【作者】 刘红涛；

【Author】 LIU HongTao Institute of Geology and Geophysics.Chinese Academy of Sciences,Beijing 100029.China

【机构】 中国科学院地质与地球物理研究所；

【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=59）,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（∑REE=151.26～288.43μg/g）、轻稀土富集(La_CN/Yb_CN=8.42～21.77)和轻微-中等程度的负铕异常(（Eu=0.25～0.82）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208Ma和222Ma,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,因而推断在印支晚期昆仑洋壳曾经向塔里木陆块发生过强烈的B型消减事件,导致在塔里木陆块南缘发育大规模弧岩浆活动;本研究认为,这套陆相火山岩与本区的同时代（或稍晚）花岗岩类共同构成了塔里木陆块南缘的印支期弧岩浆带。更多还原

【Abstract】 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,Al₂O₃, Fe₂O₃ and CaO,decrease in a linear manner with silica contents.Large ion lithophile elements,such as alkalis,correlate positively with SiO₂.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 ⁸⁷Sr/⁸⁶Sr ratios ranging from～0.708 to～0.709 and initialε_Nd（t） values ranging from -2.09 to -2.36 imply that the magma was derived from crustal sources or mixed crustal-mantal sources.It has been recognized that the volcanic suite is underlain by pre-Triassic metamorphic and non-metamorphic sequences including the Precambrian crystalline basement.In combination of the geochemical and isotopic characteristics of the volcanics rocks with regional geologic setting,it is proposed that the QTVS was formed upon an active continental margin during the late Triassic north-directed subducting event.Both the volcanics and the closely-related coeval granitoid plutons collectively make up the late Triassic arc magmatic belt on the paleogeographically southern margin of the Tarim continent.Therefore,the QTVS is an essential component of the Kunlun orogenic belt.更多还原