Geological And Geochemical Characteristics And Tectonic Significance Of Tanluo Granite In Xidamingshan,Guangxi

The southwestern boundary between the Yangtze and Cathaysia blocks is one of the fundamental geological problems in the study of tectonic evolution of the South China continent.Based on the study of petrography,petrochemistry and geochronology of Tanluo granite veins in the Xidamingshan region of Guangxi,the geotectonic background of its formation is discussed.By comparing the inherited zircon ages of granites and detrital zircons ages in the surrounding rock strata in the Xidamingshan region with those of northern Guangxi on the southeast margin of Yangtze block and Yunkai area in the southeast of Guangxi of the western Cathaysia block,This paper constraint the southwestern boundary between the Yangtze and Cathaysia blocks.Tanluo granite is characterized by high Si,peraluminous,low Fe,Mg,Ti and Ca,and poor P.T,which is calc-alkaline series rocks.The differentiation index（DI）is 80.61～87.82,showing the characteristics of high differentiation.The aluminum saturation index（A/CNK）ranges from 1.05 to 1.51,with an average value of 1.25;CIPW standard mineral calculation contains corundum,which is a strong peraluminous rock.Tanluo granite has a right-dipping chondrite normalized distributiong curve and a medium negative europium anomaly（δEu:0.40～0.63）,enriched in Rb,Th,U,and depleted of Ba,Sr,Nb,Ta,P and Ti.The characteristics of major elements and rare earth trace elements show that Tanluo granite is a strong peraluminous S-type granite and a partial melting product of clay-poor sandy rock in the upper crust,it was formed in a collisional tectonic setting.The five chronological samples have consistent U-Pb age distribution characteristics,and the ²⁰⁶Pb/²³⁸U weighted average value of the youngest magmatic zircon is 100.9±0.8 Ma,indicating that the Tanluo granite vein is the result of magmatic event at the turn of early and late Cretaceous in South China.During this period,the Xidamingshan area was generally affected by SN compression,which was caused by the remote effect of subduction of the Neo-Tethyan oceanic plate.The Western Daming Mountain area where Tanluo granite dyke is located at the turn of the early Cretaceous belongs to the Tethyan tectonic domain.The five chronological samples all contain a large amount of inherited zircon,and their mainly distributed in 390～492 Ma,502～594 Ma and 600～900 Ma.The inherited zircons from600～900 Ma account for 65.05%,suggesting that the Xidamingshan region may have the tectonic attributes of the Yangtze block.The U-Pb age distribution of captured zircon in Tanluo granite vein is consistent with those of the southeastern margin of the Yangtze block,but significantly different from those of Yunkai area of western Cathaysia blocks（900～1 300Ma）,indicating that the southwestern boundary between the Yangtze and Cathaysia blocks should be located in the east of the Xidamingshan region of Guangxi.In addition,the U-Pb age distribution of detrital zircon in the surrounding rocks strata of Tanluo granite vein is also consistent with those of Precambrian strata of the southeastern margin of the Yangtze block（700～900 Ma）,but different from those of Yunkai area of western Cathaysia blocks（900～1300 Ma）,and also supports that the southwestern boundary between the Yangtze and Cathaysia blocks is in the east of the Xidamingshan region of Guangxi,South China.