Neoproterozoic Magmatism In Liansandao Area,North Of The Sulu Orogen(China) And Its Geological Implication

During the Triassic,the Yangtze plate subducted and collided with the North China plate to form a high-ultrahigh pressure metamorphic belt,the Sulu orogenic belt,which has natural advantages for the study of continental orogeny and geodynamic processes.At the same time,the sulu orogen experienced a complex geological processes during the neoproterozoic period,It is also one of the most complete regions in the Yangtze plate to preserve the Neoproterozoic tectonic-magmatic events,and as a great carrier for Rodinia supercontinent break-up mechanism.Liansan island meta-volcanic rocks as the research objects in the study,and zircon U-Pb dating,zircon Lu-Hf isotope analysis and total trace element analysis were performed,and emphatically discussed the genesis,source characteristics and tectonic setting of the meta-volcanic rocks in the study area.The research content and main results are summarized as follows:1.Precise LA-ICP-MS zircon U-Pb dating results yield the formation ages of 747±12Ma(MSWD=0.91,n=13),767±12Ma(MSWD=5.4,n=19)and 765±21Ma(MSWD=1.7,n=14)for granite porphyry(LSD-1),gneissic monzonite diorite(LSD-2)and garnet-bearing granitic gneiss(LSD-3)respectively in Liansan island meta-volcanic rocks.The ages of the three samples are consistent within error,and reflects that they are the contemporaneous magmatism products.2.According to the study of zircon Hf isotopes,the zircon ?Hf(t)values of the three meta-volcanic rocks granite porphyry(LSD-1),gneissic monzonite diorite(LSD-2)and garnet-bearing granitic gneiss(LSD-3)were-7.2?-10.5,-6.0?-17.6,and-6.8?-12.0,respectively.And the corresponding two-stage model ages were 2.1?2.3Ga,2.0?2.7Ga and 2.1?2.4Ga respectively,peak of 2.2-2.3Ga.Thus,the Liansan island meta-volcanic rocks derive from partial melting of the Paleoproterozoic Yangtze continental crust under the extensional background.3.Major and trace element analyses shows,granite porphyry and garnet-bearing granitic gneiss have high SiO2?Al2O3?Na2O?A/CNK,and A/CNK mostly>1.1,display LREE enrichment and HREE depletion,with negative Eu anomalies,the enrichment of large ion lithophile element(Rb?Ba?K)and high field-strength elment(Sr?Zr?Y),and depleted in Th?U?Ta?P?Ti,showing the characteristics of S-type granite.The gneissic monzonite diorite have low SiO2?TFeO,high Al2O3?CaO,and the content Na2O>K2O,for the lower REE and higher LaN/YbN,indicating that the LREE and HREE fractionation is obvious.enrichment of large ion lithophile element(Rb?Ba?K)and high field-strength elment(Sr?Zr?Y),and depleted in Th?U?Ta?P?Ti.These geochemical characteristics indicate that Liansan island meta-volcanic rocks have the characteristics of island arc magma and also support their formation in the marginal environment of active continents.4.The reserarch shows northeastern margin of the Yangtze Block belongs to an active continental margin environment with oceanic subduction during the Neoproterozoic period,and the arc magmatic activity lasted until 820Ma or earlier,then the Rodinia continent started to break-up and the northeastern margin of the Yangtze Block entered the extensional environment.As the oceanic crust retreated,the lower crust thickened,and the decompression led to the upwelling of mantle source materials and the melting of the thickened lower crust resulted in the formation of granitic magma.In this process,a small amount of mantle materials were assimilated to form the intermediate basic xenolith.