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

Lanthanide kinked shape,similar to Tetrad effect,observed in sub-volcanic rocks from Qiaga,southern Tibet,China

恰嘎次火山岩侵位于特提斯喜马拉雅沉积岩系的东南部,雅拉香波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、Al2O3、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（∑REE）.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.