extent of disorder in iron bearing albite and anorthite melts insights from multi nuclear 29si 27al and 17o solid state nmr study of iron bearing naalsi3o8 and caal2si2o8 glasses abstract knowledge of the detailed structure of iron bearing aluminosilicate glasses is essential to unraveling the effects of iron on the macroscopic properties of natural silicate glasses and melts however the local structures around the framework cations such as al and si and the extent of network polymerization in iron bearing aluminosilicate glasses are not well understood this is mainly because the high resolution solid state nmr one of the effective probes of the structures of aluminosilicate glasses has limited utility in delving into the local configurations of oxide glasses with paramagnetic elements i e iron here we use multi nuclear 29si 27al and 17o high resolution solid state nmr to investigate the effect of iron content on the local structure around framework cations and oxygen species in na al1 xfex si3o8 glasses with varying x fe al fe fe3 fe 0 8 naalsi3o8 glasses with varying fe2o3 naalsi3o8 fe2o3 fe3 fe 0 5 and caal2si2o8 glasses with varying fe2o3 caal2si2o8 fe2o3 fe3 fe 0 3 the nmr results for na al1 xfex si3o8 glasses reveal preferential interaction between fe3 and si rich framework and thus an extensive mixing between 4 si and 4 fe3 the presence of a small amount of 5 al in na al0 5fe0 5 si3o8 glass suggests that the extent of disorder around al increases by al fe3 substitution the nmr results for naalsi3o8 fe2o3 glasses suggest that fe2 interacts more strongly with al rich framework and reveal the presence of a non negligible fraction of na nbo confirming fe2 induced structural disorder the absence of 5 al in naalsi3o8 fe2o3 glasses indicates that fe3 is more likely to form a highly coordinated framework i e 5 fe3 when excess fe3 is added to charge balanced glasses the nmr spectra for caalsi2o8 fe2o3 glasses showed an iron induced increase in the 5 al fraction and confirmed that the overall degree of disorder increases with increasing fe2o3 content the noble nmr results for iron bearing aluminosilicate glasses revealed the nature of diverse aspect of iron induced structural disorder mainly characterized by the intermixing between si and fe3 preferential interaction between al and fe2 and formation of highly coordinated 5 fe3 the detailed iron induced structural evolution is largely dependent on the composition of melts as well as the valence state of iron the observed increase in the extent of structural disorder in charge balanced na and ca aluminosilicate glasses with increasing fe2 and fe3 can account for the iron induced decrease in viscosity of the corresponding iron bearing aluminosilicate melts

subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

provenance of the beihuaiyang lower grade metamorphic zone of the dabie ultrahigh pressure collisional orogen china evidence from zircon ages the beihuaiyang zone of the dabie ultrahigh pressure collisional belt which was formed by the early mesozoic collision of the north and south china yangtze blocks is diversely interpreted as an active or a passive sedimentary sequence it comprises the luzhenguan and foziling complexes six granitoid rocks of the luzhenguan complex are dated at 770 ma to 720 ma and one schist sample also contains detrital zircons of 760 ma to 720 ma these data indicate that this complex was of the south china affinity and probably originated from the northern yangtze block two quartzite samples of the foziling complex contain abundant detrital zircons of archean to paleozoic ages clustering around 2 5 ga 1 9 1 8 ga 1 0 0 7 ga and 0 5 0 4 ga this age spectrum demonstrates a mixed source of these two blocks the youngest zircons suggest a paleozoic deposition along an active continental margin south of the north china block while these late proterozoic zircons are characterized for the yangtze block subsequently it is suggested that a terrain drifted from the northern yangtze block in early paleozoic must have been situated to the southern margin of the north china block to provide material source prior to the final collision in early mesozoic q 2003 elsevier ltd all rights reserved

tracking partial melting and protolith nature by zircon u pb and hf o isotope compositions of migmatites in the north dabie terrane with emphasis on paleozoic low δ18o magmatism

ichnological evidence for quadrupedal locomotion in pterodactyloid pterosaurs trackways from the late jurassic of crayssac southwestern france abstract more than 30 well preserved pterosaurian trackways have been excavated from the site of crayssac lower tithonian southwestern france along with hundreds of isolated imprints the general pattern of pterosaurian trackways is described and corresponds to the morphology of the ichnogenus pteraichnus stokes 1957 a crocodilian origin for pteraichnus is rejected the morphology of the pes and manus prints is described and discussed and exhibits a great dynamic variability the three manus digit prints are confirmed as the marks of digits i iii but the high angle of divarication may reflect strong digit abduction as well as a backward folding of the digit iii the manus gait width is as much as 3 the pes gait width the narrowest manus trackways corresponds to low velocity walking in a semi erect stance whereas the widest manus trackways corresponds to higher velocity walking perhaps even running with a subhorizontal body

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

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

tracks and trackways of myotragus balearicus bate artiodactyla caprinae in pleistocene aeolianites from mallorca balearic islands western mediterranean tracks and trackways of the ruminant goat myotragus balearicus bate 1909 are abundant in the late pleistocene aeolianites of the south coast of mallorca spain the carbonate aeolianites form impressive dunes and related sand ramps that were initiated as echo dunes in front of a 20 40 m high paleocliff of miocene reefal limestone where the dunes built up to the cliff top they covered it as an aeolian ramp the dunes and sand ramps were active during the dry summer months but were passive during the wet winter period the small goat like animals were abundant on the island and for unknown reasons crossed the aeolian accumulations in large numbers this study deals with the tracks formed in the dune deposits all preserved trackways indicate impression into moist sand special features of the tracks include the structure produced by the withdrawal of the foot here called the axis and a disturbance zone of plastic deformation around this on dune crests the disturbance zone surrounds the axis more or less symmetrically however in addition a pressure pad of dislocated slightly rotated sediment bound by curved microfaults is commonly produced posterior to the axis by propulsive pressure of the foot on steep stoss and lee slopes the pressure pad becomes oriented in a down slope position as a result of gravitational slip of the walking animal combination of disturbance of the sediment in this way by manus followed by overprinting of similar disturbance by pes produces highly complicated track structure this structure may be characteristic of artiodactylous mammals in soft sand particularly aeolian deposits the structure is designated as bifidipes aeolis isp nov

low 18o scourie dike magmas from the lewisian complex northwestern scotland the scourie dike swarm of the lewisian complex northwestern scotland comprises a dominant suite of quartz tholeiites and lesser volumes of norites olivine gabbros and bronzite picrites that were emplaced at depths of 14km in two episodes at 2 0 and 2 4 ga unaltered quartz tholeiite dikes have low and homogeneous whole rock 18 o values 2 04 0 14 relative to standard mean ocean water and concordant mineral 18 o values plagioclase 3 1 to 4 1 clinopyroxene 1 5 to 1 9 magnetite 0 3 to 0 6 the dikes intrude granulite facies gneisses that have heterogeneous 18 o whole rock values 5 8 to 9 7 and show no evidence of 18 o depletion low 18 o intrusive igneous rocks are generally believed to have exchanged with high temperature meteoric or oceanic fluids after emplacement the heterogeneous high 18 o values of the country rocks suggest that the lewisian complex has not been pervasively infiltrated by such fluids it is also unlikely that fluids were channeled along the dikes because 1 the gneisses immediately adjacent to the dikes have unaltered mineralogies and stable isotope ratios 2 the dikes preserve unaltered igneous textures and mineralogies 3 the dike minerals do not show open system oxygen isotope resetting and 4 surface derived fluids generally do not penetrate to the depths at which the dikes were intruded it is more likely that these dikes were emplaced as primary low 18 o magmas derived from the subcontinental lithosphere the geochemistry of the scourie dikes suggests that they have not assimilated large volumes of continental crust and the best explanation of the data is that the dikes were derived from a low 18 o mantle source that possibly contained the remains of subducted hydrothermally altered oceanic crust

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence

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

true tracks undertracks and eroded tracks experimental work with tetrapod tracks in laboratory and field abstract to be able to distinguish between true tracks undertracks and tracks altered by erosion is of great importance in tetrapod track ichnotaxonomy this paper reports three experimental approaches to the study of undertracks using the footprints of an emu dromaius novaehollandiae experiment 1 describes successive horizontal sections down through a plaster cast of an emu footprint emplaced in soft mud revealing a steady downward decrease in the area of the track particularly the length this is the result of the movements of the trackmaking limb during impact forward swing of the body and final kick off experiments 2 and 3 describe vertical sections cut through footprints emplaced in packages of layered coloured cement admixed with water to produce different consistencies firm and semi fluid after hardening the cement block was serial sectioned vertically and removal of a lightly cemented layer gave access to two undertrack bedding plane views successive undertracks downward show an increase in horizontal dimensions and decrease in vertical topography of the structure representing gradual degradation of the track anatomy with depth in experiment 3 the true track at the tracking surface was deformed by collapse and flow of the substrate after withdrawal of the foot in this case the undertracks that were formed in layers subjacent to the foot reproduced the morphology of the foot more faithfully than did the true track

geochemistry and geochronology of eclogites from the northern dabie mountains central china abstract recent discoveries of uhp eclogites from the northern dabie complex ndc in central china provide an opportunity to improve our understanding of the tectonic setting of the ndc and exhumation mechanism of uhpm rocks the petrology geochemistry sr nd o isotopic compositions and sm nd isotopic chronology of some eclogites from the ndc were obtained in this study the major elemental compositions and relatively low nd t of about 10 for the eclogites suggest that the protoliths of most eclogites were ancient basaltic rocks developed in continental crust and thus these eclogites were produced by subduction of the yangtze continental crust however some of the eclogites within foliated peridotites with relatively high nd t values up to 2 4 are interpreted as pieces of lithospheric mantle scraped off the overlying mantle by subducting continental crust and then subducted together into mantle depths therefore the metamorphosed mafic ultramafic belt containing eclogite and meta peridotite blocks along the southern part of the mozitan xiaotian fault zone may represent the tectonic melange produced by the subduction of the yangtze continental crust the sm nd age of 212 4 ma defined by garnet omphacite is interpreted to represent a cooling age corresponding to in the retrograde stage of granulite facies metamorphism during eclogite exhumation the relationship between the 18o values of the eclogitic minerals such as garnet and omphacite from an eclogite sample suggests oxygen isotope equilibrium pointing to sm nd isotope equilibrium between garnet and omphacite according to comparability between o and sm nd diffusivities however the sm nd isochrons defined by whole garnet or omphacite zonal garnet yield a large age range of 187 266 ma which may be produced by nd isotopic disequilibrium between garnet and retrograded minerals in whole rocks therefore the eclogites in the ndc also underwent triassic uhp metamorphism like the uhp eclogites in the southern dabie mountains the 18o values of whole rocks for the eclogites are 2 40 to 4 32 indicating that protolith of the eclogites in the ndc underwent hydrothermal alteration at high temperature by surface water before subduction of the yangtze continental crust during the triassic period in a manner similar to protoliths of uhp rocks in the southern dabie mountains

upb zircon age and geochemical constraints on tectonic evolution of the paleozoic accretionary orogenic system in the tongbai orogen central china abstract the tongbai orogen is located in a key tectonic position linking the qinling orogen to the west and the dabie sulu orogen to the east this orogen comprises a paleozoic accretionary orogenic system in the north and a permo triassic collisional orogenic system in the south hence it may serve as an ideal place to unravel the tectonic evolution from the initial oceanic subduction accretion to the final continental collision between the sino korean and yangtze cratons u pb zircon geochronology of metasedimentary and metaigneous rocks and geochemical characterization of metabasalts indicate a close genetic relationship between the individual tectonic units of the paleozoic accretionary orogenic system 1 the kuanping group as a neoproterozoic ordovician subduction accretion complex which received sediments from the qinling microcontinent 2 the erlangping group and huanggang diorite granodiorite complex as an ordovician silurian island arc evolving from backarc basin 3 the qinling group as a precambrian microcontinent converted into continental arc during the ordovician 4 the guishan complex as a mixture of the qinling continental arc and devonian forearc sediments and 5 the nanwan flysch as a devonian forearc sequence deposited on the newly accreted active margin of the sino korean craton consequently the northern and southern boundaries of the paleozoic accretionary orogenic system represent two sutures of the qinling microcontinent with the sino korean and yangtze cratons the new and previously published data reveal that the tectonic evolution of the tongbai orogen involved a series of events from ordovician early silurian ca 490 440 ma oceanic subduction and arc magmatism silurian early devonian ca 440 400 ma arc continent collision carboniferous ca 340 310 ma oceanic subduction and accretion late permian triassic ca 260 200 ma continental subduction and collision to cretaceous ca 140 90 ma extension and lateral eastward extrusion the entire accretionary orogenic processes until the final collision between the sino korean and yangtze cratons may have lasted more than 200 ma

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

dinosaur tectonics a structural analysis of theropod undertracks with a reconstruction of theropod walking dynamics abstract a dinosaur trackway in the middle jurassic eolian entrada sandstone of southern utah u s a exposes three undertracks that we have modeled as isolated tectonic regimes showing the development of fold thrust ramp systems induced by the dinosaur s feet the faulted and folded sequence is comparable to crustal scale tectonics associated with plate tectonics and foreland fold thrust belts a structural analysis of the dinosaur tracks shows the timing and direction of the forces exercised on the substrate by the animal s foot during the stride based on the structural analysis we establish a scenario for foot movements and weight distribution in the feet during the end of the weight bearing phase of the stride the weight of the animal was transferred to the front of the digits creating a rotated disk below the foot that was bounded by an extensional fault at the front and a thrust ramp toward the back as the body accelerated the foot was forced backward the rotated disk was forced backward al

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

age and nature of eclogites in the huwan shear zone and the multi stage evolution of the qinling dabie sulu orogen central china abstract in situ la icpms u pb trace element and hf isotope data in zircon demonstrate a carboniferous age for eclogite facies metamorphism in siluro devonian protoliths in the huwan shear zone dabie mountains central china this age contrasts with the more prevailing triassic age for high to ultrahigh pressure hp to uhp metamorphism in the qinling dabie sulu orogen metamorphic zircon in two eclogite samples from sujiahe is characterized by low th u ratios small negative eu anomalies flat hree patterns and low 176 lu 177 hf ratios these geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar furthermore temperatures of 655 and 638 c calculated using the ti content of zircon are consistent with their formation during eclogite facies metamorphism the weighted mean 206 pb 238 u age of 309 4 ma 2 for this zircon improves previous age estimates for eclogite facies metamorphism in the huwan shear zone ranging from 420 to 220 ma metamorphic zircon from one eclogite sample from hujiawan most likely formed during prograde metamorphism yields an equivalent age estimate of 312 11 ma magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 7 to 406 5 ma and post dating the middle paleozoic collision of the north china and the qinling terrain the zircon crystals in the three eclogite samples display a large variation of e hf t values of 4 9 to 21 3 the metamorphic zircon overgrowths show the same range of e hf t values as those of the inherited magmatic crystal interiors this suggests that the metamorphic zircon overgrowths may have formed by dissolution reprecipitation of pre existing magmatic zircon thereby preserving their original hf isotopic composition the high e hf t values suggest that the protoliths were derived from depleted mantle sources most likely paleotethyan oceanic crust while the low e hf t values are attributed to crustal contamination some eclogites in the huwan shear zone have a distinctive signature of continental crust most probably derived from the yangtze craton the coexistence of paleozoic oceanic crust and neoproterozoic continental crust with similar metamorphic ages in the huwan shear zone implies that paleozoic paleotethyan oceanic crust was produced within a marginal basin of the northern yangtze craton the opening of the paleo tethyan ocean was slightly younger than the collision of the north china craton and the qinling terrain during the late paleozoic in the qinling dabie sulu orogen subduction of the paleo tethyan oceanic crust and associated continental basement resulted in the 309 2 ma 2 eclogite facies metamorphism in the huwan shear zone the subsequent triassic continent continent collision led to the final coalescence of the yangtze and sino korean cratons amalgamation of the yangtze and north china cratons was therefore a multistage process extending over at least 200 ma

a thick tethyan multi bed tsunami deposit preserving a dinosaur megatracksite within a coastal lagoon barremian eastern spain abstract a thick multiple bed tsunami deposit consisting of sandstones and conglomerates has been discovered and investigated in the camarillas formation 130 6 128 4 ma barremian age in eastern spain the tsunami deposit is interbedded within red mudstones deposited in mud flats of a back barrier system it crops out along 7 km in length and at its base a great number of dinosaur tracks assigned to sauropods ornithopods and theropods have been preserved as natural casts then constituting an exceptional regional megatracksite associated with tsunami deposits on the basis of sedimentological features and the lateral and vertical architecture of the involved lithofacies up to five couplets of inflow backflow deposits formed by a tsunami wave train have been recognized overlying the tracks although sedimentation mainly took place during backflow currents inflows led to the removal of sand from a fronting barrier island and the rip up of lagoonal carbonate and clay pebbles depositing them in the protected back barrier lagoon its unusually great thickness is interpreted among others as being the result of the filling of the previous low topography of the back barrier lagoon

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

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

ferric ferrous ratio in silicate melts a new model for 1 atm data with special emphasis on the effects of melt composition the effect of mgo and total feo on ferric ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300 1500 c at 1 atm total pressure in air we demonstrate that the addition of these weak network modifier cations results in an increase of fe3 fe2 ratio in both mafic and silicic melts based on present and published experimental data a new empirical equation is proposed to predict the ferric ferrous ratio as a function of oxygen fugacity temperature and melt composition in contrast to previous equations the compositional effect of melts on the fe3 fe2 ratio is not only modeled by the sum of the molar fraction of the individual oxide components additional interactions terms have also been incorporated the main advantage of the proposed model is its applicability for a wide compositional range however its application to felsic melts 68 wt sio2 is not recommended other advantages of this equation and differences when compared with previous models are discussed

vertebrate track bed stratigraphy at new megatrack sites in the upper wellenkalk member and orbicularis member muschelkalk middle triassic in carbonate tidal flat environments of the western germanic basin this article examines the high resolution track horizon and track bed stratigraphy at two new middle triassic megatrack sites in nw germany the first megatrack site consists of the localities hagen and detmold and spans over a distance of about 30 km in the upper wellenkalk member of the uppermost lower muschelkalk pelsonian the second megatrack site was found between niederlistingen and ostheim over a distance of 10 km in the orbicularis member of the lowermost middle muschelkalk illyrian at all track sites the track beds are mud cracked microbial laminates and yellow dolomites products of microbial mats in carbonate tidal flat environments vertebrate tracks rhynchosauroides tirolicus abel procolophonichnium haarmuehlensis holst smit veenstra as well as scratch marks and undichna isp are abundant recently 11 track beds have become known from the upper roet member upper bunter to the marl dolomite member middle muschelkalk in the germanic basin six of these track beds build megatrack sites with an extent of 10 400 km the result is a completely different picture of the bathymetry during bithynian to illyrian in the germanic basin a flatter basin with a surrounding carbonate tidal flat belt changing its position periodically

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

discovery of metamorphic diamonds in central china an indication of a 4000 km long zone of deep subduction resulting from multiple continental collisions the central orogenic belt cob of china is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt here we report discovery of microdiamonds from both eclogites and felsic gneisses in the north qinling zone in the central portion of the cob this discovery demonstrates that the country rocks of continental affinity shared in the ultra high pressure metamorphic uhpm event and provides a bridge connecting the two previously recognized uhpm terranes thereby establishing the existence of a uhpm belt extending more than 4000 km geochronological dating yields early palaeozoic ages in the west and early mesozoic ages in the east recording two separate continental collisions overprinted within the cob occurrence of uhp metamorphism during recurrent continental collision here and in the alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional with significant implications for processes of plate tectonic reorganization and mantle mixing over time

three dimensional preservation of foot movements in triassic theropod dinosaurs dinosaur footprints have been used extensively as biostratigraphic markers environmental indicators measures of faunal diversity and evidence of group behaviour1 2 3 4 trackways have also been used to estimate locomotor posture gait and speed6 7 8 9 10 11 but most prints being shallow impressions of a foot s plantar surface provide little evidence of the details of limb excursion here we describe late triassic trackways from east greenland made by theropods walking on substrates of different consistency and sinking to variable depths that preserve three dimensional records of foot movement triassic theropod prints share many features with those of ground dwelling birds but also demonstrate significant functional differences in position of the hallux digit i foot posture and hindlimb excursion

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

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

tectonic controls on metamorphism partial melting and intrusion timing and duration of regional metamorphism and magmatism in the nigde massif turkey protracted heating and melting of orogenic middle crust followed by emplacement of a syn extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central turkey new geochronologic data for the nigde massif of the central anatolian crystalline complex document an interval of melting crystallization and pb loss at elevated ambient temperature 85 92 ma determined by u pb shrimp within a single sample of peraluminous granite crustal melting was syn to post peak metamorphism 725 c dated at 91 0 2 0 ma by shrimp analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton metamorphic monazite in sillimanite schist yields a younger age of 84 7 0 7 ma for matrix grains and inclusions in garnet hornblende 40ar 39ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to 12 km and 550 c the similarity of 40ar 39ar ages for biotite in the pluton 79 ma and hornblende 79 81 ma biotite 74 76 ma and muscovite 78 83 ma in the metamorphic country rock document rapid cooling 100 c my following intrusion the coincidence of the youngest u pb magmatic zircon ages and the oldest 40ar 39ar ages combined with structural evidence for normal sense shearing and tectonic unroofing in the late cretaceous suggests that intrusion was driven by extension we propose that crustal melting occurred in the middle crust during prograde metamorphism and or initial decompression and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks

paleozoic peralkaline a type magmatism of the tongbai orogen central china petrogenesis and tectonic implications abstract the precise timing petrogenesis and geodynamic significance of the paleozoic magmatic rocks in the southern qinling tongbai orogenic belt qtob central china are poorly constrained we performed an integrated study of petrology zircon u pb ages geochemistry and hf o nd isotopic compositions of the huangyangshan pluton from the southern qtob to constrain its genesis and tectonic implications a granite sample from the pluton yields a weighted mean zircon 206pb 238u age of 445 6 ma representing the age of the pluton the huangyangshan pluton is dominated by alkaline granite and quartz syenite has a peralkaline signature a nk 0 82 0 96 and belongs to the shoshonite and high k calc alkaline series both the alkaline granite and quartz syenite have geochemical signatures similar to a type granite with high sio2 and total alkali na2o k2o contents high fe numbers and tio2 mgo and ga al ratios and high field strength elements hfses along with low mgo cao and p2o5 contents these samples are enriched in light rare earth elements lrees and have flat heavy rare earth element hree patterns and strongly negative eu anomalies they are enriched in most large ion lithophile elements liles and hfses with significant negative eu ba sr and ti anomalies in primitive mantle normalized trace element diagrams the samples have relatively high zr contents 175 776 ppm yielding minimum magmatic temperatures of 768 c 891 c the huangyangshan pluton has initial zircon ehf t values of 6 6 to 10 0 and whole rock end t values of 2 86 to 3 62 the analyzed zircons yield a mean 18o value of 5 14 0 26 indicating negligible incorporation of hydrothermally altered crustal materials the geochemical signatures of the huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks combined with previous results we suggest that the paleozoic huangyangshan peralkaline a type granites resulted from the rifting of a passive continental margin in the northern south china block followed by the opening of the paleotethyan ocean

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

molybdenum isotope fractionation between mo4 and mo6 in silicate liquid and metallic mo abstract previous work has shown that mo isotopes measurably fractionate between metal and silicate liquids even at temperatures appropriate for core formation however the effect of variations in the structural environment of mo in the silicate liquid especially as a function of valence state on mo isotope fractionation remained poorly explored we have investigated the role of valence state in metal silicate experiments in a gas controlled furnace at 1400 c and at oxygen fugacities between 10 12 7 and 10 9 9 i e between three and 0 2 log units below the iron wustite buffer two sets of experiments were performed both with a silicate liquid in the cao al2o3 sio2 system one set used molybdenum metal wire loops as the metal source the other liquid gold alloyed with 2 5 wt mo contained in silica glass tubes x ray absorption near edge spectroscopy analysis indicates that mo6 mo in the silicate glasses varies between 0 24 and 0 77 at oxygen fugacities of 10 12 0 and 10 9 9 in the wire loop experiments and between 0 15 and 0 48 at 10 11 4 and 10 9 9 in the experiments with au mo alloys double spiked analysis of mo isotope compositions furthermore shows that mo isotope fractionation between metal and silicate is a linear function of mo6 mo in the silicate glasses with a difference of 0 51 in 98mo 95mo between purely mo4 bearing and purely mo6 bearing silicate liquid the former is octahedrally and the latter tetrahedrally coordinated our study implies that previous experimental work contained a mixture of mo4 and mo6 species in the silicate liquid our refined parameterisation for mo isotope fractionation between metal and silicate can be described as 98 95 mo metal silicate 1 43 0 14 10 6 mo 6 mo 8 6 10 4 t 2 molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of mo6 mo variation with oxygen fugacity is expanded for instance to include iron bearing systems on earth literature data indicate that the upper mantle is depleted in heavy mo isotopes relative to the bulk earth as represented by chondrites as previously highlighted this difference is most likely not caused by core formation which either enriches the mantle in heavy mo isotopes or causes no significant fractionation depending on temperature and as we determined here mo6 content we reaffirm that core formation does not account for the mo isotope composition of the modern upper mantle which may instead reflect the effect of fractionation during subduction as part of global plate recycling

origin of a cretaceous low 18o granitoid complex in the active continental margin of se china abstract low 18o felsic igneous rocks with 18o values of 1 low sr y 7 23 49 5 and la yb ratios 6 87 46 0 and relatively low zr saturation temperatures 730 c in addition the diorites and granodiorites contain a variety of inherited high ca plagioclase an 60 87 we demonstrate that the xiaocuo granitoid rocks were derived from preexisting subduction related metabasic source rocks their low 18ozrc values 3 8 5 2 are reflective of their source compositions the formation of the low 18o source rocks is most likely formed in a subduction induced lithospheric extension regime during early jurassic to early cretaceous time when a basin and range like province developed in se china intensive lithospheric extension could have increased the fracture permeability and facilitated deep circulated surface waters for exchanging with the source rocks at high temperature at depth making the metabasic source rocks depleted in 18o remelting of the 18o depleted metabasic intrusive source rocks produced the xiaocuo low 18o granitoid complex at 116 ma our work indicates that in addition to the caldera and rift settings extensional active continental margin could also be a favorable setting for the formation of low 18o granitoid rocks

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

archosaur footprints cf brachychirotherium with unusual morphology from the upper triassic fleming fjord formation norian rhaetian of east greenland the orsted dal member of the upper triassic fleming fjord formation in east green land is well known for its rich vertebrate fauna represented by numerous specimens of both body and ichnofossils in particular the footprints of theropod dinosaurs have been described recently an international expedition discovered several slabs with 100 small chirotheriid pes and manus imprints pes length 4 4 5 cm in siliciclastic deposits of this unit they show strong similarities with brachychirotherium a characteristic upper triassic ichnogenus with a global distribution a peculiar feature in the fleming fjord specimens is the lack of a fifth digit even in more deeply impressed imprints therefore the specimens are assigned here tentatively to cf brachychirothe rium possibly this characteristic is related to the extremely small size and early ontogenetic stage of the trackmaker the record from greenland is the first evidence of this morphotype from the fleming fjord formation candidate trackmakers are crocodylian stem group archosaurs how ever a distinct correlation with known osteological taxa from this unit is not currently possible while the occurrence of sauropodomorph plateosaurs in the bone record links the greenland assemblage more closer to that from the germanic basin of central europe here the described foot prints suggest a pangaea wide exchange supplementary material three dimensional model of cf brachychirotherium pes manus set from mguh 31233b from the upper triassic fleming fjord formation norian rhaetian of east greenland as pdf ply and jpg files 3d model created by oliver wings photographs taken by jesper milan is available at https doi org 10 6084 m9 figshare c 2133546

geochronology and geochemistry of leucosomes in the north dabie terrane east china implication for post uhpm crustal melting during exhumation migmatites are widespread in the north dabie ultrahigh pressure metamorphic terrane ndt of dabie orogen east china idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase quartz and biotite suggesting formation of leucosome by fluid present melting of biotite plagioclase quartz bearing protoliths at p 5 7 kbar t 700 800 c precise sims zircon u pb dating indicates that migmatization of dabie orogen initiated at 140 ma and lasted for 10 ma coeval with the formation of low mg adakitic intrusions in dabie orogen based on mineralogical petrographic and geochemical data leucosomes in ndt can be subdivided into three groups 1 high la yb n medium sr y group group i whose high dy yb n but medium sr y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses 2 low la yb n low sr y group group ii whose flat hree patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss 3 high la yb n high sr y and eu group group iii whose extremely high sr and eu but low other ree concentrations are caused by accumulation of plagioclase and quartz although group i and iii fall in the adakitic fields on la yb n yb n and sr y y diagrams they are chemically distinct from contemporary high pressure adakitic intrusions in dabie orogen in a series of geochemical indexes for example lower dy yb n and or sr y ratios at given la yb n ratio lower sr cao ratios lower rb concentration but higher k rb ratios therefore leucosomes are produced by anatexis of the exhumed ultrahigh pressure metamorphic rocks at middle crustal level instead of partial melting of thickened lower crust with garnet rich and plagioclase poor residual the coeval occurrence of migmatites and high pressure adakitic intrusions in dabie orogen indicates large scale partial melting of middle to thickened lower crustal column in the early cretaceous the required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed

determination of fe3 σfe of olivine hosted melt inclusions using mossbauer and xanes spectroscopy iron speciation is linked to oxygen fugacity hence the fe 3 fe ratio of glasses can reveal the oxygen fugacity of the system where they last equilibrated previous studies using x ray absorption near edge structure xanes spectroscopy in silicate glasses and olivine hosted melt inclusions have shown that arc basalts are more oxidized than mid ocean ridge basalts morb and oceanic island basalts oib however cottrell et al 2018 recently demonstrated that hydrous glasses can experience beam induced oxidation during xanes analysis leading to an overestimation of their fe 3 fe ratios here we determined fe 3 fe ratios in olivine hosted melt inclusions from various arc morb and oib localities by mossbauer and xanes spectroscopy we carried out a careful evaluation of beam damage during xanes measurements that included detailed analysis of consecutive xanes spectra and collection of time series on hydrous basaltic and basanitic glasses using different radiation doses fe 3 fe ratios obtained using both spectroscopy methods on the same sample set allowed us to confirm the oxidation state of those magmas and the reliability of our approach to avoid the effect of beam induced oxidation during xanes analysis our results show that melt inclusions from morbs oibs and arc basalts display mean fe 3 fe ratios of 0 10 0 05 2 n 5 0 13 0 05 2 n 2 and 0 25 0 15 2 n 19 respectively while mount etna melt inclusions display a mean fe 3 fe ratio of 0 26 0 05 2 n 7 these results confirm that arc magmas are more oxidized than those from hot spots and mid ocean ridges

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

earth s earliest evolved crust generated in an iceland like setting the setting in which earth s first continental crust formed is unclear because few rocks survive from that time geochemical analysis of 4 03 billion year old rocks discovered in the acasta gneiss complex canada reveals rocks that are strikingly similar to those formed in iceland today implying earth s first continental crust was generated in an iceland like setting

thermochronological constraints on two stage extrusion of hp uhp terranes in the dabie sulu orogen east central china along the qinling dabie sulu orogenic belt in china crops out the world s largest terrane composed of ultrahigh pressure uhp metamorphic rocks differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high pressure hp and uhp slices in different parts of the belt the western part of the dabie orogen western dabie terrane holds a key to understanding of the transition from oceanic to continental subduction this paper reports geochronological results to test a two stage tectonic model for the exhumation of hp uhp rocks in western dabie this model involves two different stages and types of extrusion for exhumation of the hp uhp rocks in east central china mica ar ar ages ranging from 241 to 231 ma indicate a general middle triassic cooling probably driven by early upward extrusion during the collision between the north and south china blocks late triassic early jurassic cooling was associated with later eastward extrusion ranging from 200 to 184 ma the second event is recorded also in mica in the region that was not affected by later deformation and magmatism the lateral movement along lithosphere scale faults resulted in the eastward extrusion of the hp uhp metamorphic terrane which was followed in the late triassic early jurassic time by a major compressive event these two extrusion events are correlative with the two stages of triassic exhumation of the western dabie hp uhp rocks respectively wintin the framework of the qinling dabie sulu orogenic belt it is suggested for western dabie that the subduction exhumation of blueschist facies unit is related to the mianlue suture whereas the subduction exhumation of hp uhp eclogite facies units is related to the shangdan suture

geochemistry and zircon u pb geochronology of paleoproterozoic arc related granitoid in the northwestern yangtze block and its geological implications abstract the early history of the yangtze block has not been well constrained yet due to the scarce outcrops of archean to paleoproterozoic rocks in this study we report an integrated study of zircon u pb age major and trace element data and sr nd hf isotope compositions for gray gneisses from the houhe complex in the northwestern part of the yangtze block zircon u pb dating yields a weighted mean 207 pb 206 pb age of 2081 9 ma for a gneiss this age is interpreted as the formation age of the houhe complex and thus the houhe complex represents the oldest rocks found in the western part of the yangtze block the gray gneisses from the houhe complex range in sio 2 contents from 58 63 to 68 59 and na 2 o from 3 88 to 5 28 and have relatively high fe 2 o 3 contents of 2 86 6 69 al 2 o 3 of 16 01 18 88 and mgo of 0 97 2 65 these rocks show low sr 149 390 ppm cr 9 07 45 1 ppm and ni 4 97 21 3 ppm contents but high y 12 9 32 7 ppm and yb 0 95 2 25 ppm they are characterized by a relative enrichment in liles and lrees but a depletion in hfses these features are similar to those of calc alkaline granitoids suggesting that their formation might be related to a subduction related process or remelting of preexisted arc rocks compiled age spectra of archean to paleoproterozoic zircon grains reveal that the western and eastern parts of the yangtze block have similar archean and early paleoproterozoic age ranges indicative of the occurrence of an old continental nucleus in both parts on the other hand there are large amounts of ca 2050 2400 ma zircon grains revealed in the western part of the yangtze block implying that there might be a microcontinent with an active type continental margin during the paleoproterozoic times in the western part of the yangtze block because of the poor outcrops of archean to paleoproterozoic rocks in the yangtze block it is no possible to establish the exact nature of such an active margin yet the occurrence of ca 2 0 ga khondalitic and metasedimentary rocks has been documented in the eastern part of the yangtze block implying that there might be a passive type continental margin in the eastern part of the yangtze block at ca 2 0 ga the collision between the western microcontinent and the eastern part may have resulted in the final arc continental collision and the amalgamation of the yangtze block

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

in situ xanes study of the influence of varying temperature and oxygen fugacity on iron oxidation state and coordination in a phonolitic melt iron oxidation state and environment in magmas affect their phase diagram and their properties including viscosity and density which determine magma mobility and eruptive potential in turn magma composition pressure temperature and oxygen fugacity affect iron oxidation state and coordination potentially leading to complex feedbacks associated with magma ascent degassing and eruption while equilibrium experiments and models have led to a deep understanding of the role of iron in melts our knowledge of the effects of disequilibrium processes on iron oxidation state and its structural role in lavas and magmas remains limited accordingly we performed a series of dynamic disequilibrium experiments on a natural melt composition a phonolite lava from erebus volcano antarctica at atmospheric pressure in which oxygen fugacity and temperature were controlled and varied during the experiments we continuously measured iron oxidation and coordination using fe k edge dispersive x ray absorption spectroscopy xas we found that iron oxidation state changes in the phonolite melt are reversible and well reproduced by existing models changes in iron oxidation state are driven by joint diffusion of alkali cations and oxygen anions at magmatic temperatures 1000 c for erebus phonolite however redox diffusion timescales are too slow for any significant oxygen exchange with the atmosphere at the lava air interface or via air entrainment turning to iron coordination while fe 2 and fe 3 are present mostly in an average five fold coordination complex coordination variations decoupled from redox changes were detected the data suggest transitions between fe 3 in four fold and six fold coordination prior to reduction or as a consequence of oxidation this questions the possible implication of fe coordination changes in triggering crystallisation of magnetite nanolites upon magma ascent and through such crystallisation events in promoting magma explosivity

zircon u pb ages and hf isotope compositions of migmatites from the north qinling terrane and their geological implications migmatites are predominant in the north qinling nq orogen but their formation ages are poorly constrained this paper presents a combined study of cathodoluminescence imaging u pb age trace element and hf isotopes of zircon in migmatites from the nq unit in the migmatites most zircon grains occur as new homogeneous crystals while some are present as overgrowth rims around inherited cores morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting the inherited cores have oscillatory zoning and yield u pb ages of c 900 ma representing their protolith ages the early neoproterozoic protoliths probably formed in an active continental margin being a response to the assembly of the supercontinent rodinia the migmatite zircon yields hf model ages of 1911 20 to 990 22 ma indicating that the protoliths were derived from reworking of palaeoproterozoic to neoproterozoic crustal materials the anatexis zircon yields formation ages ranging from 455 5 to 420 4 ma with a peak at c 435 ma combined with previous results we suggest that the migmatization of the nq terrane occurred at c 455 400 ma the migmatization was c 50 ma later than the c 490 ma ultra high p uhp metamorphism indicating that they occurred in two independent tectonic events by contrast the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the nq unit which collectively argue for their formation due to the northward subduction of the shangdan ocean uhp rocks were distributed mainly along the northern margin and occasionally in the inner part of the nq unit indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process

zircon u pb ages and hf o isotopic composition of migmatites from the zanjan takab complex nw iran constraints on partial melting of metasediments abstract we study migmatites and other metamorphic rocks in the zanjan takab region of nw iran and use these results to report the first evidence of oligocene core complex formation in iran four samples of migmatites associated with paragneisses including leucosomes and associated para amphibolite melanosomes were selected for u pb dating and hf o isotopic analysis zircon cores interpreted as originally detrital zircons have variable ages that peak at ca 100 110 ma but their sedimentation age indicated by the youngest 206 pb 238 u ages is ca 35 40 ma new zircons associated with incipient melting occur as overgrowths around zircon cores and or as newly grown grains morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting all four samples contain zircons with rims that yield 206 pb 238 u ages of 28 25 ma indicating that partial melting occurred in late oligocene time 18 o values for zircon rims vary between 8 2 and 12 3 significantly higher than expected for mantle inputs 18 o 6 and consistent with equilibrium with surface materials zircon rims yield ehf t between 2 2 and 12 4 and two stage hf model ages of 448 562 ma indicating that the region is underlain by cadomian caledonian crust according to the hf o isotopic values the main mechanism forming zircon rims was dissolution of pre existing detrital zircons with reprecipitation of new zircon shortly thereafter oligocene ages indicate that partial melting accompanied core complex formation in the zanjan takab region extension melting and core complex formation in south central iran are eocene in age but younger ages of oligocene miocene in nw iran and turkey indicate that extension was distributed throughout the region during cenozoic time

origin of the late paleoproterozoic low δ18o a type granites on the southern margin of the north china craton and their geodynamic mechanism abstract a type granites with 18o depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock forming geodynamic mechanism here we report a suite of late paleoproterozoic low 18o a type granites from the longwangzhuang pluton on the southern margin of the north china craton sims zircon u pb dating showed that the longwangzhuang pluton was emplaced at 1613 4 ma combined with previous data it is clear that the mineral components of longwangzhuang granites are variable and consist of perthite quartz plagioclase biotite and amphibole calcic hastingsite ferroedenite and sodic ferrogedrite geochemically they are metaluminous to peraluminous and have high alkali contents k2o na2o 8 76 wt k2o na2o 1 feot feot mgo 0 95 ratios zr nb ce y 993 ppm contents and 10000 ga al 4 31 ratios resembling a type granites in situ oxygen isotope analysis displayed low zircon 18o values between 4 0 and 4 9 with a weighted average of 4 4 based on the archean two stage zircon hf and whole rock nd model ages coupled with the low la yb cn ratios negative anomalies of eu and sr and high zr saturation temperatures and the presence of low pressures 3 3 5 7 kbar amphiboles the longwangzhuang a type granites were generated by self cannibalizing pre existing low 18o old rocks i e the taihua complex and emplaced at shallow depth combined with regional geological observations i e coeval tuffites a passive rift setting was the most plausible explanation for the generation of the late paleoproterozoic low 18o a type granites on the southern margin of the north china craton such a rifting could likely have lasted at least till 1 6 ga

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

relationships between zircon u pb shrimp ages and leucosome type in migmatites of the halls creek orogen western australia field relations and zircon zoning and shrimp u pb patterns introduction from an outcrop of deformed pelitic migmatites reveal relationships both the amount and isotopic variability of accessory between leucosome type and the amount of zircon growth during minerals incorporated into granitic magmas and their migmatization zircons from the oldest stromatic migmatites show constituent trace elements are a complex function of several pb pb intragrain analyses with 1850 1840 ma source and crystallization processes bea et al 1994 anatectic ages and distinctive cathodoluminescence cl patterns brown 1994 ayres et al 1995 barbey et al 1995 bea although the sample is dominated by inferred pre 1855 ma detrital 1996 it is recognized that initial bulk compositions x zircons subsequent thick contact sheath leucosomes and backand pressure p temperature t conditions of anatexis veins developed at 1845 1830 ma in response to intrusion of are major controls on the chemistry and subsequent mafic dykes into the stromatic migmatites zircons show cl evidence behaviour of granitic magmas e g clemens 1990 for substantial anatectic growth and overgrowths with post 1850 brown 1994 also there is increasing awareness that ma ages and a single population age 20 my younger than the deformation in the source region can also control the early other migmatites zircons from the structurally youngest shearand physical pathways and chemical evolution of magmas e g vein hosted leucosomes show few overgrowths and retain a large sawyer 1991 hand dirks 1992 brown et al 1995 detrital population correlation between shear zones and minimal rushmer 1995 watt et al 1996 oliver barr 1997 anatectic zircon growth is inferred to reflect rapid deformationwhich in turn may have a profound impact on subsequent induced melt extraction from the protolith and or rapid quenching granitic magma evolution for a given bulk source comthe earlier contact sheath and stromatic leucosomes probably had position magma extracted at low melt fractions from a combination of slow melt segregation larger interacting melt an actively deforming partially molten zone may differ volume and longer residence times at high temperatures results substantially in composition from one extracted at high suggest that rapid melt extraction by deformation will increase the melt volumes from rocks undergoing little deformation tendency for preservation of source rock isotopic patterns in some rushmer 1995 the link between frozen migmatites granites and complete granite bodies may not be direct clemens

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

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

two stage triassic exhumation of hp uhp terranes in the western dabie orogen of china constraints from structural geology abundant exposures of widely distributed hp uhp metamorphic rocks in the western part of the dabie orogen enable us to study the tectonic evolution of hp uhp terranes associated with the world s largest preserved continental subduction zone previous tectonic models for the dabie orogen were based largely on metamorphic studies most of them lacking significant structural constraints we present a comprehensive structural analysis based on detailed structural geology the results suggest that syn uhp d0 at 241 231 ma and syn hp d1 at 225 215 ma southeast vergent thrusting formed a series of stacked structural slices this was followed by southeast vergent folding under amphibolite facies conditions d2 at 215 205 ma then a third generation of flexural folding occurred at shallow levels d3 at 200 184 ma this leads us to proposes a two stage triassic exhumation model in which initially rapid vertical extrusion d0 d1 from uhp to hp conditions to lower crustal levels is followed by slow southeastward extrusion d3 from lower crustal levels to the earth s surface the tectonic model combines the early southeastward vertical extrusion with the later southeastward lateral extrusion revealing two different stages and thus different types of triassic extrusion for the exhumation of hp uhp rocks in the dabie orogen the first stage extrusion occurred in the middle triassic whereas the second stage extrusion lasted from the late triassic to early jurassic these two extrusion episodes correlate with the two stages of triassic exhumation of the dabie hp uhp rocks respectively during continental collision

diverse tetrapod trackways in the lower pennsylvanian tynemouth creek formation near st martins southern new brunswick canada abstract newly discovered tetrapod trackways are reported from eight sites in the lower pennsylvanian tynemouth creek formation of southern new brunswick canada by far the most abundant and well preserved tracks comprise pentadactyl footprints of medium size 32 53 mm long with slender digits and a narrow splay mostly

dinosaur tracks and possible lungfish aestivation burrows in a shallow coastal lake lowermost cretaceous bornholm denmark abstract the mesozoic succession of the island of bornholm in the baltic sea has hitherto yielded only very few remains after vertebrate life we describe a new find of a well exposed vertebrate trample ground and large densely spaced vertical to inclined burrows from shallow lake deposits belonging to the berriasian skyttegard member of the rabekke formation the succession is terrestrial but a coastal setting is indicated by the great abundance of pyrite concretions and framboidal pyrite organic geochemical and palynological data show that the lake was surrounded by a low diversity gymnospermous vegetation the trampling ground is exposed in a vertical section at the boundary between light grey structureless or trough cross bedded fluvial sand and dark brown organic rich lacustrine mud with densely spaced vertical rootlets wood fragments and large pyrite concretions the tracks are bowl shaped have an irregular flat or slightly conical base are on average 22 cm deep and on average 43 cm wide in the upper part and 23 cm in the lower part but some reach widths up to about 1 m and have a highly irregular shape the underlying sand in some cases shows a faint stratification that is bent downwards adjacent to the mud filled tracks which have near vertical and commonly overhanging margins the morphology of the bowl shaped depressions suggests that they were made by dinosaurs most likely sauropods they were clearly emplaced after deposition of the dark brown mud long vertical to inclined commonly slightly curved j shaped or spiraling cylindrical burrows with a diameter averaging 4 4 cm and lengths up to 45 cm occur densely spaced in the mud they are unbranched have no mud or lag linings the top of the burrows is indistinct or not preserved whereas the base is rounded concave up commonly with a slightly enlarged chamber the length is difficult to estimate precisely because of the missing top and because the burrows cross cut and go in and out of the plane of exposure the burrows have a passive infill of dark mud and at some levels of irregularly alternating laminae of white sand and mud the sand may be of aeolian origin or may represent distal crevasse splays or washovers from adjacent rivers or the nearby sea skeletal remains have not been found in or associated with the burrows which are similar to burrows referred to lungfish or crayfish and they are with hesitation interpreted as representing lungfish aestivation burrows based on the morphological characteristics this is only the second report of dinosaur tracks in denmark and the first in situ find where environmental conditions can be interpreted with some confidence possible lungfish burrows are here reported for the first time from the danish mesozoic the overall rarity of vertebrate remains in the mesozoic succession of bornholm may reflect that the area was a horst island or submerged shoal during much of the jurassic and cretaceous and may not always have had direct land connection with surrounding land areas of the baltic shield or mainland europe

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

u pb dates and trace element geochemistry of zircon from migmatite western gneiss region norway significance for history of partial melting in continental subduction article i nfo thewesterngneissregion wgr norway isdominatedbymigmatiticgneissthatcontainsinclusionsofeclogite some of which contain evidence for ultrahigh pressure metamorphism to evaluate geochemical and age relation ships between host migmatite and eclogite we obtained la icp ms u pb dates and trace element analyses for zircon from a variety of textural types of leucosome from layer parallel to crosscutting zircon textures euhedral oscillatory and sector zone grains indicate a likely magmatic origin of the leucosomes caledonian u pb zircon dates from zircon rim and near rim regions are as old as 410 406 ma coeval with previously determined ages of high and ultrahigh pressure metamorphismof wgr eclogite trace elementanalysesobtained simultaneously with u pb dates indicate crystallization of zircon under garnet present conditions in the majority of leucosomes other zircons including those from crosscutting pegmatite yield younger ages as young as 385 ma coinciding with dates determined for amphibolite facies retrogression of eclogite trace element analyses suggest that these zircons grew under plagioclase present garnet absent conditions combined age and trace element data for leucosome zircons record the transition from high pressure garnet present plagioclase absent crys tallization to lower pressure plagioclase present crystallization if the euhedral zircons that yield ages coeval with peak or near peak uhp metamorphism represent crystallization from anatectic leucosomes these results combined with field and petrographic observations of eclogite migmatite relationships are consistent with the presence of partially molten crust in at least part of the wgr during continental subduction the decreased viscosity and increased buoyancy and strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths

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

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

discrimination of protolithic versus metamorphic zircon ages in eclogites constraints from the erzgebirge metamorphic core complex germany abstract protolithic ages of meta basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans however in high grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint the mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature zircon from eclogites from the eastern erzgebirge metamorphic complex yielded both protolithic 540 ma 5 samples and metamorphic 340 ma 4 samples 207 pb 206 pb ages detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole plagioclase symplectite rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of zr nb lree and p eclogites with omphacite breakdown are very dark in colour in difference lighter coloured eclogites contain k feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite this supports additional new growth of zircon grains in some samples idiomorphic zircon grains with a lot of micro inclusions indicate a coupled dissolution reprecipitation process additional rounded resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth therefore scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth in contrast protolithic zircon ages were found in eclogites where no water or melt infiltration occurred this emphasizes the importance of water and or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies in the eastern erzgebirge most eclogites and amphibolites were derived from morb type or arc related basalts while the plagioclase rich amphibolites formed mainly from crustal sources the protolith ages of meta basites from the eastern erzgebirge 540 ma document a bimodal magmatic activity in a cadomian back arc basin

can magmatic water contents be estimated from clinopyroxene phenocrysts in some lavas a case study with implications for the origin of the azores islands abstract over the last decade there has been renewed interest in determining the water contents of basaltic magmas a commonly applied method is analysis of h 2 o from melt inclusions in olivine however it is also well known that these can rapidly lose or gain h 2 o by diffusion an alternative is to measure the h 2 o contents of clinopyroxene phenocrysts and use a partition coefficient d to estimate the original h 2 o content of the host magma this approach is not without complications and several recent studies have attempted to assess the effects of diffusive loss of h 2 o from magmatic clinopyroxenes in the ideal case these crystals should be taken from rapidly cooled tephra or lapilli but such materials are not always available in order to further assess the potential of using 5 10 mm clinopyroxenes from lavas we undertook a detailed multi analytical investigation of clinopyroxenes from an ankaramite flow on pico island in the azores we conclude that these can be trusted to preserve probably minimum magmatic h 2 o contents if the h 2 o concentrations of multiple clinopyroxenes from a single sample form a linear correlation with the al iv content that demonstrates a coupled substitution with little or no h 2 o loss conversely if h 2 o contents decrease from core to rim whereas al iv contents remain relatively constant then it is likely that those clinopyroxenes lost h 2 o during differentiation and or cooling we suspect that the olivine melt inclusions we analysed from pico and sao miguel islands also underwent diffusive loss of h 2 o using these criteria we present clinopyroxene derived magmatic h 2 o estimates for corvo flores faial pico and sao miguel islands that range from 0 28 to 2 2 wt when combined with published data these show that h 2 o contents often extend to higher values on the islands than along the adjacent mid atlantic ridge these localised elevated h 2 o contents can explain why the islands are emergent despite being situated away from the ridge and perhaps also the asymmetric nature of the bathymetry of the archipelago it is possible that this h 2 o was recycled from material subducted very early on in earth s history

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

continental subduction and exhumation of uhp rocks structural and geochronological insights from the dabieshan east china in the dabieshan the available models for exhumation of ultrahigh pressure uhp rocks are poorly constrained by structural data a comprehensive structural and kinematic map and a general cross section of the dabieshan including its foreland fold belt and the northern dabieshan domain foziling and luzenguang groups are presented here south dabieshan consists from bottom to top of stacked allochtons 1 an amphibolite facies gneissic unit devoid of uhp rocks interpreted here as the relative autochton 2 an uhp allochton 3 a hp rock unit susong group mostly retrogressed into greenschist facies micaschists 4 a weakly metamorphosed proterozoic slate and sandstone unit and 5 an unmetamorphosed cambrian to early triassic sedimentary sequence unconformably covered by jurassic sandstone all these units exhibit a polyphase ductile deformation characterized by i a nw se lineation with a top to the nw shearing and ii a southward refolding of early ductile fabrics the central dabieshan is a 100 km scale migmatitic dome newly discovered eclogite xenoliths in a cretaceous granitoid dated at 102 ma by the u pb method on titanite demonstrate that migmatization post dates hp uhp metamorphism ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming along the dome margins migmatite is gneissified under post solidus conditions and mylonitic ultramylonitic fabrics commonly develop the north and west boundaries of the central dabieshan metamorphics i e the xiaotian mozitan and macheng faults are ductile normal faults formed before late jurassic early cretaceous a cretaceous reworking is recorded by synkinematic plutons north of the xiaotian mozitan fault the north dabieshan domain consists of metasediments and orthogneiss foziling and luzenguang groups metamorphosed under greenschist to amphibolite facies which never experienced uhp metamorphism a rare n s trending lineation with top to the south shearing is dated at 260 ma by the 40ar 39ar method on muscovite this early structure related to compressional tectonics is reworked by top to the north extensional shear bands the main deformation of the dabieshan consists of a nw se stretching lineation which wraps around the migmatitic dome but exhibits a consistently top to the nw sense of shear the central dabieshan is interpreted as an extensional migmatitic dome bounded by an arched top to the nw detachment fault this structure may account for a part of the uhp rock exhumation however the abundance of amphibolite restites in the central dabieshan migmatites and the scarcity of eclogites found only in a few places argue for an early stage of exhumation and retrogression of uhp rocks before migmatization this event is coeval to the n s extensional structures described in the north dabieshan domain recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in triassic liassic times the main foliation is deformed by north verging recumbent folds coeval to the south verging folds of the south dabieshan domain an intense cretaceous magmatism accounts for thermal resetting of most of the 40ar 39ar dates a lithosphere scale exhumation model involving continental subduction synconvergence extension with inversion of southward thrusts into nw ward normal faults and crustal melting is presented

taphonomic controls on animal tracks at saline alkaline lake bogoria kenya rift valley impact of salt efflorescence and clay mineralogy abstract the morphologies of subaerial vertebrate tracks and invertebrate traces in surficial sediments can be used in paleoenvironmental reconstructions by providing details about the initial substrate conditions e g water content and any changes experienced during trace preservation a qualitative field study was undertaken to identify the factors affecting track taphonomy in saline lake margin environments supported by experiments to isolate selected factors that modify track morphology and substrate characteristics flamingo tracks ubiquitous around saline alkaline lake bogoria in the kenya rift were the focus of field study although other vertebrate tracks and surficial invertebrate trails and tunnels were also considered a complex and interacting set of environmental factors were identified such as the effect of lake level fluctuations on substrate grain size and capillary evaporation several factors affect the early taphonomic modification destruction stabilization and or cementation of tracks and their associated substrates the laboratory experiments tested the effects of pore water salinity clay mineralogy e g swelling or non swelling clays drying method e g solar radiation or wind and wetting and drying cycles on tracks these data were supported by statistical analyses of impression depth and planimetric area the results show that track morphology in smectitic substrates is altered rapidly by wetting and drying particularly if the substrates contain saline pore waters but morphology is less affected in non swelling clays the degree of morphologic alteration and or resistance to wetting and drying varies with the rate of substrate drying and the type of the resulting salt efflorescence e g interstitial vs surficial crust these short term process related taphonomic effects help to explain the morphologic variants of tracks in lake marginal settings and provide insights into the preserved distribution patterns of subaerial ichnofossil assemblages

the problem of dating high pressure metamorphism a u pb isotope and geochemical study on eclogites and related rocks of the marianske lazně complex czech republic this study presents new geochemical major and trace element nd sr isotope and u pb zircon monazite titanite and rutile data for various rock types eclogite high pressure granulite amphibolite orthogneiss leucosome of the high grade metamorphic marianske lazn complex in the western bohemian massif concordant u pb zircon analyses from the mafic to intermediate samples disclose the marianske lazn complex as a mixture of c 540 ma oceanic rocks juxtaposed at depth with lower crustal rocks of the structurally overlying tepla barrandian unit this interpretation refutes earlier models in which the marianske lazn complex was interpreted as a dismembered cambro ordovician ophiolite complex affected by variscan subduction metamorphic zircon in mafic rocks of the marianske lazn complex as well as monazite from orthogneiss and titanite in leucosome yield ages around 380 ma on the basis of detailed petrography and cathodoluminescence imaging we conclude that the c 380 ma age reflects the timing of variscan exhumation and associated decompression melting under upper amphibolite facies to granulite facies conditions thereby the data derived from metamorphic zircon of eclogites and high pressure granulite unexpectedly do not date the timing of eclogitization which could have happened just before variscan exhumation or even shortly after late cadomian protolith formation

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

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

experimental calibration of vanadium partitioning and stable isotope fractionation between hydrous granitic melt and magnetite at 800 c and 0 5 gpa vanadium has multiple oxidation states in silicate melts and minerals a property that also promotes fractionation of its isotopes as a result vanadium isotopes vary during magmatic differentiation and can be powerful indicators of redox processes at high temperatures if their partitioning behaviour can be determined to quantify the partitioning and isotope fractionation factor of v between magnetite and melt piston cylinder experiments were performed in which magnetite and a hydrous haplogranitic melt were equilibrated at 800 c and 0 5 gpa over a range of oxygen fugacities f text o text 2 bracketing those of terrestrial magmas magnetite is isotopically light with respect to the coexisting melt a tendency ascribed to the vi fold v3 and v4 in magnetite and a mixture of iv and vi fold v5 and v4 in the melt the magnitude of the fractionation factor systematically increases with increasing log f text o text 2 relative to the fayalite magnetite quartz buffer fmq from 51vmag gl 0 63 0 09 at fmq 1 to 0 92 0 11 sd at fmq 5 reflecting constant v3 v4 in magnetite but increasing v5 v4 in the melt with increasing log f text o text 2 these first mineral melt measurements of v isotope fractionation factors underline the importance of both oxidation state and co ordination environment in controlling isotopic fractionation the fractionation factors determined experimentally are in excellent agreement with those needed to explain natural isotope variations in magmatic suites furthermore these experiments provide a useful framework in which to interpret vanadium isotope variations in natural rocks and magnetites and may be used as a potential fingerprint the redox state of the magma from which they crystallise

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

a re assessment of the oxidation state of iron in morb glasses abstract the oxidation state of fe fe 3 fe where fe fe 2 fe 3 in glass samples of mid ocean ridge basalt morb from a wide range of localities was determined by xanes spectroscopy to be 0 10 2 n 42 this value is lower than that reported previously by xanes 0 16 1 n 103 but consistent with the most recent value determined by redox titrations 0 11 2 n 104 all for similar sets of samples we attribute the anomalously high xanes value of 0 16 to a calibration error resulting from the interpretation of mossbauer spectra and the resulting fe 3 fe values of the standards our alternative interpretation removes the problem of resolving fe 3 fe values 3 fe values that are consistent with the thermodynamically expected dependence on oxygen fugacity f o 2 fe 3 fe 2 is related to f o 2 for our synthetic morb composition by the temperature independent expression qfm 4 log fe 3 fe 2 4 23 5 where qfm is the f o 2 in log units relative to the quartz fayalite magnetite buffer the average f o 2 of natural morb was estimated to be qfm 0 1

paleozoic ages and excess 40ar in garnets from the bixiling eclogite in dabieshan china new insights from 40ar 39ar dating by stepwise crushing abstract the 40 ar 39 ar stepwise crushing technique is applied for the first time to date garnet from ultra high pressure metamorphic uhpm eclogites three garnet samples from the bixiling eclogites analyzed by 40 ar 39 ar stepwise crushing yield regular predictable age spectra and a clear separation between excess 40 ar and concordant plateau and isochron ages all three age spectra begin with high apparent ages followed by step by step decreasing ages and finally age plateaux with apparent ages in the range from 427 20 to 444 10 ma the data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 34 to 459 58 ma corresponding to initial 40 ar 36 ar ratios from 292 1 4 5 to 294 5 6 7 statistically indistinguishable from the modern air the high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40 ar whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40 ar the plateau ages are interpreted to approximate the time of garnet growth during initial uhpm metamorphism phengite analyzed by laser stepwise heating yielded a complicated two saddle age spectrum with a scattered isochron corresponding to age of 463 116 ma and initial 40 ar 36 ar ratio of 1843 1740 indicative of the presence of extraneous 40 ar within phengite these concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that dabie uhpm eclogites were first formed in the early paleozoic during the same event that caused the qinling northern qaidam basin altyn tagh eclogites

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