西藏当雄羊应乡地热田新生代火山岩特征及其成因探讨

本文首次较全面系统地研究了西藏当雄羊应乡地区与地热田有关的火山岩的岩石学、矿物学、地球化学、岩石化学稀土微量元素等方面的特征,并根据这些特征及同位素年代学的方法和地层构造关系,把滨区新生代火山岩划分为4个喷发期,确定其喷发类型、规模、喷发旋回及岩相特征。通过现代岩石学的研究方法及理论分析,讨论该区火山岩的成因,并确定其岩浆并非原生岩浆,而是派生岩浆。采用了Stormer和Nicholls(1978)提出的主要元素质量平衡原理,进一步计算了派生岩浆演化的特点。通过(?)Sr/(?)"Sr的初始比值的研究,认为本区火山岩浆是来源于壳源下相当于辉长岩质成分的部分熔融结果。

A RESEARCH ON PETROLOGICAL CHARACTERS ANDGENESIS OF THE CENOZOIC VOLCANIC ROCKS INTHE YANGYING VILLAGE GEOTHERMAL FIELD,DANGXIONG, TIBET, CHINA

The Yangying village geothermal field was explored after the Yangbajing geothermal field in Tibet. It is situated at the west - north of Lhasa. There in this region expose many volcanic rocks of Miocene to Pliocene. The volcanic activities may be divided into four eruptive periods, and the volcanic rocks are obviously very different in type and scale as well as in occurence, petrology and mineralogy. The materials produced in all of the four periods belong to alkaline series, with a succession of trachyandesite, pyroxene - monzonitic - porphyrite, biotite - trachyte and murbruk - quartz - trachyte. The research works in mineralogy, petrology and geochemistry indicate that the volcanic rocks in this area were derived from the primary magma through fractional crystallization. The primarymagma is of calc - alkaline series with the character of rich in alkaline. The formation of the primary magma is resulted from partial melting of gabbro which was effected by high temperature and alkaline ?rich fluid coming from the lower crust.