甘孜-理塘洋可能存在北向俯冲:来自松潘-甘孜道孚-炉霍岩浆岩的证据  

Possible northward subduction in the Ganzi-Litang ocean:evidence from Dawu-Luhuo magmatic rocks in the Songpan-Ganzi orogen

作　　者：卢雨潇 杨经绥[1,2] 许志琴 连东洋[2] LU Yuxiao;YANG Jingsui;XU Zhiqin;LIAN Dongyang(Key Laboratory of Deep-Earth Dynamics of MNR,Institute of Geology,Chinese Academy of Geological Science,Beijing 100037,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing,Jiangsu 210046,China)

机构地区：[1]自然资源部深地动力学重点实验室中国地质科学院地质研究所,北京100037 [2]南京大学地球科学与工程学院,江苏南京210046

出　　处：《地质学报》2022年第7期2380-2402,共23页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号92062215和9195520031)联合资助的成果。

摘　　要：松潘-甘孜造山带巨厚的三叠系复理石沉积盖层给探讨其基底性质、俯冲-碰撞过程和深部岩浆作用增添了难度,使得带内广泛出露的花岗质岩体和少量镁铁质岩体成为解开松潘-甘孜造山带构造演化谜团的重要研究对象。锆石U-Pb定年结果表明道孚花岗闪长岩形成于223.5~217.4 Ma,炉霍二长岩结晶年龄为219.4 Ma,辉长岩为218.9 Ma,均属于晚三叠世岩浆活动的产物。化学成分研究结果表明,花岗质岩石都表现出I型花岗岩特征,其中炉霍二长岩具有较高的Ba、Sr含量,相对较高且均一的ε_(Hf)(t)值(-3.69~-1.65),表明其可能来源于富集玄武质新生下地壳的熔融。道孚花岗闪长岩具有分散的ε_(Hf)(t)值(-13.51~0.41),野外和地球化学特征指示其形成于幔源熔体与古老壳源熔体的混合。辉长岩在微量元素蜘蛛图上具有类似的模式,不同程度富集Ba、Sr、Pb、Th和U元素,出现Nb、Ta、Ti元素亏损,具有岛弧玄武岩的特征,来源于流/熔体交代的地幔楔部分熔融。综合区域已有资料,我们对甘孜-理塘洋的演化历史提出新的认识,认为甘孜-理塘洋不仅仅存在向南俯冲的可能性,同时也具有北向俯冲的历史,晚三叠世道孚-炉霍岩浆岩的形成受控于甘孜-理塘洋向北边松潘-甘孜地块俯冲的背景之下。The extremely thick Triassic flysh sedimentary cover around the Songpan-Ganzi orogeny makes it difficult to assess its basement properties,subduction-collision process and magmatism history.Therefore,widely exposed numerous granitoid plutons and minor mafic plutons have become important research objects to understand the mystery of the tectonic evolution of the Songpan-Ganzi orogeny.Zircon U-Pb dating results show that the Dawu granodiorite was formed at 223.5~217.4 Ma,the Luhuo monzonite crystallization age is 219.4 Ma,and the age of gabbro is 218.9 Ma,all belonging to the products of Late Triassic magmatism.The chemical composition demonstrates that the granitoid rocks are I-type granitoids,in which the Luhuo monzonites have high Ba and Sr contents,relatively high;uniform ε_(Hf)(t) value(-3.69~-1.65) indicated that they are most likely formed by partial melting of the enriched juvenile crustal sources.The Dawu granodiorites have dispersed ε_(Hf)(t) value(-13.51~0.41);their field and geochemical characteristics indicate that they were formed by the mixing of mantle derived melt and ancient crust derived melt.The Luhuo gabbros are relatively enriched in large ion lithophile elements(Rb,Ba,Sr) and radioactive element U,but depleted in high strength elements(Nb,Ta,Ti),which are similar to those of island arc basalt,derived from the partial melting of mantle wedge metasomatized by flow/melt.Based on the information in the region,we propose a new understanding of the evolution history of the Ganzi-Litang Ocean and believe that it was not subducted to the south,but also has a history of north subduction.The Late Triassic Dawu-Luhuo magmatites were formed by the subduction of the Ganzi-Litang Ocean to the north under the Songpan-Ganzi block.

关 键 词：松潘-甘孜造山带 晚三叠世岩浆岩 锆石U-PB定年 地球化学 俯冲背景 道孚-炉霍 

分 类 号：P588.1[天文地球—岩石学] P542[天文地球—地质学] P597.3