班公湖中特提斯洋打开的时限:来自MOR型辉长岩的年代学制约  

Opening Time of Middle Tethys Oceanic Basin:Constrained from Zircon U-Pb Dating of MOR-type Gabbro in Bangong Lake Ophiolite

作　　者：秦雅东[1] 李德威[2] 刘德民[2] 李华亮[2] QIN Yadong;LI Dewei;LIU Demin;LI Hualiang(Chengdu Center of Chinese Geological Survey, Chengdu 610081, Sichuan, China;School of Earth Sciences,China University of Geosciences, Wuhan 430074, Hubei, 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 [2]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《大地构造与成矿学》2017年第6期1148-1157,共10页Geotectonica et Metallogenia

基　　金：中国地质调查局西藏1:50000聂拉木等4幅区域地质调查项目(1212011121242);中国地质调查局西藏阿木雄地区地质矿产综合调查项目(12120115023501)联合资助

摘　　要：班公湖?怒江结合带西段班公湖地区蛇绿岩组合较为完整,由超镁铁质岩、镁铁质堆晶岩、辉绿岩脉、铁镁质熔岩、放射虫硅质岩等组成。这套蛇绿岩所代表的洋盆打开时间一直争论不休。该蛇绿岩中MOR型辉长岩的LA-ICP-MS锆石U-Pb定年显示,18个测点的^(206)Pb/^(238)U加权平均年龄为231.5±2.6 Ma,代表了辉长岩的结晶年龄。地球化学分析表明辉长岩SiO_2含量在46.12%~47.85%之间,TiO_2(1.52%~1.71%),具低的K_2O(0.19%~0.25%)、Al_2O_3(12.97%~13.51%)和Fe_2O_3/FeO比值(0.19~0.24);微量元素蛛网图与洋中脊玄武岩类似,Zr、Nb、Ta、Hf丰度与N-MORB相当或略高,相对富集Ba、Pb、Sr等大离子亲石元素,无Ta-Nb负异常,在基性岩构造环境判别图上显示出洋脊玄武岩的亲合性;REE配分曲线具有平缓型特点,介于E-MORB与N-MORB之间;各种地球化学特征与洋脊玄武岩类似,表明该辉长岩是在班公湖洋盆扩张阶段形成的。结合前人研究成果,本文认为班公湖中特提斯洋盆初始打开时间至少为中三叠世。A complete ophiolite profile outcrops in the Bangong Lake area in the western part of the Bangong-Nujiang ophiolite zone.The Bangong Lake ophiolite is composed of ultramafic rocks,mafic cumulates,diabase dikes,mafic lavas,and aradiolarian-bearing cherts.LA-ICP-MS U-Pb dating of zircons from MOR-type gabbro in the ophiolite yielded a weighted mean206Pb/238U age of231.5±2.6Ma,which represents the crystallization age of the gabbro.The gabbro shows relatively low contents of SiO2(46.12%-47.85%),TiO2(1.52%-1.71%),K2O(0.19%-0.25%),Al2O3(12.97%-13.51%)and Fe2O3/FeO ratio(0.19-0.24).The trace element features of the gabbro are similar to those of the mid-ocean ridge basalts,N-MORB in particular,characterized by enriched large-ion lithophile elements(such as Ba,Pb and Sr)and no obvious Ta-Nb anomalies.The geochemical similarity between the gabbro and the mid-ocean ridge basalt indicates that the ophiolite was likely formed in a mid-ocean ridge setting,and therefore,we argue that the opening of Middle Tethys Ocean at the Bangong Lake area was initiated in the Middle Triassic.

关 键 词：蛇绿岩 MOR型辉长岩 洋盆初始裂解 班公湖中特提斯 

分 类 号：P597[天文地球—地球化学] P54[天文地球—地质学]