Neoproterozoic Magmatism And Tectonic Evolution In Eastern Part Of The South Qinling Orogenic Belt

The Qinling orogenic belt is a complicated orogenic belt which was resulted from the superposition and transformation process from multiple stages of tectonic movements,and is the bound between North China block and Yangtze block.Several Neoproterozoic magmatic activities has been recorded in the South Qinling belt,these records enable us to better understanding the tectonic relationship between the South Qinling belt and the Northern margin of Yangtze block.Xiaomoling complex and Lengshuigou complex in Zhashui area recorded multiple stages of magmatic events from 925-680 Ma,former study revealed an important transformation of regional tectonic setting from island to continental rift.Two gabbro samples in Lengshuigou and one gabbroic diorite sample in Nianpangou in this paper yield almost same U-Pb age?⁶21 Ma?,which is identical to gabbro defined in the Lijiabian Complex,indicating they were formed from the same magmatic event.Influenced by the intense fluid activity and greenschist facies metamorphism,most of the pyroxene were transformed into amphibole,the composition of residual pyroxene is extremely heterogeneous.Th/U ratios and CL images of zircons from three basaltic rock samples reported in this paper yield same feature as igneous zircon,thus represent the age of crystallization of source magma.The t_DMages of Lengshuigou gabbro samples are 1.1-1.35 Ga,indicating the protolith were formed in Mesoproterozoic.Combining with the depleted Hf-O isotopic feature(?_Hf?t?=-3.3⁺5.3;?¹⁸O=3.48⁵.09),suggesting it formed by the partial melting of depleted mantle.The basaltic sample above the weathering surface?LSG16-03?has comparatively wide range of Hf-O isotopic value,reflecting the source modified with enrich component.Nianpangou gabbroic diorite has extremely depleted Hf-O isotopic feature(?_Hf?t?=5.8¹1.1;?¹⁸O=-3.72^-2.58),the negative ?¹⁸Ovalue represents high temperature water-rock interaction.The t_DMage of Nianpangou basaltic rock is later than Lengshuigou samples,which is similar with the age yielded by the inherited zircon in Lijiabian gabbro,it could be explained as the⁶21 Ma magmatic activity triggered the remelting of previous igneous rocks.The CL images yielded no traces of alteration by fluid,suggesting it crystallized from low ?¹⁸Omagma which generated by the interaction between subduction low ?¹⁸Ofluid and basaltic magma.The U-Pb-Hf isotopic features of samples in this paper are comparable to late Neoproterozoic magmatic record which widely spread in the South Qinling belt and northern margin of the Yangtze block,representing it formed in a back-arc setting.In order to better understanding the tectonic transformation between the South Qinling belt and the northern margin of Yangtze block during Neoproterozoic,we also did detailed zircon U-Pb geochronology and Hf-O isotopic research of three samples located from the core to the rim of songjiawuchang diabase?gabbro?.Among three samples,sample JTS16-02 which located at the rim are supposed to be the surrounding diorite which involved in the structural fracture zone.Samples located at the core are two basic rocks,the particle size are irregular due to the assimilation of magma.Two gabbroic samples show depleted Hf isotopic feature,however,the slightly enrich O isotopic feature and the wide range of Hf-O isotope indicate a deplete source modified by the enrich materials.Diorite sample JTS16-02 shows enrich Hf-O isotopic feature,suggesting it crystallized from the arc magmatism triggered by the subduction-collision process between the South Qinling belt and the northern margin in the early stage of Neoproterozoic.Former regional research on magmatic rocks indicate a widely arc magmatism along the South Qinling belt and northern margin of Yangtze block in early Neoproterozoic?>735 Ma?.Compared with Lengshuigou and Jiutianshan samples reported in this article,we propose a tectonic transformation occurred in the South Qinling belt from island to back-arc environment.Three basaltic samples in Lengshuigou reported in this article recorded the latest magmatic event in South Qinling,thus marking the end of Neoproterozoic basic magmatic events,representing the tectonic setting of the South Qinling Orogenic belt became extend tectonic setting.