Geochemical Characteristics And Genetic Mechanism Of Gneiss In The Depth Interval 3000-3500m From The Main Drill Hole Of Chinese Continental Scientific Drilling Project

The main drill hole CCSD-MH, Chinese Continental Scientific Drilling Project (CCSD), with depth of 5000 m, is located in the Donghai area, southwestern Sulu terrane. The 3000-3500 m recovered cores in CCSD-MH are mainly comprised of orthogneiss (granitic gneiss) and paragneiss, with thin layers of eclogite, schist and amphibolite. The chatacteristic of petrography indicate that the granitic gneiss can be named as garnet-bearing K-feldspar granitic gneiss, biotite-bearing K-feldspar (monzonitic) granitic gneiss, phengite-bearing K-feldspar (monzonitic) granitic gneiss, magnetite-bearing K-feldspar (monzonitic) granitic gneiss, amphibole-bearing K-feldspar granitic gneiss, two-mica K-feldspar (monzonitic) granitic gneiss, magnetite-bearing two-mica monzonitic granitic gneiss, garnet- bearing biotite monzonitic granitic gneiss, and K-feldsapar granitic gneiss. The paragneiss can be divided into epidote biotite plagiogneiss/ monzonitic gneiss, epidote biotite amphibole plagiogneiss, garnet- bearing epidote biotite plagiogneiss/ monzonitic gneiss, garnet- bearing biotite plagiogneiss/ monzonitic gneiss, amphibole biotite plagiogneiss/ monzonitic gneiss, garnet-bearing phengite plagiogneiss, biotite plagiogneiss/ minzonitic gneiss, amphibole epidote biotite plagiogneiss, phengite plagiogneiss/ monzonitic gneiss, two-mica plagiogneiss/ monzonitic gneiss, garnet-gearing two-mica monzonitic gneiss, magnetite- bearing monzonitic gneiss, magnetitc- bearing biotite monzonitic gneiss.The SiO2 and Al2O3 contents of analyzed orthogneiss samples range from 73.56% to 79.01%, and from 10.49% to 13.75%, respectively. All the analysis show lower abundance of TiO2, Fe2O3, FeO, MnO and MgO (Fe2O3=0.26%-2.56%, FeO=0.09%-1.13% and MgO=0.01%-0.63%). The analyzed sample are rich in Na2O and K2O(Na2O+K2O = 6.20%-9.15%, K2O = 1.26%-5.67%), and poor in CaO(CaO = 0.08%-1.59%). The patterns of REE and trace element is characterized by medium to high negative Eu anomalies, light rare earth element relatively concentrated and high rare earth element deficient. In the MORB normalized spider diagram, the granitic gneiss is relatively riched in K, Rb, Th, Zr and Hf, but evident depleted with Ti, Ta and Nb. The major and trace element composition indicate that the preexisting rock of granitic gneiss in the main drill hole interval 3000-3500 m belong to A-type granite protolith, formed in the continental rift surrounding, registered the granitic and basic magmatism in Neoprotorozoic Rodinia supercontinent breakup event. The north margin characteristic of Yangtze block in the Neoprotoroic belongs to the continental rift surrounding.Comparing to the orthogneiss, the paragneiss have relatively lower SiO2 contents, higher Al2O3, FeO, Fe2O3, MgO and CaO contents, SiO2 ranges from 55.38% to 78.5%, Al2O3 from 10.99% to 20.09%, FeO from 0.4% to 6.04%, Fe2O3 from 0.14% to 3.86%, MgO from 0.05% to 4.83%, CaO from 0.22% to 7.39%. The REE and trace element patterns of paragneiss are characterized by high to slightly negative and positive Eu anomalies, with large variation range of REE. These major and trace element composition indicate that the paragneiss have different source rocks. Some of them may be come from acidic volcanic ignimbrite, which formed in interplate or volcanic arc surrounding. Some may be from pelitic sedimentary rocks, which formed in the fore-arc (back-arc) basin or strike basin surrounding.Combined study of Laser Raman, cathodoluminescence (CL) and SHRIMP U-Pb dating reveals that the zircons separated from the orthogneiss and the paragneiss recorded complicated geochronological traces. All the analyzed zircons separated from orthogneiss retain inherited magmatic crystallization cores with inherited ages (206Pb*/238U) of more than 750 Ma, indicating that the protolith of orthogneiss formed at Neoprotorozoic. Those separated from paragneiss retain inherited (detrital) cores with inherited ages (206Pb*/238U) of 659-313Ma, indicating that the detrital zircons of the protolith have the variety of soures. Coesite-bearing domains of zircons from orthogneiss and paragneiss recorded an weighted ages (206Pb*/238U) of 227±2Ma and 228±5Ma, indicating that the ultrahigh-pressure metamorphism happened in late Triassic time. The quartz- and albite- beaing rims of zircons recorded an weighted ages (206Pb*/238U) of 209±3Ma and 213±6Ma for the late amphibolite facies thermal event in exhumation of Sulu terrane. The geochronology of micro domains in zircons from orthogneiss indicate that the orthogneiss was come from the Neoprotonozoic grnites which experienced the ultrahigh-pressure and late amphibolite facies metamorphism.