Petrogenesis And Geological Significance Of Dacite In Zhesang Area,Southeast Yunnan,China

There have been many controversies about the tectonic framework of the Nanpanjiang Basin and the evolution of its surrounding Paleo-Tethyan Ocean.In this paper,the newly exposed volcanic rock,dacite,in the Zhesang area,southeast Yunnan Province,has been analyzed.Firstly,the field geological features and petrographic characteristics have been studied.On the base,the petrogenesis,source characteristics and tectonic setting of the rock have been revealed by the zircon SHRIMP U-Pb dating,the whole-rock major and trace element analyses,zircon Hf isotope analyses,and Sr-Nd isotope analyses.Then the tectonic environment of the southern margin of the Nanpanjiang Baisn has been studied,and the evolution of the Paleo-Tethyan branch ocean has been discussed in the China-Vietnam border area.The rock occurs in the Lower Triassic Luolou Formation,and its petrographic characteristics are similar to the characteristics of the middle-acid volcanic rock.Zircon SHRIMP U-Pb dating of dacite demonstrates that they were formed in the Early Triassic（247.8+1.7 Ma,MSWD=1.2）,belonging to the rock related to the evolution of the Tethyan Ocean in time.And the rock belongs to a calc-alkaline series through the major elements of the rock.All samples exhibit enrichment in LREE,and depletion in HREE.The distribution pattern of rare-earth elements is a right-leaning curve.All samples are enriched in LILE（e.g.Rb,K,and U）and depleted in HFSE（e.g.Nb,Ta,and Ti）,showing the characteristics of the volcanic rock associated with subduction.Based on petrographic and geochemical characteristics,the volcanic rock is dacite.Zircons in dacite haveε_Hf（t）values of-16.2 to-8.3,mostly concentrated in the range of-12～-8,lower than the average crustal value and closer to the lower crust.And the two-stage Hf model ages are1799～2301 Ma,mostly concentrated between 1800 and 1900 Ma,with the information of Early Proterozoic.The initial values of Sr-Nd isotope(⁸⁷Sr/⁸⁶Sr)_i of the dacite range from 0.706954 to 0.708589,and the calculatedε_Nd（t）values range from-11.77 to-10.88.All samples show high(⁸⁷Sr/⁸⁶Sr)_iratios and negativeε_Nd（t）values.The two-stage model ages of Nd isotope(T_DM2)are between 1750 and 1803Ma,which is also Early Proterozoic.Based on the geochemical characteristics of major and trace elements and the type of the parent rock,the dacite has the features of I-type volcanic rock.Moreover,combined the zircon Hf isotope and Sr-Nd isotope characteristics,the initial magma was mainly from the reconstructed the Early Proterozoic lower crust mixed with some mantle materials,and crystal fractionation process underwent in the late stage of magma migration.Combined with the geochemical characteristics of the dacite and previous studies on magmatic rocks of the same age in this area,the dacite was formed in the continental margin arc tectonic environment related to subduction.By a comparative study,the middle-acid volcanic rocks in Zhesang,Napo and Pingxiang areas which are located in the southern margin of Nanpanjiang Baisn from west to east,have very similar diagenetic ages,belonging to the Early Triassic.In addition,they have same characteristics of rare-earth and trace elements,and Sr-Nd isotope.Their tectonic settings are also active continental arc settings.What’s more,combined on the previous studies on the sedimentary characteristics,magmatic rocks,paleo-structure and paleo-geography of this area,the southern margin of Nanpanjiang Basin was a continental marginal arc tectonic environment during the Permian and Early Triassic,and the Nanpanjiang Basin was an arc-basin tectonic pattern.Summarized the studies on ophiolite assemblages,mid-ocean ridge basalts,sedimentary strata of deep water facies and arc-volcanic rocks,there had been a Late Paleozoic ocean,and the subduction period of the ocean was from Early Permian to Middle Triassic,and the ocean may have subducted northward.