Geological implications of the youngest SHRIMP U-Pb age of the alkaline basalt in the Tibetan Plateau
青藏高原最年轻碱性玄武岩SHRIMP年龄的地质意义

There is a lot of magmatic zircon crystals in the Cenozoic alkaline basalt exposed in the Kangxiwar area of the northem Tibet.The SHRIMP zircon U-Pb dating(3.8Ma)reveals that the basalt generated in the early stage of Pliocene(Zanclean stage). Comparing with the typical basalts in the world,the Kangxiwar basalt characterized by enrichment of incompatible elements,including Zr,U and Th.According to previous experimental studies,such enrichment depends on increasing concentration of such elements in fluids at high temperature and pressure.The biotite phenocryst in the basalt implies volatile enrichment in the primary liquid.The evidences of partial melting in the xenoliths in the basalt suggest that the dehydration dissolution of phlogopite could produced abundant water,which will increase the solubility of zirconium in the partial melt.While biotite begin to crystallize,consumption of water will decrease the solubility of zirconium in the partial melt,and cause crystallization of zircon.This indicates that the crystallization of water-bearing minerals as phenocryst is in favor of crystallization of zircon in silica-undersaturated melts.Therefore,the zircon U-Pb dating methods can be used to igneous rocks having similar petrographic features in Kangxiwar basalt.In addition,we propose to take this dating result as the initial time of last raising cycle of the Tibetan Plateau,because the mantle magmatic event can be seen as the trigging event of a geological cycle or stage.