Petrogenesis Of Volcanic-Intrusive Complex In Coqen,Tibet,and Their Tectonic Implications

The Coqen basin,with widespread Late Mesozoic volcanic and intrusive rocks,is tectonically located in the central Lhasa subterrane.Though numerous studies have been conducted on these volcanic and intrusive rocks,several issues,including their crystallization ages,the genetic relationship between volcanic and intrusive rocks,and the geodynamic setting of magmatism,still remain considerable controversial.As a consequence,this dissertation focused on the volcanic-intrusive complex outcropped in Coqen region,and conducted an integrated study including field geology,petrography,zircon U-Pb geochronology and Lu-Hf isotopic geochemistry,whole-rock elemental and Sr-Nd isotopic geochemistry.Based on these studies,this paper has precisely constrained their crystallization ages,revealed the genetic relationship between the volcanic and intrusive rocks,and discussed their petrogenesis and the evolution process of the magma chamber.Their implications for the subduction polarity and closing time of Bangong-Nujiang ocean have also been discussed.Therefore,these studies have enriched the understanding of the petrogenesis for the Early Cretaceous igneous rocks and enhanced the comprehension of the evolution of Tibetan Plateau during Mesozoic.The study region is geographically located around Coqen county,central Lhasa subterrane.Volcanic rocks,coexisted spatially with intrusive rocks,lie in both south and north sides of the Tanuocuo-Coqen fault.Zircon LA-ICP-MS U-Pb dating results demonstrate that the ages of volcanic rocks from south and north sides of the fault are 118?115Ma and 115?111Ma,respectively,indicating that both belong to the Lower Cretaceous Zenong Group.This result has modified the previous opinion that the volcanic rocks from the north side of the fault belong to the Lower Cretaceous Zenong Group and those in the southern side belong to the Paleocene Dianzhong Formation.The intrusive rocks were emplaced during 116-113Ma,which is close to the ages of the coexisted volcanic rocks.Besides the similar spatial-temporal distribution,the volcanic rocks also share the similar geochemical compositions with intrusive rocks,therefore,this dissertation concluded that the volcanic and intrusive rocks were formed from the same magma activity and can be identified as a volcanic-intrusive complexGeochemically,the complex is intermediate-acid to acidic,mainly belonging to high-K calc-alkaline series.According to the trace element compositions,all rocks are enriched in LREEs and LILEs(e.g.,Rb,Ba,etc.),depleted in HFSEs(e.g.,Nb,Ta,Ti,etc.),showing the signatures of arc-related igneous rocks under a subduction setting.The zircon Hf isotope compositions of samples are enriched and heterogeneous,with?Hf(t)values of-12.88?+3.57 and tDM2 values of 0.95?1.99Ga,indicating that these volcanic rocks were derived mainly from anatexis of the Paleoproterozoic to Mesoproterozoic basement of central Lhasa subterrane,with various degrees of contributions of depleted mantle components.Bulk-rock Sr-Nd isotope data reveal that the intrusive rocks and volcanic rocks from both sides of the fault underwent various degrees of magma mixing.Besides magma mixing,the complex also underwent fractional crystallization,and volcanic rocks have suffered a higher degree of differentiation than intrusive rocks.The complex shows obvious zonation of lithological association,chemical compositions and spatial-temporal distribution,indicating that the complex was likely derived from a zonal magma chamber.Both magma mixing and fractional crystallization have contributed to the formation of the zonal magma chamber in the deep crust.Based on a comprehensive analysis of geodynamic setting,it is demonstrated that the Coqen volcanic-intrusive complex was likely generated under a continental marginal arc setting with a relatively thick crust.The slab break-off of the southward subducted Bangong-Nujiang oceanic slab occurred during the middle-late stage of Early Cretaceous is the main mechanism that triggered the formation of the complex in this area.