h2o fluid saturated melting of subducted continental crust facilitates exhumation of ultrahigh pressure rocks in continental subduction zones we present two dimensional numerical models of plate subduction and collision inspired by the scandinavian caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra high pressure metamorphic rocks three possible reactions were tested low temperature solidus representing h2o fluid saturated partial melting and two end member reaction curves for dehydration melting thermo mechanical effects of partial melting were implemented as 1 a viscosity decrease as a determined rheologically critical melt percentage was reached here 0 1 2 a change in effective heat capacity and adiabatic heating cooling accounting for a latent heat term in the heat equation among the 3 tested reactions only h2o fluid saturated partial melting drastically modifies the collision dynamics from the non melting reference model holding all other parameters constant a substantially low general viscosity truncation here 1017 pa s is needed to properly resolve the effect of partial melting on deep collision processes low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment where migmatites exhume from uhp conditions at rates and with pressure temperature paths similar to the natural values acknowledged for the norwegian caledonides high temperature melting has no drastic influence on early collision dynamics while positive buoyancy remains the first order driver for the exhumation of buried continental rocks exhumation initiates in these cases with eduction subsequent to slab detachment melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 c some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone even if minor in terms of amount of magma produced h2o fluid saturated partial melting at uhp conditions could therefore have a dramatic rheological effect and actually limits continental rocks subduction and facilitates their exhumation

crystal melt equilibria involving potassium bearing clinopyroxene as indicator of mantle derived ultrahigh potassic liquids an analytical review abstract crystal liquid equilibria including phase relationships of minerals with silicate and or carbonate melts are reviewed in order to understand the occurrence of clinopyroxene with up to 2 wt k 2 o kcpx this mineral occurs as inclusions in diamond from kimberlite pipes and in garnet from garnet clinopyroxene potassium poor silicate rocks intercalated with diamondiferous silicate carbonate rocks of the kokchetav complex northern kazakhstan the analysis of the available experimental data allowed estimation of the effect of p t and compositional parameters on the equilibrium of kalsi 2 o 6 in cpx 1 4k 4 si 2 o 6 3 4al 4 3 si 2 o 6 in melt in various silicate systems a strong dependence of the k 2 o partition coefficient both on pressure and sio 2 and al 2 o 3 contents in the melt was identified and thermodynamically described the resulting thermodynamic equation allows the calculation of pressure of the kcpx formation within interval 50 100 kbar for known melt compositions a model for the formation of kcpx in deep mantle potassium rich carbonate silicate magmas was derived using the available experimental and petrologic data the formation of kcpx from any potassium poor k 2 o k p cpx l k 2 o in cpx k 2 o of melt is much smaller then 1 the presence of cpx with 1 wt of k 2 o as inclusions in grt from potassium poor rocks could be explained only by the crystallization of kcpx from potassium rich silicate or silicate carbonate magma in the presence of kcl brine at p 70 kbar this chemical zoning of potassium bearing clinopyroxene suggests its crystallization together with feldspar from silicate melt during rapid ascent of melt toward the earth s surface from depth about 200 km since tschermak type substitution has not been observed in either pyroxene we suggested that feldspar resulted from the peritectic reaction kalsi 2 o 6 sio 2 l fl kalsi 3 o 8 at relatively shallow mantle levels a hypothetical phase diagram for the system ca mg fe si 2 o 6 kalsi 2 o 6 is presented

size and exhumation rate of ultrahigh pressure terranes linked to orogenic stage abstract a growing set of data indicates a stark contrast between the evolution of two types of ultrahigh pressure uhp terranes large terranes that evolved slowly over 10 30 myr and small terranes that formed and were exhumed on timescales of

late palaeozoic palaeomagnetic data for wrangellia resolution of the polarity ambiguity wrangellia and the alaska peninsula terrane are two of the many allocthonous terranes making up southern alaska today palaeomagnetic data from these areas clearly indicate low palaeolatitudes in the early mesozoic because of local and regional rotations the mean declination of the palaeomagnetic vectors is uncertain this in addition to incomplete temporal coverage has led to an ambiguity in the polarity of the vector and hence an ambiguity in whether the palaeolatitudes determined are in the northern or southern hemisphere the data considered here have resolved the ambiguity for wrangellia and by implication for the alaska pennisula terrane and also allow the construction of a highly speculative apparent polar wander curve

boron cycling by subducted lithosphere insights from diamondiferous tourmaline from the kokchetav ultrahigh pressure metamorphic belt abstract subduction of lithosphere involving surficial materials into the deep mantle is fundamental to the chemical evolution of the earth however the chemical evolution of the lithosphere during subduction to depth remains equivocal in order to identify materials subjected to geological processes near the surface and at depths in subduction zones we examined b and li isotopes behavior in a unique diamondiferous k rich tourmaline k tourmaline from the kokchetav ultrahigh pressure metamorphic belt the k tourmaline which includes microdiamonds in its core is enriched in 11b relative to 10b 11b 1 2 to 7 7 and 7li relative to 6li 7li 1 1 to 3 1 it is suggested that the k tourmaline crystallized at high pressure in the diamond stability field from a silicate melt generated at high pressure and temperature conditions of the kokchetav peak metamorphism the heavy isotope signature of this k tourmaline differs from that of ordinary na tourmalines in crustal rocks enriched in the light b isotope 11b 16 6 to 2 3 which experienced isotope fractionation through metamorphic dehydration reactions a possible source of the heavy b isotope signature is serpentine in the subducted lithospheric mantle serpentinization of the lithospheric mantle with enrichment of heavy b isotope can be produced by normal faulting at trench outer rise or trench slope regions followed by penetration of seawater into the lithospheric mantle serpentine breakdown in the lithospheric mantle subducted in subarc regions likely provided fluids with the heavy b isotope signature which was acquired during the serpentinization prior to subduction the fluids could ascend and cause partial melting of the overlying crustal layer and the resultant silicate melt could inherit the heavy b isotope signature the subducting lithospheric mantle is a key repository for modeling the flux of fluids and associated elements acquired at a near the surface into the deep mantle

experimental evidence for diamond facies metamorphism in the dora maira massif the peak metamorphic conditions of subducted continental crust in the dora maira massif western alps have been revised by combining experimental results in the kcmash system with petrologic information from whiteschists textural observations in whiteschists suggest that the peak metamorphic assemblage garnet phengite kyanite coesite talc originates from the reaction kyanite talc garnet coesite liquid in the experimentally determined petrogenetic grid this reaction occurs above 45 kbar at 730 c at lower pressures talc reacts either to orthopyroxene and coesite or together with phengite to biotite coesite and kyanite the liberated liquid contains probably similar amounts of h2o and dissolved granitic components the composition of the liquid in the whiteschists at peak metamorphic conditions a major unknown in earlier studies was probably very similar to the liquid composition produced in the experiments therefore the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists experimentally determined si isopleths for phengite further constrain peak pressures to 43 kbar for the measured si 3 60 of phengite in the natural whiteschists all these data provide evidence that the whiteschists reached diamond facies conditions the fluid absent equilibrium 4 kyanite 3 celadonite 4 coesite 3 muscovite pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra high pressure uhp metapelites for graphite bearing metapelites and kyanite phengite eclogites forming the country rocks of the whiteschists peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of 750 c estimated from garnet phengite thermometry therefore the whole ultra high pressure unit of the dora maira massif most likely experienced peak metamorphic conditions in the diamond stability field while graphite is common in the metapelites diamond has not been found so far the absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism the absence of a free fluid phase in the metapelites and a short residence time in diamond facies conditions resulting in kinetic problems in the conversion of graphite to diamond

early permian location of western north american terranes based on brachiopod fusulinid and coral biogeography abstract brachiopod rugose coral and fusulinid faunas present in early permian rocks along northern and western margins of north america in both cratonal localities and accreted terranes have been compared statistically allowing refinement in placement of these terranes relative to the north american continental margin at that time statistical tests including conventional and probabilistic indices of similarity as well as multivariate analysis show that the faunas of wrangellia alexander are most similar to those of stikinia and quesnellia and to the yukon canadian rocky mountains part of the continental margin they also show that faunas of the eastern klamath terrane are most similar to those of stikinia quesnellia and the central cordilleran part of the continental margin these results suggest that during the early permian wrangellia alexander was the northernmost of these terranes lying west of the yukon canadian rocky mountains region and the eastern klamath terrane was the southernmost lying west of the central cordillera stikinia and quesnellia were in intermediate positions these relative latitudes are compatible with latitudinal determinations based on paleomagnetic data faunal diversity in the eastern klamath and stikine terranes is similar to that at similar latitudes on the craton whereas diversity in the wrangellia alexander terrane is considerably lower than that of the latitudinally similar yukon canadian rocky mountains area as diversity is in part a function of temperature this may suggest water around the wrangellia alexander terrane was cooler than that along the continental margin therefore we suggest that warm water currents bathed the southern terranes and the southern and central parts of coastal north america whereas cool water currents influenced the northernmost terrane wrangellia alexander revised otsuka similarity coefficients for corals show higher similarities between western north american terranes and the western continental margin than previously published but they still are relatively low this and the overall patterns of similarity and limits on distribution of tethyan corals and fusulinids in the eastern panthalassa suggest that wrangellia alexander stikinia and the eastern klamath terranes were at approximately the same distance estimated at 2000 3000 km from their latitudinally equivalent segments of the north american craton

nanometer size p k rich silica glass former melt inclusions in microdiamond from the gneisses of kokchetav and erzgebirge massifs diversified characteristics of the formation media of metamorphic microdiamond in uhp rocks due to host rock buffering abstract nanometer size p k rich silica glass former melt inclusions were identified within metamorphic microdiamonds from garnets of ultrahigh pressure uhp gneisses of the kokchetav and the erzgebirge massifs by analytical electron microscopy aem the chemical characteristics of these inclusions within microdiamonds are surprisingly similar among various gneissic rocks from both kokchetav and erzgebirge but are significantly different from the si poor ultrapotassic fluid inclusions within microdiamonds from garnets of the kokchetav uhp marble these contrasting findings not only provide constraints on the characteristics compositions of the formation media of metamorphic microdiamonds but also imply that the formation media must have been buffered by the hosting rocks resulting in the observed diversities as reported here in addition depending on the rock types and thus on the nature of the formation media from which metamorphic microdiamonds were formed the respective characteristic morphologies of the microdiamonds differ the p k rich silica melt tends to form octahedral or cubo octahedral microdiamonds within garnet in gneissic rocks whereas the si poor ultrapotassic fluid tends to form spheroids cuboid microdiamonds with rugged surfaces within garnet in marble consequently the buffered media in hosting rocks played a decisive role in determining the different morphologies and growth rates mechanisms of metamorphic microdiamonds in general

paleomagnetism of the permian wallowa terrane implications for terrane migration and orogeny a sequence of volcanic astic sedimentary and pyroclastic rocks of permian age late leonardian or early guadalupian 260 ma in the wallowa terrane of oregon and idaho has been sampled for paleomagnetic study forty six sites 222 cores were collected from folded laminated siltstone mudstone and tuffaceous sandstone beds exposed along the snake river in hells canyon several stratigraphic sections within the 1500 to 2500 m thick hunsaker creek formation were sampled over a stratigraphic interval of about 700 m thermal demagnetization revealed three components of magnetization the a component has the lowest unblocking temperatures generally less than about 200oc has normal polarity and fails the fold test the b component is removed between heating steps of about 200oc and 550oc is of normal polarity and also fails the fold test the c component observed between heating steps of approximately 550oc and 660oc is always of negative inclination the grouping of this component improves slightly upon unfolding with the optimum grouping occurring at 95 unfolding using sites with n 3 and k 10 k95 ko l 18 or 100 using sites n 3 k95 k0 l 51 this component is interpreted as the characteristic remanent magnetization acquired during deposition of the hunsaker creek formation based on the following observations the better grouping of the high temperature magnetic component after unfolding and the consistent reversed polarity seen in samples from the different sections the resulting paleolatitude calculated using inclination only statistics of 24o 12 o n n 3 k 10 or 26o 9on n 3 sites implies a northern hemisphere position of the wallowa terrane during the early permian and possible southward transport of this terrane with respect to north america if the wallowa terrane was attached to the wrangellia superterrane our result indicates a northern hemisphere location for that superterrane during the permian

precambrian geology of the kazakh uplands and tien shan an overview abstract a comprehensive review of new data on geology and geochronology of precambrian terranes in the western central asian orogenic belt reveals new insights into its evolution at the present surface these terranes mostly consist of meso to neoproterozoic sedimentary magmatic and metamorphic assemblages with insignificant paleoproterozoic rocks archean material is represented exclusively by detrital and xenocrystic zircons in younger strata meso to neoproterozoic felsic magmatic rocks were mostly sourced from neoarchean and paleoproterozoic continental crust indicating its reworking and potential wider presence at deeper crustal levels most meso to neoproterozoic assemblages are of intraplate origin the supra subduction assemblages of neoproterozoic and mesoproterozoic ages are of limited extent we propose to recognize the issedonian and ulutau moyunkum groups of terranes separated by early paleozoic z shaped ophiolitic suture based on their different tectono magmatic evolution in the mesoproterozoic and neoproterozoic distinctly different are the mesoproterozoic and early neoproterozoic assemblages with lithological variations at the beginning of the late neoproterozoic and practically no differences at the end of the neoproterozoic the issedonian group of terranes could be part of a mesoproterozoic ca 1100 ma orogen between the siberian north china and laurentian cratons the pre mesoproterozoic crust of these terranes was completely reworked during the younger events the ulutau moyunkum group of terranes appear to be lithologically and geochronologically similar to the tarim craton both the issedonian and ulutau moyunkum groups of terranes were metamorphosed during the ulutau moyunkum event at 700 25 ma the breakup into currently mappable precambrian terranes took place during end ediacaran to early paleozoic times after opening of oceanic basins whose relics are preserved in numerous paleozoic ophiolitic sutures

middle to late ordovician arc system in the kyrgyz middle tianshan from arc continent collision to subsequent evolution of a palaeozoic continental margin new geological geochronological and isotopic data reveal a previously unknown arc system that evolved south of the kyrgyz middle tianshan mts microcontinent during the middle and late ordovician 467 444 ma ago the two fragments of this magmatic arc are located within the bozbutau mountains and the northern atbashi range and a marginal part of the arc with mixed volcanic and sedimentary rocks extends north to the semizsai metamorphic unit of the southern chatkal range a continental basement of the arc indicated by predominantly felsic volcanic rocks in bozbutau and atbashi is supported by whole rock nd and hf in zircon isotopic data epsilon nd t of 0 9 to 2 6 and epsilon hf t of 1 8 to 6 0 imply melting of neo to mesoproterozoic continental sources with nd model ages of ca 0 9 to 1 2 ga and hf crustal model ages of ca 1 2 to 1 7 ga in the north the arc was separated from the mts microcontinent by an oceanic back arc basin represented by the karaterek ophiolite belt our inference of a long lived early palaeozoic arc in the southwestern mts suggests an oceanic domain between the mts microcontinent and the tarim craton in the middle ordovician the time of arc continent collision is constrained as late ordovician at ca 450ma based on cessation of sedimentation on the mts microcontinent the age of an angular unconformity within the karaterek suture zone and the age of syncollisional metamorphism and magmatism in the kassan metamorphic complex of the southern chatkal range high grade amphibolite facies metamorphism and associated crustal melting in the kassan metamorphic complex restricts the main tectonic activity in the collisional belt to ca 450 ma this interpretation is based on the age of a synkinematic amphibolite facies granite intruded into paragneiss during peak metamorphism a second episode of greenschist to kyanite staurolite facies metamorphism is dated between 450 and 420 ma based on the ages of granitoid rocks subsequently affected or not affected by this metamorphism the latest episode is recorded by greenschist facies metamorphism in silurian sandstones and granodiorites and by retrogression of the older higher grade rocks this may have occurred at the silurian to devonian transition and reflects reorganization of a middle palaeozoic convergent margin late ordovician collision was followed by initiation of a new continental arc in the southern mts this arc was active in the early silurian latest silurian to middle devonian and late carboniferous whereas during the givetian through mississippian ca 385 325 ma this area was a passive continental margin these arcs previously well constrained west of the talas ferghana fault continued eastwards into the naryn and atbashi areas and probably extended into the chinese central tianshan the disappearance of a major crustal block with transitional facies on the continental margin and too short a distance between the arc and accretionary complex suggest that plate convergence in the atbashi sector of the mts was accompanied by subduction erosion in the devonian or early pennsylvanian this led to a minimum of 50 70 km of crustal loss and removal of the ordovician arc as well as the silurian and devonian forearcs in the areas east of the talas ferghana fault c 2016 international association for gondwana research published by elsevier b v all rights reserved

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

the diamond bearing kokchetav uhp massif in northern kazakhstan exhumation structure in the kokchetav ultrahigh pressure uhp massif in northern kazakhstan diamond bearing uhp rocks occur exclusively in a western rhomb shaped domain that differs from an eastern transpressional domain with coesite bearing remnants indicating highest uhp conditions different mechanisms may have contributed to the early ascent of the uhp kokchetav massif the geometry and structure of the diamond bearing domain are interpreted as a sheath like fold coeval with early stage melting in contrast the coesite bearing domain has a sheet like geometry at mid crustal level this early difference in the ascending uhp wedge is reflected in a western rhomb horst and an eastern transpression structure respectively the latter extends to the east borovoye where the uppermost sequences of the uhp massif were defined by others sheath folding is postulated as a suitable mechanism contributing to the early buoyancy driven ascent of the subducted rocks and explains the selective spatial distribution of diamond and coesite bearing sequences preserved in the wedge

biogeographic constraints for tectonic reconstructions of the pacific region suspect terranes in western north america contain permian and triassic genera that were endemic to the tethyan region trend surface analysis of modern biogeographic data constitutes a new approach to understanding the origin of these exotic faunas we use the generic diversity of living reef corals together with paleomagnetic results from quaternary lavas to calculate the present locations of easter island hawaii and tahiti the results demonstrate that the eastward decline in coral diversity may be used as a measure of longitude similar data from the fossil record should constrain paleogeographic reconstructions of the pacific region

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism garnet peridotites from the kokchetav ultrahigh pressure uhp massif contain abundant volatile and high field strength element hfse bearing minerals such as ti clinohumite and zircon these characteristics are interpreted to be related to fluid infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area the zircons from the peridotites were dated by using sensitive high resolution ion microprobe shrimp and yielded apparent u pb ages of 554 494 ma weighted mean age 528 ma that are mostly consistent with the timing of uhp metamorphism deduced from diamond bearing country rocks in this massif these zircons have an almost flat rare earth element ree pattern and very low ree concentrations these characteristics are similar to those observed in kimberlitic zircons inherited zircon cores although only rarely preserved yielded apparently proterozoic ages and have different trace element characteristics compared to the overgrowth rims these features indicate that the mantle metasomatic events and the recrystallization of ti clinohumite and zircon were due to hfse enriched fluid infiltration during the uhp metamorphism at great depths the metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts because these rocks have characteristically high volatile and hfse concentrations such as those of the ti clinohumite bearing garnet peridotites

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

the mineralogy of the kokchetav lamproite implications for the magma evolution abstract kokchetav lamproite occurs in the east end of kokchetav massif and consists of phenocryst mainly clinopyroxene and matrix mainly feldspar the compositions of clinopyroxene magnetite and biotite phenocryst were determined using wavelength dispersive spectrometry on a jeol super probe 8900 electron microprobe for the purpose of revealing the process of magma evolution analyses revealed a core rim variation which is consistent with three stages of magmatic evolution mg rich clinopyroxene cores diopside and biotite cores phlogopite crystallized in a deep magma chamber stage i fe rich clinopyroxene rim salite and biotite rim crystallized at low pressure in a shallow magma chamber stage ii magnetite phenocryst core also crystallized in a shallow magma chamber and co exists with fe rich clinopyroxene rim and biotite rim the magnetite rims probably formed during magma eruption at the same time when groundmass crystallized stage iii the calculated temperatures for ilmenite magnetite pair range from 679 to 887 c log f o 2 values range from 11 1 to 14 9 log units these values represent the latest conditions of magma as ilmenite exsolution in magnetite probably occurred during magma eruption from the shallow chamber to surface

oxygen isotope variations of garnets and clinopyroxenes in a layered diamondiferous calcsilicate rock from kokchetav massif kazakhstan a window into the geochemical nature of deeply subducted uhpm rocks calcsilicate and garnet pyroxene rocks with dolomite and mg calcite matrices occur with uhpm dia mondiferous biotite gneisses and schists of the kokchetav massif the calcsilicates are characterized by high dia mond grade k bearing diopside and very high mg garnets mg 77 with variable ca contents ca 42 5 80 a rare calcsilicate sample with alternating layers of different bulk compositions was selected for oxygen isotope and electron probe microanalysis of garnets and pyroxenes a grain of fresh garnet with a brownish yellow luminescent inner domain mg 94 and a non luminescent outer part mg 88 was selected for in situ analysis of d 18 o by ion microprobe 10 lm spot the profile demonstrates a d 18 o gradient of 1 5 200 lm from 11 3 rim to 12 8 core vsmow additional 2 mg samples of hand picked garnet and clinopyroxene fragments from different parts of the same sample selected by color and chemical differences were analyzed for d 18 o by laser fluorination yielding even larger differences in d 18 o 6 3 10 6 in garnets and 6 1 8 1 in clinopyroxenes the zonation in d 18 o among grains of the same mineral in different lithologies may in part reflect initial heterogeneities of the finely layered sedimentary precursors the d 18 o values for the garnets are among the highest observed for uhp origin both for crustal or mantle rocks confirming a sedimentary origin for these carbonate bearing rocks and ruling out a primi tive mantle derived protolith oxygen diffusion in garnet at peak metamorphism temperature 1 000c was arrested by rapid cooling

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

on the occurrence and boron isotopic composition of tourmaline in ultra high pressure metamorphic rocks abstract the extensive p t stability and the high chemical variability of tourmaline tur together with its common occurrence in metasediments proves its high potential for petrological and isotope geochemical studies on fluid rock interaction in subduction and collision related rocks this paper reviews the occurrence major element chemistry and boron isotopic composition of tur in high and ultrahigh pressure metamorphic uhpm rocks in addition it presents a new discovery of coesite bearing tur schorl from the erzgebirge germany as well as tur dravite related to the retrograde history of coesite and diamond bearing rocks from the erzgebirge and the kokchetav massif kazakhstan the scarce data on worldwide occurrences of u hpm tur reveal a high occupation of the crystallographic x site dominated by na and the possible presence of excess b with little further distinctiveness in its major element composition when compared with tur from medium grade rocks high k 2 o contents in tur are probably not related to uhp growth or equilibration the b isotopic composition of u hpm tur ranges in 11 b from 16 to 1 with many samples in or below the range of continental crust in contrast tur formed during retrograde fluid influx typically shows high 11 b values up to 28 suggesting heavy b fluids infiltrating the exhuming u hpm units coesite inclusions in tur characterized by raman spectroscopy are regarded as the best indicator for its uhp stability supplementary material analytical methods tourmaline compositions and boron isotope values are available at http www geolsoc org uk sup18354

kokchetavite a new potassium feldspar polymorph from the kokchetav ultrahigh pressure terrane kokchetavite a new polymorph of k feldspar kalsi3o8 has been identified as micrometer size inclusions in clinopyroxene and garnet in a garnet pyroxene rock from the kokchetav ultrahigh pressure terrane kazakhstan kokchetavite has a hexagonal structure with a 5 27 1 a c 7 82 1 a v 188 09 a 3 z 1 and is found to be associated with phengite a b cristobalite or quartz siliceous glass phlogopite titanite calcite zircon occurring as multi phase inclusions in clinopyroxene and garnet it is concluded that kokchetavite could not be an exsolution phase in host minerals instead it might be metastably precipitated from an infiltrated k rich melt during rock exhumation alternatively although less likely kokche tavite might be derived from dehydration of k cymrite which in turn was formed at high pressures in either case kokchetavite is a metastable polymorph of k feldspar

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

allochthonous permian micro and macrofauna kamloops area british columbia the first waagenophyllid coral to be described from the western hemisphere has been found in permian rocks of the harper ranch group near kamloops southern british columbia it is parawentzelella miyagiella johnstonae sp nov associated with the tabulate coral multithecopora larushi sp nov numerous brachiopods and other fossils the containing rocks are considered to be allochthonous originating in the western part of the paleopacific tethyan ocean or in an island arc complex farther to the north the outcrop yielding the fossils appears to be a stratigraphic misfit both in age and origin fusulinids from adjacent areas are of early permian wolfcampian age but those associated with the fossils indicate a late permian guadalupian age these conflicting dates cannot at present be reconciled the tethyan origin of the fauna also contrasts with that of surrounding rocks which are non tethyan

the origin and formation of metamorphic microdiamonds from the kokchetav massif kazakhstan a nitrogen and carbon isotopic study abstract this study reports 13c 15n and n content values for microdiamonds from ultrahigh pressure metamorphic rocks of the kokchetav massif in kazakhstan both alluvial diamonds and in situ diamonds from a garnet clinopyroxene rock and a marble i e a garnet pyroxene dolomitic rock were investigated in situ diamonds were analysed in batches because of their small size average 40 m whereas the larger alluvial diamonds were analysed individually the latter group has 13c values ranging from 15 92 to 10 57 15n from 1 8 to 1 1 and n contents from 2300 to 3650 ppm diamonds from the garnet clinopyroxene rock yield mean values of 10 50 for 13c 5 9 for 15n and a high average nitrogen content of 11 150 ppm values for diamonds in marble are 10 19 8 5 and 2650 ppm respectively for diamonds from garnet clinopyroxene rock and marble there is more nitrogen released by bulk combustion than estimated by infrared ir spectroscopy the differences being of about 7000 and 1500 ppm respectively these differences suggest that a significant quantity of nitrogen is ir inactive and may be present as fluid inclusions their carbon and nitrogen isotopic compositions are compatible with an in situ crystallisation of diamond from dominantly metasedimentary sources suggesting that sedimentary nitrogen can be subducted to very high pressures carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures before the peak of metamorphism at temperatures probably below 700 c and deduced pressures of 3 gpa relative to the isotopic data reported for sediments metasediments and in situ diamonds the slightly 15n depleted compositions of alluvial diamonds is striking these values suggest that the contribution of any metasedimentary source is unlikely and may point toward a mafic ultramafic protolith

mesoproterozoic grenville age terranes in the kyrgyz north tianshan zircon ages and nd hf isotopic constraints on the origin and evolution of basement blocks in the southern central asian orogen abstract the north tianshan orogenic belt in kyrgyzstan consists predominantly of neoproterozoic to early paleozoic assemblages and tectonically interlayered older precambrian crystalline complexes and formed during early paleozoic accretionary and collisional events one of the oldest continental fragments of late mesoproterozoic grenvillian age occurs within the southern part of the kyrgyz north tianshan using shrimp zircon ages we document two magmatic events at 1 1 and 1 3 ga the younger event is characterized by voluminous granitoid magmatism between 1150 and 1050 ma and is associated with deformation and metamorphism the older event is documented by 1 3 ga felsic volcanism of uncertain tectonic significance and may reflect a rifting episode geochemical signatures as well as nd and hf isotopes of the mesoproterozoic granitoids indicate melting of still older continental crust with model ages of ca 1 2 to 2 4 ga the mesoproterozoic assemblages are intruded by paleozoic diorites and granitoids and nd and hf isotopic systematics suggest that the diorites are derived from melts that are mixtures of the above mesoproterozoic basement and mantle derived material their source is thus distinct from that of the mesoproterozoic rocks emplacement of these plutons into the precambrian rocks occurred between 461 and 441 ma this is much younger than previously assumed and indicates that small plutons and large batholiths in north tianshan were emplaced virtually synchronously in the late ordovician to early silurian the mesoproterozoic rocks in the north tianshan may be remnants of a once larger continental domain whose fragments are preserved in adjacent blocks of the central asian orogenic belt comparison with broadly coeval terranes in the kokchetav area of northern kazakhstan the chinese central tianshan and the tarim craton point to some similarities and suggests that these may represent fragments of a single mesoproterozoic continent characterized by a major orogenic event at 1 1 ga known as the tarimian orogeny

genesis of microdiamonds from melt and associated multiphase inclusions in garnet of ultrahigh pressure gneiss from erzgebirge germany abstract the common association of microdiamonds with the phase assemblage phlogopite apatite paragonite and quartz containing amorphous na al silicate inclusions as inclusions in garnets of quartzofeldspathic rocks from the saxonian erzgebirge germany was studied by analytical electron microscopy the assemblage implies a precedent melt which coexisted with the microdiamonds before and after entrapment in the garnet host and subsequently crystallized the formation of microdiamonds in this metamorphic rock could be explained by cotectic induced partial melting of a subducted continental slab at about 4 6 gpa and 1000 c as constrained by the occurrence of tio 2 with an pbo 2 type structure at the peak metamorphic conditions and by the catalytic effect of siderophile and chalcophile elements

oriented kokchetavite compound rods in clinopyroxene of kokchetav ultrahigh pressure rocks abstract two microdiamond bearing samples a dolomite marble and a garnet clinopyroxene rock from the kokchetav ultrahigh pressure metamorphic terrane were selected in the present study to explore the possible origin of kalsi3o8 rod inclusions oriented along the c axis of clinopyroxene host the kalsi3o8 rod inclusions at clinopyroxene cores where k2o content is high in the range of 0 5 1 0 wt are mostly fine grained with a rod width less than 1 m aem studies showed that the kalsi3o8 phase in most rod inclusions is kokchetavite k feldspar is present only in a few cases probably the result of phase transformation recrystallization from kokchetavite during rock exhumation electron diffractions further showed that kokchetavite rods are oriented parallel to clinopyroxene 0 0 1 direction and they exhibit the same epitaxial relation with the clinopyroxene host in both samples with the al si o4 tetrahedra chains along the hexagonal a axis of kokchetavite parallel to the single sio4 chain along the c axis of clinopyroxene i e 1 2 1 0 ko 0 0 1 cpx and 0 0 0 1 ko 1 0 0 cpx it is interesting to note that kokchetavite is always in association with phengite tremolite cristobalite si rich al k ca bearing low crystallinity phase si ca cl as phase calcite apatite lollingite feas2 forming compound rods furthermore all these phases are also present within submicron scale polyphase inclusion pockets in garnet within garnet clinopyroxene rock sample these kokchetavite compound rods are therefore most likely to have resulted from melt fluid clinopyroxene interactions leading to epitaxial deposition rather than exsolution sensu stricto from the clinopyroxene host the suggested melt fluid would have an external and or an internal origin related to rock partial melting involving phengite breakdown discrete phlogopite and phengite needle like inclusions with a needle width less than 1 m as well as phlogopite phengite and kokchetavite mica intergrowth needles are also not uncommon in clinopyroxene cores there are specific crystallographic orientation relationships among phases these micas might have formed earlier than kokchetavite and have exsolved from clinopyroxene host although the mass balance issue during exsolution and the temporal relation between phlogopite and phengite needles remain to be settled clinopyroxene rims generally contain mica needles only domains near fractures with coarse grained kokchetavite compound rods or with coarse grained phlogopite quartz needles are not uncommon these clinopyroxene rims domains are low in k2o

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

k2o prograde zoning pattern in clinopyroxene from the kokchetav diamond grade metamorphic rocks missing part of metamorphic history and location of second critical end point for calc silicate system abstract despite extensive studies of calc silicate rocks of the kokchetav massif there is no satisfactory explanation of the origin of potassium bearing clinopyroxene in alkali poor metamorphic rocks in this paper we report the finding of potassium bearing clinopyroxene with prograde zonation k 2 o increases from core to rim from diamond grade but diamond free uhp calc silicate rocks of the kokchetav massif we believe that the crystallization of potassium bearing clinopyroxene started on the prograde stage and slightly prior to the peak of uhp metamorphism thus prograde metamorphic history is only traceable in diamond free uhp calc silicate rocks while in diamond bearing uhpm rocks it is completely reset fluid and polyphase solid inclusions originally representing melt inclusions occur in the core of potassium bearing clinopyroxene and imply that melt and fluid may coexist in calc silicate rocks even at 1000 1100 c and 6 7 gpa

clinopyroxene phenocrysts with green salite cores in trachybasalts implications for two magma chambers under the kokchetav uhp massif north kazakhstan abstract clinopyroxene phenocrysts in the kokchetav trachybasalts are variable in composition and textures two distinctive cores are recognized diopside cores and green salite cores the diopside cores with mg of 80 90 are mantled by colorless salite rims with mg of 70 80 the green salite cores have especially low mg

oxygen carbon and strontium isotope geochemistry of diamond bearing carbonate rocks from kumdy kol kokchetav massif kazakhstan abstract diamond bearing carbonate rocks from kumdy kol kokchetav massif kazakhstan were strongly altered by fluids flowing through fractures and infiltrating along grain boundaries during exhumation alteration includes retrogradation of high grade silicate assemblages by hydrous minerals replacement of diamond by graphite and of dolomite by calcite diamond bearing carbonate rocks are among the most intensely altered isotopically with 18 o vsmow values as low as 9 13 c vpdb 9 and 87 sr 86 sr as high as 0 8050 evidence of isotopic equilibration between coexisting dolomite and high mg calcite during ultrahigh pressure metamorphism uhpm is preserved only rarely in samples isolated from infiltrating fluids by distance from fractures isotopic heterogeneity and isotopic disequilibrium are widespread on a hand specimen scale because of this lack of homogeneity bulk analyses cannot provide definitive measurements of 13 c 12 c fractionation between coexisting diamond and carbonate our study adequately documents alteration on a scale commensurate with observed vein structures but testing the hypothesis of metamorphic origin of microdiamonds has not fully succeeded because our analytical spatial resolution limited to 0 5 mm is not small enough to measure individual dolomite inclusions or individual diamond crystals

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

evidence of displaced terranes from permian to jurassic faunas around the pacific margins for the permian to jurassic time interval tethyan marine invertebrate faunas which are generally accepted as having lived at low latitude can be distinguished from less diverse higher latitude faunas displacement of these low latitude faunas to positions of high latitude around the pacific margins therefore provides key evidence for the movement of so called suspect or displaced terranes the fullest story so far has been worked out for the western margin of n america as far north as southern alaska but there is also convincing evidence for the substantial northward movement of continental segments along the ne asian margin as regards the southern hemisphere the torlesse terrane of new zealand appears to have moved a considerable distance southwards but there is no fauna1 evidence of comparable far travelled suspect terranes along the andean margin of south america not only tethyan and boreal but e and w pacific elements are detectable in the faunas and the distribution of these argues for a panthalassia studded with islands of various sizes fringing the adjacent continents rather than for the disintegration of a single continent pacifica

exhumation of high pressure rocks of the kokchetav massif facts and models abstract the exhumation of ultrahigh pressure uhp metamorphic units from depths more than 100 120 km is one of the most intriguing questions in modern petrology and geodynamics we use the diamondiferous kumdy kol domain in the kokchetav massif to show that exhumation models should take into consideration initially high uplift velocities from 20 down to 6 cm year and the absence of the deformation of uhp assemblages the high rate of exhumation are indicated by ion microprobe shrimp dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds diamondiferous rocks in the kumdy kol domain occur as steeply dipping 60 80 thin slices few hundred metres within granite gneiss using geological petrological and isotopic geochemical data we show that partial melting of diamondiferous metamorphic rocks occurred a very important factor which has not been taken into account in previous models deformation of diamondiferous rocks at kumdy kol is insignificant diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation all these facts could be explained by partial melting of metapelites and granitic rocks in the kumdy kol domain the presence of melt is responsible for an essential reduction of viscosity and a density difference between crustal rocks and mantle material and reduced friction between the upwelling crustal block the subducting and overriding plates besides the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle we construct different models for the three stages of exhumation a model similar to corner flow for the first superfast exhumation stage an intermediate stage of extension most important from structural point of view and a very low rate of exhumation in final diapir erosional uplift

rheology of plasticine used as rock analogue the impact of temperature composition and strain abstract uniaxial compression tests have been carried out to determine the temperature dependent rheology of plasticine commonly used for tectonic modelling the original plasticine types kolb brown beck s orange beck s green weible special soft are characterized by strain rate softening with power law exponents n and apparent viscosities ranging from 5 8 to 7 3 and 3 4 10 5 to 2 2 10 7 pa s respectively if e 10 4 10 3 s 1 and t 25 c beck s orange shows steady state creep whereas the other types show strain hardening the activation energy determined for 20 c t 35 c is ranging from 323 34 to 488 22 kj mol 1 a rise in temperature results in linear decreases of n and and a reduction in the degree of strain hardening steady state creep and major changes in n and have further been observed at decreasing filler matrix ratios the latter being obtained by adding oil to the original plasticine the new results suggest that plasticine can be used to model the deformation of natural rocks undergoing dislocation creep various rock analogues with strain hardening or steady state creep and prescribed stress exponents ranging from 3 4 to 12 3 can be easily produced by changing the temperature and or the filler matrix ratio of commercial plasticine types

subduction ultrahigh pressure metamorphism and regurgitation of buoyant crustal slices implications for arcs and continental growth abstract intracontinental collisional belts that retain mineralogic relics of phanerozoic ultrahigh pressure uhp metamorphism are increasingly being recognized adjacent regions generally lack evidence of coeval calcalkaline volcanism plutonism following consumption of intervening oceanic lithosphere each orogen marks the site of a deeply subducted microcontinental promontory fragments of which later returned to midcrustal levels mafic and ultramafic rocks are volumetrically minor pressures in uhp complexes approached or exceeded 2 8 gpa at 600 900 c subduction zones involve low t prograde gradients and constitute the only plate tectonic realm where such conditions exist although internal portions of descending plates may have even lower geothermal gradients crustal upper margins are typified by p t trajectories of 5 10 c km attending underflow clinoamphibole rich metabasaltic rocks dehydrate at pressures 2 2 2 5 gpa this accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of morbs to form hydrous andesitic melts devolatilization of partly serpentinized mantle lithosphere also introduces h 2 o to the magmagenic zone as large masses of continental crust enter deeper portions of subduction zones mineralogic transformations slow or cease because micas the major oh bearing phases in such rocks retain volatile contents to pressures 4 gpa persistence of hydroxyl bearing layer silicates may explain the metastable preservation of low density assemblages in continental crust subjected to uhp conditions and the dearth of calcalkaline magmatism in most collision belts exhumed uhp terranes are intensely deformed and disaggregated all consist dominantly of quartzofeldspathic compositions mafic eclogites garnet peridotites constitute only a few volume percent eventual decoupling of the relatively low density subducted crust promotes buoyancy propelled transport to shallow depths along the subduction channel with the hanging wall lithosphere acting as a stress guide extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material although equidimensional massifs must return surfaceward large volume to surface areas render the contained uhp phase assemblages virtually unquenchable surviving uhp terranes thus consist of relatively thin slabs of continental crust allochthons that retain uhp relics apparently lost heat by conduction across both upper normal fault and lower reverse fault contacts based on present exposures estimated thicknesses of uhp sheets are qinling dabie sulu belt 5 km kokchetav complex 1 3 km dora maira massif 1 2 km western gneiss region 1 km slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year back reaction attending decompression in all cases was nearly complete where uhp mineral inclusions persisted retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures the coarse grain size of host phases also disfavored complete retrogression

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes