The Tudy Of Fengzishan Pluton And Qinling Group In Qinling Orogenic Belt And Their Constraints On Neoproterozoic Magmatic Events

The Qinling orogenic belt is located in central China with elongated shape.It was formed by the multi-stage geological processes of the North China Block and Yangtze Block,and has complex tectonic evolution.Shang-Dan Fault is an important tectonic belt during the process of tectonic evolution of Qinling orogenic belt.Meta-sedimentary rocks of the Qinling Group lies to the north of Shang-Dan Fault,granite of the Douling Group lies to the south of Shang-Dan Fault,both of them recorded important information about tectonic evolution and orogeny.This study reported a series of new petrologic,zircon U-Pb dating,geochemistry and Hf isotopic data for the Fengzishan pluton in the South Qinling belt and the Qinling Group in the North Qinling belt.Results of this study are as follows:1)Zircon U-Pb dating results of three amphibolites?meta-sedimentary rocks?from the Qinling Group showed three maximum depositional ages of 971 Ma,1222Ma and 840 Ma for the amphibolites.The Qinling Group is considered to be a complex with units of the Mesoproterozoic?older components?and early Neoproterozoic?newer components?.The age peaks of the meta-sedimentary rocks from the Qinling Group are mainly clustering at Mesoproterozoic to early Neoproterozoic,which are different from the Yangtze block and North China block.The Qinling Group should be a microcontinent with distinct evolution during Mesoproterozoic to early Neoproterozoic.Combined with the geochronological characteristics and the previous research,the meta-sedimentary rocks from the Qinling Group were deposited in the arc-related tectonic environment.2)LA-ICP-MS zircon U-Pb dating results suggest that the Fengzishan pluton was formed in late Neoproterozoic?716-710 Ma?.Zircon Hf isotopic data showed that the?_Hf?t?values of-3.16-1.17 and T_DM2M2 values of 1.84-1.52 Ga.Geochemical characteristics showed that they are high in silica?62.92-73.57%?and Na?3.33-4.22%?contents and relatively enriched in light rare earth elements?LREE?and large ion lithophile elements?LILE??e.g K,Rb,Ba,Th,U?,poor in high field strength elements?HFSE??Nb,Ta,Ti,P?and slightly negative Eu anomalies??Eu=0.79-0.95?,displaying metaluminous to weak peraluminous and medium potassium calc-alkaline I-type granites.These characteristics suggest that the magma was primarily derived from partial melting of gneiss of the Douling Group.The Fengzishan granitic intrusion was produced by subduction of oceanic crust into continental crust and was formed in the tectonic setting of rift expansion coexisting with small ocean basin.3)The formation age of the Fengzishan pluton?716-710 Ma?is the same as extensive magmatism time of the South Qinling belt,representing an important tectonic thermal event after the Jinning period in the South Qinling belt.The tectonic setting of the South Qinling belt may be in the stage of transition from subduction and collision to extension in the middle Neoproterozoic,and the stage has not finished yet until the Late Neoproterozoic?⁷10 Ma?,this was probably the response to the amalgamation and separation of the global Rodinia supercontinent.