microstructures petrofabrics and seismic properties of ultra high pressure eclogites from sulu region china implications for rheology of subducted continental crust and origin of mantle reflections ultra high pressure uhp eclogites from sulu region china represent mafic components of the continental crust which were first subducted to mantle depths greater than 100 km and then exhumed to the earth s surface detailed investigation of microstructures chemical compositions petrofabrics and seismic properties of the uhp eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle we present here results from field optical and tem observations electron back scattered diffraction ebsd measurements and numerical computations of the seismic properties of uhp eclogites collected from fresh surface outcrops at the drill site maobei donghai county jiangsu province of the chinese continental scientific drilling program ccsd two types of eclogites have been distinguished type 1 coarse grained eclogites deformed by recovery accommodated dislocation creep at the peak metamorphic conditions and type 2 fine grained eclogites which are composed of reworked type 1 materials during recrystallizationaccommodated dislocation creep in shear zones which were active during the exhumation of the uhp metamorphic rocks both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the uhp metamorphic conditions plasticity of eclogites under uhp conditions can effectively facilitate channeled flow along the interplate shear zone the

crystallographic fabrics of omphacite rutile and quartz in vendee eclogites armorican massif france consequences for deformation mechanisms and regimes abstract this study aims at further understanding of the mechanisms how lattice preferred orientations lpo develop during deformation in the main eclogite minerals microstructures and textures of deformed eclogites from the les essarts complex western france were investigated using optical microscopy and electron backscatter diffraction ebsd in the scanning electron microscope microfabric analyses of eclogite facies minerals are used to identify their deformation mechanisms which define the rheology at high pressure metamorphic conditions mechanisms of intracrystalline deformation by dislocation movement dislocation creep result usually in a non linear flow law typically power law while diffusive processes diffusion creep correspond to linear flow laws general microstructural observations may suggest intracrystalline deformation dislocation creep of omphacite the omphacite lpo vary between s and l type and correlate with oblate or prolate grain shape fabrics respectively until now these lpo types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene an alternative interpretation is given in terms of anisotropic growth and dissolution with grain boundary diffusion as the rate controlling process there are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite in omphacite around a hollow garnet crystallographic and shape fabrics align with the c 001 axes parallel to the grain elongations defining the mineral lineation which rotates locally with the inferred flow direction in this part the grain sizes of omphacite and rutile are larger than in the surrounding matrix the geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime directions and ratios of the principal stresses the lpo of rutile duplicate the lpo of omphacite and a similar distinction between s and l type was used rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep quartz mainly occurs as an interstitial phase with weak lpo patterns interpreted as random no representative obliquity of the lpo in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria however the rutile lpo was slightly rotated relative to the omphacite lpo consistently in most samples the results suggest that diffusion processes are strongly involved in the deformation of eclogites a linear flow law should be taken into account in tectonic models where eclogites are incorporated

two stage collision related extrusion of the western dabie hp uhp metamorphic terranes central china evidence from quartz c axis fabrics and structures abstract the western dabie orogen also known as the hong an block forms the western part of the dabie sulu hp uhp belt central china rocks of this orogen have been subjected to pervasive ductile deformation and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones quartz textures in these mylonites contain important clues for understanding the movement sense of late collisional extrusion and exhumation of high pressure ultrahigh pressure hp uhp rocks from the lower crustal level to the upper crustal level during middle triassic and early jurassic the orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen the asymmetry of c axis patterns consistently indicates top to the southeast thrusting across the orogen in early structural stages later stages of deformation show different senses of movement in northern and southern parts of the orogen with top to the northwest sinistral shearing recorded in rocks north of the xinxian hp uhp eclogite facies belt and top to the southeast dextral shearing south of the same unit based on our study on quartz c axis fabrics and marco to micro scale structures simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the xinxian hp uhp unit is explained by upward extrusion in early stages of deformation the extrusion process has been attributed to syn and late collisional processes accounting for some earlier deformation in the western dabie orogen such as metamorphic sequences around the core of the xinxian hp uhp eclogite facies unit much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field an orogen parallel eastward extrusion of the xinxian hp uhp eclogite facies unit however occurred diachronously in later stages of deformation therefore a tectonic model combining an early upward extrusion with a later eastward extrusion is presented two different stages and types of extrusion for exhumation of hp uhp rocks are suitable to all of east central china geochronological data shows that the first upward extrusion occurred during middle triassic the second eastward extrusion occurred during late triassic to early jurassic these two extrusions are correlative with two stages of rapid exhumation of the dabie hp uhp rocks respectively these two stage late collisional middle triassic to early jurassic extrusion events bridge the gap between syn collisional early to middle triassic vertical extrusion and post collisional cretaceous eastward directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of hp uhp rocks

jurassic subsidence history in the hebrides n w scotland abstract geohistory analyses have been applied to the generally well dated triassic jurassic successions of three areas in the hebrides basin north west scotland and the importance of making corrections for decompaction palaeobathymetry and eustatic changes of sea level are assessed the thermo tectonic subsidence patterns in the skye areas north suggest a three phase history of crustal stretching though only the first phase is well developed in the mull area south episodes of relatively rapid subsidence are 1 1 late triassic c210 ma until earliest toarcian 186 ma 2 2 latest toarcian 180 ma until late bathonian c161 ma 3 3 early oxfordian 154 ma until at least early kimmeridgian incomplete these are separated by toarcian 186 180 ma and latest bathonian callovian c161 154 ma episodes of either little subsidence or of uplift but at slower rates than eustatic rise of sea level the intervals between phases at 30 ma and 26 ma are remarkably similar but the wider significance of this remains to be established

garnet plasticity in the lower continental crust implications for deformation mechanisms based on microstructures and sem electron channeling pattern analysis abstract elongated garnets which preserved deformational microstructures boudinage pinch and swell structures occur in granulite facies quartzites in the highland complex of sri lanka in spite of these microstructures and in contrast with previous reports on garnet plasticity there are no or only few signs of intracrystalline deformation like subgrains or lattice distortion this can be explained by annealing and slow static cooling from high temperatures sem electron channeling pattern analysis reveals that the garnets have a significant crystallographic preferred orientation aspect ratio grain size analysis shows that the deformation mechanism is grain size sensitive these features indicate a diffusion assisted dislocation glide mechanism with dominant 1 2 110 slip system a comparison of the deformation behavior between garnet quartz and feldspar shows that differences in flow strength are low under the high grade conditions 850 50 c garnet is still the phase with the highest flow strength but dry quartz is slightly stronger than feldspar

rheology of omphacite at high temperature and pressure and significance of its lattice preferred orientations abstract we have investigated the rheology of hot pressed polycrystalline omphacite di 58 jd 42 space group c2 c at strain rates of 10 4 10 5 s temperatures of 1300 1500 k and a pressure of 3 gpa using a 5 gpa griggs type deformation apparatus the rheological constitutive equation of omphacite is determined with a stress exponent of 3 5 0 2 and an activation energy of 310 50 kj mol our study shows that 1 the creep strength of omphacite falls between those of diopside and jadeite 2 experimental omphacite microfabrics are indistinguishable from natural ones s type fabric and its mesoscopic foliation develop under conditions of axially symmetric shortening whereas l type fabric and its mesoscopic lineation develop under conditions of general strain with l parallel to the greatest elongation 3 as in naturally deformed omphacite the deformation substructure is characterized by the dominant slip systems 110 1 2 110 110 001 and 100 001 we conclude that both s and l type omphacite fabrics are produced by these slip systems fabric differences arise solely from variation of the geometry and orientation of the finite strain ellipsoid we also confirm the previous suggestion that na bearing pyroxenes are significantly weaker than na free pyroxenes

relationships between eclogite facies mineral assemblages deformation microstructures and seismic properties in the yuka terrane north qaidam ultrahigh pressure metamorphic belt nw china to understand the relationships between eclogite facies mineral assemblages deformation microstructures and the seismic properties of subducting oceanic crust eclogites from the yuka terrane no

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

the dislocation substructures of naturally deformed omphacites abstract a transmission electron microscopy study of naturally deformed omphacite has revealed twins antiphase boundaries and a variety of dislocation substructures consisting of curved free dislocations loops isolated nodal points and networks both free dislocations and dislocation boundaries contain nodes that involve dislocations with the following burgers vectors 001 1 2 110 and 1 2 112 this implies a reaction of the type 001 1 2 110 1 2 112 by analogy with quartz and olivine the dislocation substructures in the omphacites are considered to be indicative of dislocation creep mechanisms and responsible for most of the optical defect structures using microstructural and geochemical data the metamorphic conditions during the creep deformation are estimated to be minimum 8 kbar 550 c maximum 15 kbar 625 c

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

seismic properties of subducting oceanic crust constraints from natural lawsonite bearing blueschist and eclogite in sivrihisar massif turkey abstract investigating the seismic properties of natural lawsonite lws bearing blueschist and eclogite is particularly important for constraining the seismic interpretation of subducting oceanic crust based on seismological observations to achieve this end we analyzed in detail the mineral fabrics and seismic properties of foliated lws blueschist and lws eclogites from sivrihisar massif in turkey in both blueschists and eclogites the lawsonite fabric is characterized by three different patterns 0 0 1 axes aligning sub normal to foliation and 0 1 0 axes aligning sub parallel to lineation normal type 0 0 1 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation with a girdle sub normal to lineation abnormal type and 0 0 1 axes aligning both sub normal to foliation and sub parallel to lineation 0 1 0 axes aligning sub parallel to lineation and 1 0 0 axes aligning sub normal to foliation transitional pattern in contrast glaucophane and omphacite mostly present consistent axial fabrics with the 0 0 1 axes aligning to lineation these mineral fabrics produce whole rock seismic anisotropies with similar patterns however the variations in seismic anisotropies are mainly controlled by the rock type to a lesser extent are determined by the lawsonite fabric type and to only a small extent are affected by mineral fabric strength despite the constructive abnormal type lawsonite fabric on whole rock seismic anisotropies because of their weaker mineral fabric strength or deformation degree the abnormal type lws blueschist still exhibit comparatively lower seismic anisotropies than those normal type lws blueschist from other localities based on the calculated seismic anisotropies and velocities we estimated that when oceanic crust transforms from lws blueschist to lws eclogite with increasing subduction depth 1 p wave and max s wave polarization anisotropies reduce about 70 and 40 respectively and 2 variations of vp and vs contrasts relative to mantle peridotites are about 7 to 3 and 8 to 6 respectively these results corroborate the important roles of lws bearing blueschist and eclogite in interpreting the existence and gradual weakening of low velocity layers in subducting oceanic crust during the subduction process

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

plastic deformation and recrystallization of garnet a mechanism to facilitate diffusion creep elongate and deformed garnets from glenelg nw scotland occur within a thin shear zone transecting an eclogite body that has undergone partial retrogression to amphibolite facies at circa 700 c optical microscopy back scattered electron imaging electron probe microanalysis and electron back scatter diffraction reveal garnet sub structures that are developed as a function of strain subgrains with low angle misorientation boundaries occur at low strain and garnet orientations are dispersed around rational crystallographic axes across these boundaries towards high strain areas boundary misorientations increase and there is a loss of crystallographic control on misorientations which tend towards random in high strain areas a polygonal garnet microstructure is developed the garnet orientations are randomly dispersed around the original single crystal orientation some garnet grains are elongate and ca rich garnet occurs on the faces of elongate grains oriented normal to the foliation commonly the garnet grains are admixed with matrix minerals and where in contact with other phases garnet is well faceted we suggest that individual garnet porphyroclasts record an evolution from low strain conditions where dislocation creep and recovery accommodated deformation through increasing strain where dynamic recrystallization occurred by subgrain rotation to highest strains where recrystallized grains were able to deform by diffusion creep assisted grain boundary sliding with associated rotations

dynamic cause of marginal lithospheric thinning and implications for craton destruction a comparison of the north china superior and yilgarn cratons we present a comparative tectonic analysis of the north china craton ncc which has lost parts of its root with the yilgarn and superior cratons which preserve their roots we compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots while others retain them based on the comparison and analysis of geological geophysical and geochemical data it is clear that the lithospheric thinning beneath craton margins is a common phenomenon which may be caused by convergence between plates however craton destruction is not always accompanied by lithospheric thinning except for cratons that suffered subduction and collision from multiple sides the western block also known as the ordos block of the ncc yilgarn and superior cratons have not experienced craton destruction the common ground among them is that they are surrounded by weak zones e g mobile belts or orogens that sheltered the cratons from deformation which contribute

crystal plasticity of natural garnet new microstructural evidence scanning electron microscope sem orientation contrast images of mantle nodules show that garnets contain cellular domains of different crystallographic orientation electron backscatter diffraction shows small crystallographic mismatches

experimental investigation of eclogite rheology and its fabrics at high temperature and pressure eclogite plays an important role in mantle convection and geodynamics in subduction zones an improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology however the deformation processes and associated fabrics in eclogite are not well understood incompatible views of deformation mechanism have been proposed for both garnet and omphacite we present here deformation behaviour of eclogite at temperatures of 1027 1427 c confining pressures of 2 5 3 5 gpa and strain rates of 1 10 5 s 1 to 5 10 4 s 1 we obtained a power law creep for the high temperature and pressure deformation of a dry eclogite 50 vol garnet 40 omphacite and 10 quartz with a 103 3 1 0 n 3 5 0 4 e 403 30 kj mol 1 and v 27 2 cm3 mol 1 the two principal minerals of eclogite have greatly different strengths progressive increase of garnet results in a smooth increase in strength analysis by electron back scattered diffraction shows that 1 garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions 2 omphacite shows pronounced lattice preferred orientations lpos suggesting a dominant dislocation creep mechanism further investigation into the water effects on eclogite show 3 water content does not influence the style of omphacite fabric but increases slightly the fabric strength 4 grain boundary processes dominate the deformation of garnet under high water fugacity or high shear strain conditions yielding a random lpo similar to that of non deforming garnet despite the strong shape preferred orientation spo observed 110 001 slip may dominate the deformation of rutile quartz displays complicated and inconsistent lpos in eclogite these results are remarkably similar to observations from deformed eclogites in nature

sm nd and rb sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at shuanghe in the dabie mountains central china abstract ultrahigh pressure metamorphic uhpm rocks at shuanghe area in the dabie mountains occur as a uhpm block within the regional granitic gneiss a sm nd isochron defined by garnet omphacite rutile from coesite bearing eclogite yield an age of 226 3 3 2 ma another sm nd isochron defined by garnet two phengites form uhp gneiss yield a similar age of 226 5 2 3 ma the two consistent sm nd ages defined by three uhpm minerals suggest that nd isotopic equilibrium between uhpm minerals in these rocks during uhp metamorphism has been achieved this may correspond to the age of peak metamorphism with an average metamorphic temperature of 800 c it is demonstrated that the retrograde metamorphism of uhpm rocks occurred in an open chemical system whereas the nd and sr isotopic systematics of uhpm minerals may have remained closed nd and sr isotopic disequilibrium between uhpm minerals and retrograde metamorphic minerals has been observed therefore the tie line of garnet or phengite and whole rock containing retrograde metamorphic minerals gives an old sm nd age and a young rb sr age respectively with no geologic significance however a rb sr ages of 219 0 6 6 ma defined by phengite garnet from a uhpm gneiss indicates that the uhpm rocks at shuanghe cooled down to 500 c at that time it suggests that the uhpm rocks at shuanghe experienced the first rapid cooling during 226 219 ma on the other hand the rb sr ages of 174 7 8 to 169 2 3 3 ma defined by retrograde minerals amphibole or biotite with closure temperature ranging from 450 to 300 c and intensely retrograded metamorphic rocks reflect a second rapid cooling during this time interval this is consistent with the rapid cooling time of 190 170 ma of the orthogneiss in the dabie su lu uhpm belt obtained by ar ar dating method in contrast to uhpm rocks the nd isotopic composition of the garnet in granitic gneiss has been reset during retrograde metamorphism and is in equilibrium with those of retrograde epidote and biotite which yields a sm nd isochron age of 213 5 ma indicating the retrometamorphic time corresponding to amphibolite facies in addition the biotites from the granitic gneiss yield rb sr ages of 171 173 ma similar to those of the uhpm rocks these data suggest that the country rocks granitic gneiss may have a similar cooling history to the uhpm rocks at shuanghe two stages of rapid cooling of uhpm rocks at shuanghe may correspond two stages of fast uplift the initial rapid uplifting and cooling of uhpm rocks during 226 219 ma may be caused by compression tectonics during subducting time of the continental crust whereas the later rapid cooling may reflect the exhumation of the entire subducted continental crust by extension during the early middle jurassic

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

implications to garnet clinopyroxene geothermometry of non ideal solid solution in jadeitic pyroxenes garnet clinopyroxene geothermometry has been applied to numerous rock types from the blueschist and eclogite terrain of the sesia zone in the western alps discrepancies in apparent temperatures of up to 600 c have been found suggesting substantial deviation from ideal behaviour of the garnet clinopyroxene equilibria in certain assemblages assemblages containing jadeitic pyroxenes rather than omphacitic or diopsidic pyroxenes yield very low kd 2 0 values and correspondingly high apparent temperatures 1 000 c the garnets coexisting with high na pyroxenes have compositions similar to those coexisting with omphacitic pyroxenes departure from ideal behaviour of these garnets is likely to be minor and is accomodated in the formulation of the geothermometer numerous examples of coexisting garnet clinopyroxene pairs indicate that there is no obvious relationship of kd with jadeite content in pyroxenes with jadeite content less than 70 but at higher values kd varies inversely with the jadeite content the dependence of kd upon the jadeite content in the high na pyroxenes is believed to be due to the preference of m2 sites for fe2 in the enstatite jadeite substitution fe2 mgna 1al 1 this substitution is usually very limited and has no demonstrable effect upon x fe cpx when the m1 fe2 mg content of the pyroxenes is high as in omphacitic and diopsidic pyroxenes however when the m1 fe2 mg content is low as in jadeitic pyroxenes the non ideality of the enstatite type exchange has a marked effect on x fe cpx and consequently on garnet clinopyroxene calculations

plastic flow strength of jadeite and diopside investigated by microindentation hardness tests abstract microindentation hardness tests were performed on jadeite and diopside being end members of the omphacite solid solution series at temperatures between 300 and 750 c the hardness of jadeite ranges from 7 4 to 8 5 gpa that of diopside from 4 9 to 6 1 gpa jadeite is significantly stronger than diopside in the low temperature plasticity regime normalization of the hardness derived yield stress with respect to the shear modulus considerably reduces the strength contrast the normalized peierls stress is identical for jadeite and diopside this indicates that jadeite and diopside belong to the same isomechanical group the hardness derived yield stress for jadeite as well as for diopside is used to estimate flow law parameters for the low temperature plasticity regime

structural geometry of an exhumed uhp terrane in the eastern sulu orogen china implications for continental collisional processes high precision 1 200 1 1000 mapping of yangkou bay eastern sulu orogen defines the structural geometry and history of one of the world s most significant uhp ultrahigh pressure belts at least four stages of folds are recognized in uhp eclogites and associated quartzo feldspathic gneiss uhp eclogite facies rootless f1 and isoclinal f2 folds are preserved locally in coesite eclogite mylonitic to ultramylonitic quartzo feldspathic and coesite eclogite shear zones separate small scale 5 10 m thick nappes of ultramafic mafic uhp rocks from banded quartzo feldspathic gneiss these shear zones are folded and progressively overprinted by amphibolite greenschist facies shear zones the prograde to retrograde d1 d5 deformation sequence is explained by deep subduction of offscraped thrust slices of lower continental or oceanic crust from the down going plate caught between the colliding north and south china cratons in the mesozoic after these slices were structurally isolated along the plate interface they were rolled in the subduction channel during exhumation and structural juxtaposition with quartzo feldspathic gneisses forming several generations of folds sequentially lower grade foliations and lineations and intruded by in situ and exotically derived melts shear zones formed during different deformation generations are wider with lower grades suggesting that deep crustal upper mantle deformation operates more efficiently perhaps with more active crystallographic slip systems than deformation at mid upper crustal levels

crystallographic preferred orientations in eclogites a review abstract during the last decades processes taking place in subduction zones have been a topic of extensive debate there are many models on a tectonic scale but knowledge of deformation at microscopic levels is essential for an understanding of the processes involved crystallographic preferred orientation cpo investigations of minerals in eclogites yield the possibility to study deformation in the crustal part of subducted oceanic lithosphere as well as subducted continental units containing basic intrusions which are frequently exhumed during continental collision from cpo data of omphacite and garnet as well as other constituent minerals in eclogites conclusions on stress and strain at depth in currently active subduction zones can be drawn as the pressure temperature and strain path of the now exhumed rocks can be unraveled this contribution provides an overview of cpo studies of eclogites concerning slip systems deformation strain and other possible cpo forming mechanisms in all major constituent mineral phases in eclogites and the implications of these microstructural data on subduction zone processes are discussed nevertheless there are still many open questions and future research is essential

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

plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites abstract we use an anisotropic viscoplastic self consistent vpsc model to simulate the development of omphacite lattice preferred orientations lpos in response to deformation by dislocation glide in these simulations we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside simulated lpos reproduce very well the characteristic omphacite lpo pattern in naturally deformed eclogites a strong concentration of 001 axes sub parallel to the lineation and of 010 poles sub perpendicular to the foliation these models reconcile the interpretation of omphacite lpos in eclogites and tem observations of naturally deformed omphacite since they show that omphacite lpos in naturally deformed eclogites may develop by dislocation glide on 1 2 110 110 001 110 and 001 100 we also investigate the effect of the strain regime on the omphacite lpo development the simulations show that changes in deformation regime lead to second order variations in omphacite lpo patterns similar to those observed in eclogites such as an asymmetry of the lpo relative to the structural frame a stronger concentration of the 001 axes relative to the 010 pole concentration or a dispersion of 001 axes in the foliation plane this suggests that omphacite lpo patterns may carry information on the deformation regime simple shear transtension active during the high pressure events

microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates microstructures and lattice preferred orientations lpo were analysed on experimentally deformed natural and hot pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures the lpo was measured using electron backscatter diffraction techniques in the scanning electron microscope sem microstructures were observed by polarized light microscopy and by orientation contrast in the sem natural samples sleaford bay pyroxenite were deformed in axial compression stepping tests up to 16 shortening these samples deformed mainly by twinning and dislocation glide with very little recrystallization no clear lpo evolution apart from the initial lpo could be attributed to deformation synthetic clinopyroxenite samples were hot pressed from powders of the same material with three different grain size ranges and deformed in compression stepping experiments up to 28 shortening in the samples with coarse 30 m and intermediate 20 m grain sizes deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization the recrystallized grains have sizes up to 8 m and represent 15 25 of the sampled area the sample with the finest initial grain size 5 m deformed dominantly by diffusion creep accompanied by grain boundary migration all of the hot pressed samples deformed in compression have a similar texture consisting of a girdle of c 001 axes normal to compression and a point maximum of b 010 axes and a 100 poles parallel to the compression direction the lpo of recrystallized grains was separated on spatially resolved orientation maps and shows an s type fabric with b 010 parallel to the compression axis and a random a 100 distribution the bulk fabric was largely present in the hot pressed starting material and is interpreted as the result of compaction during cold pressing at moderate strains in axial compression the initial compaction fabric has been weakly overprinted by the recrystallization fabric one hot pressed sample coarse grain size was deformed in torsion to a shear strain of 0 1 the microstructure indicates dominant dislocation creep and contains about 20 recrystallized grains the texture evolved from the rotationally symmetric compaction lpo to an oblique fabric

geochemistry of organic rich cretaceous and jurassic mudstones in the west lewis and west flannan basins offshore north west scotland implications for source rock presence in the north east rockall trough the west lewis and west flannan basins serve as important calibration sites for understanding the source rock distribution and quality in the north eastern part of the rockall trough in the west lewis basin two shallow boreholes penetrated organic rich oil prone bathonian mudstones which were deposited in a fresh to brackish water environment these mudstones will generate oils rich in paraffins upon maturation organic rich oil prone ryazanian mudstones were discovered in both the west lewis and west flannan basins they are offshore marine mudstones with a predominance of algal organic matter akin to the kimmeridge clay in the north sea and basins west of shetland the bathonian mudstones will generate their hydrocarbon products at higher temperatures than the ryazanian mudstones the presence of lower cretaceous and middle jurassic oil prone mudstones in basins on the hebrides shelf is intriguing and suggests that similar source rocks may be present along the north eastern margin of the rockall trough

microfabric characteristics and rheological significance of ultra high pressure metamorphosed jadeite quartzite and eclogite from shuanghe dabie mountains china quantitative analysis of the structural evolution of jadeite quartzite a rare ultra high pressure uhp rock type from the dabie mountains of eastern china sheds light on the formation and evolution of uhp orogenic belts worldwide geological mapping of the shuanghe area where jadeite quartzites crop out was carried out to determine the spatial relationships between different uhp rocks within this orogen the deformation mechanisms of jadeite quartzite geodynamical parameters stress strain strain rate and microstructure including lattice preferred orientation lpo were determined from six jadeite quartzite samples from the shuanghe area lpos of clinopyroxene jadeite and omphacite garnet rutile and quartz from these jadeite quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the shuanghe area microstructural lpos of jadeite omphacite garnet rutile and quartz were determined using electron backscattered diffraction ebsd analysis quartz fabrics were largely recrystallized during late low grade stages of deformation whereas garnet shows no strong lpo patterns rutile fabrics show a weak ls fabric along 001 jadeite and omphacite show the strongest eclogite facies lpo patterns suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust microstructural data show that the jadeite lpo patterns are similar to those of omphacite and vary between l and s types which correlate with prolate and oblate grain shape fabrics spo quartz lpos are monoclinic microstructural analysis using tem shows that the dominant slip systems of jadeite in one sample are 100 001 110 001 and 1 1 0 1 2 110 while in another sample no dislocations are observed abundant dislocations in quartz were accommodated by the dominant slip system 0001 1120 indicating basal glide and represents regional shearing during the exhumation process this suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under uhp conditions the protoliths of jadeite quartzite metasedimentary rocks from the northern passive continental margin of the yangtze craton experienced the same deep subduction and were deformed under similar rheological conditions as other uhp eclogite marble and paragneiss experimental uhp deformation of quartzo feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths the resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km 24 gpa thus processes of deep continental subduction may be better understood through understanding the rheology and mechanical behaviour of jadeite jadeite quartzites such as those from the shuanghe may be exhumed remnants of deeply subducted slabs of continental crust other parts of which subducted past thedepth of no return and remain in the deep mantle

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

cation ordering in omphacite and effect on deformation mechanism and lattice preferred orientation lpo we present microstructural data and lattice preferred orientations lpos of omphacites from a suite of eclogites from the adula cima lunga nappe central alps our work shows a surprisingly strong correlation between the measured lpo and the ordering state of cations in omphacite estimates of deformation temperature from metamorphic petrology together with measured omphacite compositions and lpos determine the field ordering state on the omphacite phase diagram into which each sample falls lpos dominated by l type and s type signatures are developed in samples that fall in the p2 n field ordered structure and c2 c field disordered structure respectively dislocations with b 1 2 110 or b 001 are observed in the transmission electron microscope tem in all samples the former change from a perfect dislocation in the c2 c structure to a partial in p2 n any movement of a partial dislocation requires the formation or growth of a stacking fault furthermore in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation the energy to form a constriction is high in omphacite due to the large separation width thus the activity of the b 1 2 110 dislocation is hindered in the p2 n structure relative to the c2 c structure which change the balance between the two and might give rise to the different lpos

deformation of granitic rocks experimental studies and natural examples crustal deformation is inherently complex because the crust is inhomogeneous and anisotropic on a wide variety of scales advances in our understanding have come from a combination of field observations laboratory experiments and theoretical models what necessarily links all these investigations is a focus on process crystal plasticity dislocation creep is one of several grain scale deformation processes or mechanisms each process is dominant over a certain range of conditions in the crust has a particular form of the flow law and produces a characteristic set of microstructures characterizing the deformation mechanisms for different minerals over a range of conditions allows one to link the mechanical behavior or flow law associated with that process to the microstructures it produces the information on microstructures can be used to help interpret the thermomechanical history of naturally deformed rocks and flow laws can be used to create more realistic models predicting crustal behavior in different tectonic settings iterations among field theoretical and experimental approaches are critical for making progress in our understanding the deformation process of crystal plasticity is the main focus of this volume crystal plastic deformation of the crust is in many ways more complex than that of the mantle because most crustal rocks are polyphase aggregates and the strengths of common crustal silicates such as quartz feldspar and mica are very different at a given set of conditions yield strength envelopes for the crust as shown schematically in figure 1 are commonly plotted using the dislocation creep flow law for quartz which inherently assumes that quartz is the controlling phase and undergoes steady state flow but is the flow law for dislocation creep of quartz a good approximation for the deformation of a granite let alone a crustal plum pudding of regions and layers of different lithologies the crust

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

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

crystal preferred orientations of garnet comparison between numerical simulations and electron back scattered diffraction ebsd measurements in naturally deformed eclogites observations of dislocations sub grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites to evaluate the crystal preferred orientations cpo of garnet formed in axial shortening pure shear and simple shear we performed numerical simulations of cpo development during plastic flow using the visco plastic self consistent model as input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens although in garnet 66 slip systems are available slip on the k111l 110 system provides over 86 of the total strain in the simulations characteristic cpo distributions are produced for the three deformation paths with the cpo being strongest for axial shortening and weakest for simple shear compared with low symmetry minerals the pole figure densities of garnet which has cubic symmetry are weak k100l axes tend to align with the shortening direction in all three deformation modes the simulations are compared with cpo of naturally deformed garnet from nine eclogite samples from the alps norway and mali which contain 20 40 garnet all samples show weak garnet cpo only two samples have a cpo pattern similar to the simulations for simple shear no samples are similar to simulations for axial strain or pure shear the presence of other weaker minerals such as omphacite and quartz with volume fractions higher than garnet probably prevented garnet from becoming highly strained and developing characteristic cpos in these eclogites higher volume fractions of garnet and higher temperature conditions may however allow the development of garnet cpo in the mantle transition zone particularly within subducted oceanic material morb q 2004 published by elsevier ltd

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

petrogenesis of early cambrian granitoids in the western kunlun orogenic belt northwest tibet insight into early stage subduction of the proto tethys ocean

kinematic and thermochronological constraints on the xincheng huangpi fault and mesozoic two phase extrusion of the tongbai dabie orogen belt abstract the tongbai dabie orogenic belt dob is composed of several fault bounded terranes with different metamorphic and deformation intensities these tectonically stacked terranes were extruded along their boundary faults from depths of 120 km during the mesozoic systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen in this paper we focus on the xincheng huangpi fault xhf which is located along the southern boundary of the central dob ultrahigh pressure complex unit uc comprehensive macro and micro structural u pb zircon and 40 ar 39 ar geochronological data are presented here to constraint the structural chronology of the xhf the xhf represents the lower detachment surface formed during extrusion of the uhp slab and experienced two episodes of ductile shearing the xhf experienced early southwestward thrusting around 234 ma and was exhumed to middle crustal levels during 234 195 ma with an average exhumation rates of ca 1 5 mm yr the uhp rocks then exhumed to the surface by erosion later nw se trending dextral strike slip shearing of the xhf occurred between 145 and 140 ma and was generally contemporaraneous with sinistral oblique slip of the xiaotian mozitan fault along the northern margin of the uc coeval dextral and sinistral oblique shearing along the southern and northern margins of the uc would have caused southeastern lateral extrusion this lateral extrusion has no relationship with the uhp exhumation

creep behavior during the eutectoid transformation of albite implications for the slab deformation in the lower mantle abstract in order to investigate effects of the post spinel transformation on rheological behavior of descending slabs we conducted simultaneous deformation and eutectoid reaction experiments using an analogue reaction from albite to jadeite and quartz by a deformation dia apparatus flow stress and transformed fraction were quantitatively obtained by in situ x ray observations during the constant strain rate deformation the microstructures of recovered samples as well as the flow and kinetic data suggest sequential variation of the creep mechanism from dislocation creep of the transformed eutectoid colony followed by the grain size sensitive creep in the degenerated eutectoid structure our study demonstrated that the creep behavior during the eutectoid transformation involves various processes than previously thought the slabs may not be weaken promptly after entering into the lower mantle when the size of eutectoid colony is enough large and keep their strength or harden over a period of time depending on the degeneration kinetics of the colony