【作者】 翟庆国；

【导师】 肖序常；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2008， 博士

【摘要】 羌塘中部榴辉岩发现于2004年,是青藏高原内部首例报道的榴辉岩。榴辉岩出露点位于西藏改则县戈木日东约20km,龙木错—双湖缝合带的中部,岩石呈透镜状、块状产在石榴白云母片岩和蓝片岩构成的高压变质带中,直接同岩为石榴白云母片岩。榴辉岩块体大小不一,大者可达数百米,小者仅几个厘米。榴辉岩主要矿物成分有:石榴子石、绿辉石、多硅白云母、角闪石、金红石、榍石、绿帘石、磷灰石、斜长石和石英等。电子探针分析显示,石榴子石中铁铝榴石（Alm）含量在61～68之间,钙铝榴石（Grs）23～30之间,镁铝榴石（Pyp）在6～11之间,锰铝榴石（Sps）含量多＞2,榴辉岩为典型的C型榴辉岩。绿辉石硬玉分子（JD）在29～47之间,霓石分子在9～17之间,多硅白云母中SiO₂含量多在50～52 wt%之间,每单位晶胞（p.f.u.）Si原子数多在3.35～3.45之间。角闪石主要为冻蓝闪石、镁红闪石等,为后期变质作用的产物。榴辉岩峰期变质的矿物组合为石榴子石+绿辉石+多硅白云母+金红石,变质温度在500℃左右,压力在2.3Gpa左右。对棉辉岩的地球化学分析显示,榴辉岩SiO₂含量在46.68—50.24%之间,Na₂O+K₂O含量在3.10—4.21%之间,稀土元素总量较高,轻稀土富集,重稀土相对亏损,LREE/HREE比值在2.83—6.06之间,稀土元素球粒陨石标准化曲线呈向右倾斜的样式,微量元素原始地幔标准化蛛网图OIB类似,这些特征和龙木错—双湖高压变质带的蓝片岩类似,它们的原岩为均为洋岛型玄武岩,形成环境类似于现在太平洋中的夏威夷,榴辉岩、蓝片岩很可能是羌塘中部地区出露的二叠纪洋岛型岩石俯冲消减的产物。锆石矿物学和SHRIMP年代学研究表明,榴辉岩锆石中含有大量的绿辉石、多硅白云母、石榴子石、冻蓝闪石等矿物原生包裹体,锆石为榴辉岩相变质作用过程中从形成的变质锆石,锆石SHRIMP U—Pb年龄在230～243Ma之间,代表榴辉岩相变质作用的时代。榴辉岩及其同岩中多硅白云母Ar—Ar年龄分别为214～219Ma和217～223Ma,代表其折返到地表的时代。羌塘中部榴辉岩变质作用P—T—t轨迹显示为顺时针快速下降曲线,主要经历了中三叠世（230～243Ma）榴辉岩相变质作用阶段（T=～500℃,P～2.3Gpa）,绿帘角闪岩相变质作用阶段（T=～450℃,P＜1.0GPa）和晚三叠世（214～219Ma）绿片岩相变质作用阶段（T＜400℃,P＜0.5GPa）。结合对羌塘地区蓝片岩、蛇绿岩和岛弧型火山岩的研究,羌塘地区构造演化主要经历了前奥陶纪基底演化、中奥陶世—志留纪早期特提斯大洋、二叠纪—三叠纪古特提斯洋、中、晚三叠世板块俯冲碰撞及古特提斯洋关闭和三叠纪之后陆—陆碰撞造山等五个阶段。更多还原

【Abstract】 The eclogite from central Qiangtang area was discovered in 2004,which is the first eclogite reported within Qianghai - Tibet plateau.The eclogite lies in about 20 km east to Gemuri,Gerze county,Tibet.Eclogites occur as lenses and blocks of various sizes within the high-pressure metamorphic belt comprising garnet - muscovite schist and glaucophane - muscovite schist.The largest block is up to 100m long,intercalated in garnet muscovite schist in Longmucuo - Shuanghu suture zone.The eclogite consists of garnet + omphacite + phengite + rutile + epidote + quarta and display distinct retrograde amphibolite assemblage of hornblende + plagioclase + epidote in some blocks.Most eclogites contain 30-40%modal content of garnet,20-50%omphacite,20-35%amphibole,0 - 5% plagioclase,0-20%epidote with minor phengite,rutile,titanite,apatite and quartz.After electron microprobe analysis,the garnets have high almandite(61 - 68 mol%),middle grossular(23 - 30 mol%),low pyrope(6 - 11 mol%) and spessartite～2,and the eclogite belongs to C group eclogite.The jadeite(JD) component of the omphacite ranges between 29 and 47 mol.%with varying amounts of aegirine component(Aeg=9 - 17 mol.%).Most white mica from the eclogite is phengite and ranges in Si content from 3.35 to 3.45 cations per formula unit(p.f.u.).Most of amphiboles are retrograde phase in the eclogite,with barroisite,magnesiocatophorite and so on.The peak metamorphic minerals are garnet, omphacite,phengite and rutile with temperature of～500℃,pressure of～2.3Gpa.On the basis of geochemical analysis,the content of SiO2 in the eclogite ranges from 46.68%to 50.24%,Na2O+K2O 3.10～4.21%,and high REE with enriched LREE, LREE/HREE of 2.83～6.06.The REE pattern and trace element spidergram are similar with those of ocean island basalt(OIB),according with the blueschist from Longmucuo -Shuanghu suture zone.The protolith of the eclogite and blueschist are OIB,and they aremaybe the result of subduction of Permian OIB in central Qiangtang area.The zircon mineralogy and chronology of the eclogite show that a lot of primary inclusions of omphacite,phengite,garnet,bariosite etc.occur in the zircons,so the zircon should be formed during eclogite facies metamorphism.SHRIMP dating for zircons suggests the age of eclogite facies metamorphism range from 230 to 243Ma.The Ar - Ar ages of 214～219Ma and 217～223Ma for phengite from the eclogite and its country rock, respectively,record the final exhumation stage of the eclogite.The P - T - t path is a clockwise decline curve and according to paragenesis and compositions of minerals,three major stages of metamorphism were recognized for the eclogite from central Qiangtang area:stageⅠmid - Triassic(230～243 Ma) peak eclogite facies metamorphism(T=～500℃,P=～2.3Gpa),stageⅡretrograde amphibole facies metamorphism(T=～450℃,P＜1.0Gpa) and stageⅢLate Triassic greenschist facies metamorphism(T＜400℃,P＜0.5Gpa).Combining with the blueschists,ophiolites and ialand arc volcanic rock in Qiangtang area,we suggest the tectonic evolution of Qiangtang area had experienced five stages,that is,Pre-Odvician basement,mid-Odvician to early Silurian Tethys,Permian to Triassic Paleo - Tethys,mid to late Triassic plate subduction and collision with the closure of Paleo-Tethys and continent - continent collision after Triassic.更多还原