藏北羌塘泛非和印支事件的记录:来自俄久卖变质杂岩地球化学与锆石U-Pb年代学的证据  

The record of the Pan-African and the Indosinian tectono-thermal event in Qiangtang terrane, northern Tibet: Evidence from geochemical characteristics and U-Pb geochronology of the metamorphic complex in Ejiumai area

作　　者：郑艺龙[1] 王根厚[1] 郭志文 梁晓[1] 袁国礼[1] 王寒冻 黄波[1] 何亚东[1] 

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]中化地质矿山总局福建地质勘查院,福州350013

出　　处：《岩石学报》2015年第4期1137-1152,共16页Acta Petrologica Sinica

基　　金：中国地质调查局项目(1212011086062;1212011221081;1212011221115)资助

摘　　要：藏北羌塘盆地大地构造属性是长期争议的议题,深入研究有助于对该问题的探讨。俄久卖杂岩体产出于羌塘盆地中央隆起带北缘,包括夕线石片麻岩和片麻状花岗岩,以及侵位于其中的淡色花岗岩脉。在中央隆起北侧,高级变质岩仅发现于俄久卖杂岩体内。因此,该杂岩为研究羌塘盆地构造演化的重要窗口。在区域地质调查的基础上,本文重点对片麻状花岗岩和淡色花岗岩开展了岩石学、锆石U-Pb年代学与地球化学研究。其中,片麻状花岗岩加权平均年龄为476.6±4.8Ma,且岩石中大离子亲石元素Rb、Ba、Th和Pb相对富集,高场强元素Nb、Ta、Ti和P较亏损,显示弧火山岩的地球化学特征。淡色花岗岩中的锆石呈明显的核-幔-边环带结构,其核部测得年龄介于2052±25Ma^541±8Ma之间,幔部加权平均年龄为463.7±7.5Ma,边部内侧为214.6±2Ma,边部外侧为189.4±1.1Ma。地球化学特征表明该淡色花岗岩形成于典型的同碰撞构造环境。综合研究表明,俄久卖杂岩体包括早古生代泛非晚期岩浆事件产物(~470Ma),且在印支运动期遭受深埋作用并发生部分熔融,分别形成了~214Ma和~189Ma的淡色花岗岩。因此,羌塘盆地内泛非时期的地质记录可延伸至中央隆起带北缘。Tectonic attribute of the Qiangtang Basin is a controversial issue,and further study would be helpful to understand it.Ejiumai complexes,located in the northern margin of the ＂Central Uplift＂in Qiangtang Basin,are composed of sillimanite gneiss,gneissic granite,and tourmaline leucogranite intruded into complexes. In the northern margin of the ＂Central Uplift＂,the high-grade metamorphic rock was only discovered in Ejiumai complexes. Therefore,the complexes would be an important window to understand the tectonic evolution of Qiangtang Basin. On the basis of the geological survey, the detailed studies on petrology, U-Pb zircon geochronology and geochemistry have been conducted for gneissic granite and leucogranite respectively. In the case of gneissic granite,the weighted average age was of 476. 6 ＋ 4. 8Ma,and geochemical characteristics indicate it being arc volcanic rocks. The volcanic rocks enriched large ion lithophile elements including Rb,Ba,Th and Pb,and lost high field-strength element including Nb,Ta,Ti,and P. At the same time,the zircons in the leucogranite show obvious core-mantle-rim structure. The age for core part ranged between2052 ± 25 Ma and 541 ± 8Ma. Besides,the weighted average age was of 463. 7 ± 7. 5Ma for mantle,214. 6 ± 2Ma for the medial edge,and 189. 4 ± 1. 1Ma for lateral edge. The geochemical characteristics demonstrated that leucogranite had been formed in typical tectonic settings of collision. Comprehensively,Ejiumai complexes contained magmatic products of later Pan-African events at Early Paleozoic（ - 470Ma）,and suffered deep burying and partial melting in Indosinian in which leucogranite was formed at ca. 214 Ma and ca. 189 Ma,respectively. Therefore,the geological record of the Pan-African period could be extended at least to the northern margin of the ＂Central Uplift＂in Qiangtang Basin.

关 键 词：羌塘盆地 变质杂岩 锆石U-PB年龄 泛非运动 印支运动 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]