la icp ms monazite u pb age and trace element constraints on the granulite facies metamorphism in the tongbai orogen central china abstract granulite facies metamorphic rocks in the tongbai orogen have great implications for the evolution of the whole qinling tongbai dabie orogenic belt however the age and tectonic setting of the granulite facies metamorphism have not been well constrained yet in this contribution bse imaging trace element analysis and u pb dating of monazite were undertaken for three felsic granulites a gneiss and a leucosome from the tongbai orogen most monazite grains from the granulite and gneiss samples exhibit no zoning relatively low th u ratios significant depletion in hrees and y and negative eu anomalies indicating their formation under granulite facies metamorphic conditions their formation ages range from 423 2 to 430 3 ma with a weighted mean of 426 5 which is taken as the timing of the peak granulite facies metamorphism in the leucosome sample the monazite shows a euhedral to subhedral shape coupled with a core and rim structure besides being depleted in hrees and y both of them have significantly negative eu anomalies and variable u contents 1220 13323 ppm and th u ratios 7 2 136 7 this suggests that their formation was coeval with the crystallization of feldspar metamorphic zircon and garnet the monazites yield a weighted mean age of 426 3 ma suggesting nearly coeval partial melting during granulite facies metamorphism the relationship between metamorphic monazite and zircon within the same metamorphic sample is complex as revealed by metamorphic monazite and zircon u pb ages for the gneiss granulite and leucosome samples in the other two granulite samples the monazite yields u pb ages of 423 2 and 425 3 ma whereas only little overgrowth or recrystallization of metamorphic zircon this indicates that monazite is more sensitive than zircon in recording of metamorphic age even at the granulite facies conditions combined with previous metamorphic zircon u pb age results the prograde peak and retrograde stages of the granulite facies metamorphism in the tongbai orogen can be constrained at ca 440 426 and 415 ma respectively therefore the granulite facies metamorphism would have lasted for more than 20 myr the prolonged granulite facies metamorphism might result from the continuous northward subduction of the paleotethyan oceanic crust beneath the north qinling terrane

fluid rock interactions during uhp metamorphism a review of the dabie sulu orogen east central china comprehensive review on the characteristics of petrology oxygen isotope fluid inclusion and nominally anhydrous minerals nams for many dabie sulu ultrahigh pressure uhp metamorphic rocks including drill hole core samples reveals that fluid has played important and multiple roles during complicated fluid rock interactions attending the subduction and exhumation of supracrustal rocks we have identified several distinct stages of fluid rock interactions as follows 1 the neoproterozoic supercrustal protoliths of uhp rocks experienced variable degrees of hydration through interactions with cold meteoric water with extremely low oxygen isotope compositions during neoproterozoic snow ball earth time 2 a series of dehydration reactions took place during triassic subduction of the yangtze plate beneath the sino korean plate the released fluid entered mainly into volatile bearing high pressure hp and uhp minerals such as phengite zoisite epidote talc lawsonite and magnesite as well as into uhp nams such as garnet omphacite and rutile 3 silicate rich supercritical fluid hydrous melt existed during the uhp metamorphism at mantle depths 100 km which mobilized many normally fluid immobile elements and caused unusual element fractionation 4 the fluid exsolved from the nams during the early exhumation of the dabie sulu terrane was the main source for hp hydrate retrogression and generation of hp veins 5 local amphibolite facies retrogression at crustal depths took place by infiltration of aqueous fluid of various salinities possibly derived from an external source 6 the greenschist facies overprinting and low pressure lp quartz veins were generated by fluid flow along ductile shear zones and brittle faults during late stage uplift of the uhp terrane

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

genesis of adakitic granitoids by partial melting of thickened lower crust and its implications for early crustal growth a case study from the huichizi pluton qinling orogen central china abstract adakitic rocks are often considered as a key to deciphering the genesis of archean ttgs and the early crustal growth granites from the huichizi pluton in the north qinling nq unit have high sr y and la yb n ratios similar to adakites their relatively high sio 2 k 2 o and na 2 o and very low mgo cr and ni contents are in the range of high sio 2 adakites and early archean ttgs and are compositionally similar to experimental melts derived from metabasalt sources new sims zircon u pb dating constrains the emplacement age of the huichizi pluton at 422 5 ma rock samples from the huichizi pluton have e nd t and zircon e hf t values similar to the neoproterozoic metabasalts in the nq unit in combination with their normal mantle like 18 o zir values these adakites are best explained by partial melting of the neoproterozoic mafic crustal root due to subduction of the shangdan ocean regional geological data suggest that the crust was probably thickened by a ca 490 ma arc collision process prior to the emplacement of the huichizi pluton our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust

experimental study of zircon coarsening in quartzite h2o at 1 0 gpa and 1000 c with implications for geochronological studies of high grade metamorphism the rate and mechanism of zircon coarsening in quartzite h 2 o at 1 0 gpa and 1000 c were characterized by performing time series experiments in a piston cylinder apparatus anhydrous experiments show no significant growth of zircon or textural modifications after 24 h in the presence of 1 2 wt h 2 o zircon 2 wt in coarsely powdered quartz recrystallized in the first 8 hours so that mean crystal size decreased relative to the starting material after 8 h zircon grew by ostwald ripening and by coalescence of crystals and maintained its position on quartz grain boundaries even while quartz crystals coarsened by recrystallizing in the fluid fitting the experimental data produced the growth rate equation log n log 2 2 log t to log k n log t to 5 9 where is mean diameter in micrometers the subscript 0 denotes initial value at time zero t is experiment duration in seconds and k n is the growth rate constant with units of m 2 s five experiments r 2 0 988 a value of n 2 suggests that growth is interface controlled with the rate limited by dissolution and precipitation on the crystal surface an equivalent growth rate equation is k 1 n t k 1 n n t 1 n 1 12 x 10 3 m s 1 2 t 1 2 a parabolic rate law equation that can be used to demonstrate that zircon in the presence of h 2 o at 1000 c grows fast enough by ostwald ripening to produce micrometer thick overgrowths in 10 y rapid coarsening of zircon by ostwald ripening will occur during thermal metamorphism when fluid is present addition of fluid and possibly melt greatly increases the rate of coarsening suggesting that most metamorphic rims form nearly instantaneously during fluid influx or anatexis and that u pb rim ages date such events

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

detrital record of mountain building provenance of jurassic foreland basin to the dabie mountains 1 the huangshi foreland basin developed on the southern margin of the dabie mountains as a result of tectonic loading during triassic collisional suturing between the north china and south china cratons modal and detrital zircon data for jurassic samples within the basin suggest a multicomponent source with input from both the south china craton and dabie orogen samples are predominantly quartz arenites derived on the basis of framework compositions from a recycled orogen source detrital zircons range in age from archean to triassic with a dominant component in the late paleoproterozoic between 1 9 1 7 ga and subsidiary components at 2 6 2 2 ga 0 8 0 7 ga 0 5 0 4 ga and 0 33 0 2 ga age data integrated with cathodoluminescence and trace element data for the zircons indicate that the archean and proterozoic detritus was derived from igneous and metamorphic sources that overlap with time equivalent pulses of such activity within the south china craton phanerozoic zircon ages overlap the times of the ordovician carboniferous and triassic high pressure metamorphism in the dabie mountains the provenance record integrated with paleocurrent and regional relations enables a paleogeographic reconstruction in which the huangshi basin was fed by a major axial flowing trunk river system carrying detritus from eastern and southern sources within the south china craton and was also fed by short south flowing tributaries supplying some detritus from the evolving dabie orogen the dominance of cratonic derived detritus within the provenance record of the huangshi basin contrasts with that of the hefei foreland basin that lies to the north of the dabie mountains which is dominated by neoproterozoic mesozoic detritus derived directly from the dabie mountains and lacks any significant older paleoproterozoic or archean components easterly extensions of the dabie sulu collisional suture and of the resultant huangshi basin occur in korea and japan over an along strike length of some 2000 km

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

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

thermobaric structure of a traverse across western dabieshan implications for collision tectonics between the sino korean and yangtze cratons as the best preserved high and ultrahigh pressure hp and uhp metamorphic terrane in the qinling dabieshan sulu orogen western dabieshan is divided into six lithotectonic units along a traverse across the orogen i e from north to south the nanwan balifan huwan xinxian hong an and mulanshan units in this terrane five eclogite bearing zones i v are developed the garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning suggesting that the rims record general peak temperature and pressure thermobarometric study indicates that the peak p t conditions of eclogite are 550 570 c and 21 kbar for zone i 470 500 c and 14 17 kbar for zone ii 620 670 c and 26 29 kbar for zone iii 530 560 c and 20 22 kbar for zone iv and 490 510 c and 19 20 kbar for zone v the symmetrical thermobaric pattern in conjunction with structural and geochronological data demonstrates that the huwan and hong an units belong to the same hp slice overlying the uhp slice this pattern together with the mulanshan lt hp blueschist greenschist belt in the south roughly constitutes a normal metamorphic zonation however clear metamorphic gaps occur between different slices it is inferred that the lt hp hp and uhp slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients the successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices whereas exhumation of the deepest uhp slice was effected by underthrusting of the lower crust of the yangtze craton

40ar 39ar thermochronology from the northwestern dabie shan constraints on the evolution of qinling dabie orogenic belt east central china abstract seven structural units with distinct kinematics have been recognized in the northwestern dabie shan 114 e 115 e and 31 n 32 n east central china they are from north to south 1 the north verging mafan fold belt mfb involving early paleozoic metamorphic rocks 2 the liangting melange belt lmb which is separated from the mfb by a south dipping thrust and consists of gneissic and marble blocks surrounded by highly sheared greenschist 3 the north verging laoshan ductile fold belt lfb which was thrust northward over the liangting melange belt and involves proterozoic schists 4 the suhe thrust sheet sts which consists of phyllite and slate above a folded thrust fault dipping both to the north and south 5 the 5 8 km wide huwan shear belt hsb in the footwall of the folded thrust consisting of mylonitic gneisses and sheared ecologite 6 the kafang eclogite bearing gneiss dome kgd which is defined by broadly warped foliations and 7 the 10 km wide pendian ductile shear belt psb characterized by dextral strike slip deformation nine phengite and muscovite samples in eclogites mylonitic shear zones and metamorphic gneisses from four of the seven structural units have been dated by the 40ar 39ar step heating method phengites from deformed eclogites in the hsb yield complex age spectra that we interpret as most likely indicating closure with respect to ar loss between 350 and 430 ma the rock samples collected from the eclogite bearing gneiss display a significant static recrystallization of quartz which suggests temperatures in excess of ca 350 400 c the ages of 350 430 ma from samples 9669b and 9669c are therefore interpreted as retrograde metamorphic ages for the eclogites six additional phengites sampled from mylonitic rocks yield two groups of ages 263 249 and 236 214 ma the older group samples 96 5 9667 and 9692 were collected from mylonitic rocks with top to north shear sense in the lmb hsb and kgd respectively 9667 and 9692 exhibit a strong static recrystallization which means that ductile deformation took place at a temperature in excess of 350 400 c we therefore adopt the age of 263 249 ma as the cooling ages of white micas considering sample 96 5 shows microfabrics formed at lower temperature around or slight below the closure temperature of white mica there is a strong possibility that the age of 254 1 ma represents a crystallization age rather than a cooling age because these samples are taken from the top to north shear zones we also interpret the 263 249 ma event as a result of thrusting induced denudation the younger group samples 96106 96124 and 96113 collected from dextral shear belts in the hsb and psb exhibit a strong static recrystallization at a temperature in excess of 350 400 c their ages 236 214 ma are therefore interpreted as the cooling ages related to later dextral strike slip faulting sample 96101 collected from weakly foliated rock without penetrative deformation on the north limb of the kgd shows poorly recovered quartz microfabrics at the temperature around or slightly below the closure temperature of white mica its age of 187 1 ma is therefore interpreted as crystallization age during the downdip detachment event related to doming the 40ar 39ar data from the northwestern dabie shan suggest four distinctive cooling events on the basis of the local structural framework and regional tectonic relationships we suggest that the 410 350 ma cooling event was related to either subduction of an oceanic plate beneath the sino korean plate or collision between the sino korean plate and a microcontinental block in the middle paleozoic the three younger episodes of cooling between the late permian and the late triassic may all have been related to the collision between the sino korean and yangtze plates specifically the 263 249 ma event could have been a result of thrusting induced denudation whereas the 236 214 ma cooling event is likely related to strike slip faulting the younger ages of 195 187 ma obtained from the gneiss dome are interpreted to be a result of regional denudation related to doming during the late stage of collision between the sino korean and yangtze plates

refining the timing and mechanism of the triassic partial melting in the sulu uhp orogen china zircon and garnet evidence from a felsic vein and its host granitic gneiss abstract partial melting of high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust felsic veins in hp uhp metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in hp uhp terranes this contribution is an integrated study of petrology zircon and titanite u pb ages as well as zircon hf o and major minerals o isotope compositions for a felsic vein and its host granitic gneiss in the sulu uhp terrane to decipher the timing and mechanism of partial melting zircon u pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca 750 ma the inherited zircons had an average 18o value of 0 03 0 15 and yielded a weighted mean ehf t value of 1 16 0 54 and two stage hf model ages of 1707 65 ma these results suggest that the protolith was derived from reworking of paleoproterozoic crust in a neoproterozoic rift in the two samples broad garnet core mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin there are large differences in 18o values between the protolith magmatic zircon 18o 0 03 0 15 and metamorphic garnet 4 14 0 21 for the felsic vein and 4 13 0 11 for the host granitic gneiss suggesting that the negative 18o fingerprints might result from hydrothermal alteration or multi episodes of rift magmatism after the formation of the protolith zircon from the felsic vein yielded a u pb age of 212 3 ma titanites from the two studied samples show undistinguishable u pb ages of 227 14 ma and 212 6 ma within the analytical uncertainties these ages register the timings of the partial melting the approximately consistent 18o values of the metamorphic zircon 4 78 0 15 and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system which might be triggered by breakdown of phengite according to a compilation of partial melting in the sulu uhp terrane there exists a clear spatio temporal variation the ca 237 224 ma partial melting in the northern uhp zone might be due to its relatively high temperature metamorphism the most extensive partial melting occurred at ca 220 215 ma in both the southern and northern uhp zones which might be triggered by the detachment of the preceding subducted slab and partial melting of the northern uhp zone continued to ca 210 ma due to heat supply from the underneath mantle such a variation might be dictated by the tectonic evolution of the different uhp slices accordingly the whole partial melting process of uhp rocks in the sulu uhp terrane might have lasted for about 27 myr with two major episodes at ca 230 224 and 220 215 ma

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

zircon u pb and trace element data from rocks of the huai an complex new insights into the late paleoproterozoic collision between the eastern and western blocks of the north china craton abstract the trans north china orogen tnco is interpreted as a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the timing of its formation has not been precisely constrained yet six samples including five high grade metamorphic rocks and a pegmatite dike from the huai an complex in the northern segment of the tnco were selected for zircon u pb dating and trace element analysis a tonalite trondhjemite granodiorite ttg gneiss sample and a high pressure hp mafic granulite sample contain magmatic zircon cores yielding protolith formation ages of 2503 17 and 1964 38 ma respectively detrital zircons of magmatic origin in two metasedimentary rocks show concordant or moderately discordant u pb ages of 2683 22 to 1964 13 ma indicating that they were mainly derived from the tnco with some possibly derived from the archean continental rocks of the north china craton metamorphic zircons in the ttg gneiss and two metasedimentary rocks exhibit low hree contents flat hree patterns and negative eu anomalies implying their formation coeval with garnet and feldspar under grannulite facies conditions metamorphic zircons in an hp mafic granulite and a sample of leucosome from a ttg gneiss also have depleted hree contents flat hree patterns but only weak negative eu anomalies which may also indicate formation under granulite facies conditions whereas the weak eu anomalies may be due to the whole rock composition all these metamorphic zircons have formation temperatures of 702 749 c approaching the peak metamorphic temperatures of hp granulite rocks in this area they have identical ages of ca 1845 ma within analytical errors which is the best estimate age for hp granulite facies metamorphism of the huai an complex these results confirm previous models proposing late paleoproterozoic collision between the eastern and western blocks of the north china craton along the tnco on the other hand some metamorphic zircons in the leucosome sample show planar or polygonal zoning high th u contents and high th u ratios high ree contents and steep hree patterns indicating that they crystallized during partial melting with abundant garnet decomposition they yielded a weighted mean age of 1819 13 ma which we interpret to register the time of retrograde metamorphism combined with previously published data there are three age peaks at 1876 6 1849 2 and 1814 4 ma for metamorphic zircons from high grade metamorphic rocks in the tnco these may indicate either a protracted event or at least three peaks of metamorphism a pegmatite dike cutting the foliation of a mafic granulite boudin was dated at 1806 15 ma which signifies termination of collision between the eastern and western blocks in the huai an area

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

archean crustal evolution in the southeastern north china craton new data from the huoqiu complex abstract the southeastern margin of the north china craton ncc is characterized by zircon ages of dominantly 2 6 ga and differs significantly from most other regions in the ncc with mostly 2 5 ga zircons in order to better understand the precambrian crustal evolution of the ncc we conducted a study of in situ zircon u pb dating trace element and hf isotope analyses as well as whole rock geochemistry on a comprehensive set of core samples from three drilling holes intersecting the basement rocks of the huoqiu area at the southeastern margin of the ncc our data reveal three magmatic events at 3 02 ga 2 77 ga and 2 71 ga in addition seven analyses of possibly inherited zircon cores plot along a discordant line with an upper intercept age of 3262 35 ma indicating possible existence of a paleoarchean crystalline basement the 3 02 ga zircons have positive hf t values close to the depleted mantle growth curve and their u pb ages are only slightly younger than the tdm1 model ages hence recording a mesoarchean crust forming event the 2 77 ga zircons also register a period of crustal growth but probably with some involvement of older crust the 2 71 ga zircons have lower hf t values and older tdm1 model ages than the 2 77 ga zircons suggesting that the former were mainly derived from older continental crust in contrast to the traditional idea that 2 7 ga was a major period of crustal growth in the ncc

high pressure blueschists and eclogites in hong an a framework for addressing the evolution of high and ultrahigh pressure rocks in central china the hong an region in the qinling dabie collisional zone in eastern china hosts a series of metamorphic rocks exposing a south to north distribution from blueschist blueschist greenschist amphibolite eclogite kyanite free and kyanite eclogite to coesite eclogite facies rocks that represent progressively deeper levels of the mesozoic subduction collision complex the hong an area is interesting for three reasons 1 it escaped the thermal and structural overprint imparted on much of the dabie mountains during early cretaceous intrusion of voluminous granites and granodiorites 2 the high pressure hp hong an eclogites are widely distributed often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist blueschist greenschist rocks exposed to the south 3 the blueschist blueschist greenschist facies rocks are generally better exposed than their equivalents in the southeastern dabie mountains and offer some opportunity for simultaneous structural and metamorphic analysis the hong an area hp rocks offer perhaps the closest approximation to a preserved snapshot of mesozoic pressure temperature p t conditions attending early subduction exhumation in the region and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh pressure uhp rocks the purpose of this contribution is twofold 1 to document previously unpublished metamorphic and structural data characterizing these hp sequences and their relative continuity in hong an 2 to incorporate these data with recent geochronologic structural and paleomagnetic information in the context of protracted late paleozoic through mesozoic subduction collision and exhumation metamorphism and exhumation of some of the southern hong an hp sequences appear to have occurred concomitant with oceanic subduction immediately to the west and thus may have preceded widespread continental subduction collision moreover all of the hp uhp sequences in the region were exhumed before the end of collision between the sino korean and yangtze cratons at ca 160 ma exhumation of hp uhp rocks both before and during continental plate collision is neither novel for central china nor for other hp uhp zones but is important to take into account when reconstructing the evolution of such orogens

generation of post collisional normal calc alkaline and adakitic granites in the tongbai orogen central china abstract post collisional granites are generally generated by partial melting of continental crust during orogenic extension the occurrence of normal calc alkaline granites following adakitic granites in a collisional orogen is frequently supposed as a sign of tectonic regime transition from compression to extension which has been debated yet in this paper we present a comprehensive study of zircon u pb ages hf o isotopes as well as whole rock major and trace elements and sr nd isotopes for tongbai and jigongshan post collisional granitic plutons in the tongbai orogen zircon u pb dating yields intrusion ages of ca 140 and 135 ma for the tongbai and jigongshan plutons respectively suggesting they are post collisional granites these granites are high k calc alkaline series metaluminous to weakly peraluminous with a cnk ratios of 0 85 1 08 the tongbai gneissic granites are normal calc alkaline granite having variable sio2 61 93 76 74 wt and sr y 2 9 38 9 and la yb n 1 7 30 1 ratios with variably negative eu anomalies 0 41 0 92 they have relatively high initial sr isotope ratios of 0 707571 to 0 710317 and low end t 15 74 to 11 09 and ehf t 17 6 to 16 9 values their nd and hf model ages range from 2 2 to 1 8 ga and 2 3 to 2 2 ga on the contrary the jigongshan granites show higher sio2 66 56 72 11 wt and sr y 30 1 182 0 and la yb n 27 4 91 4 ratios with insignificant eu anomalies 0 73 1 00 belonging to adakitic granite they have isr 0 707843 0 708366 end t 19 83 to 17 59 and ehf t 26 0 to 23 5 their nd and hf model ages vary from ca 2 5 to 2 4 ga and ca 2 8 to 2 6 ga the tongbai and jigongshan granites are characterized by mantle like zircon 18o values 5 17 5 46 these geochemical features suggest that the tongbai and jigongshan granites were derived from partial melting of paleoproterozoic and archean continental crust respectively fractional crystallization affected the geochemical compositions of the tongbai gneissic granites while the compositions of the jigongshan granites were mainly controlled by partial melting the adakitic signatures of the jigongshan granites were either inherited from their source or ascribed to more garnet in their residues the voluminous post collisional granites might form by the collapse of the thickened tongbai orogenic root the normal tongbai gneissic granites occurred slightly earlier than the jigongshan adakitic granites in the tongbai orogen suggesting that it is not a mandate to sign the tectonic transition from adakitic to normal calc alkaline granites in post collisional settings therefore this study provides another example supporting the burst of voluminous post collisional granites with different compositions as a consequence of the collapse of the thickened orogenic roots of collisional orogens

40ar 39ar geochronology and exhumation of high pressure to ultrahigh pressure metamorphic rocks in east central china new 40 ar 39 ar ages from rocks in the high pressure and ultra high pressure hp uhp metamorphic complex in the hong9an and dabie mountains areas east central china document a two phase cooling and exhumation history following triassic continental collision and metamorphism phengite ages from blueschist through kyanite bearing eclogite facies rocks in hong9an record initial exhumation from the collision zone between 230 and 195 ma biotite and hornblende ages from migmatites and eclogite bearing gneisses from the dabie mountains record a cooling event between 128 and 117 ma corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons the triassic through early jurassic 40 ar 39 ar cooling ages corroborate u pb and sm nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid emplacement of granitic melts within the hp uhp sequences 80 m y after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial rapid exhumation episode

seasonal cycle of orographic gravity wave occurrence above small islands in the southern hemisphere implications for effects on the general circulation 1 orographic gravity waves generated by flow over the topography of small islands in the southern oceans have been observed from orbit with the atmospheric infrared sounder on the aqua satellite we examine the occurrence frequencies of these waves in the stratosphere at 40km above 14 islands and examine geographical and seasonal changes our results show that these small island mountain waves occur commonly in the stratosphere in the may september season though not every day differing seasonal variations are evident at different islands and the seasonal variations are closely related to latitude and prevailing wind patterns we also examine interannual variability in 2years of data and the relationships between occurrence frequencies momentum fluxes and stratospheric and surface winds the results suggest that stratospheric winds have a first order limiting effect on the observations of these island mountain waves in atmospheric infrared sounder airs data surface wind direction and island orographic relief have an additional but secondary influence on the island mountain wave occurrence frequencies in airs data the implications are that these wave events are extremely common and that on many days when the waves are not observed in airs data they have likely dissipated and induced a drag force on the atmosphere below the 40km observation level observations of momentum flux during these wave events also permit a first estimate of their importance to the general circulation of the southern hemisphere

geochronology geochemistry and isotope compositions of piaochi s type granitic intrusion in the qinling orogen central china petrogenesis and tectonic significance abstract s type peraluminous granite can crop out in various tectonic settings the paleozoic piaochi granitic intrusion is a large s type peraluminous intrusion located in the north qinling orogen and has important implication for its tectonic evolution though intensively studied the age and tectonic setting of this intrusion are still controversial in this study we carried out an integrated study of zircon u pb age major and trace elements and sr nd hf o isotope compositions for the piaochi intrusion with aims to constrain its petrogenesis and tectonic significance sims and la icpms u pb zircon data obtained from magmatic zircons indicate that the piaochi intrusion was emplaced at 473 4 ma and contained ca 490 ma inherited zircons the granite samples from the piaochi intrusion show typical s type characteristics such as the relatively moderate aluminum saturation index 1 00 1 08 low zirconium saturation temperatures occurrences of muscovite and high zircon 18 o values 8 70 to 9 54 the relatively low sm nd 0 13 0 20 and feo t mgo ratios 2 21 3 9 suggest that the granites are unfractionated the geochemical compositions reveal that they were derived from partial melting of a clay poor but plagioclase and biotite rich psammitic source similar to metasedimentary rocks outcropping in the north qinling unit moreover the piaochi granites are moderately depleted in eu and heavy rare earth elements and have relatively high la yb n values 22 8 56 8 implying a high pressure plagioclase poor and garnet rich residual assemblage the piaochi granites have high and variable initial 87 sr 86 sr ratios 0 72167 0 72511 and low e nd t from 10 23 to 8 71 values with two stage nd model ages t dm2 of 1 91 2 03 ga which are comparable with those of supracrustal materials in the north qinling unit in addition our results show that the ca 470 ma magmatic zircons display a narrow e hf t array within those of the inherited cores these imply that the piaochi granites inherited their isotopic characteristics wholly from their sources and the hf isotopes might have been differently homogenized during the partial melting our new zircon u pb ages indicate that the piaochi granites were emplaced after the climax of the uhp metamorphism coeval with later retrograde metamorphism during exhumation therefore we suggest that the formation of the early paleozoic piaochi intrusion was triggered by the exhumation of the uhp rocks

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

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

early paleozoic ca 465 ma eclogites from beishan nw china and their bearing on the tectonic evolution of the southern central asian orogenic belt abstract the beishan orogen is situated in the southern margin of the central asian orogenic belt caob it is connected with the tianshan orogen to the west and the mongolia xing anling orogen to the east this orogen exhibits a well preserved neoproterozoic to late paleozoic sequence containing ophiolites accretionary complexes and eclogites in this paper we provide new geochronological data for eclogites and their country paragneiss to constrain the tectonic evolution of the beishan orogen u pb zircon dating of eclogites yielded two concordant ages 886 4 ma for the cores and 465 10 ma for the rims the zircon cores display bright luminescence with homogeneous or planar sector zoning intermediate th u ratios 0 17 0 47 and hree enriched patterns with negative eu anomalies eu eu 0 16 0 58 these features coupled with their formation temperatures of 680 710 c suggest that the core age represents the time of an earlier upper amphibolite facies metamorphic episode by contrast the zircon rims have dark luminescence with homogeneous or fir tree zoning low th u ratios 12 kbar for the eclogite facies metamorphism on the other hand zircon dating of a paragneiss near the eclogite produced two major groups of ages ca 1450 ma for a magmatic event and ca 900 ma for a metamorphic event it seems that eclogites and their country rocks have suffered the same earlier metamorphic episode based on the available age data obtained for eclogites ophiolites and granitoids from the beishan area a tectonic scenario can be constructed as follows the ca 1000 ma oceanic crust representing by protoliths of the eclogites was involved in early neoproterozoic orogenesis to have formed a microcontinental block during the early paleozoic island arcs and microcontinental blocks were progressively accreted to the northern margin of the tarim craton dunhuang block this was followed by the closure of paleoasian ocean and final amalgamation between the tarim craton and kazakhstan block in the late paleozoic

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

the lu hf isotope geochemistry of chondrites and the evolution of the mantle crust system earth planet sci lett 148 1997 243 258 1

zircon u pb and hf evidence for coupled subduction of oceanic and continental crust during the carboniferous in the huwan shear zone western dabie orogen central china both oceanic and continental hp rocks are juxtaposed in the huwan shear zone in the western dabie orogen and thus provide a window for testing the buoyancy driven exhumation of dense oceanic hp rocks the hp metamorphic age of the continental rocks in this zone has not been well constrained and hence it is not known if they are of the same age as the exhumation of the hp oceanic rocks in situ laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms u pb trace element and hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the huwan shear zone u pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 4 ma the zircon in this sample displays ehf t values of 6 1 to 14 4 the positive ehf t values up to 14 4 suggest that the protolith was derived from a relatively depleted mantle source most likely palaeotethyan oceanic crust a granitic gneiss and the other eclogite yield protolith u pb ages of 738 6 and 700 14 ma respectively which are both the neoproterozoic basement rocks of the yangtze block the zircon in the granitic gneiss has low ehf t values of 14 2 to 10 5 and old tdm2 ages of 2528 2298 ma suggesting reworking of palaeoproterozoic crust during the neoproterozoic the zircon in the eclogite has ehf t values of 1 0 to 7 4 and tdm1 ages of 1294 966 ma implying prompt reworking of juvenile crust during its protolith formation metamorphic zircon in both eclogite samples displays low th u ratios trace element concentrations relatively flat heavy rare earth element patterns weak negative eu anomalies and low 176lu 177hf ratios all these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar and thus under eclogite facies conditions the metamorphic zircon yields u pb ages of 310 3 and 306 7 ma therefore both the oceanic and continental type eclogites share the same episode of carboniferous eclogite facies metamorphism this suggests that high pressure continental type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic type eclogites through buoyancy driven uplift

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

formation of paleoproterozoic eclogitic mantle slave province canada insights from in situ hf and u pb isotopic analyses of mantle zircons abstract in situ hf isotope analyses and u pb dates were obtained by laser ablation mc icp ms for a zircon bearing mantle eclogite xenolith from the diamondiferous jericho kimberlite located within the archean slave province nunavut canada the u pb zircon results yield a wide range of ages 2 0 to 0 8 ga indicating a complex geological history of importance one zircon yields a u pb upper intercept date of 1989 67 ma providing a new minimum age constraint for zircon crystallization and eclogite formation in contrast hf isotope systematics for the same zircons display an intriguing uniformity and corresponding hf depleted mantle model ages range between 2 1 0 1 and 2 3 0 1 ga the youngest hf model age is within error to the oldest u pb date the jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust and their formation related to paleoproterozoic subduction regimes along the western margin of the archean slave craton during the wopmay orogeny hf isotope compositions and u pb results for the jericho zircons reported here are in good agreement with a paleoproterozoic subduction model suggesting that generation of oceanic crust and eclogite formation occurred between 2 0 and 2 1 ga the slightly older hf depleted mantle model ages 2 1 to 2 3 ga may be reconciled with this model by invoking mixing between crustal derived hf from sediments and more radiogenic hf associated with the oceanic crust during the 2 ga subduction event this results in intermediate hf isotope compositions for the jericho zircons that yield fictitiously older hf model ages

the formation of foliated garnet bearing granites in the tongbai dabie orogenic belt partial melting of subducted continental crust during exhumation foliated garnet bearing fgb granites are associated closely with and are usually the major wall rocks of the high pressure hp and ultrahigh pressure uhp metamorphic rocks in the tongbai dabie region the mid segment of the qinling dabie sulu orogenic belt in central china these granites appear either as small plutons or as veins which commonly intrude into or surround the hp and uhp metamorphic eclogites or gneisses the veins of fgb granites usually penetrate into the retrograded eclogites or gneisses along the foliations condensation rims can occasionally be found along the margins of granite veins these granites are rich in si and alkali with high ga al ratios and depleted in ca mg al ti sc v ni co cr and sr which are similar to a type granites in a chondrite normalized diagram the samples are light rare earth elements enriched with different extent of negative eu anomaly moreover rb nb ta sr p and ti show different degrees of negative anomalies whereas ba k la zr and hf show positive anomalies in the primitive mantle normalized diagram negative anomalies of eu and sr indicate strong influence of plagioclase in conventional discrimination diagrams these fgb granites belong to the a type granite with geochemical characteristics affinitive to post collisional granites the end 230 ma values 15 80 to 2 52 and tdm values 1 02 2 07 ga suggest that magma for the fgb granites were derived from a heterogeneous crustal source therefore the fgb granites may provide clues for deciphering the formation of post collisional granites it is proposed that the magma of the fgb granites both in the hp and uhp units was formed in an extensional tectonic setting slightly post dating the hp and uhp metamorphism most likely as a result of decompressional partial melting of uhp retrograded eclogites during exhumation

carboniferous and triassic eclogites in the western dabie mountains east central china evidence for protracted convergence of the north and south china blocks shrimp u pb dating and laser ablation icp ms trace element analyses of zircon from four eclogite samples from the north western dabie mountains central china provide evidence for two eclogite facies metamorphic events three samples from the huwan shear zone yield indistinguishable late carboniferous metamorphic ages of 312 5 307 4 and 311 17 ma with a mean age of 309 3 ma one sample from the hong an group 1 km south of the shear zone yields a late triassic age of 232 10 ma similar to the age of ultra high pressure uhp metamorphism in the east qinling dabie orogenic belt ree and other trace element compositions of the zircon from two of the huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase namely in the eclogite facies an interpretation supported by the presence of garnet omphacite and phengite inclusions zircon also grew during later retrogression zircon cores from the huwan shear zone have ordovician to devonian 440 350 ma ages flat to steep heavy ree patterns negative eu anomalies and in some cases plagioclase inclusions indicative of derivation from north china block igneous and low pressure metamorphic source rocks cores from hong an group zircon are neoproterozoic 780 610 ma consistent with derivation from the south china block in the western dabie mountains the first stage of the collision between the north and south china blocks took place in the carboniferous along a suture north of the huwan shear zone the major triassic continent continent collision occurred along a suture at the southern boundary of the shear zone the first collision produced local eclogite facies metamorphism in the huwan shear zone the second produced widespread eclogite facies metamorphism throughout the dabie mountains sulu terrane and a lower grade overprint in the shear zone

empirical calibration of the clinopyroxene garnet magnesium isotope geothermometer and implications the large equilibrium mg isotope fractionation between clinopyroxene and garnet observed in eclogites makes it a potential high precision geothermometer but calibration of this thermometer by natural samples is still limited here we report mg isotopic compositions of eclogite whole rocks as well as mg and o isotopic compositions of clinopyroxene and garnet separates from 16 eclogites that formed at different temperatures from the dabie orogen china the whole rock 26mg values vary from 1 20 to 0 10 among them 11 samples display limited 26mg variations from 0 36 to 0 17 similar to those of their protoliths the mineral separates exhibit very different 26mg values from 0 39 to 0 39 for clinopyroxenes and from 1 94 to 0 81 for garnets the clinopyroxene garnet mg isotope fractionation 26mgclinopyroxene garnet 26mgclinopyroxene 26mggarnet varies from 1 05 to 2 15 the clinopyroxene garnet o isotope fractionation 18oclinopyroxene garnet 18oclinopyroxene 18ogarnet varies from 1 01 to 0 98 equilibrium mg isotope fractionation between clinopyroxene and garnet in the investigated samples is selected based on both the 26mgclinopyroxene versus 26mggarnet plot and the state of o isotope equilibrium between clinopyroxene and garnet the equilibrium 26mgclinopyroxene garnet and corresponding temperature data obtained in this study together with those available so far in literatures for natural eclogites are used to calibrate the clinopyroxene garnet mg isotope thermometer this yields a function of 26mgclinopyroxene garnet 0 99 0 06 106 t2 where t is temperature in kelvin the refined function not only provides the best empirically calibrated clinopyroxene garnet mg isotope thermometer for precise constraints of temperatures of clinopyroxene and garnet bearing rocks but also has potential applications in high temperature mg isotope geochemistry

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

refining the timing of eclogite metamorphism a geochemical petrological sm nd and u pb case study from the pohorje mountains slovenia eastern alps high pressure metamorphism in the pohorje mountains of slovenia austroalpine unit eastern alps affected n morb type metabasic and metapelitic lithologies thermodynamic calculations and equilibrium phase diagrams of kyanite phengite bearing eclogites reveal pt conditions of 2 1 gpa at t 750 c but within the stability field of quartz metapelitic eclogite country rocks contain the assemblage garnet phengite kyanite quartz for which calculated peak pressure conditions are in good agreement with results obtained from eclogite samples the eclogites contain a single population of spherical zircon with a low th u ratio combined constraints on the age of metamorphism come from u pb zircon as well as garnet whole rock and mineral mineral sm nd analyses from eclogites a coherent cluster of single zircon analyses yields a 206pb 238u age of 90 7 1 0 ma that is in good agreement with results from sm nd garnet whole rock regression of 90 7 3 9 and 90 1 2 0 ma end 8 for two eclogite samples the agreement between u pb and sm nd age data strongly suggests an age of approximately 90 ma for the pressure peak of the eclogites in the pohorje mountains the presence of garnet omphacite and quartz inclusions in unfractured zircon indicates high pressure rather than ultrahigh pressure conditions the analysed metapelite sample yields a sm nd garnet whole rock scatterchron age of 97 15 ma these data probably support a single p t loop for mafic and pelitic lithologies of the pohorje area and a late cretaceous high pressure event that affected the entire easternmost austroalpine basement including the koralpe and saualpe eclogite type locality in the course of the complex collision of the apulian microplate and europe

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event

late jurassic petroleum geology of the dutch central north sea graben the callovian to ryazanian late middle jurassic to earliest cretaceous here referred to as late jurassic sediments of the dutch central north sea graben comprising the central graben and scruff groups form several potential reservoirs source rocks are present within both these groups and the carboniferous and late triassic early jurassic

timing of uhp metamorphism in the hong an area western dabie mountains china evidence from zircon u pb age trace element and hf isotope composition the hong an area western dabie mountains is the westernmost terrane in the qinling dabie sulu orogen that preserves uhp eclogites the ages of the uhp metamorphism have not been well constrained and thus hinder our understanding of the tectonic evolution of this area la icpms u pb age trace element and hf isotope compositions of zircons of a granitic gneiss and an eclogite from the xinxian uhp unit in the hong an area were analyzed to constrain the age of the uhp metamorphism most zircons are unzoned or show sector zoning they have low trace element concentrations without significant negative eu anomalies these metamorphic zircons can be further subdivided into two groups according to their u pb ages and trace element and lu hf isotope compositions one group with an average age of 239 2 ma show relatively high and variable hree contents 527 lun 14 and 176lu 177hf ratios 0 00008 0 000931 indicating their growth prior to a great deal of garnet growth in the late stage of continental subduction the other group yields an average age of 227 2 ma and shows consistent low hree contents and 176lu 177hf ratios suggesting their growth with concurrent garnet crystallization and or recrystallization these two groups of age are taken as recording the time of prograde hp to uhp and retrograde uhp hp stages respectively a few cores have high th u ratios high trace element contents and a clear negative eu anomaly these features support a magmatic origin of these zircon cores the upper intercept ages of 771 86 and 752 70 ma for the granitic gneiss and eclogite respectively indicate that their protoliths probably formed as a bimodal suite in rifting zones in the northern margin of the yangtze block young hf model ages tdm1 of magmatic cores indicate juvenile mantle derived materials were involved in their protolith formation