zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

a transitional eclogite to high pressure granulite facies overprint on coesite eclogite at taohang in the sulu ultrahigh pressure terrane eastern china abstract a transitional eclogite to high pressure granulite facies paragenesis omp pl qtz grt after peak coesite eclogite facies metamorphism and predating the later amphibolite facies overprint is identified in coesite eclogite from the taohang area of the sulu ultrahigh pressure uhp terrane in eastern china these minerals were equilibrated at 17 kbar and 820 c this reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the uhp rocks the tectono metamorphic significance of the unusually high pressure overprint is also discussed

analytical and numerical studies of the beta effect in tropical cyclone motion part i zero mean flow abstract the effect on tropical cyclone motion is studied using an analytical as well as a numerical model in a nondivergent barotropic framework the analytical model and the linear version of the numerical model give essentially the same result the linear effect causes a westward stretching of the model vortex but no significant movement of the vortex center an east west asymmetry in the meridional wind field is also created it is the inclusion of the nonlinear term that produces the northwestward movement of the vortex previously found by other investigators e g kitade 1981 this northwestward movement increases with both the maximum wind speed and the radius of maximum wind in a constant shape vortex a wind maximum is also found to the northeast of the vortex which appears to be consistent with the observational findings of shea and gray this asymmetry plays an important role in the vortex motion

u pb age trace element and hf isotope compositions of zircon in a quartz vein from eclogite in the western dabie mountains constraints on fluid flow during early exhumation of ultrahigh pressure rocks quartz veins in high pressure hp to ultrahigh pressure uhp rocks are the products of fluid rock interaction and thus provide insight into fluid processes in subduction zones in this paper we report an integrated study of mineral inclusion trace element u pb age and lu hf isotope compositions of hydrothermal zircon grains from a quartz vein within an uhp eclogite outcrop from the hong an area western dabie mountains these data are used to decipher the age conditions of formation and source of fluid for zircon formation during the exhumation of uhp rocks zircon grains from the vein have perfect euhedral shape and show sector zoning or weak zoning indicating that they precipitated from the aqueous fluid responsible for the vein formation raman spectroscopy analysis reveals that the zircon grains contain inclusions of garnet omphacite rutile quartz and h2o implying that they crystallized from aqueous fluid under hp eclogite facies conditions the zircon grains show low th u and lu hf ratios nearly flat hree patterns absent eu anomalies and low lree contents these characteristics are consistent with their precipitation in the presence of garnet and epidote and absence of feldspar and thus suggest that trace element concentrations in hydrothermal zircon are controlled by co precipitation of mineral assemblages crystallization temperatures of 670 to 712 c which were calculated using the ti content of zircon are consistent with their formation under eclogite facies conditions and may correspond to the temperature of the infiltrating fluid the weighted mean 206pb 238u age of 224 7 1 3 ma is taken as the best estimate for the age of quartz vein formation and records aqueous fluid flow during the early exhumation stage of uhp rocks the zircon grains in the quartz vein have hf compositions similar to those in the host eclogite which demonstrates isotopic equilibrium between fluid and rocks and that the fluid rock ratio was likely low

eclogite facies quartz veins within metabasites of the dabie shan eastern china pressure temperature time deformation path composition of the fluid phase and fluid flow during exhumation of high pressure rocks

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

ultrahigh pressure metamorphic rocks from the chinese continental scientific drilling project ii oxygen isotope and fluid inclusion distributions through vertical sections in order to reconstruct the formation and exhumation mechanisms of uhp metamorphic terrains the chinese continental scientific drilling program ccsd has been carried out in donghai of the dabie sulu ultrahigh pressure uhp metamorphic belt east china eclogite gneiss amphibolite retrograded from eclogite ultramafic rocks and minor schist and quartzite have been drilled aiming to reveal the fluid behaviour in a vertical sequence of an uhp slab we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot holes pp2 and zk 703 of the ccsd more than 540 laser ablation oxygen isotope analyses on garnet omphacite quartz kyanite amphibole phengite rutile epidote amphibole plagioclase and biotite from various rocks in the depth range of 0 3 000 m mainly eclogite and gneiss show that the investigated rocks can be divided into two groups 18o depleted rocks as low as 18o 7 4 for garnet indicate interaction with cold climate meteoric waters whereas 18o normal rocks with bulk 18o 5 6 have preserved the o isotopic compositions of their protoliths meteoric water rock interaction has reached depths of at least 2 700 m oxygen isotope equilibrium has generally been achieved isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology but heterogeneous on the scale of a few metres oxygen isotope distributions in the vertical sections favour an in situ origin of the uhp metamorphic rocks the very negative 18o eclogites usually have higher hydroxyl mineral contents than the normal 18o rocks indicating higher water content during uhp metamorphism fluid inclusion data suggest that rocks with depleted 18o compositions have had different fluid histories compared to those with normal 18o values rocks with depleted 18o mainly have primary medium to high salinity inclusions in omphacite kyanite and quartz and abundant secondary low salinity or pure water inclusions in quartz indicating a high salinity brine dominated fluid system during peak uhp metamorphism no carbonic inclusions have been identified in these rocks by contrast primary very high density co2 inclusions are commonly found in the rocks with normal 18o values these observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the dabie sulu uhp metamorphic rocks

high pressure veins in eclogite from new caledonia and their significance for fluid migration in subduction zones abstract this paper describes the petrology and geochemistry of high pressure veins and associated metasomatic selvages that are hosted by eclogite in the pouebo eclogite melange of northern new caledonia isotopic and geochemical evidence indicates that the mafic host rock represents seafloor altered fractionated morb that underwent eclogite facies metamorphism in a subduction zone within the host rock are cm thick garnet quartz phengite veins that are enveloped by garnet poor omphacite rich selvages or bleach zones petrography thermometry oxygen isotope characteristics and mass balance calculations are used to show that the veins largely formed by fluid mediated mass transfer from the bleach zones during prograde metamorphism minerals in the veins are free of inclusions but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets vein garnets have mn and hree zoning patterns that are indicative of progressive garnet growth during prograde metamorphism whereas mg and ca contents reveal prominent sector zoning and fine scale intergrowth features we propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock fluid circulation may have been driven by episodic microcracking sealing around garnet porphyroblasts which led to significant mass transfer and progressive vein growth mass balance calculations and phengite trace element compositions also require the additional of pelite derived components to the veins these components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux this model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases the delay of fluid migration after hydrous mineral breakdown may provide an important source of water for arc magmas as free fluid is transported to deeper levels in subduction zones

geochronology of fluid induced eclogite and amphibolite facies metamorphic reactions in a subduction collision system bergen arcs norway rb sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes granulite facies rocks of the lindas nappe bergen arcs norway were subducted and exhumed during the caledonian orogeny the rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation along fractures and shear zones the first occurred at eclogite facies 15 21 kbar 650 750 c and a later one at amphibolite facies conditions 8 10 kbar 600 c the presence of fluids enabled local metamorphic equilibration only near fluid pathways in fluid absent domains preexisting assemblages were metastably preserved this resulted in a heterogeneity of metamorphic signatures on meter to m scales well preserved granulite facies rocks preserve their proterozoic rb sr mineral ages as does the u pb system of zircon in most lithologies six rb sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid induced eclogitization at 429 9 3 5 ma 2 weighted average mswd 0 39 an eclogite facies vein has yielded metamorphic zircon with concordant u pb ages of 429 3 ma identical to the u pb age of 427 4 0 9 ma for zircon xenocrysts in an amphibolite facies vein seven rb sr mineral isochron ages date amphibolite facies fluid infiltration at 414 2 2 8 ma mswd 1 5 an age value testifying to residence of the rocks in the deep orogenic crust at temperatures 600 c for nearly 15 ma the new data show that rb sr mineral isochron ages effectively date fluid induced re crystallization events rather than stages of cooling the direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the p t reaction time space which is essential for understanding exhumation histories and the internal dynamics of orogens in general

multiphase solid inclusions in zoisite bearing eclogite evidence for partial melting of ultrahigh pressure metamorphic rocks during continental collision abstract multiphase solid ms inclusions in both garnet and omphacite were investigated for zoisite bearing eclogite from the ultrahigh pressure uhp metamorphic zone in the sulu orogen the results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust there are three types of ms inclusions 1 plagioclase quartz 2 plagioclase quartz k feldspar and 3 barite plagioclase k feldspar zoisite epidote the host minerals mostly exhibit radial fractures surrounding the ms inclusions the first and second types of ms inclusions were analyzed for their bulk compositions yielding high sio 2 and na 2 o but very low feo mgo tio 2 with variable k 2 o for the second type trace element analyses of representative ms inclusions yield generally very low concentrations except such large ion lithophile elements as sr ba and rb these features suggest different origins for the three types of ms inclusions the first type of ms inclusions would be primarily derived from dehydration melting of paragonite whereas the second type of ms inclusions would be derived from dehydration melting of both paragonite and phengite the third type of ms inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz providing components for growth of garnet during the exhumation therefore the anatexis of zoisite bearing uhp eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite the major and trace element compositions of ms inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel

oxygen isotope equilibrium between eclogite minerals and its constraints on mineral sm nd chronometer abstract sm nd and oxygen isotope analyses were carried out for mineral separates of ultrahigh pressure eclogites from the sulu terrane in eastern china the results show a direct correspondence in equilibrium or disequilibrium state between the oxygen and sm nd isotope systems of eclogite minerals the omphacite garnet pairs of oxygen isotope equilibrium at eclogite facies conditions yield meaningful triassic sm nd isochron ages whereas those of oxygen isotope disequilibrium give non triassic ages of geological meaninglessness this can be reasonably interpreted by the fact that the rates of oxygen diffusion in garnet and pyroxene are lower than or close to those of nd diffusion and thus attainment of isotopic equilibrium in the omphacite garnet o system suggests achievement of nd isotope equilibrium in the same mineral pairs the presence or absence of fluid in the eclogite protoliths is a major rate controlling factor for isotopic equilibration during high grade metamorphism it appears that the state of oxygen isotope equilibrium between cogenetic minerals can provide a critical test for the validity of the sm nd mineral chronometer in addition the exact timing of the ultrahigh pressure metamorphism in the dabie sulu terranes is constrained at early triassic rather than late triassic

metamorphic growth and recrystallization of zircons in extremely 18o depleted rocks during eclogite facies metamorphism evidence from u pb ages trace elements and o hf isotopes abstract a combined in situ sims and la mc icpms study of u pb ages trace elements o and lu hf isotopes was conducted for zircon from eclogite facies metamorphic rocks in the sulu orogen the two microbeam techniques sampled various depths of zircon domains revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique this enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization magmatic cores with u pb ages of 769 9 ma have positive 18o values of 0 1 10 1 high th u and 176lu 177hf ratios high ree contents and steep mree hree patterns with negative eu anomalies they are interpreted as crystallizing from positive 18o magmas during protolith emplacement in contrast newly grown domains have concordant u pb ages of 204 4 to 252 7 ma and show negative 18o values of 10 0 to 2 2 low th u and 176lu 177hf ratios low ree contents and flat hree patterns with weak to no eu anomalies they are interpreted as growing from negative 18o fluids that were produced by metamorphic dehydration of high t glacial hydrothermally altered rocks during continental subduction zone metamorphism differences in 18o between different domains within single grains vary from 0 8 to 12 5 suggesting different degrees of o isotope exchange between the positive 18o magmatic core and the negative 18o metamorphic fluid during the metamorphism the magmatic zircons underwent three subtypes of metamorphic recrystallization depending on their accessibility to negative 18o fluids the zircons recrystallized in solid state maintained positive 18o values and ree and lu hf isotopes of protolith zircon but their u pb ages are lowered the zircons recrystallized through dissolution exhibit negative 18o values similar to the metamorphic growths almost completely reset u pb ages and partially reset ree systems the zircons recrystallized through replacement show variably negative 18o values and partially reset ree and u pb and lu hf isotopic systems therefore this study places robust constraints on the origin of metamorphic zircons in eclogite facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

the anatectic effect on the zircon hf isotope composition of migmatites and associated granites zircon hf isotope composition is widely used to trace the growth and evolution of continental crust however it is controversial whether the hf isotope composition of magmatic zircons can faithfully reflect that of their sources especially for s type granites in order to provide an insight into this issue we have revisited the published lu hf isotope data of zircons from well studied migmatites and associated granites in the sulu orogen and the cathaysian terrane respectively the results show greatly elevated 176hf 177hf ratios by more than 10e units for newly grown zircon domains compared to the relict zircon domains this indicates considerable contributions from non zircon hf to anatectic melts during crustal anatexis and subsequent magmatism furthermore this more radiogenic hf isotope signature was not erased during magmatic processes such as crystal fractionation during melt ascent and emplacement the budget of hf isotopes in source rocks with respect to mineral lu hf ratios suggests the involvement of hf bearing major minerals in anatectic reactions by dissolving hf bearing major minerals into the anatectic melts the significant hf isotope variations in some anatectic and magmatic zircon domains from the migmatites and granites suggest not only the source heterogeneity but also the variable non zircon hf contributions as such the hf isotope compositions of anatectic and magmatic zircons are substantially dictated by the mass balance between the non zircon hf from anatectic reactions and the zircon hf from the dissolution of protolith zircons into the anatectic melts they are primarily controlled by p t conditions and mechanism of crustal anatexis and the magmatic processes during melt evolution the present study highlights the important contribution of non zircon hf to the anatectic and magmatic zircon domains in this regard the greatly elevated 176hf 177hf ratios for newly grown zircon domains in the migmatites and granites cannot reflect the hf isotope compositions of their source rocks such a deviation must be kept in mind when linking relatively depleted hf isotope compositions to the nature of source rocks in petrogenetic interpretation and to the growth of continental crust the contribution of non zircon hf can also better account for the variable hf isotope compositions often shown by zircons from s type granites

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

dehydration melting of ultrahigh pressure eclogite in the dabie orogen evidence from multiphase solid inclusions in garnet several types of multiphase solid ms inclusions are identified in garnet from ultrahigh pressure uhp eclogite in the dabie orogen the mineralogy of ms inclusions ranges from pure k feldspar to pure quartz with predominance of intermediate types consisting of k feldspar quartz silicate plagioclase or epidote barite the typical ms inclusions are usually surrounded with radial cracks in the host garnet similar to where garnet contains relict coesite barite aggregates display significant heterogeneity in major element composition with total contents of only 57 73 and highly variable sio2 contents of 0 32 25 85 that are positively correlated with bao and so3 contents the occurrence of ms inclusions provides petrographic evidence for partial melting in the uhp metamorphic rock the occurrence of barite aggregates with variably high sio2 contents suggests the coexistence of aqueous fluid with hydrous melt under hp eclogite facies conditions thus local dehydration melting is inferred to take place inside the uhp metamorphic slice during continental collision this is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust as a consequence the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction zone metamorphism

melting induced fluid flow during exhumation of gneisses of the sulu ultrahigh pressure terrane abstract hydrothermally altered rocks are products of fluid rock interactions and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks thus quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure in order to decipher the fluid activity in the sulu ultrahigh pressure uhp terrane in eastern china quartz veins together with an adjacent eclogite lens and the host gneiss were studied in one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens the amphibolite facies overprinting of the eclogite lens decreases from the rim to the core of the lens with fresh eclogite preserved in the core the foliated biotite gneiss contains felsic veins and residual phengites zircon rims from the gneiss are characterized by melt related signatures with steep hree patterns high hf contents and negative eu anomalies and a pool of weighted average 206 pb 238 u analyses reveal an age of 219 3 ma 2 which is younger than the uhp metamorphic age 236 2 ma 2 recorded by zircons from the eclogite lens this suggests that the gneiss in the sulu uhp terrane could have suffered from partial melting due to phengite dehydration during the hot exhumation stage the formation age of the quartz vein 219 2 ma 2 defined by zircon rims agrees well with the partial melting time 219 3 ma 2 of the host gneiss the initial 176 hf 177 hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss these observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss on the other hand amphibole bearing and hree rich zircon rims from the amphibolite pool an amphibolite facies metamorphic age of 217 5 ma 2 overlap with the formation age of the quartz vein this implies that retrogression of the eclogite lens could have been caused by melting induced fluid flow based on the above observations we speculate that partial melting of the gneiss in the continental subduction related uhp belt could have induced a significant fluid flow during the exhumation stage and thus contributed significantly to the extensive retrogression of eclogites in the sulu uhp terrane

stable isotope geochemistry of marbles from the coesite uhp terrains of dabieshan and sulu china abstract marbles from dabieshan and sulu china suffered ultra high pressure uhp metamorphism in the coesite eclogite facies at approximately 700 c and 30 kbars during triassic continental collision and subduction the marbles range in isotopic composition from 7 to 25 18 o vsmow and from 0 to 6 13 c vpdb high 13 c values are representative of unmodified protoliths and are similar to those of 13 c enriched sinian carbonate rocks from the yangtze craton high oxygen isotope ratios reflect pristine protoliths but the low values may have been caused by infiltration of low 18 o meteoric water during diagenesis and dolomitization by fracture controlled infiltration of water during subduction by metamorphic mineral reactions or by a combination of these processes no evidence of regional isotopic transport during uhp metamorphism has been found sampling on scales of 1 to 100 m shows marbles to be inhomogeneous in both carbon and oxygen isotopes only samples separated by less than 10 cm have equilibrated oxygen and carbon isotope compositions limited isotopic equilibration between adjacent rocks is consistent with the preservation of unaltered uhp minerals and indicates that the metamorphic fluid rock system was rock dominated during and following peak metamorphism a freely flowing pervasive fluid phase was not present during uhp metamorphism there is no evidence of isotopic exchange between marble and the upper mantle into which it was subducted correlation of geochemical similarities of uhp marbles with sinian limestones implies that the subducted edge of the yangtze craton extends at least as far north as the coesite eclogite facies rocks of dabieshan deposition of protolith carbonates may have taken place in a cold climate either preceding or following but not coincident with neoproterozoic glaciation

fluids in deeply subducted continental crust petrology mineral chemistry and fluid inclusion of uhp metamorphic veins from the sulu orogen eastern china abstract the complex vein associations hosted in southern sulu ultrahigh pressure uhp eclogites contain quartz omphacite or jadeite kyanite allanite zoisite rutile garnet these minerals have chemical compositions similar to those of host eclogites inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet and coesite inclusions were found in vein zircon these facts suggest that the veins together with host eclogites have been subjected to synchronous uhp metamorphism the vein minerals contain relatively high concentrations of rare earth elements ree high field strength elements hfse and transition metal elements tme a kyanite quartz vein has a whole rock composition similar to adjacent uhp metamorphic granitic gneisses abundant primary multi solid fluid inclusions trapped within uhp vein minerals contain complex daughter minerals of muscovite calcite anhydrite magnetite pyrite apatite celestite and liquid and gas phase of h 2 o with solids up to 30 70 of the inclusion volume the presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas these characteristics imply that the uhp vein minerals were crystallized from supercritical silicate rich aqueous fluids that were in equilibrium with peak uhp minerals and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as hree hfse and tme such fluids might have resulted in major fractionation between nb and ta i e the uhp fluids have subchondritic nb ta values whereas the host eclogites after extraction of the fluids have suprachondritic nb ta values therefore voluminous residual eclogites with high nb ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

h2o contents and d h ratios of nominally anhydrous minerals from ultrahigh pressure eclogites of the dabie orogen eastern china abstract garnet and omphacite from ultrahigh pressure eclogites from the dabie orogen eastern china were investigated by micro ftir the results show that all garnet and omphacite grains contain structural water occurring as hydroxyl oh with h2o contents varying from 14 to 1915 ppm h2o wt and from 105 to 695 ppm respectively within the same sample the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation the d values of omphacites are between 108 4 and 114 2 and independent of water contents heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during uhp metamorphism was very limited the estimated whole rock water content based on mineral h2o contents is between 260 and 750 ppm thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle at least several hundreds ppm water to mantle depths the preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span and imply that they were exhumed shortly after subduction the water released during decompression might represent the early stage retrograde fluid

fluid inclusion rb sr isochrons for dating mineral deposits the precise dating of mineral deposits has important implications for understanding the lifetime of ore forming systems and their temporal relationship to various geological events not all deposits provide material suitable for conventional radiometric dating or data which are amenable to unequivocal interpretation nor is it easy to verify that such samples have remained isotopically closed since their formation fluid inclusions provide an alternative closed system approach by analysing the isotopic composition of rband sr in inclusions within quartz from the carrock fell tungsten vein deposits cumbria we demonstrate here the feasibility of obtaining rb sr fluid inclusion isochrons the age for the mineralization 392 5 myr is in good agreement with the mean k ar age 387 6 myr for the vein micas and confirms the significance of fluid inclusions as precise rb sr geochronometers

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

petrogenesis of metatexite and diatexite migmatites determined using zircon u pb age trace element and hf isotope data higo metamorphic terrane central kyushu japan metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the higo low p high t metamorphic terrane here we report data from an outcrop in the highest grade part of the granulite facies zone in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic structured metatexite within pelitic gneiss the migmatites and gneiss contain the same peak mineral assemblage of biotite plagioclase quartz garnet k feldspar with retrograde chlorite muscovite and some accessory minerals of ilmenite rutile titanite apatite zircon monazite pyrite zinc sulphide calcite calculated metamorphic p t conditions are 800 900 c and 9 12 kbar zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning but no core rim structure zircon from the gneiss and metatexite forms euhedral subhedral grains comprising inherited cores overgrown by thin rims the overgrowth rims in the metatexite have lower th u ratios than zircon in the diatexite and yield a 206pb 238u age of 116 0 1 6 ma which is older than the 110 1 0 6 ma 206pb 238u age derived from zircon in the diatexite zircon from the diatexite has variable ree contents with convex upward patterns and flat normalized hree whereas the overgrowth rims in the metatexite and gneiss have steep hree enriched patterns however both types have similar positive ce and negative eu anomalies 176hf 177hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite based on u pb ages trace element and hf isotope data the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt following partial dissolution of inherited protolith zircon during anatexis whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component by integrating zircon and petrographic data for the migmatites and pelitic gneiss the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source whereas the diatexite migmatite included an externally derived juvenile component the cretaceous high temperature metamorphism of the higo metamorphic terrane is interpreted to reflect emplacement of mantle derived basalts under a volcanic arc along the eastern margin of the eurasian continent and advection of heat via hybrid silicic melts from the lower crust post peak crystallization of anatectic melts in a high t region at mid crustal depths occurred in the interval c 116 110 ma as indicated by the difference in zircon ages from the metatexite and diatexite migmatites

distinct zircon u pb and o hf nd sr isotopic behaviour during fluid flow in uhp metamorphic rocks evidence from metamorphic veins and their host eclogite in the sulu orogen china

deep root of a continent continent collision belt evidence from the chinese continental scientific drilling ccsd deep borehole in the sulu ultrahigh pressure hp uhp metamorphic terrane china abstract the chinese continental scientific drilling ccsd deep borehole which reached a depth of 5158 m in the sulu ultrahigh pressure uhp metamorphic terrane provides a new window into the deep root of a continent continent collision belt and the tectonic processes by which supracrustal material is recycled into the mantle by subduction and then uplifted to the surface major research themes of the ccsd project were to 1 determine the three dimensional composition structure and geophysical character of the deep root of this orogenic belt 2 investigate the nature and timing of the uhp metamorphism 3 investigate the processes of crust mantle interaction involved in the formation and exhumation of the uhp rocks 4 study the process of fluid circulation and mineralization during subduction and exhumation 5 study the rheological properties of the various rocks during subduction and exhumation 6 develop and refine dynamic models for deep subduction and exhumation of crustal rocks and 7 establish a long term natural laboratory for the study of present day crustal dynamics e g stress strain fluid activity the ccsd has developed precise oriented profiles of the main borehole in terms of lithology geochemistry oxygen isotopes zircon shrimp u pb ages 40ar 39ar ages deformation rheology mineralization physical properties of the rocks petrophysical logs seismic reflections and underground fluids the present paper summarizes the integrated research results of this project especially the new findings concerning the deep root of a continent continent collision

record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon u pb age and hf o isotope compositions abstract geochemical features of island arc rocks require two slab derived components of a melt and a fluid added to their sources these two liquids may unite to an intermediate aqueous melt at the base of volcanic arc direct evidence of this process has been scarce yet here we present an integrated study of in situ u pb age trace element and o hf isotope for zircons from a suite of a quartz vein a felsic vein their host eclogite and surrounding gneiss at the xitieshan uhp terrane of the north qaidam metamorphic belt nqmb oscillatory zoned zircons from the quartz vein are enriched in u li nb and ree with a formation age of 442 6 ma consistent with the timing of the peak uhp metamorphism this demonstrates there is no temporal gap between the uhp metamorphism and fluid flow in the xitieshan terrane in particular the oscillatory zoned zircons of the quartz vein have similar hf isotope compositions to zircons from the surrounding gneiss but variable o isotope compositions between the host eclogite and surrounding gneiss the fluid from surrounded gneiss was thus an aqueous melt with high transport ability for lu and hf and o while the fluid from the host eclogite was an aqueous fluid only with the ability to transport o according to the similar behaviors of granitic rock and sediment it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer where an aqueous fluid can be formed from underlying basalt and or peridotite layers at the sub arc depth the two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock by contrast the zircons in the felsic vein yield a weighted mean 206pb 238u age of 420 4 ma which can be regard as the timing of partial melting during the exhumed stage for the xitieshan uhp rocks the zircon o and hf isotopic features of the felsic vein are between the country gneiss and host eclogite compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the uhp rocks this indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

element fluxes associated with subduction related magmatism destructive plate margin magmas may be subdivided into two groups on the basis of their rare earth element ree ratios most island arc suites have low ce yb and remarkably restricted isotope ratios of 87 sr 86 sr 0 7033 143 nd 144 nd 0 51302 206 pb 204 pb 18 76 207 pb 204 pb 15 57 and 208 pb 204 pb 38 4 however they also have rb sr 0 03 th u 2 2 and ce yb 8 5 ratios which are significantly less than accepted estimates for the bulk continental crust the high ce yb suites have higher incompatible element contents more restricted heavy ree and much more variable isotope ratios such rocks are found in the aeolian islands grenada indonesia and philippines and their isotope and trace element features have been attributed both to contributions from subducted sediment and or old trace element enriched material in the mantle wedge it is argued that for isotope and trace element models the slab component can usefully be taken to consist of subducted sediment and altered mid ocean ridge basalts since these may contain ca 80 of the water in the subducted slab and the distinctive trace element features of arc magmas are generally attributed to the movement of material in hydrous fluids the isotope data indicate that not more than 15 of the sr and th in an average arc magma were derived from subducted material and that the rest were derived from the mantle wedge the fluxes of elements which cannot be characterized isotopically are more difficult to constrain but for most minor and trace elements the slab derived contribution in arc magmas is too small to have a noticeable effect on the residual slab

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons