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

the origin and evolution of low δ18o magma recorded by multi growth zircons in granite we for the first time reported low 18o granites within craton in bengbu uplift southeast margin of north china craton integrated ion microprobe study of 18o and u pb age on single zircon grains and zircon populations gives direct evidence for the origin and later magmatic evolution of the low 18o granites three types of zircon domains are recognized inherited magmatic core type i inherited metamorphic core mantle type ii and overgrown magmatic rim type iii type i zircons were formed in neoproterozoic and have the average 18o values of 5 3 0 6 around mantle value triassic type ii zircons have extremely low and highly heterogeneous 18o values ranging from 9 4 to 8 6 consistent with the values and variations exhibited by metamorphic zircons from dabie sulu orogen this feature argues for a triassic metamorphic origin for the 18o depletion signature of zircons from ultrahigh pressure metamorphic rocks in dabie sulu orogen jurassic type iii zircons have isotopically homogeneous 18o values of 3 3 0 5 collectively zircon populations recorded the 18o variations from source to low 18o granite the primary protolith of jurassic granites in bengbu uplift is a neoproterozoic granitic intrusion in south china block later high temperature meteoric hydrothermal alteration has lowered the whole rock 18o values in various degrees the low 18o protolith was then buried to the middle lower crust by triassic continental subduction and jurassic anatexis produced the low 18o granitic magma moreover 18o values of type iii zircons are isotopically lighter than previously reported values of magmatic garnets in jurassic granites indicating later magmatic evolution of low 18o magma toward high 18o values probably as a result of infiltration of high 18o fluid from wall rocks

high pressure eclogites in northern jiangsu southern shandong province eastern china abstract eclogites are distributed for more than 500 km along a major tectonic boundary between the sino korean and yangtze cratons in central and eastern china these eclogites usually have high p assemblages including omphacite kyanite and or coesite or its pseudomorph and form a high p eclogite terrane they occur as isolated lenses or blocks 10 cm to 300 m long in gneisses type i serpentinized garnet peridotites type ii and marbles type iii type i eclogites were formed by prograde metamorphism and their primary metamorphic mineral assemblage consists mainly of garnet pyrope prp 15 40 mol omphacite jadeite jd 34 64 mol pargasitic amphibole kyanite phengitic muscovite zoisite an sio2 phase apatite rutile and zircon type ii eclogites characteristically contain no sio2 phase and are divided into prograde eclogites and mantle derived eclogites the prograde eclogites of type ii are petrographically similar to type i eclogites the mantle derived eclogites have high mgo feo fe2o3 and cr2o3 compositions in bulk rock and minerals and consist mainly of pyrope rich garnet prp 48 60 mol sodic augite jd 10 27 mol and rutile type iii eclogites have an unusual mineral assemblage of grossular rich grs 57 mol garnet omphacite jd 30 34 mol pargasite rutile pargasitic and taramitic amphiboles calcic plagioclase an68 epidote zoisite k feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites this suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and or amphibolite facies rocks with near isothermal compression reflecting crustal thickening during continent continent collision of late proterozoic age equilibrium conditions of the prograde eclogites range from p 26 kbar and t 500 750 c in the western part to p 28 kbar and t 810 880 c in the eastern part of the high p eclogite terrane the prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part subsequent reactions manifested by 1 narrow rims of sodic plagioclase or paragonite on kyanite and 2 symplectites between omphacite and quartz are interpreted as an effect of near isothermal decompression during the retrograde stage the conditions at which symplectites re equilibrated tend to increase from west p 9 kbar and t 680 c equilibrium temperatures of type ii mantle derived eclogites and type iii eclogite are 730 750 c and 680 c respectively

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

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

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

defect microstructures of minerals as a potential indicator of extremely rapid and episodic exhumation of ultrahigh pressure metamorphic rock implication to continental collision orogens exhumation of subducted continental crust to the earth s surface was presumably rapid though at an uncertain rate to retain coesite diamond and pbo2 type tio2 here we report unique defect microstructures of minerals as a potential indicator of a rapid and episodic exhumation process in a fossil fracture zone of coesite eclogite from the sulu terrain eastern china having the most negative 18o value ever reported for eclogite facies metamorphic rock analytical electron microscopy indicates that semi brittle deformation occurred in kyanite omphacite spinel with extensive and unusual fine scale twin lamellae and that brittle deformation occurred in garnet with hardly healed 110 microcleavages these unique defect microstructures can be rationalized by a high strain rate at local weakening and deep faulting of continental collision orogens

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

stable isotope geochemistry of high to ultrahigh pressure metamorphic rocks in the dabie mountains the identification of coesite and micro diamond in the metamorphic rocks of the dabie and s u u terranes implies that the crustal materials canbe subducted to mantle depths of a t least 100 km okay et al 1989 wang et al 1989 xu et al 1992 stable isotope analyses of high to ultrahigh pressure uhp metamorphic rocks can be used to determine whether isotopic compositions are largely inherited from pre metamorphic protoliths or alternatively if compositions have been modified by the infiltration of external fluids previous investigations show that minerals in eclogites from qinglongshan in the western s u u have extreme negative 6i8o values of 10 to 8 whereas eclogites from shuanghe in the eastern dabie have 6180 values of 3 to 7 0 yui et al 1995 zheng et al 1996 1998 preservation of oxygen isotope equilibrium between the eclogite minerals suggests that the eclogite protoliths underwent hydrothermal alteration by fluids of meteoric origin prior to subduction limited crust mantle interaction has been inferred between the o depleted eclogites and the mantle when the slab containing the eclogite protoliths was subducted to mantle depths implying a short residence time of the eclogite containing slab in the mantle zheng et al 1998 concordant oxygen isotope temperatures are obtained from different mineral pairs suggesting great cooling rates of the eclogite slab at the initial stage of exhumation in this study the carbon oxygen and hydrogen isotope compositions of mineral separates have been determined for marble eclogites and two types of gneiss biotite gneiss and granitic gneiss from the dabie mountains the marble eclogites and biotite gneiss from the eastern dabie contain coesite and thus belong to the uhp slab whereas there is no mineralogical evidence for the marble and eclogite from the western dabie and the granitic gneiss from the eastern dabie for having experienced the uhp metamorphism the results place constraints on the sedimentary environment of marble protolith the scale of fluid rock interaction during plate subduction and tectonic relationship between uhp and non uhp units

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

low δ18o zircons u pb dating and the age of the qinglongshan oxygen and hydrogen isotope anomaly near donghai in jiangsu province china zircons from metamorphosed granites exposed near qinglongshan have 18ovsmow values of 7 to 0 in both grain rims and cores the concordant 238u 206pb ages of zircon cores are 684 to 754 ma with rims at 221 ma discordant 238u 206pb ages range from 242 to 632 ma results demonstrate a neoproterozoic age for the origin of the qinglongshan oxygen and hydrogen isotope anomaly the low 18o values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate groundwater circulation was driven by heat from cooling granitic magma the geologic age of the hydrothermal system correlates with that of the nantuo tillite in the sinian strata of the south china block suggesting that qinglongshan s cold climate may be a manifestation of neoproterozoic snowball earth

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

stable isotope characteristics of eclogites from the ultra high pressure metamorphic terrain east central china abstract a stable isotope study was made for the ultra high pressure uhp metamorphic rocks in the dabie and sulu terrains east central china 18 o values of minerals in eclogites cover a wide range from 4 to 6 the o isotope fractionations between minerals i e omphacite phengite and or amphibole and garnet in eclogites are mostly not in isotopic equilibrium the large range of o isotope composition of garnet may approximate the 18 o variations of eclogites during uhp metamorphism and must have been largely inherited from their pre subduction basaltic precursors which might have interacted with meteoric waters under variable temperature conditions and water rock ratios before uhp metamorphism the o isotope disequilibrium between minerals and garnet in eclogites was caused by retrograde hydration reactions the fluids required for these hydration reactions must have low or even negative o isotope compositions it is postulated that such fluids were either originated from meteoric deep infiltration or were derived from low 18 o para gneisses

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

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

numerical experiments on the seasonal variations of the oyashio near the east coast of japan abstract seasonal variations of the oyashio near the east coast of japan are examined using a simple layered model with a localized diapycnal transport due to the strong tidal mixing around the kuril islands the modeled southernmost latitude is located at around 40 n in spring and 42 n in autumn the southwestward oyashio transport in the upper layers and depth integrated transport become large small during winter spring summer autumn these results are in agreement with observations the seasonal variation of the southernmost latitude is mainly caused by the baroclinic velocity anomalies off tohoku and hokkaido japan which have been excited by the wind stress changes along the west coast of north america the kamchatka peninsula and the model northern boundary this is different from the previous studies that emphasized the importance of direct barotropic response of the ocean to the basin scale wind stress changes for comparison an additional experiment in which the diapycnal transport around

evolution of the blueschist and greenschist facies rocks of sifnos cyclades greece the metamorphism on the island of sifnos is characterized by the eocene development of a coherent highpressure blueschist terrane and an early miocene greenschist facies overprint this study documents the metamorphic evolution of the blueschist assemblages still preserved in the northern parts of the island and their subsequent transformation into greenschists in the central and southern parts the oxygen isotope geothermometry is based on calibrations for quartz pyroxenes and magnetite matthews et al 1983a augmented by revised calibrations for the minerals muscovite qz mu 1 55 106 t 2 epidote qz ep 1 56 1 92 ps 106 t 2 and rutile qz ru 4 54 106 t 2 oxygen isotope analyses of minerals from the blueschist unit of northern sifnos give consistent fractionations which are independent of rock type an average temperature of 455 c was obtained although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred the minerals and whole rocks of the greenschists of central sifnos are systematically enriched in 18o relative to the blueschist assemblages chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of ca2 and co2 the petrologic isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18o co2 enriched aqueous solutions it is reasonable to assume that this is connected with the miocene magmatic activity observed throughout the cyclades the marbles separating the blueschist from the greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation the pressure of the blueschist metamorphism is estimated at 14 2 kbar corresponding to a depth of ca 50 km the structural style and stratigraphy of sifnos are suggestive of the subduction of a continental margin sequence it is clear that the considerable tectonic depression may be associated with continental collision and underthrusting

oxygen isotope geothermometers for metamorphic rocks the chicago mineral carbonate oxygen isotope fractionation curves have been combined with mineral water fractionation data for jadeite zoisite and rutile and new data for grossular water to provide a set of self consistent mineral pair calibrations the a coefficients in the equation 1000 in a 106t 2 of the new mineral pair fractionations are jadeite zoisite grossular rutile quartz 1 69 2 00 3 03 5 02 jadeite 0 31 1 34 3 33 zoisite 1 03 3 02 grossular 1 99 the isotopic fractionation properties of natural pyralspite garnet ca fe mg mn 3al2si3o12 can be approximated by those of the grossular end member appropriate substitutions also yield coefficients for the solid solution minerals sodic pyroxene and epidote e g aquartz sodic pyroxene 2 75 1 06xjd aquartz epidote 2 00 0 75xps where xjd and xps are the mole fractions of the jadeite and pistacite components respectively the new data set is particularly suitable for the geothermometry of metamorphic rocks 18o data from minerals of the high pressure metamorphic rocks of the sesia zone of italy and cyclades complex of greece yield well constrained mean temperatures of 572 and 478d c respectively type iii blueschist metabasalts of the franciscan formation of california give mean quartz garnet temperatures of 354d c

anomalous nitrogen isotopes in ultrahigh pressure metamorphic rocks from the sulu orogenic belt effect of abiotic nitrogen reduction during fluid rock interaction modern nitrogen n fixation is primarily mediated by biological processes however in the early earth where biological activity was absent or limited abiotic n reduction in hydrothermal systems is thought to be a key process to transform atmospheric n2 and nox to ammonium an essential nutrient to support the emergence of life and also an n form that can be incorporated into rocks surprisingly evidence for abiotic n reduction in the rock record has not been clearly identified in this study we reported anomalously low n isotope compositions n15 values as low as 15 8 15 8 of mica samples in ultrahigh pressure metamorphic rocks from the donghai area in the sulu orogenic belt eastern china compared with mica samples with typical crustal n15 values 3 9 3 9 in similar metamorphic rocks from the western dabie orogen the 15n depleted mica samples from the sulu orogen are characterized by significant n enrichment 10 times higher and extreme 18o depletion o18 values as low as 9 9 these features can be best explained by assimilation of n from a source characterized by extremely low n15 values less than 16 16 the extremely low n15 value would be produced by abiotic n reduction during reaction of a meteoric hydrothermal fluid with crustal rocks before subduction this observation provides a clue to the occurrence of abiotic n reduction in continental supracrustal rocks and infer that abiotic n reduction process could be a fundamental process driving the geological n cycling in early earth

regional ultrahigh pressure coesite bearing eclogitic terrane in central china evidence from country rocks gneiss marble and metapelite the dabie mountains represent a fault bounded coesite bearing eclogite gneissic terrane within the collisional belt between the sino korean and yangtze cratons the following geologic evidence indicates that this terrane has been subjected to regional ultrahigh pressure metamorphism 1 the country rocks have a metamorphic pressure temperature evolution compatible with that of the eclogites on the basis of the occurrence of quartz pseudomorphs after coesite calcite pseudomorphs after aragonite relict high pressure assemblages in different rock types of the country rocks and consistent comparable compositional zonations of garnets in both country rocks and eclogites 2 peak metamorphic pressure temperature conditions consistently decrease both from east to west in the collisional belt and from north to south in the dabie mountains and clockwise pressure temperature paths are common for eclogites over the region 3 layered eclogites interbedded with gneisses and amphibolites contain omphacite kyanite and garnet indicating a minimum pressure of 20 kbar at 600 c for formation of these rocks

fluid rock interactions during ultra high pressure metamorphism dabie shan china oxygen isotope analyses of garnet separates and whole rock samples of gneiss and marble hosted eclogites from the ultra high pressure uhp dabie shan terrain fall in the range 6 76 to 10 76 comparison of the isotopic compositions of the eclogites with those of their host rocks leads to the following conclusions 1 the basaltic protolith of the eclogites had isotopic compositions in the range 2 15 to 4 99 it is proposed that this is the result of contamination of the parent basaltic magma with hydrothermally altered18o depleted source materials 2 18o enriched eclogite compositions resulted from local scale 3 m interaction between this basaltic eclogite protolith and fluids produced by decarbonation of impure marble lithologies during uhp metamorphism 3 18o depleted eclogite and gneiss compositions resulted from regional scale meteoric hydrothermal alteration of the basalts and their host rocks prior to uhp metamorphism epidote separates and whole rocks from the cores and rims of mineralogically zoned eclogite boudins within amphibolite grade biotite gneisses show no distinct isotopic differences this suggests that the preferential recrystallisation of the gneisses and the associated amphibolite grade overprint of the rims of eclogite bodies could not have resulted from regional scale infiltration of significant volumes of fluid during the exhumation of the dabie shan terrain

zircon u pb dating of water rock interaction during neoproterozoic rift magmatism in south china abstract a combined study of zircon u pb dating and mineral o isotope analysis was carried out for neoproterozoic granite gabbro and gabbrodiorite from the beihuaiyang zone that marks a rifted margin between the south china block and the supercontinent rodinia the results provide precise constraints on timing of water rock interaction resulting in unusual 18 o depletion in igneous minerals cl imaging reveals that zircons mostly show clear oscillatory and or sector zoning with or without thin alteration rims no metamorphic zircon occurs in the igneous intrusives although they suffered greenschist facies metamorphism during triassic continental collision la icpms zircon u pb dating yields consistent ages of 739 8 to 754 5 ma which are interpreted as timing of magma crystallization mineral separates have highly variable 18 o values of 0 6 to 6 8 for zircon 1 2 to 12 5 for quartz and 0 3 to 8 4 for plagioclase oxygen isotope heterogeneities exist within single intrusions and between different intrusions magmatic zircons with low 18 o values and a weighted mean u pb age of 748 6 ma crystallized from low 18 o magmas that were derived from melting of hydrothermally altered low 18 o rocks both low and normal 18 o intrusives in the beihuaiyang zone are shown to form at the same time contemporaneous with the rodinia breakup thus tectonic evolution from supercontinental rift to breakup is proposed to create conditions for alternating processes of high t hydrothermal alteration and low 18 o magmatism such processes are a characteristic feature of high t water rock interaction in volcanic rift margins associated with localized water saturated melting while the occurrence of low 18 o magmas demonstrates penetration of meteoric water into the rift tectonic zone of crustal melting the existence of normal 18 o magmas indicates that their emplacement may serve as a thermal engine to drive the second stage of meteoric water rock interaction resulting in the unusual 18 o depletion in adjacent rocks the concurrent occurrence of both low and normal 18 o magmas appears to indicate highly heterogeneous high t water rock interaction during rift magmatism

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

low 18o terranes tracking mesozoic polar climates in the south pacific abstract substantially negative 18 o values of altered rocks are an unfailing guide to 18 o depleted meteoric water of low mean annual temperature and therefore of cold climates at times of hydrothermal alteration however because water rock interaction is often incomplete even where it has occurred the lack of depleted values is poor evidence for lack of cold climate a now established average surface rock composition of 6 6 c 18 o with a lower limit of 16 for an igneous and metamorphic terrane of some 6 000 km 2 in west antarctica indicates cretaceous meteoric water of less or much less than 20 c this new anomaly and similar anomalies in new zealand date from before rifting along the antarctic pacific rise and are tracking terranes originating from the mesozoic south polar archipelago in this area subaerial or subglacial hydrothermal isotope exchange has been particularly effective and or meteoric waters were isotopically unusually depleted the discovery mapping and dating of further isotopically depleted zones in the geologic record will improve constraints for paleozoic and mesozoic greenhouse climates

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

reply to the comments of s karato on petrofabrics and seismic properties of garnet peridotites from the uhp sulu terrane china by xu et al tectonophysics 421 2006 111 127 abstract we thank karato for his interest in our recent paper in tectonophysics xu z q wang q ji s c chen j zeng l s yang j s chen f y liang f h wenk h r 2006 petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab tectonophysics 421 111 127 however karato is incorrect to take water or water related species herein referred to generically as water as an almost unique cause for any olivine fabric transition in nature no matter what geological conditions are in this reply we will first give a succinct introduction to the context of the ongoing debate and then provide some additional arguments in favour of our conclusion that the olivine c type fabric of the zhimafang garnet bearing peridotites was formed under the conditions of high pressure low temperature and low water content during the subduction of cold and dry supracrustal rocks of the yangtze plate the map of olivine slip systems as a function of differential stress and water content drawn by karato and coworkers cannot be reliably extrapolated to earth s interior until relevant scaling laws have been established the role of high water content on the fabric transitions is more likely to modify recrystallization kinetics than to switch the dominant slip systems

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

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

clay illuviation in calcareous soils of the semiarid part of the indo gangetic plains india abstract in view of diverse understanding on the movement and accumulation of clay particles in calcareous parent material a micromorphological study on 28 alfisols of the semiarid part of the indo gangetic plains igp was undertaken the study indicates that the identified clay pedofeatures are typically of the type impure clay pedofeatures which have resulted from the impairment of the parallel orientation of the clay platelets induced by dispersion of both clay and silt size layer silicates in slightly to highly sodic environment the study also indicates that the illuviation of clay particles and their subsequent accumulation in the bt horizons have occurred in sodic environment caused by the precipitation of soluble ca 2 ions as calcium carbonate caco 3 thus discounting any role of soluble ca 2 ions and the presence of caco 3 in preventing the movement and accumulation of clay particles the study thus suggests that the formation of impure clay pedofeatures and pedogenic caco 3 are two pedogenic processes occurring simultaneously in soils of the igp as contemporary pedogenic events in the semiarid climate since the last 4000 years b p

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

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

polyphase growth of accessory minerals during continental collision geochemical evidence from ultrahigh pressure metamorphic gneisses in the sulu orogen abstract an integrated study of petrology geochronology and geochemistry was performed for ultrahigh pressure metamorphic gneisses in the sulu orogen the results are used to elucidate the polyphase growth of such metamorphic minerals as zircon titanite and garnet in response to pressure temperature changes and fluid melt action during continental collision this provides insights into the property of metamorphic fluid melt and their effects on trace element mobility a combined result from ree patterns mineral inclusions and ti in zircon temperatures indicates three stages of zircon growth prograde growth occurred at 237 ma primarily at eclogite facies retrograde growth at 222 ma mostly at eclogite facies and the last growth at 205 ma at granulite facies the three stages of zircon growth are deciphered by distinct ree patterns and trace element compositions reflecting the differences in the property of metamorphic fluid melt the episodic growth of metamorphic zircon is primarily dictated by the episodic concentration of zr and si in metamorphic fluid melt relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive ree patterns and trace element compositions the metamorphic titanite exhibit variably elevated nb contents and nb ta ratios suggesting nb ta fractionation due to breakdown of amphibole and or biotite during metamorphism polyphase growth of garnet is suggested by an integrated analysis of mineral inclusions and major and trace elements in large garnet grains trace element abundances vary in different zones of garnet which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet forming reactions at different p t conditions therefore the metamorphic growth of zircon titanite and garnet would have occurred not only during prograde subduction but also during retrograde exhumation in the continental collision zones both metamorphic dehydration and partial melting would have taken place episodically during the collisional orogeny the breakdown of hydrous minerals at high to ultrahigh pressure conditions is a key to fluid liberation and element supply for the growth of these accessory minerals

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

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

the paleozoic metamorphic history of the central orogenic belt of china from 40ar 39ar geochronology of eclogite garnet fluid inclusions abstract the pressure temperature time evolution of the uhp eclogites of dabie sulu in the eastern sector of the central orogenic belt of china shows a complex pattern of predominantly triassic and to a lesser extent early paleozoic ages 40 ar 39 ar stepwise crushing analysis of eclogitic garnets yielded consistently paleozoic ages because our initial results from the bixiling eclogite were of limited areal extent their significance required further testing data for garnet bearing gneiss and eclogites from new locations in northern and southern dabieshan shuanghe ganghe and hongmiao confirm the paleozoic garnet results obtained from bixiling eclogite we also tested the technique on a garnet sample of known paleozoic age from altyn tagh in the western central orogenic belt these new results indicate that light colored garnets from dabieshan and altyn tagh yield concordant 40 ar 39 ar ages ranging from 445 465 ma by in vacuo crushing and confirm that a paleozoic u hp metamorphic event took place in the central orogenic belt from altyn tagh in western china to dabieshan in eastern china in contrast dark colored garnets from mafic metavolcanics do not yield 40 ar 39 ar plateaux or isochrons ages

eclogite facies fluid infiltration constraints from δ 18 o zoning in garnet in situ analysis reveals that eclogite facies garnets are zoned in 18o with lower values in the core and rims that are 1 5 to 2 5 higher this pattern is present in 9 out of 12 garnets analyzed by sims from four orogenic eclogite terranes and correlates with an increase in the mole fraction of pyrope and mg fe ratio from core to rim indicating prograde garnet growth at the maximum temperatures and the time scales experienced by these garnets calculated intragranular diffusion distances for oxygen are small 5 from two kimberlites are homogeneous in 18o possibly due to diffusive exchange which is possible for prolonged periods at higher mantle temperatures homogeneity of 18o in garnets outside the normal mantle range 5 6 may be common in kimberlitic samples

oxygen isotope thermometry based on a refined increment method and its application to granulite grade rocks from sri lanka abstract in this paper we first present a revision of the increment method a semi empirical technique which allows the calculation of o isotope fractionation factors between all silicates provided their structure and chemical composition is known for the temperature dependency the experimentally calibrated high pressure mineral calcite fractionation factors of chiba et al 1989 are taken as a basis then this method is applied to granulite facies parageneses from sri lanka including quartz k feldspar sillimanite garnet pyroxenes amphiboles cordierite magnetite and ilmenite it can be shown that o isotope equilibration occurred at granulite facies conditions at about 830 c due to static recrystallization locally retrograde re equilibration occurred also at much lower temperatures due to late d4 in most cases however o isotope disequilibria resulted from retrograde oxygen intracrystalline diffusion it can be proved by model calculations that the o diffusion data for water saturated systems reported in the literature are too high by two orders of magnitude to explain the measured isotopic compositions the fractionation patterns allow the discrimination between open and closed system behaviour the regional distribution of open and closed system patterns corresponds to the frequency of occurrence of late stage migmatization western part and to differences in amount and composition of fluid inclusions

kinetics of the coesite to quartz transformation abstract the survival of coesite in ultrahigh pressure uhp rocks has important implications for the exhumation of subducted crustal rocks we have conducted experiments to study the mechanism and rate of the coesite quartz transformation using polycrystalline coesite aggregates fabricated by devitrifying silica glass cylinders containing 2850 h 10 6 si at 1000 c and 3 6 gpa for 24 h conditions were adjusted following synthesis to transform the samples at 700 1000 c at pressures 190 410 mpa below the quartz coesite equilibrium boundary reaction proceeds via grain boundary nucleation and interface controlled growth with characteristic reaction textures remarkably similar to those seen in natural uhp rocks we infer that the experimental reaction mechanism is identical to that in nature a prerequisite for reliable extrapolation of the rate data growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation time data fitting the rates to turnbull s equation for growth therefore yields two distinct sets of parameters with similar activation energies 242 or 269 kj mol but significantly different pre exponential constants extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375 400 c the survival of coesite has previously been linked to its inclusion in strong phases such as garnet that can sustain a high internal pressure during decompression other factors that may play a crucial role in preservation are low fluid availability possibly even less than that of our nominally dry experiments and the development of transformation stress which inhibits nucleation and growth these issues are discussed in the context of our experiments as well as recent observations from natural rocks

geochemistry of garnetiferous ti clinohumite rock and talc kyanite phengite almandine schist from the kokchetav uhp terrane kazakhstan an insight to possible origins of some chemically unusual uhp rocks abstract major element trace element and o isotope analyses were carried out for garnetiferous ti clinohumite rocks and eclogites from the kumdy kol area as well as talc kyanite phengite almandine schists previously named as whiteschists and eclogites from the kulet area of the kokchetav ultrahigh pressure uhp terrane to explore their possible origins both garnetiferous ti clinohumite rocks and talc kyanite phengite almandine schists are possibly genetically related to eclogites as they occur in close association with eclogites enclosed within gneiss metapelite in these two areas most eclogites from the kumdy kol and the kulet areas exhibit chemical characteristics similar to that of ocean floor basalt although a few show arc affinity with slight ta and nb depletions garnetiferous ti clinohumite rocks from the kumdy kol region are characterized by very high mg low si ca na and k contents compared with eclogites they also exhibit lree depleted patterns with negative eu anomalies their o isotope compositions are less than 5 6 on the other hand the rocks show comparable al ni cr co and hree contents as eclogites the mg enrichment of these garnetiferous ti clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock ultramafic rock fluid interactions talc kyanite phengite almandine schists from the kulet area are typically low in ca and na but high in mg k rb ba cs th u and o isotope compositions 6 to 10 relative to eclogites this peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high temperature enrichment of mg and low temperature enrichment of k rb ba cs th u and o isotope compositions seawater basalt interactions one talc kyanite phengite almandine schist sample klw 7 is distinctly high in si and k but low in mg and fe which can be attributed to more severe low temperature hydrothermal alterations however except for mg the enrichment of rb ba cs th u and o isotope compositions in these schist samples could also be enhanced by or otherwise be interpreted as a result of fluid influx sourced from enclosing metapelite during subduction

oxygen isotopes in coexisting garnets clinopyroxenes and phlogopites of roberts victor eclogites implications for petrogenesis and mantle metasomatism abstract 18 o values of coexisting garnet clinopyroxene and phlogopite for twelve compositionally and texturally diverse roberts victor eclogite xenoliths range from 3 8 to 7 1 4 0 to 7 4 and 5 9 to 7 4 respectively differences between the 18 o values of coexisting garnets and clinopyroxenes are normally zero however there is some variation in the 18 o values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite amphibole and celsian the 18 o value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet the large range in 18 o values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the roberts victor eclogites represent metamorphosed altered basalts the eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite and the source of this fluid was probably unrelated to the eclogite

oxygen isotopes in indian plate eclogites kaghan valley pakistan negative δ18o values from a high latitude protolith reset by himalayan metamorphism abstract oxygen isotope compositions are reported for the first time for the himalayan metabasites of the kaghan valley pakistan in this study the highest metamorphic grades are recorded in the north of the valley near the india asia collision boundary in the form of high pressure hp group i and ultrahigh pressure uhp group ii eclogites the rocks show a step wise decrease in grade from the uhp to hp eclogites and amphibolites the protoliths of these metabasites were the permian panjal trap basalts ca 267 2 4 ma which were emplaced along the northern margin of india when it was part of gondwana after the break up of gondwana india drifted northward subducted beneath asia and underwent uhp metamorphism during the eocene ca 45 1 2 ma at the regional scale amphibolites group i and ii eclogites yielded 18o values of 5 84 and 5 91 1 66 to 4 24 and 2 25 to 0 76 respectively relative to vsmow on a more local scale within a single eclogite body the 18o values were the lowest 2 25 to 1 44 in the central the best preserved least retrograded parts and show a systematic increase outward into more retrograded rocks reaching up to 0 12 these values are significantly lower than the typical mantle values for basalts of 5 7 0 3 the unusually low or negative 18o values in group ii eclogites potentially resulted from hydrothermal alteration of the protoliths by interactions with meteoric water when the indian plate was at southern high latitudes 60 s the stepwise increase in 18o values among different eclogite bodies in general and at single outcrop scales in particular reflects differing degrees of resetting of the oxygen isotope compositions during exhumation related retrogression

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

crustal recycling processes in generating the early cretaceous fangcheng basalts north china craton new constraints from mineral chemistry oxygen isotopes of olivine and whole rock geochemistry the early cretaceous fangcheng basalts were erupted in the peak stage of lithospheric thinning of the north china craton ncc they have olivine phenocrysts with forsterite fo contents ranging from 74 to 92 clinopyroxene with high mg from 76 to 89 and contain pyroxenite xenoliths both the basalts and pyroxenite xenoliths have coexisting high ca and low al clinopyroxene and high mg and low ca olivine indicating that the parental magmas of the basalts have high water contents comparable with arc basalts they also show similar mineral chemistry and olivine o isotopic compositions 18o 5 5 7 2 trace element patterns with large ion lithophile elements liles and light rare earth elements lrees enrichments and high field strength elements hfses depletions as well as enriched sr nd hf isotopic features therefore the pyroxenite xenoliths are cognate with the basalts high 18o values average 6 5 of fresh olivine crystals from the basalts and xenoliths indicate the involvement of 18o rich crustal materials in the mantle source incorporation of 18o rich crustal materials to the mantle source is consistent with the highly evolved sr nd hf isotopic signatures of the basalts and the pyroxenite xenoliths modeling results based on the sr nd o isotopic compositions of the basalts indicate that both upper and lower crustal materials of the yangtze block may have been incorporated into the mantle source therefore the 18o rich crustal materials were likely derived from the subducted yangtze block in triassic time however neither the ncc subcontinental lithospheric mantle nor the deeply subducted yangtze crustal materials can provide sufficient water to produce hydrous basaltic magmas instead exotic fluids may have been derived from the subducted paleo pacific slab dehydration of the subducted paleo pacific slab may have released enough fluids to trigger extensive hydrous melting of the enriched mantle source beneath the fangcheng and its surrounding regions

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

synexhumation anatexis of ultrahigh pressure metamorphic rocks petrological evidence from granitic gneiss in the sulu orogen abstract petrological evidence is provided for partial melting of ultrahigh pressure uhp metamorphic granitic gneiss in the sulu orogen petrographic observations show the occurrence of elongated highly cuspate feldspars in grain boundaries interstitial cuspate feldspars in triple junctions felsic veinlets mainly consisting of k feldspar quartz and feldspar crystal faces against quartz these features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss zircon domains grown from these melts were identified based on cl images mineral inclusions and ree patterns some large zircon grains 100 m contain small relict domains of magmatic origin suggesting nearly complete dissolution of the protolith zircon during the anatexis all newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions one group of domains contain no coesite inclusion and exhibit high u contents but low th u ratios pb ages of 217 2 to 224 2 ma the other group of domains contain coesite inclusions and exhibit low u contents and very low th u ratios pb ages of 221 5 to 226 3 ma the two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative eu anomalies that are absent for metamorphic zircons grown from aqueous fluids the zircon u pb ages of 217 2 to 226 3 ma are close to but slightly younger than known ages for major uhp metamorphism in the sulu orogen therefore the uhp gneiss experienced incipient melting during the initial exhumation subsequent to the peak uhp metamorphism and extensive anatexis later at lower pressures muscovite relicts coexist with cuspate feldspars suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the uhp granitic gneiss as the degree of partial melting increases with temperature significant fractions of the anatectic melts would be produced in the regional gneiss provided that the uhp rocks experienced hot exhumation such melts can be gathered together being eventually emplaced as synexhumation granitic intrusions

petrogenesis of uhp metamorphic rocks from qinglongshan southern sulu east central china abstract five kinds of uhp metamorphic rocks including eclogite orthogneiss paragneiss schist and quartzite are exposed in the qinglongshan roadcut southern sulu orogenic belt of eastern central china they comprise metamorphic supracrustal rocks with bimodal volcanic characteristics and continental affinity and granitic intrusive associations the preservation of coesite inclusions and or its pseudomorphs in eclogite and other rocks indicate that they have been subjected to in situ uhp metamorphism four stages of metamorphism were recognized by combining petrographic observations and compositions of minerals from various uhp rocks prograde epidote amphibolite facies uhp coesite eclogite facies post uhp quartz eclogite facies and retrograde amphibolite facies assemblages delineate an inferred p t path with a clockwise trajectory and a retrograde event characterized by the coupling of decompression with a temperature decrease garnet porphyroblasts in uhp eclogites display a complex growth zoning and mineral distribution and record a crucial segment of the prograde and retrograde metamorphic evolution the preservation of growth zoning in eclogitic and gneissic garnets suggests that the uhp rocks had a short residence time before retrograde metamorphism and a very high uplift rate in order to preserve the prograde growth zoning

low δ 18 o mantle derived magma in panjal traps overprinted by hydrothermal alteration and himalayan uhp metamorphism revealed by sims zircon analysis abstract we report two generations of low 18o zircons from the himalayan eclogites and their host gneisses in situ sims 18o analyses on single zircon crystals with known age and hf isotope ratios from two populations of chemically distinct zircons demonstrate a complex history 1 an early low 18o mantle derived magma 2 followed by post emplacement high temperature meteoric water alteration and finally 3 crystallization of new low 18o minerals during the ultrahigh pressure metamorphism magmatic zircon 269 ma in group i eclogites yielded 18o values from 1 9 to 4 6 vsmow with an average value of 4 0 0 2 n 35 the error is 2sd analytical precision and n represents number of analyzed spots which is lower than the typical mantle values 5 3 0 6 2sd in contrast metamorphic zircons 45 ma in group ii eclogites preserve unusually low negative 18o values from 3 9 to 2 7 average 3 4 0 4 n 35 2sd zircons in felsic gneiss that surround group ii eclogites have inherited magmatic cores ca 260 ma with 18o values of ca 2 9 which decrease to 0 1 in metamorphic ca 45 ma rims these zircons preserve lower 18o values than would be equilibrated with typical mantle the low 18o values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative 18o values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons this study reports evidence for melting of subducted low 18o ocean crust to form low 18o mantle derived mafic magmas as previously proposed by cartwright and valley 1991 geology 19 578 581 for proterozoic scourie dikes

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

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

formation of wollastonite by chemically reactive fluid flow during contact metamorphism mt morrison pendant sierra nevada california usa quartz calcite sandstones experienced the reaction calcite introduction quartz wollastonite co2 during prograde contact metaflow of chemically reactive fluid may control the minmorphism at p 1500 bars and t 560 c rocks were in eralogy stable isotope composition and trace element equilibrium during reaction with a co2 h2o fluid with xco2 chemistry of rocks during contact metamorphism e g 0 14 the transition from calcite bearing wollastonite free to labotka et al 1988 jamtveit et al 1992a 1992b nabelek wollastonite bearing calcite free rocks across the wollastonite isograd labotka 1993 bowman et al 1994 roselle et al is only several millimeters wide the wollastonite forming reaction 1999 a number of aspects of metamorphic fluid flow was driven by infiltration of quartz calcite sandstone by chemically however remain the subject of debate including the reactive h2o rich fluids and the distribution of wollastonite directly direction of flow labotka et al 1988 ferry dipple images the flow paths of reactive fluids during metamorphism the 1992 ferry 1995a hanson 1995a whether numerical mapped distribution of wollastonite and modeling of an o isotope models that assume uniform rock properties adequately profile across a lithologic contact indicate that the principal direction predict the geometry of fluid flow in aureoles that contain of flow was layer parallel directed upward with any cross layer a variety of anisotropic structural features hanson 1992 component of flow 0 1 of the layer parallel component fluid 1995b cook et al 1997 and the degree to which local flow was channeled at a scale of 1 100 m by pre metamorphic mineral fluid equilibrium is attained lasaga rye dikes thrust and strike slip faults fold hinges bedding and 1993 bolton et al 1999 lasaga et al 2000 these and stratigraphic contacts limits on the amount of fluid based on related questions were addressed in an investigation of minimum and maximum estimates for the displacement of the contact metamorphism in the mt morrison pendant wollastonite reaction front from the fluid source are 0 7 1 9 eastern sierra nevada california the location is an 10 cm fluid cm rock the sharpness of the wollastonite isograd exceptional site for the study of metamorphic fluid flow the consistency of mineral thermobarometry the uniform measured for several reasons first the mt morrison sandstone o o fractionations between quartz and calcite and model the focus of this study is ideal for application of concalculations all argue for a close approach to local mineral fluid tinuum models of coupled fluid flow and reaction because equilibrium during the wollastonite forming reaction it is unusually homogeneous in mineralogy and texture