祁漫塔格陆相火山岩:塔里木陆块南缘印支期活动大陆边缘的岩石学证据  

Qimantage terrestrial volcanics: petrologic evidence of active continental margin of Tarim Plate during late Indo-China epoch.

作　　者：刘红涛[1] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2001年第3期337-351,共15页Acta Petrologica Sinica

基　　金：中国科学院资源环境领域知识创新工程重大项目 (KZCX1-0 7)

摘　　要：在东昆仑北部的祁漫塔格山地区发育一套晚三叠世陆相火山岩。这套火山岩主要由英安 -流纹质熔岩和火山碎屑岩构成 ,并含有少量的玄武安山岩和安山岩夹层 ,岩石组合类似于消减带之上的玄武岩 -安山岩 -英安岩 -流纹岩组合 ,其岩相特征与安第斯火山岩带发育的“灰流凝灰岩”组合基本一致。岩石地球化学研究表明 ,它属于典型的钙碱系列岩石 (CA=5 9) ,并具有富钾特点 ;以富集大离子亲石元素、亏损高场强元素为重要特征。具有稀土总量较高 (Σ REE=15 1.2 6～ 2 88.4 3μg/ g)、轻稀土富集 (L a CN/ Yb CN=8.4 2～ 2 1.77)和轻微 -中等程度的负铕异常 ((Eu=0 .2 5～ 0 .82 )的球粒陨石标准化稀土元素配分图谱。根据上述岩石地球化学特征并结合其发育的区域地质条件 ,认为这套火山岩类似于中安第斯的弧火山岩 ,其形成的大地构造环境应为活动大陆边缘。采集于火山岩东、西两个剖面中的两组同位素测年样品 ,获得的全岩 Rb- Sr等时线年龄分别为2 0 8Ma和 2 2 2 Ma,指示火山岩的喷发时代相当于晚三叠世晚期 (印支晚期 )。根据同位素测年样品的地理分布 ,推断该期火山活动在西部喷发较早 ,东部则相对较晚。由于这套火山岩发育在昆仑造山带北侧与塔里木陆块的接合部位的祁漫塔格山地区 。The Qimantage terrestrial volcanic suite (QTVS) of late Triassic age lies in the middle segment of the Qimantage Mt. region, the Kunlun orogenic belt that extends along the northern margin of the Tibetan plateau, western China. The QTVS is dominated by dacite to rhyolite lavas and minor basaltic andesite and andesite, with vast accumulations of pyroclastic rocks (including ignimbrites, volcanic breccias and agglomerates) of dacitic and rhyolitic compositions. Petrological and geochemical studies indicate that the volcanic rocks are of typical calc alkaline character (alkali lime index is about 59) with silica contents ranging between ～53 and ～80 wt% and display relatively high K feature. Major element variations for the volcanic rocks define rather smooth compositional trends, and compatible elements, such as MgO, Al 2O 3, Fe 2O 3 and CaO, decrease in a linear manner with silica contents. Large ion lithophile elements, such as alkalis, correlate positively with SiO 2. Relative variations in trace element abundances on the chondrite normalized data show high Rb, Ba and Th relative to other trace elements and Nb and Ti are depleted. Compatible trace elements, such as Ni, V and Co, as well as some incompatible trace elements such as Th, Ta, Nb and Sr, diminish with increasing differentiation, while other trace elements ( e.g. Rb, La and Ce) generally show increasing trends with the increasing silica contents. The chondrite normalized REE patterns for different rocks of the QTVS are pseudo paralleled to each other and exhibit highly fractionated LREE/HREE spectra with small to pronounced negative Eu anomalies indicating plagioclase involving fractional crystallization was also well developed in crustal differentiation. Two whole rock Rb Sr isochrons yield ages of ca.208 and ca.222 Ma respectively, indicating the volcanic rocks were formed during the late Triassic. The isotope signatures of initial 87 Sr/ 86 Sr ratios ranging from ～0.708 to ～0.709 and initial ε Nd

关 键 词：钙碱性火山岩 地球化学 铷-锶 全岩等时线年龄 活动大陆边缘 祁漫塔格 昆仑山 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]