Source Of Muchang Alkaline Granite In Southern Baoshan Block,Southwest "Tgree-river"

Muchang alkaline granite is located in the southern part of Baoshan-Zhenkang block in the eastern Tethys tectonic belt.The study on it is of great significance to the study of crust-mantle interaction and the extension of Baoshan block.On the basis of previous work,this paper summarizes the following conclusions through systematic sampling of Muchang alkaline granite and subsequent analysis of geology,petrology,electron probe analysis,principal and trace elements analysis,zircon U-Pb chronology analysis,whole rock Sm-Nd isotope and zircon Hf isotope:?1?The Muchang alkaline granite belongs to A-type granite,which has the characteristics of high Al₂O₃,SiO₂ and FeOT/MgO ratio,low CaO and MgO.The rare earth element distribution pattern of the Muchang granite is LREE enrichment,and the morphology is a special seagull type,similar to the typical A-type granite.They are obviously rich in lithophile elements?such as Rb,K,U and Th?and high field strength elements?such as Zr,Hf,Nb,Y,Ga?,have high 10000×Ga/Al ratio,and are poor Sr,Ba,Ti,Cr and Ni.?2?Analysis of the pyroxene family and the amphibole minerals contained in the Muchang granite by electron probes,and found typical alkaline dark minerals such as aegirine,aegitite augite,riebeckite,and arfvedsonite,further confirming that the Muchang granite belongs to A-type Granite.?3?Through the La-ICP-MS zircon U-Pb data test of some Muchang granites,they are relatively concentrated in age,231.3±0.6 Ma,234.0±1.6 Ma,231.4±2.1 Ma,238.1±1.2 Ma,232.4±1.1 Ma,228.8±1.5 Ma,indicating that they formed in the late Early Triassic.?4?The Sm-Nd isotope and zircon Hf isotope results of the whole rock indicate that the source area of the A-type granite in the Muchang should be composed of the mixture of the lower crust base and the mantle material.The OIB-type basalt in the Niuhetang Formation in the same era is also suggesting that the OIB magma is simultaneously baked in the crust,resulting in partial melting of the crust to produce A-type granite and mixing to some extent.?5?The formation environment of the alkaline granite in Muchang is the extension stage of the early Middle Triassic collision,which means that the consumption of the ancient Tethys Ocean occurred before 233Ma.The Baoshan block,where the Muchang alkaline granite is located,also shows that the post-orogenic extension is not limited to the subduction of the upper plate,which expands the understanding of the spatial extent of the post-orogenic extension.