西藏冈底斯带中西部措麦地区林子宗火山岩地球化学特征及意义  

Geochemistry and implication of the Linzizong volcanic succession in Cuomai area,central-western Gangdese,Tibet

作　　者：于枫[1] 李志国[2] 赵志丹[1] 谢国刚[3] 董国臣[1] 周肃[1] 朱弟成[1] 莫宣学[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室中国地质大学地球科学与资源学院,北京100083 [2]中国黄金集团公司,北京100011 [3]江西省地质矿产勘查开发局,南昌330002

出　　处：《岩石学报》2010年第7期2217-2225,共9页Acta Petrologica Sinica

基　　金：国家自然科学基金重点项目(40830317);国家自然科学基金项目(40873023;40973026);国家重点基础研究发展规划项目(2009CB421002);教育部新世纪优秀人才支持计划(NCET-10-0711);111计划(B07011);中国地质调查局综合研究项目(1212010610104)联合资助

摘　　要：广泛发育在西藏冈底斯带南部的林子宗火山岩被认为是印度与亚洲大陆碰撞的火山作用响应,对于揭示大陆碰撞的时限和过程具有重要的意义。但迄今对林子宗火山岩的系统研究主要局限于拉萨东北部的林周盆地林子宗火山岩的典型剖面,缺乏对冈底斯带中西部地区林子宗火山岩的系统研究。本文在已有区域地质填图研究的基础上,对冈底斯带中西部的措麦地区林子宗年波组火山岩(65～70Ma)进行了岩石学、元素与Sr-Nd-Pb同位素地球化学研究。措麦年波组主要为酸性火山岩、钙碱性和高钾钙碱性系列岩石,比林周盆地年波组更偏酸性,更富碱,显示出更明显的碰撞后加厚背景的火山岩特征。岩石富集大离子亲石元素Rb、Ba、Th、U,富集Pb,亏损高场强元素Nb、Ta、Ti、P,与林周盆地年波组相似,都具有典型的岛弧火山岩特征。措麦年波组火山岩具有更为富集的Sr-Nd-Pb同位素特征(87Sr/86Sr为0.709～0.727;εNd(0)为-9.7～-6.9),相对于东部林子宗火山岩,措麦的岩浆源区中更加富集地壳基底组分,表明冈底斯带南部地壳基底组成的不均一性。若冈底斯带南部的林子宗火山岩底部年龄可以代表印度与亚洲的碰撞起始时间,则措麦地区发生更早,可能预示着印度与亚洲大陆沿着雅鲁藏布缝合带发生的碰撞具有穿时性,中西部地区碰撞时间略早于东部。The widely spreading Linzizong volcanic succession(LVS)is regarded as the magmatism response to the continental collision between India and Asia,which are essentially important in discovering the initial collisional time and processes.However,the present research on LVS is only limited on the typical cross-section of LVS in Linzhou basin to the northeast of Lhasa City,while that in the central-western part of Gangdese belt is rare.This study,based on the regional geological mapping results,present a systemic work of petrology,major,trace and Sr-Nd-Pb isotopic geochemistry on the Nianbo Formation of LVS in Cuomai area(65~70Ma)in central-western Gangdese belt.The Cuomai Nianbo Formation is calc-alkalic to high-potassic calc-alkalic series,rhyolitic lavas,with much higher silica contents and higher alkaline compositions in comparing with the Nianbo Formation in Linzhou basin.This suggests a typical volcanic series which are occurring in the tectonic setting with postcollisional thickening continental crust.The Cuomai volcanics are enriched in Rb,Ba,Th,U,and Pb,depleted in Nb,Ta,Ti,and P,which are similar to that in Linzhou basin,both of which indicate the arc-related volcanics.The Cuomai Nianbo rocks have much enriched Sr-Nd-Pb isotopes,with 87Sr/86Sr ranging from 0.709 to 0.727,εNd(0)ranging from-9.7 to-6.9,which revealing an large portion involvement of mature continental materials in the source region of the magmas,relative to the LVS in Linzhou basin.This also suggests the inhomogeneous distribution of the continental basement in southern Gangdese belt.If the age of the lower most part of LVS can be used to represent the initial collisional time between India and Asia,then one can conclude that this initial collisional time is asynchronous,with that in central-western Gangdese(Cuomai,65~70Ma)earlier than that in the eastern part(Linzhou basin,60~65Ma).

关 键 词：西藏 冈底斯带 措麦地区 林子宗火山岩 SR-ND-PB同位素 

分 类 号：P588.141[天文地球—岩石学]