晚三叠世金沙江结合带碰撞作用:贡觉石英二长岩年代学、地球化学及Hf同位素证据  

Late Triassic Collision of Jinshajiang Suture Belt:Geochronological,Geochemical and Hf Isotope Evidences from Quartz Monzonite in Gonjo Area

作　　者：巩小栋[1] 唐渊[1] 秦雅东[1] 王保弟[1] 王冬兵[1] 刘函[1] 段瑶瑶 Gong Xiaodong;Tang Yuan;Qin Yadong;Wang Baodi;Wang Dongbing;Liu Han;Duan Yaoyao(Chengdu Center of Chira Geological Survey,Chengdu 610081,China)

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610081

出　　处：《地球科学》2020年第8期2905-2919,共15页Earth Science

基　　金：中国地质调查局项目(No.DD20190053)。

摘　　要：金沙江结合带是三江特提斯构造域重要的结合带之一,是研究金沙江古特提斯洋陆俯冲-碰撞演化过程的重要窗口.然而,关于金沙江古特提斯碰撞闭合的准确时限争议颇多.选择位于藏东地区金沙江结合带西侧的贡觉花岗岩体为研究对象,对其中大规模出露的石英二长岩进行了年代学、地球化学和Hf同位素分析.LA-ICP-MS锆石U-Pb测年结果显示石英二长岩形成于231±1 Ma,代表了金沙江结合带晚三叠世岩浆活动事件.锆石Hf同位素分析获得石英二长岩锆石εHf(t)为-8.3^-5.5,二阶段模式年龄TDMC为1611~1788 Ma.岩石地球化学特征表明,石英二长岩为钾玄岩-高钾钙碱性系列的I型花岗岩,富集K、Th、Rb等大离子亲石元素,亏损Nb、Ta、Zr、Hf等高场强元素.此外,地球化学特征显示石英二长岩形成于碰撞环境,贡觉地区在晚三叠世早期(~231 Ma)处于碰撞挤压向后碰撞伸展环境的转换阶段,石英二长岩为下地壳中基性变质火成岩部分熔融的产物.结合前人研究,认为金沙江古特提斯洋是由南向北逐渐闭合的,区域地质背景的差异性和古特提斯洋斜向碰撞的复杂性是导致金沙江结合带不同地区碰撞闭合时限不一致的主要原因.As one of the important suture belts in the Sanjiang Tethyan tectonic domain,the Jinshajiang suture belt has long been the focus of study on the process of Paleo-Tethys ocean-continent subduction and collision.However,controversies remain about the exact time for the collision and closure of the Jinshajiang Paleo-Tethys.In this paper,the Gonjo granite body was selected as the research object,which is located on the west side of the Jinshajiang suture belt in the eastern Tibet.Based on the chronological,geochemical and Hf isotope analyses of the large-scale exposed quartz monzonite,the geochemical characteristics and tectonic setting of the Gonjo granite body are revealed.LA-ICP-MS zircon U-Pb dating results yield a weighted mean206 Pb/238 U age of 231±1 Ma for the sample,indicating that quartz monzonite was emplaced in Late Triassic.Zircon from the quartz monzonite exhibits a narrow range of initial Hf isotope ratios(εHf(t)=-8.3 to-5.5),with corresponding TDMCof 1611-1788 Ma.Geochemical studies show that the quartz monzonite is typical I-type granite,belonging to shoshonite and high K calcalkali series,enriched in Rb,Th,K,and LILEs,but depleted in Nb,Ta,Zr,Hf and HFSEs.In addition,geochemical features show that the quartz monzonite was formed in the geological background of continent collision,suggesting that the Gonjo area was in a transition phase of syn-collision compressional to post-collisional extension stage in the Early Triassic(~231 Ma)and the quartz monzonite was a product of partial melting of lower crustal intermediate-basic igneous rocks.Combined with previous studies,it can be proposed that the Paleo-Tethys ocean of Jinshajiang suture was gradually closed from south to north,and the differences of regional geological background and the complexity of the Paleo-Tethys ocean oblique collision are the main reasons for the different collision-closure time of different regions of the Jinshajiang suture belt.

关 键 词：金沙江结合带 石英二长岩 锆石U-Pb定年 HF同位素 晚三叠世 古特提斯 

分 类 号：P588.12[天文地球—岩石学]