| This thesis focuses on fluid or anatectic melt activities as well as fluid-rock interaction in the continental subduction zones from Sulu orogen and Western Alps.Several conclusions have been obtained.(1):ultrahigh pressure(UHP)eclogites and associated metamorphic veins at Chizhuang in the Sulu Orogen could reflect the composition and evolution of internal metamorphic fluids in the eclogite.Petrographic observations indicate that the veins are mainly composed of quartz,garnet,omphacite,phengite,kyanite,zoisite,rutile and zircon similar to the host eclogites.Compared to the host eclogites,the garnets in the veins contain relatively high concentrations of heavy rare-earth elements(HREE)and the zoisites in the veins contain strongly high concentrations of light rare-earth elements(LREE).Minerals from both vein and eclogite have similar O isotopic compositions within errors(Quartz?18O values are 2.42‰ and 2.79‰,garnet-0.30‰ and 0.01‰;omphacite 0.25‰ and 0.071‰,respectively).This indicates that the vein-forming fluids are internally derived.Compilation of published data for veins and their host eclogites from different localities in the Dabie-Sulu orogenic belt indicates highly heterogeneous O isotopic compositions,suggesting that fluid activity in deeply subducted continental crust is limited.Mineral geothermobarometry yields 692 ± 65? and 3.6±0.3GPa for the peak metamorphic conditions of vein formation in Chizhuang,while zircons from the vein yield U-Pb ages of 218 ± 2.4 Ma.This indicates that the vein formed during the initial exhumation of deeply subducted continental crust under UHP conditions.Mineral assemblages in the veins combined major and trace element concentrations of these minerals indicate that the vein-forming fluids are enriched in Si,Al Ca,K,LILE,REE,and HFSE.This suggests that such fluids are probably supercritical fluids which have a great capacity to dissolve and transport elements.(2)Based on petrographic observation zircon U-Pb geochronology,and the chemical composition of minerals and melt inclusions,we demonstrate that,a suite of alkali granites in the ultra-high pressure(UHP)metamorphic belt of the southwest Sulu orogen,originated from partial melting of deeply subducted continental crust at an initial exhumation stage.The Tuofeng granites are enriched in Si and alkalies but depleted in Al,Ca,Mg,Ti,and Sr.Melt inclusions are recognized and analysed by Laser-ablation(LA)-ICP-MS documented the initial melt composition with higher alkali,Ca,Mg and LILE contents but lower Fe,HFSE and REEs compared to the granite.Zircons show anatectic growth with U-Pb ages of 214± 4 Ma to 216 ± 3 Ma,shortly after the Triassic ultrahigh-pressure(UHP)event,suggesting syn-exhumation melting during an early exhumation stage.Coesite inclusions in the rims of anatectic zircons indicate that partial melting begun under UHP conditions.The granites have low bulk ?18O values of-2.74 to-3.57‰,and inherited cores of the zircons document Neoproterozoic U-Pb ages of the protolith.These characteristics are similar to those of UHP metamorphic rocks of the Sulu orogen.Thus,it is suggested the granites are produced from partial melting of deeply subducted continental crust at an initial exhumation stage.Syn-exhumation magmatism in the continental collision zones may occurred more widespread than previously thought.Such magmatism would significantly change the geodynamic rheology and thus may be instrumental for rapid exhumation.(3)To explore the activity of deep fluids and key trace element mobility in the continental subduction zones,we have systematically investigated a complex vein located in the boundary between eclogite and granite in the Tuofeng,Sulu orogen.Petrographic observations show that the vein are mainly composed of quartz+kyanite+phengite+epidote+omphacite with minor zircon,allanite and rutile.The zircon in the vein gave a U-Pb age of 217±9 Ma which is similar with zircon U-Pb age of 216±4 Ma in the eclogite.Alanite Th-Pb dating for the vein gave an age at 226±13 Ma and 221±5 Ma.This suggests the deep fluid flow during exhumation in the Late Triassic subduction event.Whole rock O isotope analyses showed similar ?18O values of the vein and eclogite,indicating that the fluid was mainly derived from dehydration of eclogite.While the zircon and allanite-epidote trace element characteristics suggested involvement of hydrous melt or supercritical fluid from host granite.The mass-balance calculations demonstrate that the fluid reacted with the eclogite resulted in the significant absorption of Sr,Pb,Th,U and LREEs and liberation of LILEs.Therefore,the vein-forming fluid was suggested to be derived from contribution of hydrous melt from host granite and aqueous fluid from eclogite.This process could be an efficient agent for key trace element transport in the continental subduction zones.(4)Whole rock Lithium(Li)and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones.The whiteschists reveal relatively low ?7Li values(-6.4‰ to +2.4‰,with an average value of-2.1‰)in comparison to the country rock metagranites(?7Li=-1.2‰ to+3.6‰ with an average value of+1.4‰).The ?7Li values of whiteschists do not show any correlations with major elements(e.g.Mg,Fe),Li,or isotopes(i.e.,?26Mg and ?18O values)and cannot be explained by kinetic fractionation.Hence,the low ?7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone.Fluid-rock interaction modeling results suggest that the metasomatic fluids contain low ?7Li values(down to-4‰)and moderate Li concentrations(50 to 150 ppm),which were probably derived from the dehydration of serpentinites in the subduction channel.The presence of such low ?7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior. |
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