藏南隆子地区恰嘎流纹质次火山岩稀土元素类似四分组效应

摘要

恰嘎次火山岩侵位于特提斯喜马拉雅沉积岩系的东南部,雅拉香渡片麻岩穹隆与藏南拆离系之间的地区.通过镜下鉴定和对岩石的主量元素、微量元素组成的测试和分析,确定为流纹质次火山岩.该岩体以富SiO2、A12O3、K2O、Na2O,贫MgO、CaO、TFeO为特征,里特曼指数σ介于1.07～2.37之间,指示其属于钙碱性系列岩石.微量元素的测试结果表明,稀土元素总量较低,其配分图上产生类似于四分组的"膝折"效应;富集LILE、亏损HFSE,K/Rb和Rb/Cs比值低于一般酸性岩浆侵入岩体,Y/Ho、Nb/Ta和Zr/Hf的比值均偏离球粒陨石值.结合前人对稀土元素四分组效应的研究,认为富挥发分的流体作用于高度演化的酸性过铝质岩浆后,其稀土元素配分模式也能形成类似四分组的"膝折"配分模式.元素Gd显示正异常.%Qiaga sub-volcanic rocks intruded into the southern-eastern Tethys Himalaya, between the Yardoi gneiss dome to the north and South Tibetan Detachment System to the south. Based on the study by thin-section and whole-rock geochemical analysis,it is classified as rhyolitic sub-volcanic rock. Geochemical analysis results indicate that these rhyolitic sub-volcanic rocks are rich in silicon, aluminum, alkalis, poor in calcium, magnesium and total iron. The Rittmann index(σ) varies from 1.07 to 2.37, which shows the rhyolitic sub-volcanic rocks belong to calc-alkaline series. The trace elements data show that they are enrichment of LILE and depletion of HFSE, low total REE concentration (ΣREE). Rare earth elements Chondrite-normalized distribution pattern is like a kinked shape, similar to tetrad effect; the ratio of K/Rb and Rb/Cs are below the average ratio of acid igneous rocks, and the ratio of Y/Ho,Nb/Ta and Zr/Hf are contrasting difference from those in chondrite. Thus, combine with former tetrad effect researchs, we concluded that after magmatic-hydrothermal alteration, the REE distribution pattern in high-silica peraluminous magma can be a "kinked" shape,similar to tetrad effect, which has positive Gd anomaly.