摘要
对于东昆仑地区晚古生代‒早中生代古特提斯演化中洋盆闭合的时限目前还存在较大争议。本文对位于东昆仑造山带中段五龙沟地区鑫拓斑状二长花岗岩开展详细的LA-ICP-MS锆石U-Pb年代学、元素和同位素地球化学研究,确定其形成时代,探讨其岩石成因及成岩构造背景,限定古特提斯洋的形成演化过程。结果表明,斑状二长花岗岩LA-ICP-MS锆石U-Pb年龄为240.5±1.2 Ma,指示其侵位于中三叠世中晚期。岩石K_(2)O/Na_(2)O值为0.79~1.02,Mg^(#)为34~39,σ为1.71~1.96,A/CNK为1.00~1.05,属弱过铝质高钾钙碱性系列。岩石的轻重稀土元素分异明显((La/Yb)_(N)=20.93~32.89),具有微弱的Eu负异常(Eu/Eu*=0.88~0.93),富集大离子亲石元素Rb、K、Pb、Sr等,亏损高场强元素Nb、Ta、P、Ti等,以及具有较高的Sr含量(384~460μg/g)和Sr/Y值(46.11~61.53),较低的Y(7.20~9.30μg/g)和Yb(0.63~0.89μg/g)含量,显示埃达克质岩石的地球化学特征。锆石ε_(Hf)(t)值介于−7.7~−0.3,对应二阶段模式年龄(t_(DM2))为1157~1565 Ma。综合分析表明,鑫拓斑状二长花岗岩形成于由俯冲向同碰撞转换的构造背景,源于增厚的古老下地壳含石榴子石角闪岩的部分熔融,且在岩浆形成演化过程中,经历了一定程度的分离结晶作用,但未发生明显的壳幔混合。综合区域构造演化以及同时代岩浆岩证据,本文认为鑫拓斑状二长花岗岩侵位于古特提斯洋北向俯冲的末期、陆(弧)陆局部初始碰撞造山阶段,古特提斯洋最终完全闭合时间应为中三叠世晚期‒晚三叠世早期。
Even though many dating researches have been carried out,however,the closure time of the Paleo-Tethys Ocean remains elusive.The Xintuo porphyritic monzogranite in the East Kunlun orogenic belt may yield constraints on the evolution of Paleo-Tethys Ocean.The results including zircon LA-ICP-MS U-Pb ages,major and trace element concentrations and Lu-Hf isotope compositions are utilized to constrain the petrogenesis and tectonic setting of the Xintuo porphyritic monzogranite,as well as the evolution of the Proto-Tethys Ocean.LA-ICP-MS zircon U-Pb dating yielded a weighted mean age of 240.5±1.2 Ma,indicating that it was emplaced in the Middle Triassic.It is characterized by low K_(2)O/Na_(2)O,σand A/CNK,indicating that it belongs to the high-K calc-alkaline and weakly peraluminous series.The rocks exhibit right-dipping REE patterns with weakly negative Eu anomalies and high(La/Yb)_(N) ratios(between 20.9 and 32.9).The rocks are enriched in LILE(large ion lithophile elements,such as Rb,K,Pb,Sr),and depleted in HFSE(high field strength elements,such as Nb,Ta,P,Ti).Besides,all the samples have high Sr(vary from 384 to 460μg/g)and low Y(vary from 7.20 to 9.30μg/g),Yb(vary from 0.63 to 0.89μg/g)contents with relatively high Sr/Y ratios(between 46.11 and 61.53),displaying geochemical characteristics similar to those of adakitic rocks.Theε_(Hf)(t)values of the Xintuo porphyritic monzogranite vary from−7.7 to−0.3,with two-stage Hf model ages vary from 1157 to 1565 Ma.According to the lithological and geochemical characteristics and isotopic compositions of the rocks,we propose that the Xintuo porphyritic monzogranite was derived from partial melting of garnet amphibolite of thickened ancient lower crust,formed in a subduction-collision transition tectonic environment.Finally,in combination with the tectonic evolution of the East Kunlun orogenic belt and the geochronological results and geochemical characteristics of the contemporary intrusive rocks,it is concluded that the Xintuo porphyritic monzogranite was formed
作者
曾闰灵
魏俊浩
李欢
黄啸坤
闫茂强
张新铭
ZENG Runling;WEI Junhao;LI Huan;HUANG Xiaokun;YAN Maoqiang;ZHANG Xinming(School of Earth Resources,China University of Geosciences,Wuhan 430074,Hubei,China)
出处
《大地构造与成矿学》
EI
CAS
CSCD
北大核心
2021年第6期1233-1251,共19页
Geotectonica et Metallogenia
基金
国家自然科学基金项目(41772071)资助。
关键词
东昆仑造山带
古特提斯洋
埃达克质岩
中三叠世
俯冲‒碰撞转换
East Kunlun orogenic belt
Proto-Tethys Ocean
adakitic rocks
Middle Triassic
subduction-collision transition
