multichronometric evidence for an in situ origin of the ultrahigh pressure metamorphic terrane of dabieshan china abstract u pb zircon data on gneisses indicate a late proterozoic protolith at 700 ma at bixiling and a late archean to early proterozoic for the shuanghe sheet within the ultrahigh pressure metamorphic terrane of dabieshan china u pb zircon and sm nd dates on the gneisses of the shuanghe sheet constrain a metamorphic peak at 230 ma rb sr and 40ar 39ar analyses of phengite from five gneisses in the bixiling complex yield ages at 198 212 ma overlapping the 190 220 ma age on eclogites the host gneisses record a similar metamorphic evolution as the coesite bearing eclogites since both the peak and the retrograde evolution were contemporaneous this argues for an in situ tectonic relationship

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

multiple episodes of anatexis in a collisional orogen zircon evidence from migmatite in the dabie orogen abstract a combined study of mineral inclusions u pb ages and trace elements was carried out on zircon from migmatites in the dabie orogen the results provide insights into multistage anatexis of ultrahigh pressure metamorphic rocks in the continental collision orogen zircon grains in thin sections and mounts record four episodes of magmatic metamorphic and anatectic events 1 middle neoprotrozoic u pb ages for domains that contain qz ap pl inclusions and exhibit high th u ratios 0 1 steep hree patterns with marked negative eu anomalies dating protolith emplacement 2 late triassic u pb ages of 212 5 to 219 4 ma for domains that contain cpx grt inclusions and show low th u ratios

carbon concentrations and isotopic ratios of eclogites from the dabie and sulu terranes in china abstract both concentration and isotope composition of bulk carbon in apatite and host eclogites from the dabie sulu ultrahigh pressure uhp terranes in china have been determined along with their oxygen isotope composition the results show significant 13 c depletion in the apatite 13 c 27 7 to 20 8 with the carbon concentrations of 0 59 to 1 65 wt co 2 despite a large variation in 18 o 6 5 to 9 5 the bulk carbon in 21 of the 24 eclogites has low 13 c values of 26 1 to 17 9 with low carbon content of 500 to 1000 ppm whereas the other three samples show high 13 c values of 7 1 to 2 8 with high carbon contents of 2400 to 4300 ppm noncarbonate carbon was measured by treating the all eclogites with 5 n hcl solution yielding uniformly low 13 c values of 27 9 to 24 2 and carbon contents of 200 to 300 ppm carbonate carbon is thus calculated by mass balance to have also low 13 c values of 25 6 to 15 1 for the 21 samples but high 13 c values of 4 3 to 1 2 for the remaining three samples secondary carbonate was identified in the three eclogites that are also depleted in 13 c primarily but subjected to overprint of 13 c rich co 2 bearing fluid subsequent to the uhp metamorphism the isotopically light carbon in both eclogite and apatite is interpreted to represent the isotope composition of carbon in eclogite precursors before plate subduction and thus has an origin of organic carbon from the earth s surface the uniformly low 13 c values of apatite suggest that the co 2 of metamorphic fluid in equilibrium with the host eclogites would be derived from the oxidation of organic carbon rather than the decarbonation of underthrust carbonates during progressive metamorphism protoliths of the eclogites are inferred to be of igneous origin which underwent more extensive interaction with organic matter than with meteoric hydrothermal fluid on the subsurface of the continental crust the break off of the subducted plate containing the isotopically light carbon and subsequent interactions with the surrounding mantle could produce the mafic and or silicic magmas that are significantly depleted in 13 c relative to the primary mantle carbon this may provide evidence for a linkage of the 13 c depleted mantle carbon to a surficial source via plate subduction

simulated garnet clinopyroxene geothermometry of eclogites the fe mg thermometer widely used to infer the equilibration temperature of garnet clinopyroxene assemblages of eclogites records the progressive blocking of diffusion limited exchanges between coexisting mineral phases it is argued that equilibrium is achieved through the fast grain boundary model in which fe and mg circulate in an interstitial medium fast enough for the rate limiting step to be the volume diffusion in each mineral phase a semi analytic solution is found and the influence of the cooling history grain size distribution and rock composition on the temperature at which the fe mg exchange between garnet and clinopyroxene is frozen in are quantitatively evaluated in particular the model simulates the temperatures that would be obtained from the concentration of fe and mg in the rim of adjacent garnet and clinopyroxene crystals such as those commonly obtained by electron probe for eclogites the simulations show that correct peak temperatures are retrieved as long as the temperature of the metamorphic climax does not exceed 650 c at higher climax temperatures rim rim temperatures underestimate the peak temperature and cluster around 650 c fast cooling however strongly limits diffusive equilibration in crustal type eclogites cooling rates of a few degrees per million years at 700 c and of a few tens of degrees at 800 c preserve the record of the peak temperature it is shown that these results are largely independent of the chemical composition of the garnet and clinopyroxene on the contrary the mineral grain size and the respective proportions of garnet and clinopyroxene have a major effect on retrogressive diffusion for eclogites formed at a temperature of 800 c and under conditions of slow cooling the record of the climax temperature is preserved by crystal rims as long as the volume of clinopyroxene is smaller than that of garnet and crystal size is at least millimetric the choice of a particular set of experimental data for the diffusion coefficients is found to be immaterial we suggest that the temperature of the metamorphic climax of eclogites is best estimated from the composition of clinopyroxene rims combined with that of the inner edge of the diffusion boundary layer fringing the coexisting garnet grains

eclogite facies relics and inferred ultrahigh pressure metamorphism in the north dabie complex central eastern china mineral assemblages and microstructures of a newly identified retrogressed eclogite sensu lato in the north dabie complex ndc central eastern china indicate early very high pressure metamorphic relics preserved in a dominant amphibolite facies host where no eclogite sensu stricto has been positively identified before the investigated eclogitic rock shows distinct multistage recrystallization with granulite and amphibolite facies assemblages overprinting eclogite facies relics the minimum temperature for the eclogite facies metamorphism is estimated to be 800 820 c a spectacular microstructure of oriented quartz needles 2 20 m wide 5 200 m long in matrix ca na clinopyroxene implies the prior existence of a non stoichiometric supersilicic omphacite stabilized at ultrahigh pressure uhp 25 kbar conditions although no coesite or coesite pseudomorphs have been found in the samples the absence of coesite may be due to the lack of free silica at uhp conditions or the consumption of silica during retrograde reactions the inferred uhp conditions metamorphism is further supported by sm nd ages that are equivalent to triassic metamorphic ages from uhp eclogites in the southeastern dabie mountains this finding expands the uhp terrane northward about 50 km spatial distribution of subduction collision related uhp rocks includes parts of the ndc

exhumation of the ultrahigh pressure continental crust in east central china cretaceous and cenozoic unroofing and the tan lu fault the orogenic architecture of the world s largest ultrahigh pressure exposure the hong an dabie mountains of the triassic qinling dabie orogenic belt is dominated by cretaceous and cenozoic structures that contributed to its exhumation from 30 km depth cretaceous magmatic crustal recycling 50 for the entire dabie and heating 250 to 700 c were most prominent in dabie and exhumation magmatism and cooling were all controlled by cretaceous transtension exhumation was accomplished principally by an asymmetric cordilleran type extensional complex in the northern dabie northern orthogneiss unit between 140 and 120 ma at rates as fast as 2 mm yr and average horizontal stretching rates of up to 6 mm yr cretaceous reactivation occurred within a regional transtensional strain field as a result of far field collisions and pacific subduction the onset of crustal extension was preceded and possibly facilitated by a reheating of the hong an dabie crust 140 ma coeval with the onset of voluminous magmatism in eastern china 145 ma which resulted from a change in pacific subduction from highly oblique to orthogonal the tan lu continental scale fault was a normal fault zone in the mid cretaceous 110 90 ma and underwent 5 4 km dip slip and 4 km throw in the cenozoic during the india asia collision the qinling dabie belt acted as the structural discontinuity between the strike slip dominated escape tectonics south of the qilian qinling dabie belt and the rifting dominated tectonism north of it the most prominent cretaceous and cenozoic structures of the hong an dabie the xiaotian mozitang and the jinzhai fault zones respectively reactivated major lithospheric structures of the triassic orogen i e the huwan detachment zone and the suture

episodic growth of zircon in uhp orthogneisses from the north dabie terrane of east central china implications for crustal architecture of a collisional orogen zircon u pb dating of three orthogneiss samples from the north dabie terrane ndt is undertaken in order to reconstruct their formation and evolutionary histories and also the crustal architecture of the dabie orogen after triassic subduction and exhumation shrimp zircon u pb dating in combination with back scattered electron bse imaging and laser raman spectrometry provides accurate identification of the core mantle and rim structure for zircon growth during protolith formation and overgrowth during subduction exhumation and post collisional metamorphism concordant u pb ages of 760 730 ma and high th u ratios of 0 4 are obtained for relict oscillatory zoning fields of inherited cores that were not metamictized these features suggest that these ages represent the time of magmatic protolith formation during the breakup of rodinia the overgrown mantle domains around the metamictized cores are clean with few mineral inclusions e g quartz garnet and apatite mantle domains have low th u ratios of 0 719 similar to the values obtained from uhp orthogneisses in the cdt it is concluded that as with the cdt the orthogneisses with episodic zircon growths from the ndt should also be a part of the exhumed slice following the continental deep subduction however the orthogneisses in this study were buried at a lower level in the orogenic crust compared with those of the cdt prior to the cretaceous magmatism therefore the orthogneisses from the ndt were affected by the cretaceous magmatism whereas the cdt orthogneisses were not affected

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

partial equilibrium of radiogenic and stable isotope systems in garnet peridotite during ultrahigh pressure metamorphism a critical question in radiometric dating of metamorphic rocks is whether minerals used to define internal isochrons have achieved isotopic equilibrium during a given metamorphic event followed by preservation of the equilibrium afterward a garnet peridotite at zhimafang in the sulu terrane of eastern china shows incongruent u pb rb sr and sm nd ages zircon shrimp u pb dating yielded an isochron age of 224 8 ma pointing to zircon growth during triassic uhp metamorphism two mineral assemblages gave consistent rb sr internal isochron ages of 201 4 and 205 4 ma but much older sm nd internal isochron ages of 376 16 and 378 24 ma apparently the sm nd radiometric system fails to reequilibrate among the dated minerals during the triassic uhp metamorphic event for the garnet peridotite this result is confirmed by the state of oxygen isotope equilibrium or disequilibrium among the constituent minerals in the garnet peridotite in particular garnet is not in oxygen isotope equilibrium with any other of the analyzed minerals the degree of oxygen isotope disequilibria among the other minerals varies from pair to pair oxygen isotope equilibrium is observed only between orthopyroxene and olivine for both samples and between phlogopite and clinopyroxene for one sample it appears that the u pb rb sr sm nd and o isotope systems in minerals of the garnet peridotite are in partial equilibrium during uhp metamorphism i e equilibrium with respect to some isotopes and minerals but not all based on available experimental diffusion data our study of the combined u pb rb sr sm nd and o isotope systems in the garnet peridotite demonstrates that the time scale for the uhp metamorphism and subsequent hp eclogite facies recrystallization is possibly in the range of about 12 to 26 ma which was not long enough for the reequilibration of the sm nd and o isotope systems but just long enough for the reequilibration of the rb sr isotope systems therefore the rate of sr diffusion in phlogopite thus sr isotope reequilibration between the mafic minerals is constrained to be faster than rates of nd and o diffusion in garnet thus sm nd and o isotope reequilibration between the same minerals under conditions of subduction zone metamorphism

young ages from old garnet in the polyphase metamorphic terrane of the limpopo belt south africa abstract the p t t path of high grade metamorphic rocks is significant for understanding vertical motions and heating of the crust in general garnet is used to constrain p t paths and metamorphic zircon is used to obtain t for different metamorphic stages however this approach may be complicated by polyphase tectonothermal events the central zone of the limpopo belt in south africa is a complex precambrian metamorphic terrane that has experienced three distinct high grade metamorphic events at 3 22 ga m1 2 62 ga m2 and 2 02 ga m3 based on zircon dating a previous investigation of a metapelite enclave in the bulai pluton in the central zone showed that metamorphic zircon in the enclave recorded mainly m2 granulite facies metamorphism and was protected by the bulai pluton however titanite and rutile u pb ages of the same sample record the m3 event here we investigated garnet sm nd isotopic systematics of four metapelite samples from the evelyn enclave in the bulai pluton we constructed single mineral sm nd isochrons for garnet using micro samples in order to avoid the risk of isotopic disequilibrium between different minerals or whole rocks metamorphic zircon with sillimanite inclusions has yielded a granulite facies metamorphic age of ca 2 6 ga however the sm nd isotopic data for the garnet record much younger ages of ca 2 03 ga these ages are consistent with the m3 event in this area but were rarely identified in previous zircon dating studies this suggests that the m3 event reached sufficiently high temperatures to totally reset the rutile and titanite u pb and garnet sm nd systems moreover the decoupling of the age of garnet formation and the date it records needs to be carefully considered when reconstructing p t t paths and again highlights the usefulness of garnet in the study of polyphase metamorphic terranes

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

smnd isotopic systematics of a gabbro eclogite transition abstract sm nd isotopic systems have been studied in 1 a sequence of coronitic olivine gabbros showing different degrees of transition to eclogite 2 coronitic leuco gabbro norite and 3 country rock eclogite all from the nordoyane brattvag area western norway isochrons defined by combinations of whole rock samples and relict igneous phases give ages of 1198 56 and 1289 48 ma olivine gabbro and 926 70 ma leuco gabbro norite and have been interpreted in terms of two different episodes of igneous intrusion in gabbro with advanced corona formation relict augite tends to retain its original nd isotopic composition while plagioclase may show selective disturbance related to recrystallization and partial replacement by garnet in completely eclogitized samples there is a correlation between the degree of isotopic equilibrium and microstructural equilibrium whole rock eclogites occurring within gabbro are characterized by pseudomorphs after igneous phases and preserve original igneous nd isotopic composition results for eclogite minerals suggest caledonian ages but these phases have not achieved complete isotopic equilibrium in contrast nd isotopic systems in texturally equilibrated and strongly deformed eclogite within adjacent gneisses have been disturbed on a whole rock scale minerals from an external eclogite define a sm nd isochron with an age of 400 16 ma which is interpreted to date synkinematic eclogite equilibration at high p and t conditions

sm nd and rb sr dating of pyroxene garnetite from north dabie in east central china problem of isotope disequilibrium due to retrograde metamorphism abstract oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the sm nd and or rb sr systems in eclogites that experience retrograde metamorphism this study demonstrates that sm nd rb sr and o isotopes for minerals from two portions of a clinopyroxene garnetite from north dabie in east central china record both equilibrium peak metamorphic conditions and disequilibrium conditions resulting from anhydrous retrograde metamorphism the minerals are homogeneous in terms of major elements and show no variation across the sample but clinopyroxene shows variable degrees of alteration pure primary mineral separates that are in o isotope equilibrium yield triassic sm nd isochron ages of 219 11 and 229 13 ma that match previously published zircon u pb ages for peak metamorphism thus the sm nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism in contrast altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the sm nd isochron defined by the pure minerals implying that like oxygen the sm nd systems in these clinopyroxene were disturbed during retrogression no meaningful isochrons are found from the rb sr dating even in the pure primary minerals implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of sr is faster than those of sm nd and o in mafic minerals our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of sm nd mineral isochrons in dating peak metamorphic events in addition variations in the sm nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids

silica precipitates in omphacite from eclogite at alpe arami switzerland evidence of deep subduction observations of oriented sio2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the alpe arami garnet peridotite massif ticino switzerland and petrological studies of the eclogitic mineral assemblages suggest that this rock was subjected to higher pressure metamorphism than previously realized we employed various calibrations of the fe2 mg exchange thermometer and calculations of equilibria with thermodynamic data considering the calcium tschermak s component caal2sio6 of garnet pyroxene pairs from these calculations it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following stage i prograde c 2 4 gpa 700 c stage iia maximum recorded grade c 7 0 gpa 1100 c stage iib retrograde c 3 7 gpa 900 c stage iii retrograde c 2 1 gpa 750 c because of the preservation of stage i a relatively rapid subduction and exhumation of alpe arami eclogite is suggested the exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the alpe arami garnet lherzolite proposed previously by several authors based upon a variety of different observations although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

la icp ms u pb zircon geochronology geochemistry and kinetics of the wenquan ore bearing granites from west qinling china abstract many hydrothermal cu mo au deposits related to granitoid intrusives were recently discovered in the west qinling orogenic belt wqob these deposits were mainly formed during the late indosinian epoch ca 214 ma and the regional geological setting of cu mo ore formation in wqob during this epoch is poorly understood until now this paper describes the geochronology and geochemistry of the wenquan ore bearing pluton a composite granite body to study the geologic background of magmatic emplacement and ore formation the mo mineralisation occurs at the contact between a fine grained biotite monzogranite and a medium to fine grained porphyritic monzogranite zircon 206pb 238u ages of 223 3 ma biotite monzogranite and 225 3 ma porphyritic monzogranite were obtained geochemical analyses show that the wenquan pluton is a high k calc alkaline to shoshonite series rock with relatively high lree and low hree and a moderate to weak negative eu anomaly relatively negative anomalies of ba ti p nb ta also exist these results imply that the wenquan pluton was emplaced during a transitional process from collision to extension between the yangtze craton and north china craton during the later indosinian epoch the east qinling orogenic belt eqob and wqob had similar tectonic settings and intensive magmatic activity and mo mineralisation occurred the eqob was then involved in the mesozoic subduction of the pacific plate and its subsequent tectonic evolution was different from that of the wqob

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

individualization of textural and reactional microdomains in eclogites from the bergen arcs norway consequences for rb sr and ar ar radiochronometer behavior during polymetamorphism 1 rb sr 40ar 39ar and sm nd isotopic data are reported in caledonian eclogites from the lindas nappe bergen arcs norway in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks the bergen arcs exposes sveconorwegian 950 930 ma granulites partially overprinted by caledonian eclogite facies metamorphism at 425 ma and amphibolite facies metamorphism at 410 ma geochemical and rb sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain a few hundred microns in size in which phengite crystallized phengite crystallized after garnet or plagioclase by dissolution precipitation processes yield apparent age between 700 and 600 ma at the time of their crystallization these phengites inherited the isotopic composition of their precursor minerals at a microdomain scale phengite from quartz veins which crystallized from elements mobilized by the circulating fluid yield an age closer to the eclogite facies metamorphic age the closed system evolution of the eclogitizing fluid the segregation of textural and reactional microdomains the high sr content of the studied phengite and the short duration of the recrystallization processes 1ma are interpreted as the main factors responsible for the lack of a rb sr isotopic equilibrium at the scale of hand samples such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization the in situ single mineral ar ar datings revealed that both eclogite and amphibolite facies minerals are characterized by an excess of radiogenic argon ages between 425 and 520 ma the excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid the apparent sm nd ages from garnets inherited from the granulite facies metamorphism c 930 ma are in agreement with previous estimates this result confirms that the diffusion of nd in garnet does not occur at temperatures lower than 700 c this study highlights the complexity of radiochronometer behavior during hp metamorphism and demonstrates that coupling different radiochronometers such as ar ar and rb sr does not always guarantee the validity of the geochronological results

thermochronologic constraints on deformation and cooling history of high and ultrahigh pressure rocks in the qinling dabie orogen eastern china the hong an block is the best place to study the exhumation of high pressure hp and ultrahigh pressure uhp metamorphic rocks in the qinling dabie orogen of eastern china because it lacks the extensive cretaceous tectonic and thermal overprint observed in the dabie shan we measured timing of deformation and rate of cooling of the hp uhp rocks by 40ar 39ar analyses of synkinematic minerals from tectonites of key structural zones in the hong an and tongbai shan normal sense shear along the north dipping huwan detachment at the northern edge of the hong an block occurred between 237 and 231 ma this detachment facilitated the bulk of the exhumation of the hp uhp rocks our new 40ar 39ar ages combined with u pb zircon and sm nd ages of 245 240 ma suggest that exhumation of uhp rocks from mantle depths occurred at rates of 5 25 mm yr from 245 to 230 ma the mountain range is a warped extensional footwall within which white mica cooled from 225 to 205 ma locally younger extension is recorded by white mica recrystallization at 198 194 ma after which the entire block had cooled to below 300 c early cretaceous 40ar 39ar ages from the tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral oblique slip along the xiaotian mozitan fault along the northern boundary coeval dextral and sinistral shear zones along the northern and southwestern margins of the hong an and dabie shan would have caused eastward lateral extrusion of these two blocks perhaps driven by collision of the lhasa block with eurasia

paragonite eclogites from dabie shan china re equilibration during exhumation paragonite in textural equilibrium with garnet omphacite and kyanite is found in two eclogites in the ultrahigh pressure metamorphic terrane in dabie shan china equilibrium reactions between paragonite omphacite and kyanite indicate a pressure of about 19 kbar at c 700 c however one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet suggesting minimum pressures of 27 kbar at the same temperature the disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh pressure assemblages this hypothesis is tested by calibrating a garnet zoisite clinozoisite kyanite quartz coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh and high pressure terranes these four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite clinozoisite grossular kyanite quartz coesite h o the results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts for the paragonite eclogites in dabie shan the geobarometer indicates pressures in the quartz stability field confirming that the former coesite bearing paragonite eclogite has re equilibrated at lower pressures on the other hand garnets from other coesite bearing but paragonite free kyanite zoisite eclogites show a very wide variation in grossular content corresponding to a pressure variation from coesite into the quartz field this wide variation partly due to a rimward decrease in grossular component in garnet is caused by partial equilibration of the mineral assemblage during the exhumation

fluid regime in continental subduction zones petrological insights from ultrahigh pressure metamorphic rocks abstract intensive studies of ultrahigh pressure uhp metamorphic rocks in continental collision zones have provided petrological insights into the fluid regime during subduction and exhumation recent studies in the dabie sulu orogen of china have made important contributions to understanding fluid action in such systems hence these rocks are the main focus of this review sources of metamorphic fluid can be deduced from study of hydrous minerals structural hydroxyl and molecular water in nominally anhydrous minerals fluid and solid inclusions of uhp metamorphic minerals are also a record of fluid action in deep subduction zones the presence or absence of metamorphic fluid during subduction and exhumation dictates zircon growth and recrystallization making available materials for u pb dating isotopic dating demonstrates that after peak uhp metamorphism retrograde fluid with deuteric origin became active the decomposition of hydrous minerals and the exsolution of molecular water and structural hydroxyl during exhumation can provide sufficient amounts of aqueous fluid for quartz veining and amphibolite facies retrogression within uhp slabs aqueous fluid released by the exsolution of molecular water and structural hydroxyl from uhp nominally anhydrous minerals is characterized by low salinity and light hydrogen and oxygen isotope compositions therefore one part of the uhp slab is a source of aqueous fluid whereas another part is a sink in both cases the fluid action during subduction and exhumation is episodic rather than continuous supplementary material a description of the dabie sulu uhp metamorphic rocks analytical methods la icpms zircon u pb isotope data and rare earth element compositions are available at http www geolsoc org uk sup18355

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

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

postglacial rebound in a non newtonian spherical earth using finite elements we study the postglacial rebound in an axisymmetric earth with a non newtonian rheology our approach allows for the first time a self consistent study of the effect of a power law creep on the long wavelength deformations which cannot be performed using flat models we show that for a non newtonian upper mantle the time variations of the low degree stokes coefficients become largely insensitive to the viscosity of the lower mantle this is in contrast with previous evidence based on newtonian models which emphasized the importance of the low degree signatures for constraining the lower mantle viscosity

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

high pressure ultrahigh pressure eclogites from the hong an block east central china geochemical characterization isotope disequilibrium and geochronological controversy the hong an block western dabieshan exposes a series of hp uhp metamorphic rocks with a s to n distribution from blueschist greenschist kyanite free to kyanite and coesite bearing eclogites the available age data are inconclusive that hinder our understanding of the tectonic evolution of the block the metamorphic temperatures in the hong an block tmeta 700 to 500 c are lower by 50 150 c than that of the dabie and sulu terranes in this work we undertook new trace element and sr nd o isotopic analyses on minerals in order to gain more insight into the geochronological problems the results are as follows 1 trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium and the presence of high temperature lree rich mineral inclusions e g epidote in garnet and omphacite has contributed to isotope disequilibrium 2 sm nd isotope analyses yielded no isochron ages for the hong an eclogites 3 rb sr isotope analyses gave mixed results in some cases coexisting minerals are completely out of isotope equilibrium and in others isochron relationship is established yielding ages from 210 ma to 225 ma the pattern of rb sr isotope disequilibrium appears to be independent of the petrological and o isotope temperatures 4 in contrast to the unequilibrated sm nd isotopic systems oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near equilibrium oxygen isotope temperatures are comparable with petrological temperatures however this is an apparent feature due to mass balance constraints 5 whole rock 18o values show a large variation from 10 to 8 suggesting that their protoliths have undergone very different processes of water rock interaction in view of the overall geochronological information we conclude that the hp uhp metamorphism in the hong an block took place in the triassic at about 220 230 ma as observed in the dabie and sulu terranes the significance of published paleozoic dates 450 300 ma for the xiongdian eclogite is not clear however any hypotheses advocating two periods of uhp metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data

coesite bearing eclogite from the dabie mountains in central china coesite and coesite pseudomorphs are recognized in eclogite from the dabie mountains a collision zone between the sino korean and yangtze cratons in central china this third known occurrence of coesite in deep crustal rocks is within an archean gneiss terrane and is the first locality where coesite has been identified as an inclusion in both omphacite and garnet crystals at the same location coesite bearing eclogite is estimated to form at pressures of 28 kbar and temperatures from 600 to 710 c the dabie mountains eclogite underwent retrograde metamorphism to amphibolite facies after its formation the preservation of coesite in eclogite is found to be strongly controlled by the pressure temperature time path that coesite bearing eclogite followed during its uplift and retrograde metamorphism

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

diverse p t paths of the northern dabie complex in central china and its reworking in the early cretaceous abstract the northern dabie complex ndc in the dabie orogen central china is a high temperature migmatite terrane it is composed mainly of banded gneiss migmatite and minor amphibolite granulite retro eclogite marble and spinel peridotite and intruded by the early cretaceous massive granitoid plutons and subordinate mafic ultramafic rocks whether or not the ndc experienced a high pressure or ultrahigh pressure hp uhp metamorphic history has been a long standing issue however in the last 10 years an increasing number of retrograde uhp eclogite relics and marbles and hp granulites have been identified throughout the ndc this together with the discovery of microdiamond preserved as inclusions in zircons within eclogite relics and gneisses and numerous age dates of 220 240 ma and 130 ma from the ndc supports the notion that at least part of the ndc was subjected to triassic continental subduction and hp uhp metamorphism the youngest ages being related to an intense overprinting of ultrahigh temperature uht granulite facies metamorphism in the early cretaceous distinct peak uhp conditions 2 5 gpa and 800 c 3 5 gpa and 750 800 c 4 gpa and 900 c or 750 c have been inferred from oriented qtz or ab kfs qtz needles in ca na clinopyroxene of retrograde eclogite ti chu bearing assemblage preserved in marble rod like cpx rt ap inclusion in garnet of retrograde eclogite and microdiamond inclusions in zircon of tonalitic gneiss and retrograde eclogite from the ndc the overprinted uht 900 c or 1000 c metamorphism is indicated by osm growth in garnet pgt exsolution in clinopyroxene of retrograde eclogite and spl crd qtz symplectite in felsic granulites diverse p t paths of an overall clockwise evolution are suggested with post peak decompression or heating followed by decompression cooling and consist of two distinct paths opx pl inclusions in garnet and zircon in retrograde eclogite relics indicate that the lower continental crust in the ndc underwent triassic subduction the ndc shows overprinted uht granulite facies metamorphism thus consists of two metamorphic events in contrast to the single event of the central dabie uhp eclogite belt cdb distinct peak p t conditions may be attributed to the lack of suitable mineral assemblages for geobarometry the overall clockwise p t paths consist of a peak hp uhp event with overprinted uht event suggesting that the ndc experienced triassic subduction and intense intracontinental reworking in the early cretaceous the latter stage may be associated with a heat pulse due to asthenospheric upwelling and lithospheric thinning as a result of cretaceous mantle superwelling and the westward subduction of the pacific plate

unusually low δ18o ultra high pressure metamorphic rocks from the sulu terrain eastern china eclogite and quartzite interlayers containing ultra high pressure uhp metamorphic mineral assemblages occur at qinglongshan in the sulu terrain eastern china these rocks exhibit some of the most negative oxygen isotope compositions that have ever been reported for metamorphic rocks the 18 value of minerals in eclogite ranges from 10 4 to 9 0 o and in quartzite ranges from 10 2 to 7 3 near isotopic equilibrium between minerals and rocks suggests that these rocks must have acquired such low oxygen isotope compositions before the uhp metamorphism through near surface meteoric water rock interactions the rocks were then subducted to a great depth during continent continent collision forming uhp minerals with concomitant local oxygen isotope redistribution it is significant that the rocks have retained such negative oxygen isotope compositions despite subduction into the upper mantle

fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe dabie shan china fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe in dabie shan east central china have preserved remnants of early prograde and or peak metamorphic fluids reset during post uhp ultrahigh pressure metamorphic uplift inclusions occur in several minerals e g omphacite epidote notably as isolated primary inclusions in quartz included in various host minerals two major fluid types have been identified non polar fluid species n2 or co2 and aqueous the latter is by far the most predominant aqueous fluids cover a wide range of salinity from halite bearing brines to low salinity fluids for non polar fluids few n2 inclusions occur in undeformed eclogite whereas a number of co2 rich inclusions have been found in microshear zones of eclogite or jadeite quartzite in close proximity to marble occurrences the primary character of n2 and high salinity aqueous inclusions indicates that they are remnants from uhp metamorphic fluids and for some there is the distinct possibility that they are ultimately derived from pre metamorphic fluids this conclusion is supported by the preservation in some samples of microdomains containing synchronous inclusions of variable salinities which tend to relate to the chemical composition of the host crystal carbonic fluids may be derived from neighbouring rocks notably marble and carbonate bearing metasediments during post metamorphic uplift during post uhp exhumation a limited decrease of the fluid density has occurred with formation of new sets of fluid inclusions fluid movements however remained exceedingly limited at the scale of the enclosing crystal

tectonics of the dabieshan eastern china and possible exhumation mechanism of ultra high pressure rocks the dabieshan is divided into three tectonic domains the southern dabieshan is a stack of allochtons from top to bottom i unmetamorphosed cambrian early triassic series unconformably covered by jurassic sandstone ii weakly metamorphosed proterozoic slate iii hp rocks mostly retrogressed into greenschist facies iv uhp allochton v uhp free gneisses these units exhibit a nw se lineation and top to the nw shearing reworked by s verging folds the central dabieshan is a migmatitic dome superimposed on uhp metamorphism and bounded by a detachement fault responsible for the exhumation of the uhp unit during its retrogression into amphibolite facies in the northern dabieshan early foliation and n s trending compressional lineation are deformed by n verging folds coeval to the syn exhumation ductile structures of the central dabieshan a geodynamic model involving coeval thrusting and normal faulting is discussed

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

a granulite record of multistage metamorphism and ree behavior in the dabie orogen constraints from zircon and rock forming minerals abstract a combined study of mineral inclusions u pb ages and trace elements was carried for zircon and coexisting minerals from granulite in the north dabie terrane ndt of the dabie sulu ultrahigh pressure metamorphic uhp zone east central china the results provide insights into the exhumation history of ndt and into rare earth element ree behavior during retrogression besides inherited cores and one magmatic rim zircons separated from the granulite record three episodes of metamorphism under different p t conditions 1 223 8 2 3 ma for domains that contain grt cpx rt f ap aln inclusions without plagioclase and show flat hree patterns without negative eu anomalies representing peak eclogite facies event 2 213 3 2 1 ma for domains that contain pl cpx grt qtz ap inclusions and show rather flat hree patterns with negative eu anomalies corresponding to granulite facies retrogression 3 199 9 3 3 ma for domains that contain amp pl qtz ap inclusions and show high ree contents with steep hree patterns and remarkable negative eu anomalies representing amphibolite facies overprinting therefore the uhp eclogite in ndt experienced decompression heating during the initial exhumation with local hydration in the late stage of the triassic continental collision garnet in the granulite is composed of a corroded core with embayed outline and spongy texture and an overgrowth rim there is equilibrium distribution of hree between garnet rim and granulite facies zircon domain confirming the geological interpretation of 213 3 2 1 ma for the granulite facies metamorphism there is the prograde hree depletion in porphyroblastic garnet from core to rim and the continuous decrease of hree from the eclogitic to granulitic zircons suggesting that the metamorphic transformation from eclogite facies to granulite facies took place in a closed system on the other hand the amphibolitic zircons show steep hree patterns and significantly elevated ree contents suggesting that the amphibolite facies metamorphism took place in an open system with introduction of external fluids containing more ree to the granulite facies rocks residual clinopyroxene partially replaced by amphibole has remarkably elevated ree contents relative to euhedral primary clinopyroxene there is equilibrium distribution of ree between the residual clinopyroxene and the amphibole marking fluid action during the amphibolite facies overprinting

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

grain scale sr isotope heterogeneity in amphibolite retrograded uhp eclogite dabie terrane implications for the origin and flow behavior of retrograde fluids during slab exhumation to constrain the origin and flow behavior of amphibolite facies retrograde fluids during slab exhumation we investigate the textures trace element contents and in situ strontium sr isotopic compositions using la mc icp ms of multiple types of epidote and apatite in the uhp eclogite and amphibolites from the hualiangting area dabie terrane china the uhp epidote porphyroblasts in the eclogite ep e which formed at 28 30 kbar and 660 720 c contain high amounts of sr pb th ba and light rare earth elements lrees and have a narrow range of initial 87sr 86sr ratios 0 70431 0 00012 to 0 70454 0 00010 two types of amphibolite facies epidote were recognized in the amphibolites the first type of epidote ep ai developed in all the amphibolites and has slightly lower trace element contents than ep e the ep ai has a same initial 87sr 86sr ratio range as the ep e and represents the primary amphibolite facies retrograde product that is associated with an internally buffered fluid at 8 0 10 3 kbar and 646 674 c the other type of epidote ep aii occurs as irregular fragments veins veinlets or reaction rims on the ep ai in certain amphibolites elemental x ray maps reveal the presence of ep ai relics in the ep aii domains appearing as a patchy texture which indicates that ep aii most likely formed by the partial replacement of the ep ai in the presence of an infiltrating fluid the distinctly lower trace element contents of ep aii are ascribed to element scavenging by a mechanism of dissolution transport precipitation during replacement the ep aii in an individual amphibolite exhibits large intra and inter grain variations in the initial 87sr 86sr ratios 0 70493 0 00030 to 0 70907 0 00022 which are between those of the ep ai and granitic gneisses wall rock of the amphibolites 0 7097 0 7108 these results verify that the infiltrating fluid was externally derived from granitic gneisses the matrix apatite in the amphibolites has the same initial 87sr 86sr ratio range as the ep ai indicating that the amphibolite facies fluid involved in the apatite crystallization was also internally derived we propose that at least two separate stages of fluids were accounted for the amphibolite facies retrogression of the hualiangting eclogite the fluid responsible for the growth of most of the amphibolite minerals was locally derived and behaved in a pervasive manner whereas the influx of gneiss derived fluid was transient episodic and highly channelized with a longer transport distance 60 m the disparate origins and flow behavior of these fluids significantly influence the water budget and element transfer in exhumed hp uhp slabs this study also indicates that examining grain scale sr isotopic variations provides key information regarding the isotopic dis equilibrium fluid origins and fluid flow regimes in metamorphic or metasomatic rocks that form in subduction zone environments

excess argon in phengite from eclogite evidence from dating of eclogite minerals by smnd rbsr and 40ar39ar methods the existence of excess ar in phengites from coesite bearing ultrahigh pressure eclogites of the dabie mountains and su lu terrain china has been confirmed by dating of minerals using sm nd rb sr and 40ar39ar methods 40ar39ar ages of phengites for two samples studied are much older than the sm nd and rb sr mineral isochron ages sample ql 1 has rb sr age of 219 5 0 5 ma sm nd age of 226 3 4 5 ma and phengite 40ar39ar age of 877 5 15 9 ma the corresponding ages for sample zb 4 are 223 9 0 9 ma rb sr 228 4 6 0 ma sm nd and 943 3 12 4 ma 40ar39ar phengite an age of 225 ma based on the sm nd and rb sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the north china and yangtse blocks and formation of these eclogites we call for greater caution in 40ar39ar dating of phengites from other occurrences of ultrahigh pressure environments

geochemical age and isotopic constraints on the location of the sino korean yangtze suture and evolution of the northern dabie complex east central china the northern dabie complex in east central china lies between the sino korean plate to the north and the yangtze plate to the south the northern dabie complex has been variously proposed to represent a paleozoic magmatic arc on the sino korean plate an exhumed piece of subducted yangtze plate crust or crust produced almost entirely by cretaceous extension related magmatism trace element compositions of northern dabie complex orthogneisses and granites show arc signatures similar to those of ultra high pressure rocks in the central dabie but no mineralogical evidence of ultra high pressure metamorphism is present in the samples investigated here field relationships textures major and trace element compositions and ion microprobe u pb zircon protolith crystallization ages reveal three distinct types of gneiss diorite gneiss xenoliths 770 6 26 ma 95 confidence limit those within first genation highly deformed migmatitic grey gneisses 747 6 14 ma and those cross cut by secondgeneration cretaceous weakly foliated felsic gneisses 127 6 4 ma unfoliated cretaceous granites 117 6 11 ma monazite th pb age 5 117 6 1 ma intrude secondgeneration gneisses cretaceous second corresponding author e mail john c ayers vanderbilt edu generation gneisses and granites yield zircon inheritance ages of ca 2 ga 700 800 ma and rarely 227 271 ma indicating that the northern dabie complex is not simply a cretaceous extensional terrane the 700 800 ma zircon ages are similar to those of granitic gneisses from the central ultra highpressure zone 698 6 47 ma and are characteristic of the yangtze craton nd values suggest that cretaceous rocks in the northern dabie complex formed by partial melting of basement with very low nd and not by melting of first generation or diorite gneisses nd depleted mantle model ages are consistent with the time of formation of the yangtze craton at 1 4 2 5 ga the northern dabie complex is interpreted to be an extension of the yangtze craton that was unaffected by ultra high pressure metamorphism the sino korean yangtze suture must lie to the north of the northern dabie complex

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

geochemical and sr nd pb isotopic compositions of cretaceous granitoids constraints on tectonic framework and crustal structure of the dabieshan ultrahigh pressure metamorphic belt china abstract the dabie orogenic belt can be divided into four tectonic and lithological units they are from north to south the north huaiyang nhy unit the northern dabie complex ndc unit the ultrahigh pressure uhp metamorphic unit and high pressure hp metamorphic unit cretaceous granitoids from the four tectonic units irrespective of tectonic unit and lithology show surprisingly similar major and trace element and particularly sr nd pb isotopic compositions indicating a similar crustal source for their magma genesis they have initial 87 sr 86 sr t values of 0 7068 0 7091 and nd depleted mantle model ages of 1 6 2 4 ga and e nd t values at the time of their emplacement 120 ma ranging from 13 to 26 their present day pb isotopic compositions are 206 pb 204 pb 15 62 17 06 207 pb 204 pb 15 21 15 44 and 208 pb 204 pb 36 59 37 81 with comparison to the sr nd pb isotopic compositions of the basement rocks in the dabie orogenic belt the isotopic compositions of the cretaceous granitoids indicate a source that is similar to the northern dabie complex but distinct from the ultrahigh pressure metamorphic unit this strongly suggests that the exposed northern dabie complex extends in the deep crust southward beneath the uhp hp metamorphic units and northward beneath the north huaiyang unit the northern dabie complex is best interpreted as the core of a dome within the dabie orogenic belt the uhp hp metamorphic unit is only a thin skinned slab confined in shallower crustal levels over the northern dabie complex present tectonic framework for the uhp hp terrain was formed by extensional tectonics and post the uhp hp metamorphism the northern dabie complex has an affinity to the yangtze craton the similar source for the cretaceous granitoids also suggests that the suture between the north china and the yangtze cratons is marked by the xiaotian mozitang fault separating the north huaiyang unit and the northern dabie complex rather than the previously thought wuhe shuiko fault which separates the northern dabie complex and the uhp unit

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

u pb zircon and sm nd isotopic study of the huangtuling granulite dabie sulu belt china implication for the paleoproterozoic tectonic history of the yangtze craton sims data demonstrate archean 2 9 to 2 7 ga and paleoproterozoic 1991 43 ma ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from huangtuling dabie sulu orogenic belt china the granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the yangtze craton the rounded zircon cores show oscillatory zoning and have th u ratios 0 17 0 78 systematically higher than those of the unzoned luminescent rims 0 005 0 03 the age of the detrital zircon cores and whole rock nd isotope model age suggest that the huangtuling granulite was possibly derived from a provenance dominated by archean rocks similar to the kongling complex exposed 200 km in the southwest multiple phases of metamorphism have been identified for the granulites with the dominant granulite facies mineral assemblage defining an early clock wise metamorphic p t path orthopyroxene plagioclase and whole rock material define a linear trend in a 147sm 144nd 143nd 144nd diagram corresponding to an age of 1926 146 ma this result and the u pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the yangtze craton at 2 0 ga inviting further study on whether the yangtze craton was involved in the assembly of the recently proposed paleo mesoproterozoic supercontinent columbia

comment on paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing by qiu and wijbrans 2006 abstract qiu and wijbrans qiu h n and wijbrans j r 2006 paleozoic ages and excess 40 ar in garnets from the bixiling eclogite in dabieshan china new insights from 40 ar 39 ar dating by stepwise crushing geochim cosmochim acta 70 2354 2370 present three ar ar age spectra for fluid inclusions in garnet from eclogite at bixiling in the dabie orogen east central china these paleozoic ages of 427 20 to 444 10 ma are interpreted to represent the first formation of dabie ultrahigh pressure uhp eclogite and thus require subduction of yangtze crust to have started much earlier than previously accepted however no petrographic evidence such as mineral inclusions in the garnet relating to the particular metamorphic conditions is presented to substantiate the proposed uhp metamorphic event because garnet growth is not uniquely responsible for uhp eclogite facies metamorphism a distinction between uhp and high pressure hp metamorphic events must be made in the interpretation of geochronological results available data from mineral sm nd and zircon u pb dating of eclogites from the same area have firmly established that the uhp eclogite facies metamorphism took place at triassic neither the age of uhp metamorphism nor the timing of continental collision is reliably constrained by their presented data the fluid inclusions in garnet must contain inherited 40 ar from uhp eclogite precursor without considerable resetting of the ar ar isotopic system during triassic uhp metamorphism therefore their data are either meaningless or at best viewed as the age of garnet growth by low t hp blueschist eclogite facies metamorphism of the uhp eclogite precursor during arc continent collision in the early paleozoic furthermore it is critical for metamorphic geochronology to substantiate the timing of uhp metamorphic event by means of zircon u pb in situ dating on coesite bearing domains of metamorphically grown zircon

10be and 9be in marine sediments and their potential for dating using sequential leaching experiments we have measured the fraction of the cosmogenic isotope 10be half life 1 5 my and its stable isotope 9be co extracted with the following phases in marine sediments exchangeable calcium carbonate fe and mn oxyhydroxides organic opal and detrital the extraction of the authigenic phases typically release 70 of the 10be but only 40 of the 9be in order to test its potential for dating marine sediments we have measured the authigeniccally associated 10be 9be in 17 surface sediment samples from different ocean basins the observed spatial variability which appears to be influenced by proximity to continental input of 9be precludes the general and precise applications of leached 10be 9be for dating however the relatively constant value observed in surface sediments over large areas of the pacific indian and southern atlantic oceans shows considerable promise for approximate dating of open oceans sediments in order to test temporal variability a profile of the authigenically associated 10be 9be ratio has been measured in the independently dated central pacific core rc12 65 the observed variation in 10be 9be over the past nine my is approximately twice that expected on the basis of 10be decay alone several possible explanations for such behavior and their potential consequences for the use of leached 10be 9be for dating are discussed

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

10be spikes in plio pleistocene cyclothems wanganui basin new zealand identification of the local flooding surface lfs marine shelf sediments of the wanganui basin castlecliff section spanning oxygen isotope stages 21 17 ca 920 680 ka contain spikes of enriched 10be concentrations and enhanced 10be 9be ratios up to 10 background values the 10be spikes do not coincide precisely with peaks in carbonate content and grainsize or with changes in bulk rock chemical composition rather they appear in the sequence a little above the base of each cyclothem at the presumed location of the local flooding surface lfs they therefore originated in response to seafloor sediment starvation during transgression and not from ice sheet melting biogenic scavenging or geomagnetic intensity changes as has been suggested previously for 10be spikes in some deep sea or continental margin sequences

10be study of rapid erosion in taiwan cosmogenic 10be was measured using accelerator mass spectrometry in soils and sediments to study the erosion of taiwan which has the highest denudation rate in the world the river sediments in taiwan have very low 10be concentrations around 5 million atoms per gram about 145 the world wide average this is the direct consequence of its high sediment yield of more than 1000 mg cm2 yr 70 times the world average combining these values we found that for taiwan as a whole the 10be output to the sea only slightly exceeds the input from the rain a situation typical of many areas around the world therefore even in this example of extremely rapid erosion 10be seems to remain a useful indicator for erosion status the total 10be inventory found in a soil profile from a geologically stable area is at least 18 of the maximum possible inventory the minimum age thus estimated for the soil is 0 11 ma in reasonable agreement with estimates from other means the maximum erosion rate for this area thus estimated is at most 0 14 mg cm2 yr four orders of magnitude slower than the average value observed for taiwan we also demonstrate that the shelf sediments around taiwan have received the admixing of a 10be rich component from the ocean and would give a poor estimate for material eroded from taiwan

sm nd disequilibrium in high pressure low temperature himalayan and alpine rocks in order to decipher the causes of sm nd isotopic disequilibrium in high pressure low temperature rocks sm nd isotopic analyses were carried out on minerals from four himalayan tso morari unit and four alpine dora maira monte viso sesia lanzo eclogitic rocks of different lithologies and different intensities of post eclogitic metamorphism in most of these

relationships between o isotope equilibrium mineral alteration and rb sr chronometric validity in granitoids implications for determination of cooling rate a combined study of mineral o and rb sr isotopes was carried out for a number of mesozoic granitoids in china in order to compare the degree of o isotope equilibrium between coexisting minerals with the validity of mineral rb sr isochrons for granitoids a scrutiny of both o isotope geothermometry and rb sr internal isochron dating for corresponding minerals indicates that equilibrium o isotope fractionation between rb sr isochron minerals corresponds to geologically meaningful isochron ages if the variation in 87rb 86sr ratio is big enough to provide reasonably small uncertainties in age significant deviation of the rb sr isochron age from the actual age appears to depend on the difference in sr isotopic composition between an external fluid and the igneous minerals as a result o isotope disequilibrium is often caused by interaction between the rock and the external fluid that results in mineral alteration post magmatic alteration can cause isotope exchange between the minerals and an internally buffered fluid that is isotopically identical to the host rock the o isotope composition of coexisting minerals in studied samples changed principally due to a decrease in temperature both rb and sr concentrations and the sr isotope ratios of isochron minerals also changed due to the mixing of different sr reservoirs nevertheless the isochron age can remain unchanged if the mixing took place along the isochron chord between the internal fluid and the minerals from that newly altered minerals formed this provides an insight into the effect of internal and external fluids on the validity of mineral rb sr chronometry in addition an alternative approach is proposed to construct the cooling curve by a combined use of o isotope temperature and mineral isotope age for the granitoids of interest comparing with the traditional method using the empirical closure temperature for rb sr chronometry the proposed approach utilizes fewer variables with smaller uncertainties than the traditional way

dating of subduction and differential exhumation of uhp rocks from the central dabie complex e china constraints from microfabrics rb sr and u pb isotope systems abstract the correlation of deformation fabrics and metamorphic reactions with geochronologic data of uhp metamorphic rocks demonstrate that the multistage subduction and exhumation history of the central dabie complex requires rapid subduction and rapid initial exhumation moreover these data show that volume diffusion is not the major resetting mechanism of radiogenic isotope systems thus our age data do not simply reflect a thermal cooling history in the investigated section the maximum age for uhp is given by the 244 3 ma 2 u pb age of a pre uhp titanite phenocryst that survived uhp metamorphism and subsequent tectonometamorphic events a minimum age for uhp is set by the 238 1 ma 2 238 u 206 pb mineral isochron age of titanite and cogenetic epidote these minerals formed from local partial melts during ascent and their age suggests fast exhumation and emplacement in the middle crust in the period of ca 238 218 ma the uhp terrain records ht metamorphism local partial melting and extensive pervasive strain below the eclogite jd grt stability field exhumation was polyphase with a first phase of fast exhumation succeeded by episodes of ht metamorphism and concomitant deformation at deep mid crustal level between 238 and 218 ma slow exhumation related to the final emplacement of tectonic units along greenschist facies shear zones did not cease before ca 209 204 ma the resetting and homogenization of radiogenic isotope systems were aided by dissolution precipitation creep which was the dominant deformation mechanism in quartz feldspar rocks in combination with fluid influx

isotopic disequilibrium in ultrahigh pressure and retrograde metamorphism of eclogite and gneiss from the chinese continental scientific drilling in the sulu orogen china evidence from mineral nd sr o isotopic composition diffusion rates of sr and o in minerals are often comparable while nd has a lower diffusion rate during thermal overprint s thus the o isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of rb sr and sm nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism this study presents a combination of investigation on sm nd rb sr and o isotopic compositions of minerals separated from ultrahigh pressure eclogite and gneiss that were collected from the main hole of the chinese continental scientific drilling project located in the sulu orogen eastern china oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620 740 and 460 590 c representing diffusion cessation of isotopic exchange during the eclogite facies recrystallization and later amphibolite facies retrograde overprint rb sr mineral regressions of two eclogite samples give consistent triassic ages of 244 ma corresponding to eclogite facies metamorphism while the same minerals do not yield meaningful sm nd isochron ages this phenomenon likely suggests that rb sr isotopic equilibrium was achieved during eclogite facies metamorphism and preserved during late amphibolite facies retrogression in contrast sm nd isotopic equilibrium between the minerals of eclogite was not achieved under uhp metamorphic conditions regressions of epidote and biotite of one gneiss sample give a triassic sm nd age of 243 34 ma corresponding to the time of the eclogite facies metamorphism and a jurassic mineral rb sr age of 187 5 1 8 ma these results imply that fluids have played an important role to achievement of the sm nd isotopic equilibrium during eclogite facies metamorphism and re equilibration of the rb sr isotopic system during later retrograde overprint

average chemical compositions of post archean sedimentary and volcanic rocks from the qinling orogenic belt and its adjacent north china and yangtze cratons the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision during the late paleozoic to middle triassic with the formation of the e w trending qinling dabie mountains we report the average compositions of 13 major 16 trace and 14 rare earth elements of post archean sedimentary arenaceous pelitic and carbonate and volcanic mafic with sio2 56 and felsic with sio2 56 rocks from the qinling orogenic belt including the north and south qinling and the adjacent southern margin of the north china craton and northern margin of the yangtze craton as well as 9 tectonostratigraphic subunits the results are based on systematic sampling and analyses of 3 100 individual rock samples taken over an area of 153 200 km2 the obtained average compositions of sedimentary and volcanic rocks of the study area generally compare well with previously published averages for corresponding rock types pb is a notable exception and is 1 5 3 times as high as the earlier estimates for sedimentary and mafic volcanic rocks also the felsic volcanic rock has a bulk composition of quartz keratophyre with na2o k2o which is in marked contrast with the rhyolitic composition for both average world wide tertiary recent orogenic and anorogenic felsic volcanic rocks although there are significant intra unit chemical variations persistent inter unit distinctions are also evident especially in major elements

is the hp uhp hong an dabie sulu orogen a piercing point for offset on the tan lu fault abstract the tan lu fault is a major strike slip fault in eastern china that appears to offset the high grade rocks of the hong an dabie sulu orogen left laterally 540 km we evaluate models for the collision between the south and north china blocks published radiometric dates recording hp uhp metamorphism and exhumation in the hong an dabie and sulu terranes and the timing of sinistral motion on the tan lu fault to evaluate whether uhp rocks provide a piercing point for offset on the tan lu fault uhp metamorphism in hong an dabie was concurrent with sulu based on u pb dating of coesite bearing domains of zircon at 244 5 226 2 ma for hong an dabie and 243 4 225 2 ma for sulu retrograde metamorphism began c 220 ma for both hong an dabie and sulu but retrograde zircon growth ended c 214 ma in hong an dabie and continued until c 202 ma in sulu based on u pb dating of zircon domains external to coesite bearing domains structures in sulu are rotated 25 counter clockwise from but are broadly similar to hong an dabie suggesting the two areas have a common triassic orogenic history that pre dates motion on the tan lu fault and that is consistent with paleomagnetic studies we constructed a pre cretaceous restoration of the hong an dabie sulu belt that moves the sulu terrane south aligning the suture and the eclogite facies isograd and rotates sulu c 25 clockwise to re align structures with hong an dabie our restoration is supported by published data and shows that the hong an dabie sulu orogen is a piercing point for post collisional offset on the tan lu fault and that these regions shared a common subduction exhumation history the tan lu fault did not play a significant role in the hong an dabie sulu collision and likely developed later in the early cretaceous

hydrogen and oxygen isotope evidence for fluid rock interactions in the stages of pre and post uhp metamorphism in the dabie mountains abstract hydrogen and oxygen isotope studies were carried out on high and ultrahigh pressure metamorphic rocks in the eastern dabie mountains china the 18 o values of eclogites cover a wide range of 4 2 to 8 8 but the d values of micas from the eclogites fall within a narrow range of 87 to 71 both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some eclogite samples the 18 o values of 4 to 1 for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water rock interaction before the high to ultrahigh pressure metamorphism heterogeneous 18 o values for the eclogite protoliths implies not only the varying degrees of the water rock interaction before the metamorphism at different localities but also the channelized flow of fluids during progressive metamorphism due to rapid plate subduction retrograde metamorphism caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions and could be derived from structural hydroxyls dissolved in nominally anhydrous minerals

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

zircon u pb age and trace element evidence for paleoproterozoic granulite facies metamorphism and archean crustal rocks in the dabie orogen abstract zircon u pb ages and trace elements were determined for granulites and gneiss at huangtuling which are hosted by ultrahigh pressure metamorphic rocks in the dabie orogen east central china cl images reveal core rim structure for most zircons in the granulites the cores show oscillatory zoning relatively high th u ratios and hree enriched patterns consistent with a magmatic origin they gave a weighted mean 207pb 206pb age of 2766 9 ma interpreted as dating magma emplacement of the protolith the rims are characterized by sector or planar zoning low th u ratios negative eu anomalies and flat hree patterns consistent with their formation under granulite facies metamorphic conditions zircon u pb dating yields a weighted mean 207pb 206pb age of 2029 13 ma which is interpreted to record a metamorphic event possibly during assembly of the supercontinent columbia the gneiss has a protolith age of 1982 14 ma which is younger than the zircon age of the granulite facies metamorphism suggesting a generally delay between ht metamorphism and the intrusion of post collisional granites a few inherited cores with igneous characteristics have 207pb 206pb ages of 2 90 3 28 and 3 53 ga suggesting the presence of mesoarchean to paleoarchean crustal remnants in the yangtze craton a few cretaceous metamorphic ages were also obtained suggesting the influence of post collisional collapse in response to cretaceous extension of the dabie orogen it is inferred that the recently discovered archean basement of the yangtze craton occurs as far north as the dabie orogen

high temperature inter mineral magnesium isotope fractionation in eclogite from the dabie orogen china abstract to investigate the magnitude and mechanism of inter mineral mg isotope fractionation at high temperatures we report high precision analyses of mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from bixiling in the dabie orogen china magnesium isotopic compositions of whole rocks 26mg of 0 44 to 0 26 are similar to the estimated 26mg values of the mantle suggesting mg isotopic inheritance from a gabbroic protolith with limited mg isotope fractionation during eclogite facies metamorphism by contrast mineral separates are highly heterogeneous with 26mg values ranging from 0 30 to 0 60 in phengite from 0 16 to 0 40 in omphacite and from 0 95 to 0 74 in garnet phengite and omphacite are 1 heavier in 26mg than coexisting garnet indicating large high temperature inter mineral mg isotope fractionations the constant 26mgomphacite garnet 26mgomphacite 26mggarnet value 1 14 0 04 together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet suggests an equilibrium mg isotope fractionation controlled by the difference in coordination number of mg between omphacite six and garnet eight the 1 14 fractionation is the largest high temperature equilibrium inter mineral mg isotope fractionation observed so far and makes the omphacite garnet mg isotope fractionation a potential geothermometer by contrast mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1 25 to 1 47 and from 0 14 to 0 32 respectively this implies mg isotopic disequilibria between phengite and garnet omphacite which might result from the mg isotopic variation in phengites due to mg isotope exchange between phengites and retrograde fluids in bixiling eclogites

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

ultrahigh ϵnd eclogites from an ultrahigh pressure metamorphic terrane of china the su lu ultrahigh pressure uhp metamorphic terrane is part of the qinling dabie suture zone formed by the collision between the sino korean and yangtze cratonic blocks geochemical and isotopic investigation of the uhp metamorphic rocks has revealed a spectacular history for some eclogitic rocks retrograded coesite bearing eclogites from the weihai area in the eastern end of the su lu terrane have recorded an extraordinary nd isotopic growth with a world record high nd value of 264 range 170 to 264 such isotopic characteristics are consistent with the observed extreme lree fractionation and very high sm nd ratios the sm nd isotopic data allow us to calculate the minimum age of protolith formation at 1 7 ga assuming their evolution from a depleted or a chondritic mantle geochemical data indicate that the eclogite protoliths were of basaltic compositions originally but underwent metasomatism water rock interaction at high pressures and temperatures leading to the present apparent andesitic composition and extreme lree depletion this process took place in the proterozoic 1 7 ga and was not related to the mesozoic continental collision and ultrahigh pressure metamorphism at 210 220 ma ago because of a protracted retrograde metamorphism garnet sm nd systems were not closed until 160 ma ago 50 ma after the peak eclogite metamorphism this implies that the exhumation history is significantly different in different parts of the dabie and su lu uhp metamorphic terranes from geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late proterozoic can be identified prior to the last mesozoic continental collision and accompanying uhp metamorphism in the su lu and dabie terrane