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

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

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

late mesozoic development of the southern qinling dabieshan foreland fold thrust belt central china and its role in continent continent collision abstract the southern qinling dabieshan foreland fold thrust belt is a key region in which to investigate the continental continental collision between north china and south china we provide structural evidence of a diachronous dextral strike slip thrusting deformation from the middle triassic in the east to the late late triassic in the west which indicates a northwestward oblique subduction and suturing of the south china plate under the north china qinling dabieshan plate we also identify a later phase of intracontinental deformation that included an orthogonal intracontinental collision and south vergent thrusting during the early and middle jurassic indentation of south china into the qinling dabieshan orogen and arc shaped extrusions of the southern qinling dabieshan foreland fold thrust belt from the late jurassic to the cretaceous our results reveal that the long term intracontinental collision rotated clockwise in a northwest southeast vergent direction in the late triassic to northeast east southwest west vergent in the late cretaceous this collision may have been driven by a north vergent subduction of the meso tethys ocean in the south west vergent subduction of the izanagi plate in the east and south vergent compression of the eurasia plate in the north

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

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

ultra high pressure metamorphism of carbonate rocks in the dabie mountains central china abstract widespread ultra high p assemblages including coesite quartz pseudomorphs after coesite aragonite and calcite pseudomorphs after aragonite in marble gneiss and phengite schist are present in the dabie mountains eclogite terrane these assemblages indicate that the ultra high p metamorphic event occurred on a regional scale during triassic collision between the sino korean and yangtze cratons marble in the dabie mountains is interlayered with coesite bearing eclogite and gneiss and as blocks of various size within gneiss discontinuous boudins of eclogite occur within marble layers marble contains an ultra high p assemblage of calcite aragonite dolomite clinopyroxene garnet phengite epidote rutile and quartz coesite coesite quartz pseudomorphs after coesite aragonite and calcite pseudomorphs after aragonite occur as fine grained inclusions in garnet and omphacite phengites contain about 3 6 si atoms per formula unit based on 11 oxygens similar to the coesite bearing eclogite marble exhibits retrograde recrystallization under amphibolite greenschist facies conditions generated during uplift of the ultra high p metamorphic terrane retrograde minerals are fine grained and replace coarse grained peak metamorphic phases the most typical replacements are symplectic pargasitic hornblende epidote after garnet diopside plagioclase an18 after omphacite and fibrous phlogopite after phengite ferroan pargasite plagioclase and actinolite formed along grain boundaries between garnet and calcite and calcite and quartz respectively the estimated peak p t conditions for marble are comparable to those for eclogite garnet clinopyroxene geothermometry yields temperatures of 630 760 c the garnet phengite thermometer gives somewhat lower temperatures the minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite such estimates of ultra high p conditions are consistent with the coexistence of grossular rich garnet rutile and the high jadeite content of omphacite in marble the fluid for the peak metamorphism was calculated to have a very low xco2 0 03 the p t conditions for retrograde metamorphism were estimated to be 475 550 c at 7 kbar

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

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

clinoenstatite exsolution in diopsidic augite of dabieshan garnet peridotite from depth of 300 km clinoenstatite exsolution lamellae have been discovered in clinopyroxene grains of garnet peridotite from the bixiling massif of the ultrahigh pressure metamorphic uhpm terrane in the dabie mountains china using transmission electron microscopy we show here that although the lamellae are low clinoenstatite lclen with p 2 1 c space group symmetry they contain nanometer scale antiphase domains indicating that the originally precipitating phase had c 2 c space group symmetry the lamellae are oriented parallel to 010 and 18 to 001 of the host approximately parallel to 401 both the very cold exhumation path of the dabie uhpm terrane and the orientation of the lamellae preclude the possibility that the originally precipitating phase could have been high temperature clinoenstatite htclen we conclude that the lamellae precipitated as high pressure clinoenstatite hpclen and subsequently inverted to lclen analysis of the incomplete high pressure crystallographic data available for compositions approximating those of the lamellae and host suggests a minimum pressure of precipitation of 9 gpa for dabie and about 12 gpa for similar exsolution previously observed in the alpe arami peridotite switzerland the bixiling complex is a crustal cumulate hence this result extends the minimum depth of subduction of continental rocks in the dabie sulu orogen to 300 km p 9 gpa

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

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

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

refolded syn ultrahigh pressure thrust sheets in the south dabie complex china field evidence and tectonic implications the exhumation mechanisms of ultrahigh pressure uhp rocks have been a controversial issue ever since the discoveries of coesite and diamond in metamorphic rocks from several orogenic belts the lack of consensus principally reflects the limited number of combined structural petrologic and geochronologic studies that have been carried out in great detail within these terrains in this study we derive a complex structural history for the uhp south dabie terrain based on detailed 1 10000 mapping in the eastern dabie shan and use this to place new constraints on the exhumation process structural analysis demonstrates that syn uhp south vergent thrusting d 1 was followed by north vergent folding under amphibolite facies conditions d 2 and a third generation of folding at shallower level d 3 this supports a two stage exhumation model in which initially rapid exhumation from mantle depth to lower crustal levels is followed by slower exhumation from the lower crust level to the earth surface the remarkable structural concordance between lithological units containing uhp mineral assemblages and the enclosing gray gneiss units implies that the apparent difference in peak pressures recorded by different lithologies is not a consequence of tectonic juxtaposition but a result of differential reequilibration during decompression

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

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

geochemistry and trace element behaviors of eclogite during its exhumation in the xitieshan terrane north qaidam uhp belt nw china abstract the xitieshan terrane located in the central part of the north qaidam ultrahigh pressure uhp metamorphic belt china is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins based on their bulk rock tio 2 contents the eclogites can be subdivided into a high ti group tio 2 2 and a low ti group tio 2 e nd t and restricted i sr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment on the other hand the sr nd isotopes of high ti eclogites show mainly positive but also some negative e nd t values and relatively broadly distributed i sr values indicating minor crustal contamination of the ocean floor basalts considering available 750 877 ma protolith ages preserved in zircon cores it is inferred that some of the eclogites derived from neoproterozoic protoliths were emplaced onto the crust far ahead of the paleozoic deep subduction while the other eclogites originate from a different oceanic crust e g the paleo qilian ocean indicating multiple orogenies in the geological history of the xitieshan terrane china whole rock and in situ la icp ms mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations the mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that at the peak metamorphic stage m1 and at the earlier eclogite facies overprint m2 the fluid was internally controlled by the rock itself within a mafic lens the amount of water soluble elements e g rb sr ba u pb and lree observed in the whole rock compositions as well as in amphiboles increases from the core phengite bearing eclogite to the rim amphibolite and implies an external fluid source for the amphibolite facies retrogression m3 which should be helpful for the final exhumation of uhp eclogite

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

contrasting plate tectonic styles of the qinling dabie sulu and franciscan metamorphic belts the dabie mountains are part of the 2000 km long qinling dabie sulu suture zone juxtaposing the sino korean and yangtze cratons an eastern extension apparently crosses korea and lies along the japan sea side of honshu as the imjingang and sangun terranes respectively a northeastern segment may be present in sikhote alin russian far east this orogenic belt records late paleozoic ocean floor consumption and the triassic collision of two precambrian continental massifs in east central china coesite and microdiamond inclusions in strong refractory minerals of eclogite facies ultrahigh pressure uhp metamorphic rocks in the dabie sulu area attest to profound subduction of a leading salient of the old cold yangtze craton now recovered through tectonic exhumation and erosion northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high pressure hp and uhp metamorphism documented in the western alps in both regions subduction of narrow prongs of continental material uhp metamorphism and return toward midcrustal levels of relatively lower density buoyant microcontinental blocks resulted from delamination of these rocks from the descending higher density oceanic crust capped lithospheric plate such salients of continental crust represent an integral structural part of the downgoing slab remain intact and may be dragged to great depths before disengaging and rising differentially as coherent blocks uhp parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700 900 c and 28 35 kbar or more in contrast pacific type hp metamorphic belts as represented by the franciscan complex of western california recrystallized under physical conditions up to 200 500 c 10 3 kbar in this setting voluminous quartzo feldspathic and graywacke debris was carried downward on oceanic crust capped lithosphere choking the subduction zone with incompetent material sited between both plates and strongly adhering to neither this buoyant largely sedimentary complex decoupled at 25 30 km depth and ascended toward the surface in both alpine type intracontinental collision and pacific type underflow light sialic material displaced dense mantle thus the return to midcrustal levels was propelled dominantly by body forces

ultra high pressure uhp terrains lessons from thermal modeling the thermal history of subducted exhumed crustal fragments is explored to depths of 125 km using a forward modeling approach the finite element code uses an adaptive gridding technique that allows deformation and differential movement of subgrids to simulate tectonic mass flow the geometry and velocity of plate convergence and the large scale strain partitioning for example subduction channel mobile tectonic fragments are described explicitly as functions of time the evolution of metamorphic conditions during descent and subsequent exhumation of a terrane within the subduction channel is computed for different kinematic scenarios these include a range of velocities relative to a stable continental block for the subducting slab the exhuming uhp fragment and a subduction channel of given width though essentially generic in emphasis our models are designed to help understand real uhp occurrences most especially their exhumation history boundary conditions and the geometric set up were chosen according to three case studies of known uhp terranes eastern china western norway western alps simulations indicate that the temperature reached at a given depth during the subduction stage is crucially dependent on the convergence rate width of the subduction channel location of the fragment within the subduction channel and on rates of shear heating and convection of the subcontinental mantle the p t t path experienced by a tectonic fragment during its exhumation from pmax is determined chiefly by its size thickness and exhumation velocity the rate of ongoing subduction is less relevant given the rapid exhumation required to preserve uhp assemblages the depth at which tmax is reached by an exhuming uhp fragment and the dp dt slope of its exhumation path may critically affect the extent of irreversible phase transitions during decompression comparing data for the three uhp case studies with corresponding simulation results indicates general agreement of the p t paths predicted t t evolution paths are also compatible with well constrained mineral chronometry from the study areas this agreement between field data and simulation results leads to a number of observables that may be useful in relating uhp occurrences to their evolution for example no metamorphic field gradients are expected for fragments of less than 5 km thickness and the spread in mineral ages reflecting the high pressure stage is predicted to be less than 1 ma therefore observed variations in metamorphic conditions for the stage near tmax and p tmax may reflect real differences of evolution in very large fragments only and the same is valid for a substantial spread in metamorphic age data pertaining to stages near pmax or tmax tectonic extrusion of fragments along a subduction channel is not the only mechanism proposed to explain uhp occurrences however the broad agreement between several test cases and the present simulation results lend support to the viability of the kinematic scenario adopted where field evidence indicates a similar emplacement mechanism and allows the initial size of the uhp fragment to be inferred the melonpit model provides a powerful integrative tool to produce a quantitative picture of the dynamics of uhp terranes

ion microprobe zircon u pb age and geochemistry of the myanmar jadeitite combined geochemistry and geochronology of the myanmar jadeitite were determined bulk rock trace element compositions display u shaped ree patterns with pronounced positive eu anomalies the total ree abundances are very low less than half chondritic and the high field strength elements and some large ion lithophile elements are moderately enriched these features indicate a metasomatic origin there are three groups of zircons with different interior characteristics cathodoluminescence mineral inclusions chemical compositions and sensitive high resolution ion microprobe u pb ages group i zircons with a mean age of 163 2 3 3 ma mostly have distinct oscillatory zoning highest u and th contents and na free mg rich mineral inclusions and thus indicate an igneous formation of oceanic crust or hydrothermal serpentinization and or rodingitization event in the middle jurassic group ii zircons with a mean age of 146 5 3 4 ma have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions suggesting that formation of the myanmar jadeitites as well as subduction of the eastern indian oceanic plate occurred in the late jurassic group iii zircons have an age of 122 2 4 8 ma which represents a later unknown thermal event discovery of the middle jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern indo burman range

tracing exhumation of the dabie shan ultrahigh pressure metamorphic complex using the sedimentary record in the hefei basin china jurassic rocks in the hefei basin were deposited by braided fluvial and alluvial fan systems characterized by a general coarsening upward sequence multiproxy provenance analyses demonstrate that the sediment source areas for the hefei basin are composed of a variety of rocks including ultrahigh pressure uhp and high pressure hp metamorphic rocks and yangtze basement rocks of the axial dabie shan metamorphic complex the luzhenguan complex granite low and medium grade metamorphic rocks and the yangshan group sandstone in the north huaiyang fold and thrust belt a middle jurassic section in the western part of the basin is characterized by relatively high e nd values at 176 ma ranging from 13 8 to 11 3 whereas a section in the middle part of the basin has higher 147 sm 144 nd ratios from 0 1168 to 0 1266 and somewhat lower e nd values at 176 ma from 15 0 to 14 5 sediments in a section in the eastern part of the basin have the lowest e nd values at 176 ma ranging from 22 0 to 14 6 the highest t dm values from 1 8 to 2 4 ga and low 147 sm 144 nd ratios from 0 0937 to 0 1067 provenance analyses of detrital compositions and nd isotopic compositions of the sediments in the hefei basin clearly demonstrate that the depth of exhumation in the dabie shan orogen increases from the west to the east the unroofing ages of the uhp and hp metamorphic rocks change from early jurassic to late jurassic westward the exhumation rate during the late triassic and jurassic is inferred to have increased eastward from 1 4 mm a to 2 5 mm a on average the sediments in the basin record the episodic thrusting events and periodic unroofing in the orogen

paleomagnetism and rock magnetism of fernando de noronha brazil abstract seven sites from basaltic rocks of the quixaba formation k ar age3 4 1 7m y on fernando de noronha 3 85 s 32 4 w yield a pole at 80 3 n 234 e with a95 2 9 and k 446 after af cleaning and seven sites from the remedios formation 9 5 1 1m y yield a pole at 84 5 n 136 e with a95 7 5 and k 65 combination of all published reliable data results in poles at 80 6 n 216 e n 21 a95 3 2 k 100 and 86 0 n 81 e n 17 a95 4 9 k 53 for the quixaba and remedios formation respectively both with mixed polarity the two pole positions differ significantly from each other seven sites from volcanics k ar age28 7 2 5m y in the province of fortaleza 3 8 s 38 7 w yield a pole at 66 n 54 e with a95 19 1 and k 10 9 rock magnetic investigations and chemical analyses on samples from fernando de noronha reveal as primary phase titanomagnetites with about 60 ulvospinel in most cases the titanomagnetites have undergone high temperature oxidation and low temperature oxidation of various degrees the ferrimagnetic phases in the samples from the province of fortaleza are magnetites with low temperature alterations showing relative weak stability of remanence

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

occurrences of intergranular coesite in ultrahigh p rocks from the sulu region eastern china implications for lack of fluid during exhumation several coesite grains along garnet omphacite grain boundaries were discovered in eclogites from yangkou beach of the sulu region eastern china these grains exhibit variable extents of conversion of coesite to palisade through mosaic to granoblastic quartz aggregates the rare occurrence of intergranular and matrix coesite in ultrahigh pressure uhp crustal rocks is related to the fast rate of exhumation and the low amount of fluid infiltration required during retrograde recrystallization sluggish reactions due to the lack of fluid in both uhp and retrograde metamorphism resulted in the occurrence of intergranular coesite in these eclogites the preservation of gabbroic texture and relict igneous minerals and the metastable persistence of low p assemblages in the coesite stability field

oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at shuanghe in the dabie mountains the oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss biotite gneiss and granitic gneiss associated with ultrahigh pressure uhp eclogites in the shuanghe district of the eastern dabie mountains there are significant differences in 18o between the two gneisses the uhp biotite gneiss varying from 4 3 to 10 6 similar to the associated eclogites whereas the non uhp granitic gneiss ranges only from 3 8 to 1 2 the d values are similar in the two gneisses with 37 to 64 for epidote zoisite 92 to 83 for amphibole and 63 to 109 for biotite phengite hydrogen isotope disequilibrium among the coexisting hydroxyl bearing minerals is ascribed to retrograde exchange subsequent to amphibolite facies metamorphism oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock 18o oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals the metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss the isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite facies metamorphism however high temperatures of 550 to 650 c are obtained from those minerals resistant to retrograde oxygen isotope exchange implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers the 18o depletion of both gneisses is interpreted to result from meteoric hydrothermal exchange before during plate subduction therefore the measured 18o values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the uhp metamorphism it is inferred that the uhp unit is in foreign contact with the non uhp unit like a tectonic melange but both of them experienced the two common stages of geodynamic evolution 1 18o depletion prior to the uhp metamorphism 2 uplifting since the amphibolite facies metamorphism

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

ultrahigh pressure metamorphism and multistage exhumation of eclogite of the luotian dome north dabie complex zone central china evidence from mineral inclusions and decompression textures abstract the studied eclogite is located in the luotian dome in the southwestern part of the north dabie complex zone ndz central china and is a portion of deeply subducted mafic lower continental crust of the south china block petrologic analysis suggests that the eclogite underwent ultrahigh pressure uhp and high pressure eclogite facies metamorphism and subsequent hp granulite facies overprinting and amphibolite facies retrogression during continental subduction and exhumation as a result multiple decompression textures were produced suggesting a multistage exhumation history from eclogite to granulite to amphibolite facies conditions a striking feature of this eclogite is the widespread exsolution of different phases oriented needles of rutile clinopyroxene amphibole apatite in garnet quartz amphibole hypersthene sodic plagioclase in clinopyroxene and pyrrhotite in apatite most importantly we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite confirmed by in situ raman spectroscopy in zircon in addition there are two groups of apatite one is fluor f apatite that occurs as inclusions in garnet or in matrix which is rich in f 3 wt lakes exsolution features and formed at uhp metamorphic conditions the second is associated with the oriented pyrrhotite exsolution and formed during decompression microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the ndz are similar to eclogites from the northern segment of the ndz e g huangweihe and baizhangyan and underwent uhp metamorphism with a peak pressure 5 7 gpa these results combined with published geochronological data imply that the ndz experienced triassic uhp metamorphism as a coherent unit

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

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

petrogenesis of a high temperature metamorphic terrane a new tectonic interpretation for the north dabieshan central china the northern dabie terrane consists of a variety of metamorphic rocks with minor mafic ultramafic blocks and abundant jurassic cretaceous granitic plutons the metamorphic rocks include orthogneisses amphibolite migmatitic gneiss with minor granulite and metasediments no eclogite or other high pressure metamorphic rocks have been found granulites of various compositions occur either as lenses blocks or layers within clinopyroxene bearing amphibolite or gneiss the palaeosomes of most migmatitic gneisses contain clinopyroxene melanosomes and leucosomes are intimately intermingled tightly folded and may have formed in situ the granulites formed at about 800 830 c and 10 14 kbar and display near isothermal decompression p t paths that may have resulted from crust thickened by collision plagioclase amphibole coronae around garnets and matrix pi hbl assemblages from mafic and ultramafic granulites formed at about 750 800 c partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism amphibolite facies orthogneisses and interlayered amphibolites formed at 680 750 c and c 6 kbar formation of oligoclase orthoclase antiperthite after plagioclase took place in migmatitic gneisses at t 490 c in response to a final stage of retrograde recrystallization these p t estimates indicate that the northern dabie metamorphic granulite amphibolite facies terrane formed in a metamorphic field gradient of 20 35 c km 1 at intermediate to low pressures and may represent the sino korean hangingwall during triassic subduction for formation of the ultrahigh and high p units to the south post collisional intrusion of a mafic ultramafic cumulate complex occurred due to breakoff of the subducting slab

high pressure low temperature metamorphism in northern hubei province central china the qinling dabie accretionary fold belt in east central china represents the e w trending suture zone between the sino korean and yangtze cratons a portion of the accretionary complex exposed in northern hubei province contains a high pressure low temperature metamorphic sequence progressively metamorphosed from the blueschist through greenschist to epidote amphibolite eclogite facies the hongan metamorphic belt can be divided into three metamorphic zones based on progressive changes in mineral assemblages zone in the south is characterized by transitional blueschist greenschist facies zone is characterized by greenschist facies zone in the northern most portion of the belt is characterized by eclogite and epidote amphibolite facies sequences changes in amphibole compositions from south to north as well as the appearance of increasingly higher pressure mineral assemblages toward the north document differences in metamorphic p t conditions during formation of this belt preliminary p t estimates for zone i metamorphism are 5 7 kbar 350 450 estimates for zone iii eclogites are 10 22 kbar 500 50 the petrographic chemical and structural characteristics of this metamorphic belt indicate its evolutionin a northward dipping subduction zone and subsequent uplift prior to and during the final collisionbetween the sino korean and yangtze cratons

garnet spinel corundum quartz bearing titanohematite veins in eclogite from the sulu ultrahigh pressure terrane imprint of a short lived high temperature metamorphic stage abstract eclogite hosted garnet spinel corundum quartz bearing titanohematite veins and lenses 10 20 cm in width are described for the first time in the sulu ultrahigh pressure metamorphic terrane some of the lenses were aligned parallel to the foliation of the host eclogite suggesting that they were the product of ductile deformation of one titanohematite layer a vein composed of titanohematite ilmenite hematite spinel garnet corundum quartz k feldspar albite was studied in detail ti fe oxides account for up to 80 and al rich phases for 15 of the total volume of this vein electron microprobe analyses show that the titanohematite solid solution was made up of 0 75 hematite 0 25 ilmenite the unusual mineral assemblage of garnet spinel corundum quartz implies that this vein could have experienced high temperatures 900 c although the garnets showed well defined mg and mn diffusion zoning in the rim as a result of the high temperature event slight mg and mn growth zoning was preserved in the core thus we suggest that the sulu uhp terrane could have experienced a short lived high temperature 900 c stage during exhumation garnets in the titanohematite vein were characterized by extremely low trace element contents petrological and geochemical features of the veins suggest that they could be metamorphic products of igneous cumulates composed of magnetite plagioclase clinopyroxene

permo triassic collision subduction zone metamorphism and tectonic exhumation along the east asian continental margin abstractconvergent plate motion at 320 210 ma generated the tongbai dabie sulu east central china imjingang gyeonggi central korea renge suo southwestern japan sikhote alin orogen along the paleo pacific edge of cratonal asia this amalgamated belt reflects collision between the sino korean and yangtze cratons on the sw portion and accretion of outboard oceanic arcs sialic fragments against the ne margin subducted proterozoic paleozoic continental and oceanic crustal complexes underwent high and ultrahigh pressure metamorphism at low to moderate temperatures tectonic slices of sialic crust episodically disengaged from the downgoing plate and driven by buoyancy ascended rapidly to midcrustal levels from depths exceeding 90 200 km after continental collision in east central china plus or minus korea and from 30 50 km after arrival of far traveled oceanic terranes in sw japan and the russian far east on achieving neutral buoyancy and stalling out at 10 20 km depth later doming gravitation

petrology of very high pressure eclogitic rocks from the brossasco isasca complex dora maira massif italian western alps petrographical and mineral chemical data are given for the eclogites which occur in the garnet kyanite micaschists of the penninic dora maira massif between brossasco isasca and martiniana italian western alps and for a sodic whiteschist associated with the pyrope coesite whiteschists of martiniana the brossasco isasca bi eclogites are fine grained foliated and often mica rich rocks with a strong preferred orientation of omphacite crystals and white micas porphyroblasts of hornblende are common in some varieties whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz jadeite and garnet these features differentiate the bi eclogites from the eclogites that occur in other continental units of the western alps which all belong to type c garnet sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist sodic pyroxene in the eclogites is an omphacite often close to jd50di50 with very little acmite and virtually no aliv and impure jadeite in the leucocratic layers and in the sodic whiteschist garnet is almandine with 20 30 mol for each of the pyrope and grossular components in the eclogites and a pyrope rich variety in the sodic whiteschist white mica is a variably substituted phengite and paragonite apparently only occurs as a replacement product of kyanite amphibole is hornblende in the eclogites but the most magnesian glaucophane yet described in the sodic whiteschist quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets the p t conditions during the vhp event are constrained in the eclogites by reactions which define a field ranging from 27 28 kbar to 35 kbar and from 680 to 750 c these temperatures are consistent with the results of garnet pyroxene and garnet phengite geothermometry which suggest that the eclogites may have equilibrated at around 700 c in the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite pyrope garnet glaucophane compatibility as in the eclogites water activity must have been low such conditions are close to the p t values estimated for the early alpine recrystallization of the pyrope coesite rock and like petrographical and mineralogical features set aside the bi eclogites from the other eclogites of the western alps instead indicating a close similarity to some of the eclogite bodies occurring in the adula nappe of the central alps an important corollary is that glaucophane stability at least in na and mg rich compositions and under very high pressures may extend up to 700 c in agreement with the ht stability limit suggested by experimental studies

geology and structure of diamond bearing rocks of the kokchetav massif kazakhstan abstract two crustal settings for microdiamond formation have been described from eclogite bearing metamorphic areas 1 the economic concentration of microdiamonds in metasedimentary gneisses and calc silicate rocks northern kazakhstan and 2 microdiamonds recently found in eclogite garnet pyroxenite and jadeitite from dabie shan mountain eastern china the latter occurrence is interpreted to be the product of ultra high pressure metamorphism in a mesozoic collision zone there are however a number of discrepancies between the geological structural and geochemical data for kazakhstan microdiamond deposits and an interpretation in terms of a deep subduction zone model the geodynamic setting of the kokchetav massif can be defined as a continental rise prism environment related to a passive continental margin where rifting predominated during early palaeozoic orogeny the kumdikol microdiamond province is closely associated with a tectonic melange zone involved in ductile to semi ductile shearing abundant in graphite microdiamonds of the kumdikol area have a dual setting in the rocks they appear to be included not only in refractory garnet and zircon but also in almost all rock forming minerals as biotite phlogopite diopside quartz and secondary sericite chlorite and sericite chlorite calcite aggregates after garnet pyroxene and plagioclase and in spite of softness of the host phases microdiamonds are well preserved on the other hand the ore body extends along the shear zone and high concentrations of the microdiamonds within it are distributed without any lithological control along the local s c surfaces of the main kumdikol strike slip shear zone this duality of microdiamond settings in absence of practically all ultra high pressure minerals except diamond itself weakens the interpretation of this occurrence in terms of very deep subduction and very fast uplift and exhumation during 15 10 ma according to recent geochronological data these microdiamonds could have been formed metastably under anisotropic stress conditions in fact the available data from the kumdikol province provides certain support for either metastable and stable hypothesis

petrology geochemistry and isotope data on a ultrahigh pressure jadeite quartzite from shuanghe dabie mountains east central china abstract in the dabie ultrahigh pressure terrane of east central china coesite bearing jadeite quartzites occur locally as intercalated layers with marble and mafic eclogite this rock assemblage is in turn enclosed within quartzofeldspathic gneisses metamorphic parageneses and kelyphitic textures reveal a multistage metamorphic evolution and complex exhumation history the primary peak metamorphic assemblage consists of jadeite garnet coesite rutile apatite minor coesite and coesite pseudomorphs occur as inclusions in jadeite and garnet three stages of retrograde assemblages are observed in the jadeite quartzites stage a is represented by the polymorphic transformation of coesite to quartz aggregates stage b is characterized by formation of coronas around jadeite porphyroblasts consisting of an inner layer of oligoclase amphibole and an outer layer of albite aegirine augite the last stage stage c involved total replacement of jadeite and most garnets by taramitic amphibole albite aegirine augite peak metamorphic p t conditions were 26 kbar at 660 c and are consistent with the estimates from the adjacent coesite bearing eclogites the jadeite quartzites display clockwise p t path that matches those of the adjacent eclogites major and trace element data suggest that the protolith of the jadeite quartzite could have been an albitized siltstone enriched in na and depleted in k and ca the highly negative present day end value 24 7 indicates a very old age for the protolith its late archean model age tdm of 2 58 ga is among the oldest so far identified for rocks from the dabie uhpm terrane concordant field relations and petrogenetic considerations suggest that all mafic politic carbonate and gneissic rocks have experienced in situ uhp metamorphism during triassic continental collision between the sino korean and yangtze cratons

geochemical evidence from coesite bearing jadeite quartzites for large scale flow of metamorphic fluids in a continental subduction channel abstract metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab mantle interface in subduction channels it is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels this assumption is challenged by a combined study of whole rock geochemistry mg and o isotopes zircon u pb ages and trace elements in coesite bearing jadeite quartzites from the dabie orogen china although the target samples were collected from different outcrops in an area of 50 km2 zircon u pb dating yields similar discordia lines with not only consistent upper intercept ages of 1 9 2 0 ga but also consistent lower intercept ages of 224 235 ma this indicates the same paleoproterozoic protolith and the same triassic metamorphism for these jadeite quartzites the o isotope analysis of mineral separates and whole rock yields variable 18o values from 6 3 to 9 4 indicating involvement of supracrustal components except for one outlier at 0 43 all the other rocks give variable 26mg values from 0 16 to 0 61 much higher than normal mantle values the whole rock mg isotopes show significant positive correlations not only with mgo contents but also with rb la rb gd and rb nb ratios but a negative correlation with na2o contents these observations indicate that the middle paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high 26mg values during the continental subduction in the triassic the metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh pressure metamorphism they would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the ttg provenance in view of the spatial occurrences of the target samples the metamorphic fluids would have flowed inside the continental subduction channel on a large scale this is the first report of the large scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels

submicron sized fluid inclusions and distribution of hydrous components in jadeite quartz and symplectite forming minerals from uhp jadeite quartzite in the dabie mountains china tem and ftir investigation abstract fluid inclusions and clusters of water molecules at nanometer to submicron scale in size have been investigated using transmission electron microscopy tem and fourier transform infrared spectroscopy ftir in jadeite quartz and symplectite aegirine augite albite taramite and magnetite corona minerals from ultrahigh pressure uhp jadeite quartzite at shuanghe the dabie mountains china fluid inclusions from 0 003 m to 0 78 m in size occur in jadeite and quartz crystals and a small number of fluid inclusions from 0 001 m to 0 25 m have also been detected in symplectite forming minerals most of the fluid inclusions have round or negative crystal morphology and contain aqueous fluids but some contain co 2 rich fluids they are usually connected to dislocations undetectable at an optical scale the dislocations represent favorable paths for fluid leakage accounting for non decrepitation of most fluid inclusions when external pressure decreased at later stages although there was partial decrepitation of some fluid inclusions unconnected to defect microstructures resulting from internal overpressure non decrepitation and partial decrepitation of fluid inclusions resulted in changes of original composition and or density it is clear that identification of hidden re equilibration features has significant implications for the petrological interpretation of post peak metamorphic processes micro ftir results show that all jadeite and quartz samples contain structural water occurring as hydroxyl ions oh and free water h 2 o in the form of clusters of water molecules the h 2 o transformed from oh during exhumation and could have triggered and enhanced early retrograde metamorphism of the host rocks and facilitated plastic deformation of jadeite and quartz grains by dislocation movement and thus the h 2 o released during decompression might represent early stage retrograde metamorphic fluid the nominally anhydrous mineral nam jadeite is able to transport aqueous fluids in concentrations of at least several hundred ppm water along a subduction zone to mantle depths in the form of clusters of water molecules and hydroxyl ions within crystals

coesite inclusions in dolomite from eclogite in the southern dabie mountains china the significance of carbonate minerals in uhpm rocks coesite inclusions have been discovered in dolomite from carbonate bearing eclogite in the dabie uhp terrane the eclogite boudins are enclosed in calc silicate rocks that contain quartz aggregatesafter coesite in the matrix the carbonate bearing eclogite is composed of garnet coesite or quartz dolomite magnesite omphacite rutile tminor phengite apatite and zircon and was formed at 760 dc 28 kbar and 0 01 100 km

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

a coherent lithostratigraphic unit in the coesite eclogite complex of dabie shan china geologic and petrologic evidence abstract as a contribution to the discussion about the in situ vs exotic origin of the ultrahigh pressure metamorphic uhpm rocks and to the real size of the exhumed tectonic units the wuhe pailou unit wpu in eastern dabie shan was studied the wpu typically consists of two micas epidote gneiss marble with minor eclogite and jadeite bearing granofels with rare garnetite and eclogite nodules these lithologies are enclosed within leucocratic granitic gneiss jadeite bearing granofels is the most peculiar lithology of the wpu being the only felsic rock in dabie shan still preserving extensive relics of the uhpm mineral assemblage the uhp peak assemblage is coesite jadeite garnet epidote phengite kyanite with accessory rutile apatite and zircon all eclogites consist of garnet omphacite quartz coesite rutile or high al titanite phengite clinozoisite kyanite two micas epidote gneiss associated with the jadeite bearing granofels typically consists of epidote muscovite biotite plagioclase k feldspar rare garnet and accessory rutile titanite apatite and zircon locally the wpu paragneiss contains garnet with inclusions of phengite kyanite rutile and amphibole na plagioclase symplectite after omphacite a clockwise p t path was inferred for the wpu which consists of an early prograde part up to coesite eclogite facies conditions p 3 3 gpa and t 720 c an early decompression stage and a late part characterised by a near adiabatic decompression down to medium pressure p 1 1 gpa t 670 c and to low pressure amphibolite facies conditions followed by a final cooling to greenschist facies conditions both field and laboratory data consistently suggest that the wpu characterised by the close association of paragneiss marble eclogite and granofels is a supracrustal metasedimentary sequence which underwent subduction uhpm and exhumation as a coherent tectonic unit therefore within collisional orogens large coherent slices of continental crust may be subducted at depths greater than 100 km

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections

distribution of hydrous components in jadeite of the dabie mountains two kinds of jadeite occur in the jadeite quartzite from the dabie mountains eastern china associated either with weak or strong deformation the former shows a uniform composition while the latter shows both uniform and zoning composition these jadeites were examined with infrared ir spectroscopy all jadeite displays hydroxyl oh stretching bands implying that hydrous components commonly exist in jadeite the concentration of the hydrous components in the jadeite with weak deformation is homogenous whereas the concentration of the hydrous components in strongly deformed samples is variable the correlation between the intensity of oh band absorbance and compositional zoning of jadeite with strong deformation indicates that na rich jadeite can accommodate more oh than na poor jadeite its interpretation is that there is more na associated with greater oh availability in the initial crystallization environment our data of the comparatively high oh concentrations recorded in the core of the jadeite which contains relative high vacancies in m2 site imply that both oh content and the vacancies in m2 site could decrease during exhumation of the jadeite quartzite the decrease of oh solubility in jadeite would result in the formation of h2o fluid during the early exhumation of uhp metamorphic rocks the h2o fluid transformed from oh during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep

cluster of water molecules in garnet from ultrahigh pressure eclogite through the use of transmission electron microscopy and infrared spectroscopy hydrous components have been identified as hydroxyl and free water h 2 o in elongate garnet of ultrahigh pressure eclogite from the dabie mountains eastern china the h 2 o occurs as clusters of water molecules and as hydroxyl in garnet the clusters of water molecules are interpreted to have promoted elongation of garnet through hydrolytic weakening the presence of hydrous species in ultrahigh pressure garnet from continental crust protoliths indicates that such protoliths are able to carry water into the mantle during the subduction of continental lithosphere

discovery and implication of p21 n crystal structure on a nano scale in single jadeite crystals we present the results of high resolution transmission electron microscope hrtem studies on jadeite crystals of jadeite quartzite from the dabie ultra high pressure uhp metamorphic terrane china although jadeite is known to have only c2 c crystal structure 100 110 112 010 diffraction patterns and hrtem images reveal c2 c and p2 1 n structures in 500 nm microdomains of single jadeite crystals that we recognized in all the above four orientations the cell parameters of c2 c crystal structure correspond to those of jadeite whereas the cell parameters of p2 1 n structure are equivalent to those of omphacite the presence of the p2 1 n structure shows that jadeite partially changed into omphacite on a nano scale during retrograde metamorphism and that this reaction failed to reach equilibrium it suggests uplift of the host rock during exhumation accompanied by complex non equilibrium retrogressive process in the jadeite with significant implications for the derivation of p t paths and tectonic interpretation of the dabie uhp metamorphic terrane c 2002 elsevier science b v all rights reserved

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

provenance of rift sediments in a composite basin mountain system constraints from petrography whole rock geochemistry and detrital zircon u pb geochronology of the paleocene shashi formation southwestern jianghan basin central china the paleocene shashi formation the host formation of potassium rich brines and solid potash in the jianghan basin central china is characterized by rift sediments in a composite basin mountain system however the provenance of the paleocene jianghan basin remains poorly understood a combined study of petrographic examination geochemical analysis and zircon u pb dating was conducted to characterize the provenance of the shashi formation the petrographic analysis of the shashi formation reveals that these samples have predominantly quartz lithic compositions with minor contributions from other minerals whole rock geochemical data indicate that this formation experienced only slight to moderate alteration the chondrite normalized rare earth element ree patterns of these samples are essentially parallel to those of the upper continental crust ucc as their negative eu anomalies and th sc and la th ratios suggest an average source composition similar to that of the ucc these values together with plots of their th u vs th ratios suggest that certain samples have undergone moderate degrees of weathering and sedimentary recycling the analysis of major and trace elements i e la th hf th sc zr sc la y zr sc tio2 fe2o3t mgo al2o3 sio2 fe2o3t mgo indicates that their sedimentary sources were felsic rocks from an active continental margin or a continental arc containing a minor amount of recycled sediments derived from a passive continental margin the shashi formation contains detrital zircons that primarily have u pb ages of 2 5 1 8 ga 725 965 425 475 310 350 and 200 260 ma these results reveal that the pre ordovician zircons are derived from recycled sediments of the yangtze block and thus originate from the surrounding ranges the magmatic rocks of the south qinling belt and the jiangnan orogen are responsible for supplying the ordovician to triassic detrital zircons these provenance data combined with paleocurrent analyses can be used to identify multiple source characteristics in the jianghan basin during the early paleocene most clastic material was derived from a southern source during the late paleocene northern and western sources contributed most of the detritus to the basin the supply of clastic material was thus broadly variable and widely dispersed throughout the basin this study shows that the architecture of the region and the rifting process exerted important influences on this sediment dispersal

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

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