The Petrology Of The Xindaduo Eclogite In Tibet And Its Constrain On The Evolutionary Of The Paleo-Tethys Subduction Zone

The Sumdo HP(high pressure)-UHP(ultra-high pressure)metamorphic belt is another huge orogen in Himalayan-Tibetan Plateau after the discovery of the Qiangtang and Himalaya continent-continent collision belt.The eclogites and its country rocks have been studied for last ten years,which are recognized from Jilang,Bailang,and Sumdo area in the central Lhasa terrane.However,there is no relevant research paper about the Xindaduo eclogite,located in the eastern part of the Sumdo HP-UHP metamorphic belt.In order to investigate the metamorphic processes of the Xindaduo eclogites and constrain on the evolutionary of the Paleo-Tethys subduction zone,detail Petrological,Geochemical,Geochronological investigations have been made on the eclogites from the Xindaduo area.Normal bimineralic,glaucophane-bearing and phengite eclogites have been recognized within mafic boudins(or lenses)enclosed in epidote amphibolites and schists in the Xindaduo area,eastern part of the Lhasa terrane,Tibet.The general mineral assemblage of the glaucophane-bearing eclogite is garnet + omphacite + glaucophane + phengite,with minor epidote,hornblende,and quartz.The general mineral assemblage of the normal bimineralic eclogite is garnet + omphacite + quartz,with minor epidote,hornblende,rutile,ilmenite and sphene.The normal bimineralic eclogites can be divided into two type eclogites,the garnet porphyroblast shows typical compositional zonation,which is the difference between the type I and type II eclogites.The general mineral assemblage of the phengite eclogite is garnet + omphacite + phengite,with minor epidote,hornblende rutile and quartz.Two different thermodynamic techniques were applied:traditional geothermobarometer and P-T pseudosections calculated with Domino.The estimated peak P-T conditions of the glaucophane-bearing,bimineralic(type ? and type ?)and phengite eclogite are 615±5?,33±0.5kbar;630±10?,550??580?(the peak metamorphic pressure of the bimineralic eclogite can't be constrained because of the simple mineral assemblage);635?690?,37?39kbar.According to the peak metamorphic temperature,the eclogites from the Xindaduo area are attributed to the low-T eclogite.On the basis of the investigations on the whole-rock major element,trace element and Sm-Nd,Rb-Sr isotope of the Xindaduo eclogites,we have identified the Xindaduo eclogites are characterized by flat REE distribution pattern,which is similar to N-MORB.The protolith of the eclogites are considered as the depleted oceanic crust material.Combined with the peak metamorphic pressure ofXindaduo eclogites(33±0.5kbar,37-39kbar)and index mineral like coesite pseudomorph,which indicate the partial eclogites in the Xindaduo area in the eastern part of the Sumdo metamorphic belt underwent UHP metamorphism,the evidence suggest the low-T Xindaduo eclogites were caused by deep cold oceanic plate subduction.The P-T path of the different types of eclogites present within the Xindaduo area of the Sumdo HP-UHP metamorphic belt shows the similar evolution,revealing fast subduction and related exhumation processes forms in a typical intra-oceanic subduction-zone setting.Zircon U-Pb LA-ICP-MS dating shows that the bimineralic eclogite(type ?)has a protolith zircon recrystallization age of 277.9±3.6Ma,the phengite eclogite has a protolith zircon recrystallization age of 274.5±3.4Ma,and an eclogite facies metamorphism of 266.3±2.9Ma.Combined the previous data,we builds the evolution of the Sumdo HP-UHP metamorphic belt:the opening of the Paleo-Tethys Ocean,located in-between the North and South Lhasa segments,was initiated prior to 280Ma.Then the eclogites were caused by oceanic plate subduction during late Permian(c.266Ma)and early Triassic(225Ma),the ultimate closure and final collision of the two segments was not earlier than 200Ma.