青藏高原最年轻碱性玄武岩SHRIMP年龄的地质意义

Geological implications of the youngest SHRIMP U-Pb age of the alkaline basalt in the Tibetan Plateau.

青藏高原北缘康西瓦地区的新生代碱性玄武岩含有大量的岩浆锆石,SHRIMP锆石U-Pb定年结果(3．8Ma)揭示该玄武岩形成于上新世早期(赞克尔期)。与世界典型地区玄武岩相比,该区玄武岩具有富集包括Zr,U,Th在内的不相容元素的特征。根据前人的实验成果,这种富集与这类元素在高温高压条件下在流体相中浓度增加有关。玄武岩中发育黑云母斑晶可以作为熔浆富含挥发分的证据,而导致熔浆富含挥发分的原因则可能是幔源橄榄岩包体中金云母的脱水熔融。当黑云母开始晶出时,水流体的消耗造成熔浆中ZrO_4的浓度降低,同始晶出锆石。这表明,含水矿物呈斑晶产出有利于锆石在硅酸不饱和熔浆中的结晶。因此,锆石U-Pb定年方法可以应用于具有类似岩石学特征的火成岩中。另一方面,由于幔源岩浆事件可以作为一个地质旋回或阶段开始的触发事件,建议将该测年结果看作是青藏高原最新一期隆升的起始时间。

There is a lot of magmatic zircon crystals in the Cenozoic alkaline basalt exposed in the Kangxiwar area of the northern 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.