Metamorphic Evolution And Tectonic Implication Of The Sumdo (U)HP Metamorphic Belt

High pressure-ultrahigh pressure(HP-UHP)metamorphic belts are important symbols for the ancient convergent plate boundary.They record a series of dynamic processes of the oceanic and continental materials that experienced deep subduction and exhumation to the Earth surface.The Sumdo eclogite-bearing(U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet,which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes,leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny.Therefore,Studying the peak metamorphic P-T conditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane.In this thesis,eclogite in the Jilang and Xilang areas of the Sumdo belt are chosen as examples to study the metamorphic evolution of the Sumdo belt.By means of detailed petrographic studies,we found that the mineral assemblage of eclogite in the Jilang area is garnet+omphacite+phengite+hornblende+epidote+quartz and minor biotite.Garnet has a "dirty" core with abundant inclusions such as epidote,amphibole,plagioclase and a "clear" rim with few inclusions of omphacite and phengite.From the core to the rim,pyrope content in garnet increases while grossular content decreases,showing typical growth zoning.The main mineral assemblage of the eclogite in the xilang area is garnet+amphibole+epidote+phengite+plagioclase+quartz with minor biotite,chlorite and titanite.There are a lot of inclusions in the core and mantle of garnet.According to the composition of the electron microprobe analysis,few grains with relatively euhedral rim preserved compositional zoning with"bell-shape" spessartite from garnet core to rim indicating a prograde metamorphic growth.In this thesis,we combined with petrographical observation and phase equilibrium modelling,three stages of metamorphism have been identified in Jilang and Xilang area,respectively.Jilang area:(1)prograde stage,represented by the core of garnet and mineral inclusions therein;(2)peak stage,represented by the garnet rim,omphacite.lawsonite,phengite,and quartz;(3)retrogra de stage characterized by decomposition of lawsonite to zoisite.followed by symplectite of omphacite and corona rimmed garnet.Xilang area:(1)pro grade stage,represented by the garnet core and mineral assemblage in it such as plagioclase,hornblende,epidote and quartz:(2)near peak stage.represented by the garnet rim,omphacite,phengite,quartz and epidote;(3)retrograde stage characterized by the plagioclase.chlorite.biotite.hornblende,zoisite,quartz and epidote.A P-T pseudosection contoured by isopleths of grossular and pyrope contents in garnet and Si content in phengite is used to constrain the near peak P-T condition of the eclogite in Jilang and Xilang regions as 2.85 GPa.575 ?;1.84 GPa,539 ?,respectively.On the basis of the P-Tpath,peak P-T conditions and the output state of eclogite in the two regions,we infer that the Jilang eclogite experienced fast exhumation to the Moho depth and got stagnate to acquire the amphibolite facies mineral assemblage.After that,the Xilang eclogite subducted to relatively shallow depth,and captured the eclogite in Jilang area here,and finally returned to the surface with the continental margin material.