Geochemistry And Its Geological Implication Of The Basalt In The Angwu District,Northern Tibetan Plateau

The Angwu basalt is located in the middle section of the Bangong-Nujiang suture zone?BNSZ?,which is located in the sedimentary rocks of the Mugakangri rock group,representing the relicts of the Bangong-Nujiang Tethyan Ocean.This paper presents petrography,whole-rock geochemical and Sr-Nd isotope data for the basalt from Angwu area.The basalt in this area can be divided into two types according to their field and geochemical characteristics:the first type is pillow basalt,the second type is massive bansalt.The pillow basalt is characterized by low SiO₂,P₂O₅,MgO,MnO,but with hign CaO and Al₂O₃ content,showing strongly high sodium content characteristics.The total amount of rare earth elements??REE?is 123.43×10^-6152.95×10^-6,showing the enrichment of LREE and HREE depletion,?La/Yb?_N=6.32⁷.49.The high field strength elements?HFSEs?such as Nb,Ta,Pb,Zr,Hf and Y is enriched,but the large ion stone elements such as Rb,Th,Sr and Nd is depleted.The pillow basalt indicates mildly positive Eu anomaly,so it is no fractionation of plagioclase.The characteristics of petrology and geochemistry that the pillow basalt might be derived from a hot spot in an intra-oceanic setting.The characteristics of La/Sm,Sm/Yb and La/Yb show that they are derived from partial melting?10%-15%?of the garnet peridotite.The second type is massive basalt,which is characterized by low SiO₂,P₂O₅,and MnO,high TiO₂,CaO,MgO and Al₂O₃ content,and strongly high sodium content characteristics.The total amount of rare earth elements??REE?is342.76×10^-6437.31×10^-6,indicating the enrichment of LREE but HREE deleption,?La/Yb?_N=3.39⁴.21.The basalts are characterized by high Nb,Ta,Zr,Hf,depletion in Rb,K,Ba,Sr.The basalts have a slightly negative anomaly of Eu,which indicates the fraction of plagioclase.The characteristics of trace elements indicate basaltic magma experienced fractional crystallization of olivine,pyroxene and plagioclase during the rising process.The character of petrology and geochemistry indicate Angwu massive basalt is formed in the initial stage of the Bangong-nujiang ocean.The Rb-Sr isotopic result of the five pillow basalt samples shows that the isochronous age is 239±19Ma.The discovery of the Angwu alkaline oceanic island basalts is of great significance to reveal the evolution history of Bangong-Nujiang ocean in the middle Triassic.