哀牢山造山带南段仰宗岩体地球化学特征及其构造环境

GEOCHEMICAL FEATURES AND TECTONIC SETTING OF THE YANGZHONG ROCK MASS IN THE SOUTH PART OF AILAO MOUNTAINS OROGEN

依据仰宗岩体野外地质特征及主量元素地球化学特征分析表明,仰宗岩体岩性为流纹斑岩,岩石系列为高钾钙碱性过铝质系列。仰宗岩体A/CNK值为1.2～2.3,碱度率指数AR值为2.69～4.28,为S型花岗岩,仰宗岩体物质来源于地壳,属造山期花岗岩类型。仰宗岩体轻、重稀土的质量比为4.75～9.47,LaN/YbN>4,稀土分配型式为负铕的右倾海鸥式,微量元素地球化学相对富集Rb,Th,Ce和Sm(Sm富集不强烈),相对亏损Ta,Nb,Zr,Hf(Ba的贫化不强烈)。主量元素、稀土元素、微量元素地球化学综合分析表明,仰宗岩体形成于造山期后阶段的后碰撞应力松弛伸展环境。研究区砾石碎屑成分、二叠系地层沉积相变化、岩浆活动表明,从中二叠世早期或中期开始,哀牢山洋盆扩张已经停止,开始进入俯冲碰撞阶段。仰宗岩体侵入时间为263.1 Ma±2.3 Ma,表明,研究区内的哀牢山洋盆在中二叠世晚期可能已经碰撞闭合。

According to geochemical characteristics and geological characteristics,the Yangzhong rock mass is mainly rhyolite porphyry, belongs to potassium-rich calc-alkali series. It is showed that the Yangzhong rock mass has A/CNK ratios ranging from 1.2 to 2.3, and AR ratios from 2.69 to 4.28, suggesting that it belongs to typical S-type granite of orogenic granitic magma. REE diagram shows rightward incline and obvious negative Eu abnormity with the Y.REE/S.HREE ra- tios ranging from 1.2 to 2.3. Trace element abundance shows that Ta,Nb,Zr, Hf is of negative abnormality while Rb, Th, Ce and Sm of obvious positive abnormality. Analysis of major element, REE elements,trace element abundance shows that the Yangzhong rock mass formed in the extensional stage of post orogenic collision environment. The study of composition of sedimentary clasts, variation of sedimentary facies and magmatic activity demonstrates that the Ailao Mountains oceanic basin ended its extension in the early to middle Permian and entered the stage of collision,which is evidenced by dating of Yangzhong rock mass with 263.1 Ma±2.3 Ma age.