～0.33Gpa. For the garnet amphibole plagiogneiss, during the peak metamorphic stage (M2), the PT condition is770-830℃,0.82-0.92Gpa; during the after peak retrograde metamorphic stage (M3), the PT condition is580-630℃,0.52-0.73Gpa. For the garnet two-pyroxene granulite, during the peak metamorphic stage (M2), the PT condition is850-900℃,0.92-1.10Gpa; the PT condition of after peak retrograde metamorphic stage (M3) is650-700℃,0.58-0.65Gpa.The three representative metamorphic rocks mentioned above all recorded PT paths of clockwise style. After peak metamorphic stage, cooling happenned as well as decompression. But the rocks decompressed to a greater extent than that of cooling. Such a process usually happens during the fast thinning of thickened crust.SHRIMP and LA-ICPMS zircon-U-Pb-dating of peraluminous gneiss, meta-acid-igneous rock, meta-mafic rocks and marble show the early precambrian metamorphism occoured around1.85-1.88Ga. Zircons of peraluminous gneiss show source rocks with the age of2002±29Ma and2451±63Ma, which suggests that the its protolith, the sediments, was from mainly paleoproterozoic terrains. The acid granite, which formed around1923±7.6Ma, experienced metamorphism at about1.87Ga. The zircons of the granite also contain inherited cores ranging from2.0-2.8Ga, with the oldest of2803±34Ma.The paleoproterozoic granulite facies metamorphism recorded in the supracrustal rocks of the Radu group in southwestern Zhejiang province and the widespread of S-and A-type granites in the region show that this area experienced the whole process of a paleoproterozoic orogeny, which might be resulted from the amalgamation of the supercontinent Columbia between1.8and2.1Ga around the world.