calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

metamorphic history and geodynamic significance of high grade metabasites from the ophiolitic melange beneath the yarlung zangbo ophiolites xigaze area tibet abstract blocks of highly foliated amphibolite are locally embedded within a serpentinite melange underlying the yarlung zangbo ophiolites in the xigaze area of southern tibet the ophiolites are remnants of an early cretaceous back arc basin within the permo cretaceous tethys ocean which are exposed along in the yarlung zangbo suture zone yzsz these amphibolites are interpreted as fragments of a dismembered dynamothermal sole three types of amphibolite are present 1 common amphibolite with assemblages of hbl pl ep ap ttn 2 clinopyroxene bearing amphibolite with hbl pl cpx ep ttn qtz ap and 3 garnet clinopyroxene bearing amphibolite characterized by the assemblages hbl cpx grt pl rt and grt hbl pl corona assemblage in all three types plagioclase is pseudomorphed by late albite prehnite retrograde cataclastic veins containing assemblages of prh ab ep chl are also present p t estimates indicate that the amphibolites reached peak metamorphic conditions of 13 15 kbar and 750 875 c partial replacement of pyrope rich up to 35 mole garnet by al tschermakite al2o3 up to 21 wt reflects a high pressure 18 kbar 600 c metamorphic event followed by rapid exhumation soon after exhumation the amphibolites were intruded by very fine grained diabase dykes that were then hydrothermally altered the field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back arc basin prior to obduction of the ophiolites onto the indian passive margin

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

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

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

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

effect of catchment scale subsurface mixing on stream isotopic response a 3 8 ha watershed on the west coast of new zealand was instrumented with suction lysimeters and automatic water samplers to determine the relationship between subsurface isotopic and chemical concentrations to those of rainfall and resulting streamflow a t test showed that 2 represented a significant difference between successive sample deuterium values eleven rainfall episodes were subdivided into two categories 1 two events where stream isotopic composition did not deflect 2 from prestorm values and 2 four events which demonstrated new water flushing detailed analysis of one 47 mm rainfall 9 8 mm runoff event showed that old water dominated stream water exiting the watershed by 90 using a standard two component hydrograph separation for deuterium corroborated by cl and electrical conductivity three component hydrograph separation indicated that 12 16 was in the form of soil water with 5 as on channel precipitation and 80 groundwater analysis of over 1000 water samples revealed systematic trends in soil water and groundwater isotopic composition both in a downslope and downprofile direction between storm suction lysimeter deuterium data showed a systematic dampened response to temporally variable rainfall deuterium concentrations multivariate cluster analysis revealed three distinct soil water groundwater groupings with respect to soil depth and geographic position within the watershed within storm suction lysimeter sampling preserved similar groupings indicating that the subsurface reservoir is poorly mixed on short time scales understanding subsurface mixing response to rainfall should greatly improve models of episodic stream response and partitioning of storm flow into waters of different age

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

new insights into the india asia collision process from cretaceous paleomagnetic and geochronologic results in the lhasa terrane abstract to further constrain the india asia collisional process a combined paleomagnetic and geochronologic study has been carried out on the upper cretaceous jingzhushan formation redbeds and the lower cretaceous dianzhong formation volcanic rocks dated at 121 117 ma from the cuoqin area in the central lhasa terrane stepwise thermal demagnetization successfully isolated reliable characteristic remanent magnetization chrm directions that include dual polarity and pass positive fold tests at 95 and 99 confidence levels indicating prefolding primary magnetizations the tilt corrected site mean direction for 33 redbed sites is d 316 8 i 30 2 with 95 5 4 corresponding to a paleopole at 49 0 n 344 3 e with a 95 5 3 and the other for 12 volcanic sites is d 350 5 i 25 5 with 95 7 7 corresponding to a paleopole at 70 5 n 292 9 e with a 95 7 4 our new paleomagnetic results together with reliable cretaceous paleomagnetic data obtained from the lhasa terrane demonstrate that the southern margin of asia was located at 15 1 n during the cretaceous comparison with the apparent polar wander paths apwp of india and the cretaceous paleocene paleopoles of the himalayan terrane suggests that the india asia collision was likely a complex process which consists of the collision of the lhasa and himalayan terranes at 54 0 2 1 ma the longtime subduction of an intra oceanic basin between the himalayan terrane and the indian craton from 54 0 to 40 4 ma and the collision of the himalayan terrane and the indian craton at 40 4 4 1 ma comparing with the late cretaceous average pole of the east asia apwp reveals that a latitudinal convergence of 780 240 km has taken place between the lhasa terrane and east asia the hexi corridor since the india asia collision the amount of latitudinal shortening deduced from paleomagnetic data is very consistent with the 600 1000 km accommodated by the cenozoic fold and thrust belts between the lhasa terrane and hexi corridor

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

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

iron isotopic systematics of uhp eclogites respond to oxidizing fluid during exhumation the iron stable isotope compositions 56fe and iron valence states of ultrahigh pressure uhp eclogites from bixiling in the dabie orogen belt china were measured to trace changes of geochemical conditions during vertical transportation of earth materials for example oxygen fugacity the bulk fe3 fe ratios of retrograde eclogites determined by mossbauer spectroscopy are consistently higher than those of fresh eclogites suggesting oxidation during retrograde metamorphism and fluid infiltration the studied eclogites five samples display limited morb like 0 10 56fe values which are indistinguishable from their protoliths i e gabbro cumulates formed through differentiation of mantle derived basaltic magma this suggests that fe isotope fractionation during continental subduction is limited garnet separates display limited 56fe variation ranging from 0 08 0 07 to 0 02 0 07 whereas coexisting omphacite displays a large variation of 56fe values from 0 15 0 07 to 0 47 0 07 omphacite also has highly variable fe3 fe ratios from 0 367 0 025 to 0 598 0 024 indicating modification after peak metamorphism omphacite from retrograde eclogites has elevated fe3 fe ratios 0 54 0 60 compared to that from fresh eclogites 0 37 whereas garnet displays a narrow range of ferric iron content with fe3 fe ratios from 0 039 0 013 to 0 065 0 022 the homogenous 56fe values and fe3 fe ratios of garnet suggest that it survived the retrograde metamorphism and preserved its fe isotopic features and ferric contents of peak metamorphism because of similar diffusion rates of fe mg in garnet and omphacite and constant 26mgomphacite garnet values 1 14 0 04 equilibrium iron isotope fractionation between garnet and omphacite was probably achieved during peak metamorphism elevated fe3 fe ratios of omphacite from retrograde eclogites and variant 56feomphacite garnet values of the studied eclogites 0 13 0 10 to 0 48 0 10 indicate that oxidized geofuild infiltration resulted in elevation of 56fe values of omphacite during retrograde metamorphism this article is protected by copyright all rights reserved

stratigraphic and sedimentological constraints on the age and tectonic evolution of the neotethyan ophiolites along the yarlung tsangpo suture zone tibet

prograde metamorphism decompressional partial melting and subsequent melt fractional crystallization in the weihai migmatitic gneisses sulu uhp terrane eastern china abstract in order to decipher fluid melt activity during subduction and subsequent exhumation of felsic continental crust we carried out a combined study of zircon internal structure zircon u pb age zircon trace element composition and whole rock geochemistry on migmatitic gneisses from the weihai region in the sulu ultra high pressure uhp metamorphic terrane eastern china the weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase pl rich leucosome and k feldspar kfs rich pegmatite veins whole rock geochemistry suggests that mgo fe2o3t cao tio2 p2o5 rare earth elements ree and large ion lithophile elements hfse are nearly completely retained in the melanosome whereas significant amounts of lile e g rb ba k sr pb and u are partitioned into the pl rich leucosome and the kfs rich pegmatite veins the kfs rich pegmatite vein has much higher k2o u pb and rb but lower cao and na2o contents than those of the pl rich leucosome layer protolith magmatic zircon domains with an upper intercept age of about 780 ma are common in the cores of zircon grains from the melanosome and the pl rich leucosome layers indicating that the protolith of the migmatitic gneiss is mid neoproterozoic magmatic rock metamorphic zircons with concordant ages ranging from 243 to 256 ma occur as overgrowth mantles on the protolith magmatic zircon cores they are characterized by remarkably flat heavy rare earth element hree patterns without obviously negative eu anomalies and have low y and th contents low th u and high hf y ratios indicating a prograde metamorphism in the hp eclogite facies during subduction recrystallized rims of zircons 228 2 ma in the pl rich leucosome layer are similar to magmatic zircons in terms of their hree enriched patterns with negative eu anomalies high y and low th contents and low hf y and th u ratios this indicates that the gneiss has experienced decompressional partial melting and the pl rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation zircon grains 219 2 ma in the kfs rich veins are characterized by aqueous fluid melt related signatures with strongly hree enriched patterns high u pb and low th contents and low th u and high lu hf ratios indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of pl rich leucosome layers during later stage cooling during exhumation our results suggest that the migmatitic gneiss successively recorded information concerning the protolith prograde hp metamorphism during subduction fractional crystallization of primary melt and crystallization of the residual melt during exhumation

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

the capillary fringe and its effect on water table response abstract shallow water tables are often observed to respond in a highly disproportionate manner to precipitation events that is the magnitude of the response is often much greater than would be predicted on the basis of the specific yield of the geologic material and the amount of rainfall physical arguments are presented to show that if the capillary fringe the zone of tension saturation extends to ground surface then the addition of a very small amount of water can result in an immediate and large rise in the water table the arguments are supported by a field experiment in which the addition of 0 3 cm of water caused the water table to rise 30 cm in 0 25 min thus demonstrating the large and highly transient influence of the capillary fringe on the position of the water table the evidence in this study as well as recent work of others suggests that the capillary fringe effect could have a major influence on the process of stream flow generation contaminant transport to surface waters and contaminant transport in hydrogeologic regimes having shallow water tables it is further shown that calculations of groundwater recharge and consumptive use based on water table response and an assumed specific yield could be substantially in error

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

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

geochronology of fluid induced eclogite and amphibolite facies metamorphic reactions in a subduction collision system bergen arcs norway rb sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes granulite facies rocks of the lindas nappe bergen arcs norway were subducted and exhumed during the caledonian orogeny the rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation along fractures and shear zones the first occurred at eclogite facies 15 21 kbar 650 750 c and a later one at amphibolite facies conditions 8 10 kbar 600 c the presence of fluids enabled local metamorphic equilibration only near fluid pathways in fluid absent domains preexisting assemblages were metastably preserved this resulted in a heterogeneity of metamorphic signatures on meter to m scales well preserved granulite facies rocks preserve their proterozoic rb sr mineral ages as does the u pb system of zircon in most lithologies six rb sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid induced eclogitization at 429 9 3 5 ma 2 weighted average mswd 0 39 an eclogite facies vein has yielded metamorphic zircon with concordant u pb ages of 429 3 ma identical to the u pb age of 427 4 0 9 ma for zircon xenocrysts in an amphibolite facies vein seven rb sr mineral isochron ages date amphibolite facies fluid infiltration at 414 2 2 8 ma mswd 1 5 an age value testifying to residence of the rocks in the deep orogenic crust at temperatures 600 c for nearly 15 ma the new data show that rb sr mineral isochron ages effectively date fluid induced re crystallization events rather than stages of cooling the direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the p t reaction time space which is essential for understanding exhumation histories and the internal dynamics of orogens in general

zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh pressure jadeite quartzite in the dabie orogen abstract a combined study of mineral inclusions u pb ages trace elements and hf o isotopes was carried out for zircons from a coesite bearing jadeite quartzite in the dabie orogen the results provide insights into the action of ultrahigh pressure uhp metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone the zircons mostly show core rim structures in cathodoluminescence images the overgrown rims contain rare mineral inclusions and exhibit concordant u pb ages of 225 to 246 ma and flat hree patterns with negligible eu anomalies in contrast the relict cores contain uhp metamorphic mineral inclusions such as coesite jadeite and rutile and show discordant u pb ages ranging from 983 to 2045 ma and steep ree patterns with significant negative eu anomalies the u pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000 43 ma and 234 18 ma respectively we interpret the rims with triassic ages as the new growth of metamorphic origin and the cores with precambrian ages as the protolith relics of magmatic origin the relict magmatic zircons underwent two subtypes of metamorphic recrystallization i e solid state transformation and metasomatic alteration the solid state recrystallized zircons exhibit slightly discordant u pb ages close to the protolith age steep mree hree patterns and almost unchanged hf isotope ratios these observations point to the lowest degree of resetting to the geochemical composition of protolith zircons in contrast the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon ree composition and u pb and lu hf isotopic systems all of the zircon domains regardless of the rims and cores show relatively consistent 18 o values of 4 0 0 2 such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the uhp metamorphic fluids of triassic age the metasomatic recrystallization of protolith zircons is indicated by the occurrence of uhp inclusion minerals such as coesite rutile and jadeite in sealed microcracks in this regard the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the uhp conditions as such the metasomatic recrystallization has heterogeneously reset the u pb and lu hf isotope systems of protolith zircons the composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in si ti na and al these elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss therefore the jadeite quartzite would be precipitated from the uhp metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction zone metamorphism

forearc hyperextension dismembered the south tibetan ophiolites suprasubduction zone ophiolites are relics of oceanic upper plate forearcs and are typically preserved as discontinuous belts with discrete massifs along suture zones ophiolites usually contain an incomplete condensed section compared to average modern oceanic lithosphere the incompleteness and discontinuity of ophiolites are frequently attributed to dismemberment but tectonic causes remain poorly constrained here we show new paleomagnetic and field geological evidence for the preservation of extensional detachment faults that thinned and dismembered the south tibetan ophiolite belt during the early cretaceous similar to those documented in modern slow and ultraslow spreading ridges these detachments exhumed lithospheric mantle and subophiolitic melange to the seafloor which became unconformably covered by asia derived forearc strata we call this mechanism forearc hyperextension whereby widespread detachment faults accommodate upper plate extension above a subduction zone we propose that hyperextension is the key mechanism responsible for dismemberment of the south tibetan ophiolitic belt shortly after its magmatic accretion

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

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

fore arc basalts and subduction initiation in the izu bonin mariana system recent diving with the jamstec shinkai 6500 manned submersible in the mariana fore arc southeast of guam has discovered that morb like tholeiitic basalts crop out over large areas these fore arc basalts fab underlie boninites and overlie diabasic and gabbroic rocks potential origins include eruption at a spreading center before subduction began or eruption during near trench spreading after subduction began fab trace element patterns are similar to those of morb and most izu bonin mariana ibm back arc lavas however ti v and yb v ratios are lower in fab reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid ocean ridges and back arc basins some fab also have higher concentrations of fluid soluble elements than do spreading center lavas thus the most likely origin of fab is that they were the first lavas to erupt when the pacific plate began sinking beneath the philippine plate at about 51 ma the magmas were generated by mantle decompression during near trench spreading with little or no mass transfer from the subducting plate boninites were generated later when the residual highly depleted mantle melted at shallow levels after fluxing by a water rich fluid derived from the sinking pacific plate this magmatic stratigraphy of fab overlain by transitional lavas and boninites is similar to that found in many ophiolites suggesting that ophiolitic assemblages might commonly originate from near trench volcanism caused by subduction initiation indeed the widely dispersed jurassic and cretaceous tethyan ophiolites could represent two such significant subduction initiation events

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

coesite and clinopyroxene exsolution lamellae in chromites in situ ultrahigh pressure evidence from podiform chromitites in the luobusa ophiolite southern tibet abstract we report in situ petrological evidence of deep mantle origin from podiform chromitites in the luobusa ophiolite southern tibet analytical transmission electron microscopy measurements reveal that chromites in podiform chromitites from the luobusa ophiolite have numerous exsolution lamellae of diopsidic clinopyroxene and coesite which indicate an ultrahigh pressure origin of over 3 gpa 100 km deep the presence of these lamellae coupled with abundant micro inclusions of clinopyroxene requires high solubility of sio2 and cao in the host chromite and suggests a precursor of chromite a cafe2o4 structured high pressure polymorph stable at pressures over 12 5 gpa 380 km deep these nano scale observations and geological occurrence indicate that the mantle peridotite under the tibetan mid ocean ridge was transported from the deep mantle at least 100 km probably more than 380 km deep by the mantle convection it implies that the root of mantle upwelling has a much deeper origin than previously believed

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

composition and pre metamorphic geodynamic setting of the ultrahigh pressure metabasic rocks from dabie shan e china as part of the yangtze plate segments of the dabie shan terrane of central china underwent ultra high pressure metamorphism during triassic subduction we studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths and their geodynamic setting although some previous geochemical work exists the analyses and interpretation herein are based on a new subdivision of the ultra high pressure sequence into basement and cover units changpu and ganghe unit revealing new and important results in addition eclogites of the so called hp unit south of the uhp units were studied whereas the large ion lithophile elements indicate postmagmatic metasomatic changes of some samples the high field strength elements and the rare earth elements display original magmatic trends the geochemical characteristics of the eclogites of the ultra high pressure areas display a strong dependence on their structural and geographic position the eclogites of the basement and the changpu unit indicate melt intrusion and extrusion in a continental rift system i e during extensional tectonics in contrast the ganghe unit is characterized by a pronounced chemical homogeneity the composition of the eclogites indicates generation from a mantle source highly influenced by slab derived fluids those of the hp unit show similar characteristics magmatism of the ganghe and hp unit probably occurred in a continental arc setting a similar age for both units geographically and or tectonically separated is possible the geodynamic interpretation based on the geochemistry of the four units points to a neoproterozoic scenario in which the protoliths of the hp and the cover units could have been of similar age and deposited in one evolving geological system a rift related larger scale basin might have formed e g a continental back arc basin behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc alternatively magmatism occurred in independent geodynamic settings distinct in time and space the units were juxtaposed during exhumation after subduction to varying depths

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

calculation of oxygen isotope fractionation in metal oxides abstract a theoretical model calculating the thermodynamic isotope factors of oxygen in metal oxides of geochemical interest is presented for temperatures between 0 and 1200 c on the basis of the modified increment method the following order of 18o enrichment is obtained cassiterite rutile magnetite ilmenite ulvospinel hematite uraninite chromite corundum spinel the results are expressed as 103 in a 106 t2 b 103 t c which compare well with existing theoretical experimental and empirical calibrations on quartz oxides and oxides water systems the present calculations enable not only estimation of the oxygen isotope composition of fluids from which the metal oxides were precipitated when independent temperature information is available but can also be applied as oxygen isotope geothermometers to igneous and metamorphic rocks

the subduction initiation rule a key for linking ophiolites intra oceanic forearcs and subduction initiation we establish the subduction initiation rule sir which predicts that most ophiolites form during subduction initiation si and that the diagnostic magmatic chemostratigraphic progression for sir ophiolites is from less to more hfse depleted and lile enriched composi tions this chemostratigraphic evolution reflects formation of what ultimately becomes forearc lithosphere as a result of mantle melting that is progressively influenced by subduction zone enrichment during si the magmatic chemostratigraphic progression for the izu bonin mariana ibm forearc and most tethyan ophiolites is specifically from morb like to arc like volcanic arc basalts or vab boninites or bon because si progressed until establishment of a mature subduction zone morb like lavas result from decompression melting of upwelling asthenosphere and are the first magmatic expression of si the contribution of fluids from dehydrating oceanic crust and sediments on the sinking slab is negligible in early si but continued melting results in a depleted harzburgitic residue that is progressively metasomatized by fluids from the sinking slab subsequent partial melting of this residue yields typical ssz like lavas in the latter stages of si if si is arrested early e g as a result of collision morb only ophiolites might be expected consequently morb and ssz only ophiolites may represent end members of the si ophiolite spectrum the chemostratigraphic similarity of the mariana forearc with that of ophiolites that follow the sir intimates that a model linking such ophiolites oceanic forearcs and si is globally applicable

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

effects of a beaver pond on runoff processes comparison of two headwater catchments abstract natural variations in concentrations of 18 o d and h 4 sio 4 in two tributary catchments of woods lake in the west central adirondack mountains of new york were measured during 1989 1991 to examine runoff processes and their implications for the neutralization of acidic precipitation by calcium carbonate treatment the two catchments are similar except that one contained a 1 3 ha beaver pond evaporation from the beaver pond caused a seasonal decrease in the slope of the meteoric water line in stream water from the catchment with a beaver pond wo2 no corresponding change in slope of the meteoric water line was evident in stream water from the other catchment wo4 nor in ground water nor soil water from either catchment indicating that evaporative fractionation was not significant application of a best fit sine curve to 18 o data indicated that base flow in both catchments had a residence time of about 100 days ground water from a well finished in thick till had the longest residence time 160 days soil water from the o horizon and b horizon had residence times of 63 and 80 days respectively water previously stored within each catchment pre event water was the predominant component of streamflow during spring snowmelt and during spring and autumn rainfall events but the proportion of streamflow that consisted of pre event water differed significantly in the two catchments the proportion of event water rain and snowmelt in wo2 was smaller than at wo4 early in the spring snowmelt of march 13 17 1990 but the proportions of source water components for the two catchments were almost indistinguishable by the peak flow on the third day of the melt the event water was further separated into surface water and subsurface water components by utilizing measured changes in h 4 sio 4 concentrations in stream water during the snowmelt results indicated that subsurface flow was the dominant pathway by which event water reached the stream except during the peak flow of a rain on snow event on the last day of the melt streamflow from a spring rain storm with dry antecendent conditions two months later may 16 18 1990 was less than 5 event water at peak flow in wo2 and 26 in wo4 this change from the runoff pattern in march is attributed to retention of event water in the beaver pond favored by relatively low pre event storage and isothermal nonstratified conditions in the pond that allowed mixing streamflow during several autumn storms was about 15 25 event water at peak flow in wo4 the highest values for event water were associated with wet antecedent moisture conditions these results indicate that a beaver pond can significantly affect the downstream delivery of event water through evaporation and mixing but provides minimal retention during large runoff events such as snowmelt beaver ponds are expected to provide greater opportunity for neutralization of acidic waters during most of the year in catchments treated with calcium carbonate but little neutralization effect during snowmelt

subduction zone decoupling retreat modeling explains south tibet xigaze and other supra subduction zone ophiolites and their uhp mineral phases abstract the plate tectonic setting in which proto ophiolite oceanic lithosphere is created remains controversial with a number of environments suggested recent opinions tend to coalesce around supra subduction zone ssz forearc extension with a popular conceptual model in which the proto ophiolite forms during foundering of oceanic lithosphere at the time of spontaneous or induced onset of subduction this mechanism is favored in intra oceanic settings where the subducting lithosphere is old and the upper plate is young and thin we investigate an alternative mechanism namely decoupling of the subducting oceanic lithosphere in the forearc of an active continental margin followed by subduction zone trench retreat and creation of a forearc oceanic rift basin containing proto ophiolite lithosphere between the continental margin and the retreating subduction zone a template of 2d numerical model experiments examines the trade off between strength of viscous coupling in the lithospheric subduction channel and net slab pull of the subducting lithosphere three tectonic styles are observed 1 c continuous subduction without forearc decoupling 2 r forearc decoupling followed by rapid subduction zone retreat 3 b breakoff of subducting lithosphere followed by re initiation of subduction and in some cases forearc decoupling b r in one case ba b r where ba denotes backarc subduction zone retreat follows backarc rifting subduction zone decoupling is analyzed using frictional plastic yield theory and the stefan solution for the separation of plates containing a viscous fluid the numerical model results are used to explain the formation of xigaze group ophiolites southern tibet which formed in the lhasa terrane forearc likely following earlier subduction and not necessarily during subduction initiation either there was normal coupled subduction before subduction zone decoupling or precursor slab breakoff subduction re initiation and then decoupling rapid deep upper mantle circulation in the models during subduction zone retreat can exhume and emplace material in the forearc proto ophiolite from as deep as the mantle transition zone thereby explaining diamonds and other 10 15 gpa uhp phases in tibetan ophiolites

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

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

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

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

transition of uhp eclogites to gneissic rocks of low amphibolite facies during exhumation evidence from the dabie terrane central china the shuanghe ultrahigh pressure uhp slab in the dabie mountains consists of layered coesite bearing eclogite jadeite quartzite marble and biotite gneiss and is fault bounded against hosting orthogneiss representative assemblages of eclogite are grt omp coe rt ky phn mgs it formed at p 27 kbar and 680 720 50 c during exhumation these uhp rocks experienced multistage retrograde metamorphism coesite was overprinted by quartz aggregates phengite by biotite muscovite and rutile by titanite garnet was successively replaced by a thin rim of amp amp pl and amp ep bt pl minor omphacite and kyanite were replaced by amp pl cpx or bt and by zo pl ms mrg bt respectively secondary calcite occurs as irregular pockets in some layers an outcrop near the uhp slab border is composed of 20 thin concordant layers of foliated eclogites amphibolite and gneissic rocks of variable bulk composition these layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low amphibolite facies through hydration metasomatism and recrystallization retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals but the fluid for retrograde reactions was internally buffered in the stable isotope compositions retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of co2 rich and low salinity aqueous intensity of deformation and mineral resistance to alteration the fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites and or as isolated pockets in extensive or less altered eclogite layers

the zedong terrane a late jurassic intra oceanic magmatic arc within the yarlung tsangpo suture zone southeastern tibet an overturned sequence of igneous and volcaniclastic rocks crops out along the yarlung tsangpo suture zone ytsz in southeastern tibet these rocks are remnants of an intra oceanic island arc the zedong terrane that once lay between india and asia 40ar 39ar dating and u pb ion microprobe analyses reported here reveal that this arc was active during the jurassic u pb dating of zircon from a dacite breccia from the middle portion of the zedong terrane yields an age of 161 2 3 ma 1 40ar 39ar dating of hornblende from a cross cutting andesite dyke yields the youngest age of 152 2 3 3 ma 1 40ar 39ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156 0 3 ma 1 additional u pb and 40ar 39ar dating of zircon and hornblende from quartz diorite yields a mean age of 156 8 0 8 ma 1 geochronological data reported here and other published work indicate that the intra oceanic subduction system to which the zedong terrane belonged was active from at least late jurassic to early to mid cretaceous

characteristics of oxygen 18 and deuterium composition in waters from the pecos river in american southwest abstract the pecos river situated in eastern new mexico and western texas receives snowmelt from winter storms in the headwater region of the southern rocky mountains and runoff from warm season monsoonal rainfall in the lower valley the isotopic composition of the two water sources differs from each other due to their different geographical origins in the pacific north and the gulf of mexico to better assess the physical features of the pecos river oxygen and hydrogen isotopic compositions 18 o and d major ion concentrations and other physical variables e g water temperature and electrical conductivity were measured on water samples collected from the main stem and its selected tributaries during a growing season march may and july in 2005 the results of this work indicate that stream water from the pecos river contains a relatively large magnitude of variations in 18 o and d with 18 o ranging from 8 9 to 3 6 and d from 64 5 to 1 6 the average value of 18 o is around 3 which is significantly larger than that of the snowmelt but almost identical to that of the mexican monsoonal rainfall on the other hand the average value of d is around 30 which is significantly larger than that of the snowmelt but lower than that of the mexican monsoonal rainfall application of a dual isotope index deuterium excess d excess allows us to assess the relative contribution of various hydrologic components and processes that shape the stream hydrology of the pecos river the river water from the upper valley is characterized by relatively low values of 18 o and d and relatively high values of d excess d 10 documenting the isotopic fingerprint of the snowmelt the middle basin is topographically gentle and its water quality has been severely affected by anthropogenic disturbances e g water impoundments and diversions as a result chemical and isotopic composition of the water from the middle basin is highly variable depending on its time location and degree of disturbances both 18 o and d increase significantly from upstream to downstream and from cold to warm seasons because of evaporation induced isotopic enrichments the average 18 o value of the heavy isotope enriched waters from the middle basin is identical to that of the waters from the lower valley in contrast d excess of the waters from the middle basin usually is negative and substantially lower than that of the waters from the lower valley using a simple d excess based hydrologic model we estimated that there was up to 85 of stream flow which was derived from local freshwater sources mainly from the mexican monsoonal rainfall in the lower valley and that there was up to 33 of stream water which was lost through evaporation occurring in stream channels and fields of the middle basin additionally the correlation of d excess and electrical conductivity further highlights the role of evaporative enrichments in regulating stream chemistry and isotope hydrology this study demonstrates the usefulness of combined isotopic and geochemical data especially the applicability of d excess for watershed baseline assessments

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

hydrograph separation using stable isotopes silica and electrical conductivity an alpine example abstract hydrograph separation of runoff events in two nested alpine subalpine basins in the coast mountains of british columbia was carried out using electrical conductivity specific concentration of silica and the stable isotopes oxygen 18 and deuterium as hydrological tracers the methods predicted consistent high pre storm water contribution for the subalpine site 60 90 but more variable contribution at the alpine basin outlet 25 90 the pre storm water contribution is much larger than had previously been expected precipitation is believed to run off as overland flow due to the steep slopes in combination with the hydrophobic soils until it can enter the subsurface environment the rapid influx of stored water is possibly caused by pressure propagation in the macropore system which could be enhanced by the heavily fractured bedrock and permeable landslide debris acting as efficient hydrological conduits the study suggests that alternative hydrological tracers such as electrical conductivity and silica concentration can be used under certain hydrological and lithological conditions these alternative tracers should however be verified against more conventional tracers before use as the behaviour depends on specific characteristics of each basin at the upper basin outlet both electrical conductivity ec and silica underestimated the pre storm contribution at the lower station silica and ec showed a similar pattern to deuterium and oxygen 18 tracers the calculated pre storm component using ec was however 10 20 lower than the calculated values from the other three tracers the advantage of using these alternative tracers is that hydrograph separation results can a priori be anticipated

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

oxygen isotope geochemistry of ultrahigh pressure metamorphic rocks from 200 4000 m core samples of the chinese continental scientific drilling abstract oxygen isotope studies were carried out for ultrahigh pressure metamorphic rocks from the main hole of the chinese continental scientific drilling in the sulu orogen the samples of interest include various types of lithology mainly eclogite and gneiss in a depth of 200 to 4000 m containing five continuous core segments between contrasting lithologies the results show a large variation in 18o value from 10 41 to 9 63 for constituent minerals distinct 18o depletions are observed in the frequently alternated layers of eclogite and gneiss in which the variations of 18o values are gradual regardless of lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals special attention was paid to the relationship between distance petrography and 18o value of adjacent samples the results show o isotope heterogeneities between the different and same lithologies on scales of about 20 to 50 cm corresponding to the maximum scales of fluid mobility during the continental collision amphibolite facies retrograde metamorphism during exhumation caused mineral reactions and o isotope disequilibria between some of the minerals considerable changes occur in 18o and petrography at the contact between eclogite and gneiss suggesting that the contact between different lithologies is the most favorable place for fluid activity despite the widespread retrogression retrograde fluid was internally buffered in the stable isotope compositions the retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl although locally external fluids became available along fault zones and lithological layers it still acted within the exhumed slabs with internal origins fluid flow also took place after the amphibolite facies retrogression but it only affects the o isotope compositions of feldspar and mica premetamorphic protoliths are deduced to have heterogeneous 18o values due to varying degrees of meteoric water rock interaction before continental collision minimum depth of the 18o depletion is up to 3300 m together with areal 18o depletion in surface outcropped rocks along the dabie sulu orogenic belt at least 66 000 km3 of supracrustal rocks were interacted with meteoric water along the northern margin of the south china block during the neoproterozoic

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

geochemical and u pb age constraints on the occurrence of polygenetic titanites in uhp metagranite in the dabie orogen abstract relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in uhp metagranite in the dabie orogen la icpms u pb dating gave neoproterozoic ages for the magmatic titanite but triassic ages for the metamorphic titanite the magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions the magmatic titanite occurs as residual cores that show bright bse the presence of allanite and quartz inclusions low contents of cao al2o3 and tio2 but high contents of fe2o3 and mgo in trace elements the magmatic titanite exhibits high ree and hfse contents distinctly negative eu anomalies with flat mree hree patterns and high th u ratios in contrast the metamorphic titanite occurs as rims and grains of homogeneously dark bse that contain inclusions of epidote quartz k feldspar rutile biotite and phengite and have relatively high contents of cao al2o3 and tio2 but low contents of fe2o3 and mgo and relatively low ree and hfse contents slightly negative eu anomalies with hree depletion relative to mree and low th u ratios the zr in titanite thermometry yields 727 to 877 c at 0 5 to 1 0 gpa for the magmatic titanite and 729 to 870 c at 1 5 to 2 0 gpa for the metamorphic titanite the neoproterozoic u pb chronometric system of magmatic titanite survived the triassic continental subduction zone hp uhp metamorphism this suggests a relatively high closure temperature of 800 c for the titanite u pb system the metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from hp eclogite facies to amphibolite facies therefore titanite holds a great potential to geochronological and petrogenetic studies of continental subduction zone metamorphic rocks

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

protolith control on fluid availability for zircon growth during continental subduction zone metamorphism in the dabie orogen abstract different episodes of zircon growth are recognized by a combined study of cl images mineral inclusions u pb ages trace elements and lu hf isotopes for ultrahigh pressure uhp eclogite facies metamorphic rocks in the dabie orogen the results provide insights into the effect of protolith property on fluid liberation during continental collision fluid availability from premetamorphic protoliths of different origins is recognized as a key to the zircon growth zircon u pb dating for uhp metabasalt and metasediment eclogite and its host paragneiss yields two groups of ages at 244 3 and 225 2 ma respectively mineral inclusion trace element and lu hf isotope analyses also suggest that these two groups of zircon grew from hydrous melt during the final subduction and supercritical fluid during the initial exhumation respectively in contrast zircon u pb dating for uhp metaintrusive rocks granitic orthogneiss and its hosted eclogite gave only one group of age at 222 2 ma mineral inclusion trace element and lu hf isotope analyses suggest that the metamorphic zircon grew from aqueous fluid during the initial exhumation the difference between the two groups of zircon u pb dates is attributed to the difference in their protolith origin volcanic and sedimentary rocks contain large amounts of water primarily in the form of molecular water pore fluid so that considerable amounts of aqueous fluid can be released from them during subduction this episode of fluid action was recorded by the growth of anatectic zircon at 244 ma in contrast intrusive rock only contains small amounts of water primarily in the form of structural hydroxyl in crystalline minerals so that little fluid can be released from them during subduction nevertheless large amounts of retrograde fluid were released from uhp metamorphic rocks regardless of their protolith origin during decompression exhumation this episode of fluid action was recorded by the growth of metamorphic and anatectic zircons at 220 225 ma therefore the protolith property is a key to the liberation of aqueous fluid from metamorphosing rocks during subduction which has great bearing on partial melting element transport and mineral growth during continental subduction zone metamorphism

mineral oxygen isotope and hydroxyl content changes in ultrahigh pressure eclogite gneiss contacts from chinese continental scientific drilling project cores a combined study of mineral o isotopes and hydroxyl contents was carried out for the contacts between ultrahigh pressure eclogite and gneiss from main hole of the chinese continental scientific drilling project in the sulu orogen while there is a large d 18 o variation from 8 3 to 7 3 for all minerals different styles of mineral pair fractionation occur at the boundaries of different lithologies both equilibrium and disequilibrium o isotope fractionations are observed between quartz and the other minerals with reversed fractionations between omphacite and garnet in some samples of eclogite this suggests that both eclogite and gneiss acquired their negative d 18 o values by meteoric hydrothermal alteration of their protoliths at high temperatures before subduction and that fluid assisted o isotope exchange did take place across the boundary of different lithologies at local scales during amphibolite facies retrogression fourier transform infrared spectroscopy analysis yielded h2o concentrations of 50 to 1144 p p m by weight for garnet and 139 to 751 p p m for omphacite the state of equilibrium or disequilibrium o isotope fractionations between omphacite and garnet are correlated with variations in their water content at local scales indicating that the internally derived fluid plays a critical role in retrograde metamorphism during exhumation the retrograde metamorphism results in mineral reactions and o isotope disequilibria between some of the minerals but the fluid for retrogression was derived from the decompression exsolution of structural hydroxyl and thus internally buffered in the o isotope composition a quantitative estimate suggests that a hand specimen 3 6 9 cm of eclogite composed of 70 garnet and 30 omphacite can release 0 316 g water by the decompression exsolution of structural hydroxyl which can form 14 4 g amphibole during exhumation this provides sufficient amounts of water for the amphibolite facies retrogression

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

hydrograph separation using continuous open system isotope mixing we propose a new hydrograph separation method for runoff source modeling based on continuous open system isotope mixing using a variable source area and three isotopic reservoirs the three reservo

basement cover sequences within the uhp unit of the dabie shan field data define two lithologically distinct basement cover sequences within the ultrahigh pressure metamorphic uhpm unit of the dabie shan eastern china one of the cover units the changpu unit comprises calc arenitic metasediments in stratigraphic contact with a basement consisting of gneisses of the yangtze craton the second cover unit the ganghe unit consists of felsic intermediate metavolcanics and clastic metasediments basement exposure of the ganghe unit is not known fold axes in the ganghe unit are oblique to those of the changpu unit which are parallel to those of the yangtze gneisses preservation of primary textures in some volcanic rocks and tectonic separation from the yangtze gneisses by a greenschist facies mylonite support an interpretation of the ganghe unit as a low strain domain protolith associations in the ganghe and changpu units are compatible with deposition in a rift setting and along a passive continental margin respectively a u pb single zircon age of 761 33 ma for volcanoclastic rocks of the ganghe unit demonstrates a neoproterozoic deposition age concordant with inferred rifting at that time eclogite facies parageneses in the gneisses and both cover units along with p t data demonstrate regional uhpm in the dabie shan

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

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

storage outflow modelling of streamflow recessions with application to a shallow soil forested catchment abstract the objectives of this study were to introduce an alternative to the matching strip correlation and parameter averaging methods for deriving master recession characteristics from a set of recession segments and to apply the method to streamflow recession segments for a small forested catchment in which baseflow is provided by drainage of the saturated zone in the shallow permeable soil the method involves fitting conceptual storage outflow models using an iterative algorithm diagnostic plots revealed that the recessions were non linear and that the recessions did not follow a common single valued storage outflow relation a model involving two linear reservoirs provided substantially better fit than three single reservoir models indicating that the form of the recession curve probably depends not just on the volume of subsurface storage but also on its initial distribution amongst reservoirs

behavior of calcium isotopes during continental subduction recorded in meta basaltic rocks abstract in order to investigate the behavior of ca isotopes during subduction we report high precision ca isotopic data of a suit of well characterized meta basaltic rocks from the dabie sulu orogen including six greenschists six amphibolites and seven eclogites except two samples that may have been affected by carbonation alteration greenschists amphibolites and eclogites yield comparable 44 42ca values ranging from 0 36 to 0 41 0 32 to 0 39 and 0 32 to 0 42 respectively overall dabie sulu meta basaltic rocks have an average 44 42ca of 0 37 0 06 2sd n 17 19 comparable to terrestrial basalts previously reported 0 36 0 05 2sd n 34 combined with no correlation of 44 42ca with h2o and rb tio2 this study suggests that prograde metamorphism dehydration cannot significantly fractionate ca isotopic compositions of meta basaltic rocks given the comparable cao contents in meta basaltic rocks with increasing metamorphism grade isotope fractionation may be limited by insignificant ca loss during dehydration two heavy greenschists with 44 42ca of 0 46 and 0 49 also have high co2 concentrations indicating carbonation alteration that is evidenced by 0 04m hcl leaching experiments the limited 44 42ca variation observed in the other metamorphic samples is most likely inherited from their igneous protoliths considering that terrestrial basalts have 44 42ca systematically lower than the upper mantle subduction of mafic lithologies e g oceanic slabs thus could reflux light ca isotopes back into the mantle and may create mantle heterogeneity

numerical simulation of unsaturated flow along preferential pathways implications for the use of mass balance calculations for isotope storm hydrograph separation an objective common to many watershed studies is to separate storm hydrographs into two components water that was present in the watershed prior to a storm event soil moisture and groundwater and water which fell on the watershed during the storm to use this approach a number of assumptions must be made including that the composition of water in the soil moisture and groundwater reservoirs are constant and known the objective of this paper is to show that in settings where flow and transport are dominated by preferential pathways for flow steady state mass balance calculations for quantitative hydrograph separation may be in error we present field data from a site where flow and transport are dominated by preferential pathways relict fractures in saprolite of sedimentary rocks which indicate that the 18o content of the water in the unsaturated and shallow saturated zones is not constant over the course of a storm event we use a numerical model to further explore the interactions between the fractures and surrounding matrix both the field data and modeling results indicate that the 18o of the previous storm event s has a strong influence on water in the fractures on the time scale of a storm event only the water in the matrix immediately surrounding the fracture mixes with water in the fracture while the bulk of the matrix is isolated from fracture flow the spatial and temporal heterogeneity of the 18o in the subsurface and the isolation of the most of the matrix water from flow in fractures make the measurement of a singular 18o value for subsurface reservoirs problematic and the assumption of a constant value doubtful since most near surface geologic materials have preferential flow paths we suggest that quantitative hydrograph separation using mass balance techniques is not possible in most situations future field and modeling investigations using the approach outlined here could be designed to explore the important temporal and spatial scales of variability in watersheds and lead to a more quantitative approach to storm hydrograph separation

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

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

geodynamic regimes of intra oceanic subduction implications for arc extension vs shortening processes abstract 40 of the subduction margins of the earth are intra oceanic they show significant variability in terms of extension and shortening we investigated numerically the physical controls of these processes using a 2d petrological thermo mechanical intra oceanic subduction model with spontaneous volcanic arc growth and deformation we varied the fluid and melt related weakening the ages of both the subduction slab and the overriding plate the subducting plate velocities and the cohesive strength of rocks three main geodynamic regimes were identified retreating subduction with opening of a backarc basin stable subduction and advancing compressive subduction the main difference between these regimes is the degree of rheological coupling between plates which is governed by the intensity of rheological weakening induced by fluids and melts retreating subduction regimes require plate decoupling which results from strong weakening due to both fluids and melts spreading centers nucleate either in forearc or in intraarc regions episodic trench migration is often due to variations of plate coupling with time which is caused by fore arc deformation stable subduction regime with little variation in the trench position forms at an intermediate plate coupling and shows a transient behavior from the retreating to advancing modes the advancing subduction regime results from strong plate coupling at the mature stage this subduction mode is associated with both partial fragmentation and subduction of the previously serpentinized forearc region forearc subduction is typically associated with a magmatic pulse which is caused by dehydration of subducted serpentinized forearc fragments our models demonstrate distinct differences in thermal and lithological structure of subduction zones formed in these different geodynamic regimes results compare well with variations observed in natural intra oceanic arcs

paleomagnetism and u pb zircon geochronology of lower cretaceous lava flows from the western lhasa terrane new constraints on the india asia collision process and intracontinental deformation within asia to better understand the india asia collisional process and intracontinental deformation within asia a paleomagnetic study has been conducted on the qushenla formation lava flows dated at 132 120 ma from the yanhu area in the western lhasa terrane stepwise thermal demagnetization isolates stable characteristic remanent magnetizations which include dual polarity and pass fold tests at a 99 confidence level indicating primary magnetizations the tilt corrected site mean direction for 51 sites is d 28 2 i 34 5 and k 74 3 with 95 2 3 corresponding to a paleopole at 61 4 n 192 9 e a95 2 1 our new paleomagnetic data combined with previous cretaceous volcanic paleomagnetic data from the lhasa terrane show that the precollisional southern margin of asia was at 16 8 n with a relatively e w alignment and that the lhasa terrane did not experience significantly discrepant north south movement although local vertical axis rotations did take place after the cretaceous comparison with the apparent polar wander paths apwps of india and the cretaceous paleocene paleopoles of the tethyan himalaya shows that the india asia collision was at 54 3 ma and the greater india during the paleocene had a large northern extent of 2000 km 18 1 beyond its present northern margin comparison with the cretaceous eurasian apwp indicates that a latitudinal convergence of 1000 km has taken place between the lhasa terrane and eurasia since the india asia collision the amount of north south shortening deduced from cretaceous paleomagnetic data is consistent with that accommodated by the cenozoic fold and thrust belts between the lhasa terrane and the hexi corridor

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

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

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks

recharge sources and hydrochemical evolution of an urban karst aquifer sete lagoas mg brazil the current water supply for the city of sete lagoas brazil is almost entirely groundwater from the sete lagoas karst aquifer with a small contribution from a fractured basement rock aquifer characterizing the hydrogeologic processes is important for proper water resource management avoiding contamination and other future issues the main goal of this study is to identify active hydrogeological processes such as possible surface ground water interactions sources of recharge and the hydrochemical evolution of groundwater through the use of stable isotopes 18o and 2h and major ion data the groundwater samples were collected from the karst aquifer central urban area and from the fractured aquifer southern part of the city surface water samples were collected from the seven main lakes and rainwater was sampled over a period of one year the stable isotope results indicated the groundwater origin is directly from local precipitation having a limited recharge period and locally receiving surface water contributions groundwater quality in the central urban area may be influenced by surface water infiltration where the karst aquifer is in contact with overlying unconsolidated sediments some samples with nitrate concentration of 10 mg l or higher indicates the natural composition of the groundwater has been altered by urban sewage contamination the highest concentrations of major ions were found in the central region where the most karstified area from the sete lagoas formation is located these data suggest a longer residence time for this water indicating more mineralization in accordance with the geological information

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

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

u pb zircon ages for yarlung tsangpo suture zone ophiolites southwestern tibet and their tectonic implications ophiolite complexes preserved along the yarlung tsangpo suture zone ytsz and obducted onto the northern continental margin of india in southern tibet represent the remnants of the once extensive permian mesozoic neo tethyan ocean that separated india from asia complete ophiolite successions are preserved near xigaze whereas the rest of the belt is essentially represented by mantle rocks with subordinate disrupted lower crustal rocks u pb zircon la mc icp ms geochronology on two gabbro samples from the luobusa ophiolite yielded concordant data with mean 206pb 238u ages of 149 9 1 4 2 ma and 150 0 5 0 ma these ages are in contrast to a younger age of 131 8 1 0 ma obtained from a pegmatitic gabbro in xigaze five u pb zircon tims ages from gabbroic samples in the western portion of the ophiolite belt reveal that the dangxiong ophiolite formed between 126 7 0 4 ma and 123 4 0 8 ma zircons from the jungbwa ophiolite have similar ages of 123 4 0 8 ma and 123 9 0 9 ma a single zircon analysed from a gabbro in kiogar has an age of 159 7 0 5 ma geochronological data reported here show ytsz ophiolites formed in association with intra oceanic subduction zone systems and are related a significant tectonic episode within the tethyan ocean during late jurassic to early to mid cretaceous times