Petrology, Geochemistry And Tectonic Evolution Of The Eclogite From Central Qiangtang Area, Northern Tibet

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.