西藏措勤地区典中组火山岩地球化学特征及构造背景

措勤盆地位于西藏冈底斯构造-岩浆岩带的西段北侧,该区古新世火山活动十分强烈,形成了一套厚度大于1 000 m的中酸性火山岩地层,火山岩K-Ar和Rb-Sr同位素年龄为63.9～61.0 Ma,区域上可与古近纪林子宗群下部层位相对比,归为古新统典中组.火山岩岩石化学、地球化学特征分析表明,该火山岩系属高钾钙碱性-钙碱性系列,轻稀土富集,负Eu异常明显.与原始地幔相比,微量元素Rb,Ba,K,Th,U富集;Ti,P,Sr,Ta(Nb)亏损.锶、氧同位素组成显示岩浆来源与陆壳物质关系密切,推测是来自于俯冲带的幔源基性岩浆与陆壳重熔酸性岩浆以不同比例混合所形成.综合研究认为,这套火山岩形成于陆缘弧构造环境,与喜马拉雅特提斯洋壳向北俯冲消减作用有关.

Lithogeochemical characteristics and tectonic setting of volcanic rocks of Dianzhong Formation in the Coqen area, Tibet, China

The Cuoqin basin is located on the north side of the western Gangdise structural magmatic belt in Tibet.The volcanic activities were very intensive in this area during Paleocene era,resulting in a set of intermediate-acid volcanic rock strata with a thickness of up to 1000 m.The volcanic rocks dated as 63.9~61.0 Ma by K-Ar and Rb-Sr isochron method,can be assigned to the Paleocene Dianzhong Formation by comparing with the lower part of the Linzizong Group in the region.The lithogeochemical characteristics of the volcanic rocks indicate that they belong to high-K calc-alkaline and calc-alkaline series.The rocks with rich LREE and distinct negative Eu anomalies are enriched with trace elements such as Rb,Ba,K,Th,U,and relatively depleted in Ti,P,Sr,Ta(Nb).According to Sr and O isotopic compositions,it is suggested that the source of volcanic rocks was closely related with continental materials and the magma might have resulted from mixing of basic magma and crustal anatectic acid magma in different proportions.On the basis of an integrated analysis,the authors consider that the volcanic rocks formed in continental-margin magmatic arc environment,associated with the northward subduction of the Tethyan oceanic crust.