中文摘要：

雅鲁藏布江结合带在东构造结地区形成弧形展布的蛇绿混杂带,此前对该带蛇绿岩的地球化学特征和成因已有研究,但年代学研究十分薄弱。本文报道对该带蛇绿岩的地质年代学研究成果。从变玄武岩和变辉长岩中分选出两类锆石。一类锆石为自形的柱状双锥,具有清晰的同心韵律环带,较高的Th/U比值（主要为0.63~2.79）,SHRIMP II测出的年龄为185.8±1.7 Ma（MSWD=0.64,n=11）。另有3颗此类锆石经SHRIMP II和LA-ICP-MS分别测出207±29 Ma（MSWD=6.5）、209±20 Ma（MSWD=3.0）年龄值。另一类锆石为它形、半自形长柱-短柱状双锥,内部呈扇状分带、弱分带或无分带,具有较低Th/U比值（主要为0.005~0.033）,经SHRIMP II测出32.74±0.65 Ma（MSWD=0.92,n=11）年龄值。还有一些变质锆石测出82.8~70.8 Ma等年龄。测年结果表明蛇绿岩形成时代为早侏罗世,局部可能为晚三叠世。笔者认为新特提斯洋在晚三叠世至早侏罗世发生向北的俯冲消减,在晚白垩世闭合,蛇绿岩构造侵位。

英文摘要：

The Yarlung Tsangpo suture forms a bended ophiolitic melange in the eastern syntaxis area.Its geochemistry and genesis has been studied previously,but the chronology study is very poor.This paper reports the results of geochronology study of the ophiolite.Two types of zircon were selected from metamorphosed basalt and gabbro.One type of zircon is cylindrical-shaped double cone,with clear rhythm concentric rings,that yields a weighted mean age at 185.8±1.7 Ma（MSWD = 0.64,n=11） and high Th/U ratios（mainly 0.63~2.79） by SHRIMP II analyses.Another three such zircon grains give 207±29 Ma（MSWD=6.5） and 209±20Ma（MSWD=3.0） ages by SHRIMP II and LA-ICP-MS analyses respectively.Another type of zircon grains are semi-euhedral columns to short cylindrical double cones,with weak internal zoning and lower Th / U ratios（mainly 0.005~0.033） and yield a weighted mean age at 32.74±0.65 Ma（MSWD = 0.92,n=11）.There are also some metamorphosed zircon grains that give ages ranging from 82.8 Ma to 70.8 Ma.Geochronological dating suggests that the ophiolite formed in early Jurassic and locally in late Triassic.We argue that the Neo-Tethys began northward subduction in the Early Jurassic to Late Triassic time and closed in the Late Cretaceous time,thus causing ophiolite tectonic emplacement.