Evolution Of Metamorphic Melts/fluids During Deep Continental Subduction And Exhumation

Metamophic dehydration and partial melting are two important processes during continental subduction and exhumation,which may significantly affect the rheological strength of the subducted lithosphere,promote mineral reactions,exhumation,and may also induce considerable crust–mantle interactions.Therefore,decoding the formation and evolution of metamorphic melt/fluid is crucial in better understanding the geological processes occurring in continental subduction channel.However,deeply subducted continental crust most likely underwent multi-stage deformation,extensional retrogression and re-equilibration within mineral aggregates,it is still of hot debate on the stages,nature,mechanism,sources and crystallization products of metamorphic melts/fluids during continental subduction and exhumation.Even within a single UHP orogenic belt,significant different conclusions on the timing,scale,metamorphic conditions,mechanism and sources of melts/fluids were drawn,and consequently,more detailed research need to be conducted.The Dabie–Sulu orogenic belt in eastern China is one of the world‘s known largest UHP metamorphic belts,where UHP metamorphic rocks that recorded multi-stage deformation and metamorphism were exposed.In the Dabie Sulu belt,partial melting of UHP gneiss is extensively reported,whereas large-scale melting of UHP eclogite is rarely studied due to the lack of evidence.Recently,our research group recognized contrary scales of deformation,metamorphism and anatexis within UHP eclogite at Yangkou and nearby(1–2 km)General‘s Hill from the Sulu belt.Therefore,these two localities open an important window to study complicated fluid–rock interactions during subduction and exhumation of continental crust.Moreover,lateral comparison on degree of deformation,retrogression and scale of melt/fluid flow in these two eclogites will help unravel the differences on deformation–metamorphism in various localities within a single subduction channel and how they can be linked to melt/fluid flow.Thus,this Ph D dissertation conducted detailed field-based studies of microstructure,petrology,geochemistry,geochronology,phase equilibria modelling and thermobarometry on eclogite at Yangkou and General‘s Hill.At first,eclogite at Yangkou is fresh,which preserves intergranular coesite that is rare in UHP metamorphic rocks.It mainly shows micro-to hand specimen-scale folding,local retrogression and intergranular melt crystallization products,and therefore provides an unique opportunity to decipher close-system fluid generation and evolution during continental collision.Barite is a common accessory mineral in UHP metamorphic rocks,which can indicate an episode of high-salinity fluid flow.However,it has attracted rare attention to geologists,crystallization stages,conditions and mechanisms of which has not been systematically studied.Petrological observation shows that five microstructural types of barite are developed in coesite-bearing UHP eclogite at Yangkou.Type I barite occurs as equant inclusions with rutile and clinopyroxene in the center of garnet and omphacite,indicating that they were trapped as primary inclusions at almost the same time during the growth of their hosts.Zr-in-rutile thermometry on the primary inclusions of rutile yields T = 658–699 oC at P = 2.5–4.5 GPa,which is significantly lower than that of the peak UHP metamorphism at Yangkou(>900 oC).Thus,barite inclusions were likely precipitated from an internally buffered high-salinity fluid during the late prograde evolution at eclogite facies.Moreover,some type I barites have high Sr O content(~25 wt%),indicating that the prograde fluid is enriched in Sr.Type II barite mostly occurs in multiphase solid inclusions(MSI)located towards the rims of garnet and omphacite,with no sign of visible cracks,suggesting that these MSI were trapped as primary inclusions by their hosts during the later stage of growth.The MSI are mainly composed of barite + biotite + epidote + Ba-bearing/Ba-poor K-feldspar ± ablate ± zircon,which are namely enriched in Si,Al,Na,K,Ca,Fe,Mg,Ba,Sr,SO42-and Zr,indicating of their crystallization from a solute-rich fluid trapped by garnet and omphacite during the late prograde evolution close to the metamorphic peak.Since peak pressure was above the second critical endpoint for basaltic compositions,the solute-rich fluid maybe ?supercritical?.Type III barite occurs in multi-mineral pseudomorphs composed of K-feldspar + quartz + plagioclase + barite ± biotite that are in turn as inclusions in garnet.Based on the morphology and mineralogy of the pseudomorphs,phengite is inferred to be the most likely precursor mineral.Therefore,the replacement phase assemblage was inferred to be formed during exhumation,by in-situ melting of phengite.Phase equilibria modelling shows phengite broke down at T = 655–710 oC and P = 1.2–1.7 GPa.Type IV barite forms coarse-grained irregular patches associated with sub-solidus replacement of omphacite by hornblende and albite symplectites along grain boundaries,while type V barite occurs as grain-boundary veinlets and sometimes shows closed ring structure around pyrite that is partly replaced by hematite and goethite.Microstructural observation indicates no evidence of fluid penetration,suggesting that the fluid responsible for the crystallization of types IV and V barite is internally buffered.Therefore,we interpret the types IV and V barite to have precipitated from an internally generated grain-boundary aqueous fluid,which is expected to be a response to H2 O exsolving from garnet and omphacite during low-pressure amphibolite facies conditions.The oxidation reactions from pyrite to hematite/goethite argue for an oxygen enriched environment.Although Sr was likely modified or leached from solid phases during metamrophism,high Sr O content contained in some type I barite grains suggests that the prograde fluid is characterized by high Sr/Ba;while on the other hand,all the other four types of barite show consistently low Sr O(mostly lower than 5 wt%),indicating that the peak to retrograde fluids are characterized by low Sr/Ba ratio.Thus,the detailed microstructural and chemical analyses help determine the crystallization sequences and conditions of multi-stage barite in UHP eclogite for the first time;in addition,multi-stage barite crystallization provides insight into the evolution of fluid systems as P–T contions evolved thorough the late prograde stage of subduction to the peak of UHP metamorphism and subsequently during exhumation.Compared to the eclogite at Yangkou,eclogite at General‘s Hill shows complex folding,shearing and strong foliation,and it underwent extensive retrogression.Moreover,Meter-scale leucosome dikes and composite granite–quartz veins are developed within the retrogressed eclogite.Therefore,this locality provides an effective target to decode the timing,mechanism and source of large-sacle melt/fluid flow during continental collision.As thus,this Ph D dissertation focuses on the composite granite–quartz veins and conducts a detailed study on their petrogenesis,which provides a comprehensive understanding on the generation and evolution of large-scale melt/fluid during exhumation of deeply subducted continental crust.The granite in the veins has a high-pressure(HP)mineral assemblage of dominantly quartz + phengite + allanite/epidote + garnet that yields pressures of 2.5–2.1 GPa(Si-in-phengite barometry)and temperatures of 850–780 oC(Ti-in-zircon thermometry)at 2.5 GPa(~20 oC lower at 2.1 GPa).On the other hand,the vein quartz is almost composed of quartz(>99 vol.%),with few accessory minerals including biotite.Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at ca.221 Ma.This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt,but is younger than the peak of UHP metamorphism in the Sulu belt at ca.230 Ma.Therefore,the combination of zircon U–Pb dating and thermobarometry shows that the composite granite–quartz veins were formed at HP eclogite facies during exhumation.The granite of the composite vein is enriched in light rare earth elements(REE)and large ion lithophile elements,but depleted in heavy REE and high field strength elements,consistent with crystallization from a crust-derived hydrous melt.By contrast,the vein quartz of the composite vein has >99 wt% Si O2 and very low abundances of all other oxides and trace elements,reflecting that it was precipitated from an aqueous fluid.The initial Hf isotope values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite,whereas ?18O values of new zircon are similar in the veins and the crustal rocks.These data are consistent with zircon growth from a blended melt/fluid generated internally within the gneiss and the eclogite,without any ingress of fluid from an external source.However,at the peak metamorphic pressure which could have reached 7 GPa at Yangkou and General‘s Hill,although micro-scale fluid could have been generated,the rocks were generally fluid absent.During initial exhumation under UHP conditions,exsolution of H2 O from nominally anhydrous minerals(possibly including hydrous minerals)generated a grain boundary solute-rich supercritical fluid in both gneiss and eclogite.As exhumation progressed,the volume of fluid increased(by dissolution of the silicate mineral matrix)allowing it to migrate by diffuse porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite.This produced a blended fluid intermediate in its isotope composition between the two end members,as recorded by the composite veins.During exhumation from UHP(coesite)eclogite to HP(quartz)eclogite facies conditions,the supercritical fluid intersected the solvus for the granite–H2O system,allowing phase separation to hydrous melt and aqueous fluid and crystallization of the granite and vein quartz,respectively,in the composite veins.In addition,in the granite and country gneiss,phengite is surrounded by aggregates of variable thickness composed of plagioclase + biotite + K-feldspar from which thin films,cuspate veinlets and patches of K-feldspar with low dihedral angles extend along grain boundaries.These microstructural evidence show that after crystallization of the composite veins,phengite breakdown melting has occurred in both the granite and the gneiss during ongoing exhumation.Phase equilibria modelling of the granite indicates that this late-stage melting records P–T conditions towards the end of the exhumation during the transition from HP eclogite to amphibolite facies conditions.The modelled amount of melt is ~3–4 mol.% from 1.5 to 1.1 GPa,before decreasing to the solidus;the predicted amount of phengite decreases by a few mol.%.,The final equilibration of the granite in the composite veins is 0.7–1.1 GPa at T <670 °C.Yangkou and General‘s Hill are spatially close to each other,with a distance of only ~2 km.However,it can be seen from that there are significant differences on the geological features of deformation and retrogression between the two eclogites,which exactly correspond to the scales of melt/fluid flow.This Ph D dissertation posits that the eclogites at Yangkou and General‘s Hill were located at the core and rim domains,respectively,of a mafic block in the northern margin of the Yangtze Craton during subduction beneath the North China Craton.Differential scales of melt/fluid penetration and stresses in these two eclogites caused decoupling in the degree of deformation and retrogression.The eclogite at Yangkou at the core was survived from fluid penetration and shearing from the outside of the mafic block,which led to the small-scale deformation and retrogression at Yangkou,with internally buffered micro-scale melt/fluid flow.However,the eclogite at General‘s Hill at the rim underwent strong shearing and melt/fluid penetration from its exterior,inducing large-scale fluid flow with a mixed source.Hydroweakening aided in extensive retrogression and complicated deformation of the eclogite at General‘s Hill,while growing degree of deformation provided channel for melt/fluid migration.Conclusively,important conclusions can be drawn from the detailed analyses of eclogite at Yangkou and composite granite–quartz veins at General‘s Hill within the central Sulu belt in this Ph D dissertation,as follows:(1)This PhD dissertation comprehensively defines the crystallization stages and conditions of barite in UHP eclogite.Five microstructural types of barite are recognized in UHP eclogite at Yangkou,which are: type I-barite inclusions in the core of garnet and omphacite,which were likely precipitated from aninternally buffered high-salinity fluid during the late prograde evolution at eclogite facies;Type II-barite-bearing multiphase solid inclusions located at the rim of garnet and omphacite,which were likely precipitated from a supercritical fluid during peak of UHP metamorphism;Type III-barite-bearing multi-mineral pseudomorphs,which were crystallizaed from a hydrous melt induced by in-situ melting of phengite at T = 655–710 oC and P = 1.2–1.7 GPa;Types IV and V-barite aggregates surrounded by amphibole and albite symplectite and intergranular veins,respectively,which were formed from an aqueous fluid during the low-pressure amphibolite facies;(2)The multi-stage barite crystallization within UHP eclogite at Yangkou shows that episodic metamorphic fluids could have occurred during late prograde to the peak of UHP metamorphism and subsequently during exhumation,which might have caused local recombination of trace elements including Ba,Sr and S;(3)The occurrence of composite granite–quartz veins within retrogressed eclogite at General‘s Hill indicates formation of supercritical fluid by dissolution of nominally anhydrous minerals(possibly including hydrous minerals)during initial exhumation and its separation during HP eclogite facies conditions;(4)During ongoing exhumation from HP eclogite to amphibolite facies conditions,minor phengite breakdown melting has occurred,which generated a second-stage hydrous melt flow;(5)Episodic metamorphic fluids were generated by different mechanisms;dissolution of nominally anhydrous minerals was an unappreciated but important for melt/fluid flow during early-stage exhumation;while phengite breakdown was the main mechanism for hydrous melt flow during late-stage exhumation.