Geochronology And Geochemistry Of The Qinling Complex And Shahewan,Caoping Granites In South Qinling And Their Significance

The Qinling Orogenic Belt is formed by the superposition of multiple stages of different tectonic movements between the North China Block and the Yangtze Block and its micro-plates.It is an important part of the Central Orogenic belt.The Qinling Orogenic Belt can be divided into the southern margin of the North China Block,North Qinling Belt,South Qinling Belt and the northern margin of Yangtze Block by the Luonan-luanchuan fault,Shangdan structural zone and Mianlue strutural zone from north to south.Due to the complex tectonic characteristics,the tectonic affinity and the sedimentary age of the North Qinling,and the tectonic setting of the Southern Qinling in the Mesozoic are still controversial.This contribution presents a set of new petrologic,geochemical,zircon U-Pb dating and Hf isotopic data for the Qinling Complex in the North Qinling Belt,and Shahewan granite pluton and Caoping pluton in the South Qinling Belt in order to discuss the formation and metamorphism of these rocks,which provides a new basis of tectonic evolution in the Qinling Orogenic Belt.The results are as follows:(1)Protoliths of the meta-sedimentary rocks from the Qinling Complex were graywackes and were sourced from intermediate-acid igneous rocks.The mate-sedimentary rocks were metamorphosed under greenschist facies and the main metamorphic event occurred between ca.516-509 Ma.Detrital zircons from nine samples show that protoliths of the mate-sedimentary rocks probably formed between the Late Mesoproterozoic and Early Neoproterozoic,and the supra-crustal succession can be divided into three units,1182 to 1158 Ma for the lower unit,957 to 955 Ma for the middle unit and 917 to 840 Ma for the upper unit.According to the geochemistry characteristics,these rocks were formed in an arc-related tectonic setting or active continental margin during the Early Neoproterozoic.(2)The magmatic crystallization age of the Shahewan pluton is 214-207 Ma while the Caoping pluton is 215-212 Ma.The two plutons belong to metaluminous I-type granite of high-K calc-alkaline series.The samples show evident fractionation of light rare earth elements(LREE)and heavy rare earth elements(HREE),as the samples are enriched in LREE and large ion lithophile elements(LILE,e.g.Rb,Ba and Sr)and relatively depleted in HREE and high-field-strength elements(HFSE,e.g.,Nb,Ta,Ti,Y,and Yb).This study indicates that the parental magma of the two plutons is a mixture of Mesoproterozoic crust and mantle and the plutons formed during the extensional delamination stage of the post-collisional stage.(3)During Meso-to Neo-proterozoic,the Qinling Ocean between the North China Block and Yangtze Block subducted toward north underneath the North China Block,which resulted a series of huge magmatic event and formation of the North Qinling Belt.In the Early Paleozoic,the Yangtze Block started to subduct toward the north underneath the North Qinling Belt along the Shangdan suture,which led to strong metamorphism in North Qinling Belt and reached the peak of metamorphism at 516-480 Ma.In the Late Paleozoic,the Mianlue Ocean between the Yangtze Block and South Qinling Belt subducted toward north underneath the South Qinling Belt.The consumption of the Mianlue oceanic crust led to collision between the Yangtze Block and South Qinling Belt.During the post-collision stage,the delamination of lower crust dued to mantle uplift,and the stress state change from compression to extension.And the early Mesozoic granite,like the Shahewan and Caoping pluton,formed under the influence of mantle thermal and decompression melting.