滇西南澜沧江结合带中段云县花岗岩的地质特征及形成环境

GEOLOGICAL CHARACTERISTICS AND TECTONIC SETTING OF THE YUNXIAN GRANITE IN THE MIDDLE PART OF SOUTH LANCANGJIANG CONVERGENT MARGIN, WESTERN YUNNAN PROVINCE

南澜沧江结合带云县花岗岩体的岩石类型主要为黑云二长花岗岩,SiO2平均为68.57%,K2O/Na2O平均为1.67,相对富钾,岩石属高钾钙碱性系列。其岩石酸、碱度低于同类岩石平均值;而镁铁组分高于平均值,显示岩石偏中性,与同碰撞构造环境形成的花岗岩特征类似。Al2O3平均为13.66%,含量较高,ACNK平均为1.1,呈铝过饱和。CIPW计算结果,都出现标准矿物刚玉分子(>1%)。岩石总体上相对富集大离子亲石元素,亏损高场强元素。稀土元素总量(ΣREE)较高,平均240.75×10-6。岩体轻稀土元素富集,重稀土亏损,(La/Yb)N为8.88～9.41,分异不是很大;Eu为中等负异常,δEu为0.52～0.57,经多种相关图解判别,岩石属S型花岗岩。其构造环境相当于大陆碰撞花岗岩类(CCG)。锆石颗粒U-Pb测年结果显示,源岩的形成年龄最晚是晋宁期(>778Ma),岩体主体形成于华力西晚期-印支期。0206024-5k号锆石是岩浆成因锆石,该样品的206Pb/238U和207Pb/235U两个年龄(49Ma和61Ma)反映在云县岩体中可能存在喜山期岩浆活动。

Yunxian granite in the South Lancangjiang convergent margin is mainly diotite monzonite granite and belongs to potassium-rich calc-alkali series with SiO2=68.57 % and K2O/Na2O=1.67 on average. Both acidity and alkalinity of the rock are lower than the average and its mafic ingredient is higher than the average of homogeneous rocks which indicates that the rock tends to neutrality and is similar to syn-collision type granite. Yunxian granite is peraluminous with A1203=13.66% and ACNK=1.1 on average and corundum molecule〉l% according to CIPW. As a whole the rocks are relatively rich in LILE but poor in HFSE and shows moderate negative Eu anormalies with δEu=0.52-0.57, ∑REE=240.75×10^-6 on average and （La/Yb）N=8.88-9.41. According to geochemical characteristics and related discrimination diagrams, it is suggested that Yunxian granite should belong to S type granite and its tectonic setting corresponds to CCG. The results of zircon U-Pb Age show that the forming period of source rocks is Jinnin phase （〉778Ma） at latest and mainly is late Hereynian to Indo-China phase. The No. 0206024-5k zircon is a magmatic origin one and its two ages of 206^pb/238^U and 207^Pb/235^U （49Ma and 61Ma）indicate that there likely exists magmatic activity in Himalayan period of the Yunxian granite.