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

high pressure eclogites in northern jiangsu southern shandong province eastern china abstract eclogites are distributed for more than 500 km along a major tectonic boundary between the sino korean and yangtze cratons in central and eastern china these eclogites usually have high p assemblages including omphacite kyanite and or coesite or its pseudomorph and form a high p eclogite terrane they occur as isolated lenses or blocks 10 cm to 300 m long in gneisses type i serpentinized garnet peridotites type ii and marbles type iii type i eclogites were formed by prograde metamorphism and their primary metamorphic mineral assemblage consists mainly of garnet pyrope prp 15 40 mol omphacite jadeite jd 34 64 mol pargasitic amphibole kyanite phengitic muscovite zoisite an sio2 phase apatite rutile and zircon type ii eclogites characteristically contain no sio2 phase and are divided into prograde eclogites and mantle derived eclogites the prograde eclogites of type ii are petrographically similar to type i eclogites the mantle derived eclogites have high mgo feo fe2o3 and cr2o3 compositions in bulk rock and minerals and consist mainly of pyrope rich garnet prp 48 60 mol sodic augite jd 10 27 mol and rutile type iii eclogites have an unusual mineral assemblage of grossular rich grs 57 mol garnet omphacite jd 30 34 mol pargasite rutile pargasitic and taramitic amphiboles calcic plagioclase an68 epidote zoisite k feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites this suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and or amphibolite facies rocks with near isothermal compression reflecting crustal thickening during continent continent collision of late proterozoic age equilibrium conditions of the prograde eclogites range from p 26 kbar and t 500 750 c in the western part to p 28 kbar and t 810 880 c in the eastern part of the high p eclogite terrane the prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part subsequent reactions manifested by 1 narrow rims of sodic plagioclase or paragonite on kyanite and 2 symplectites between omphacite and quartz are interpreted as an effect of near isothermal decompression during the retrograde stage the conditions at which symplectites re equilibrated tend to increase from west p 9 kbar and t 680 c equilibrium temperatures of type ii mantle derived eclogites and type iii eclogite are 730 750 c and 680 c respectively

partial transformation of gabbro to coesite bearing eclogite from yangkou the sulu terrane eastern china an ultra high pressure uhp metamorphic slab at yangkou beach near qingdao in the sulu region of china consists of blocks of eclogite facies metagabbro metagranitoid ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss a gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra high pressures was identified in a single 30 m block metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins the metagabbro contains an igneous assemblage of pl aug opx qtz bt ilm ti mag it shows relict magmatic textures and reaction coronas fine grained garnet developed along boundaries between plagioclase and other phases primary plagioclase broke down to ab ky ms zo grt amp augite is rimmed by sodic augite or omphacite whereas orthopyroxene is rimmed by a corona of cum act and omp qtz layers or only omp qtz in transitional rocks augite and orthopyroxene are totally replaced by omphacite and the lower pressure assemblage ab ky phn zo grt coexists with domains of omp jd70 73 ky phn in pseudomorphs after plagioclase both massive and weakly deformed coesite bearing eclogites contain omp ky grt phn coe qtz rt and preserve a faint gabbroic texture coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz some intergranular coesite and quartz pseudomorphs after coesite also occur assemblages of the coronal stage transitional and uhp peak occurred at about 540 50 c at c 13 kbar 600 800 c at 15 25 kbar and 800 850 c at 30 kbar respectively garnet from the coronal through the transitional to the eclogite stage rocks show a decrease in almandine and an increase in grossular pyrope components garnet in low grade rocks contains higher mno and lower pyrope components the growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex the application of garnet zoning to estimate p t should be carried out with caution some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries these quartz aggregates as well as other primary and low p phases persisted metastably at uhp conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde p t evolution

the ultra high pressure coronitic and pseudomorphous reactions in a metagranodiorite from the brossasco isasca unit dora maira massif western italian alps a petrographic study and equilibrium thermodynamic modelling metagranodiorite samples from the brossasco isasca unit dora maira massif western alps show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra high pressure uhp metamorphic assemblages the original magmatic paragenesis consisted of quartz plagioclase k feldspar biotite and minor phases during uhp metamorphism the plagioclase site p was replaced by zoisite jadeite quartz k feldspar and kyanite and coronitic reactions developed between biotite and adjacent minerals at the original igneous biotite quartz contact site a a single corona of poorly zoned garnet is developed whereas at the biotite k feldspar site b and biotite plagioclase site c contacts composite coronas are formed integration of results from petrographic observations calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions which reached as high as 24 kbar and 650 c accurate microanalysis by energy dispersive spectroscopy eds and statistical analysis of the data allowed us to identify for the first time in a natural na pyroxene of metagranitoid rocks the end member ca eskola

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

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences buatier m van r rmund h l m drury m r and lardeaux j m 1991 deformation and recrystallization mechanisms in naturally deformed omphacites from the sesia lanzo zone geophysical consequences teetonophysrc c 195 11 27 naturally deformed omphacites clinopyroxenes from the sesia lanzo zone western italian alps have been studied using light and transmission electron microscopy techniques deformation and recrystallization of omphacite took place at 5 50 600 c and minimum pressures of 14 16 kbar tem reveals isolated free dislocations loops nodal points tiltwalls complex dislocation networks minor twins stacking faults and antiphase domain boundaries the activated slip systems are l z llo llo l llo and al assisted by climb in nodal points and complex dislocation networks dislocations with burgers vector l 2 112 are present in low strain domains most of the subgrain transitional and new grain boundaries subparallel to wl develop from 110 tiltwalls which accommodate lattice bending produced by the l 2 110 110 slip system a component of the misorientation across such newly formed sub grain boundaries is however also formed by sub rain boundary migration grain rotations in the intermediate and high strain domains are more complex but can be explained by the activated slip systems the omphacite microstructures indicate that initial strain induct rec stallisation occurred by a rotation mechanism where single crystals become polycrystals by the development of numerous high angle grain boundaries at a subsequent more advanced stage migration recrystallization is also activated and a different kind of recrystallization mechanism occurs which involves the discontinuous development of new strain free grains these grains are derived from the extremum of the orientations produced by heterogeneous strain and slip system activity the evident ductility of omphacite during low to intermediate eclogite metamorphism is inferred to be facilitated by hydrolitic weakening effects as such the plastic behaviour of omphacite should be incorporated in geophysical flow models dealing with subducted oceanic and or lithosperic plate margins

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

composition and origin of nuevo laredo trend eucrites new bulk rock analyses are reported for the eucritic meteorites alha81001 lakangaon and nuevo laredo alha81001 resembles ibitira as both have far lower concentrations of na than any other eucrite lakangaon and nuevo laredo are the two most ferroan low mgfe of all eucrites monomict eucrites have previously been divided into three geochemical classes a main group and the nuevo laredo and stannern trends the nuevo laredo trend which is characterized by limited variations of incompatible element contents despite considerable variations in mg mg fe ratio and v content clearly formed out of melt s undergoing fractional crystallization probably as partial cumulates the less distinctive main group eucrites are often assumed to be primary basalts geochemically but one step equilibrium partial melting removed from the bulk composition of the eucrite parent asteroid this partial melting model seems more appropriate for the stannern trend which is characterized by moderate high mg mg fe despite moderate high incompatible element contents our new data and a review of literature data suggest that most of the so called main group eucrites actually belong to the nuevo laredo trend while few if any belong to the stannern trend or any other partial melting trend the nuevo laredo trend may be part of a series of fractional crystallization products that began with orthopyroxenite cumulates diogenites the bulk mgo mgo feo ratio of the eucrite parent asteroid is generally estimated at 0 67 by assuming that typical eucrites are primary basalts but if typical eucrites are diogenite comagmatic fractional crystallization products the bulk mgo mgo feo of the parent asteroid must be considerably higher probably about 0 81

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

shrimp zircon u pb ages of eclogite and orthogneiss from sulu ultrahigh pressure zone in yangkou area eastern china we report shrimp u pb age of zircons in four samples of eclogite and one sample of orthogneiss from sulu ultrahigh pressure uhp zone in yangkou area eastern china uhp rocks are distributed along the sulu orogenic belt suturing north china block with south china block in yangkou area uhp unit is well exposed for about 200 m along yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para and orthogneiss which are highly sheared partly zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in cl images and most of the grains have high th u ratio ranging from 0 8 to 2 1 indicating an igneous origin the weighted mean 206pb 238u ages of zircons from the four samples range from 690 to 734 ma these ages can be correlated to the magmatic stage of the protoliths in rare cases zircon grains possess a narrow rim with very low th u ratio 0 02 epma u th total pb dating of such rim yields younger ages that range from 240 to 405 ma marking the metamorphic stage on the other hand zircons from the orthogneiss show irregular shape and zoning with inclusion rich core and inclusion free rim these grains of zircon yield u pb discordia intercept ages of 226 63 ma and 714 110 ma mswd 0 78 bulk of the areas of the rims rim of the zircons demonstrate younger 206pb 238u ages close to the upper intercept with low th u ratio 0 20 indicating their metamorphic origin in contrast the cores show older 206pb 238u ages close to lower intercept and high th u ratio of 0 14 5 25 indicating their igneous origin the upper intercept age is also commonly noted in zircons from eclogite our results suggest a bimodal igneous activity along this zone during the neoproterozoic probably related to the rifting of the rodinia supercontinent

the dabie uhp unit central china a cretaceous extensional allochthon superposed on a triassic orogen petrological geochronological and structural data show that the eastern dabie metamorphic complex resulted from two orogenic stages precursor rocks of the ultrahigh pressure uhp and high pressure hp units in the present hanging wall tectonic position were buried penetratively deformed and subsequently exhumed by distributed ese directed shearing during triassic time in contrast rocks of the dabie orthogneiss domes now in a footwall tectonic position were penetratively deformed during temperature dominated early cretaceous tectonic events that are likely related to magmatic underplating the dabie orthogneiss domes and the uhp hp units were juxtaposed during early cretaceous exhumation of dabie orthogneiss domes by the formation of an ese directed low angle ductile normal shear zone consequently the uhp hp units now represent an extensional allochthon in the hanging wall of the younger normal shear zone the cretaceous extensional structures are limited by boundary strike slip faults consistent with the south china regional tectonic framework ese directed lateral extrusion is considered to be the driving mechanism for extension and the present day structure of the dabie metamorphic complex

high to ultrahigh pressure uhp ductile shear zones in the sulu uhp metamorphic belt china implications for continental subduction and exhumation high to ultrahigh pressure hp uhp metamorphic rocks that resulted from deep continental subduction and subsequent exhumation in the sulu orogenic belt china have experienced multiphase deformation and metamorphic overprint during its long journey to the mantle and return to the surface hp uhp shear zones are strain localized weak zones on which the uhp slab is transported over long distances hp uhp shear zones are well exposed along a 200 km belt in the sulu uhp metamorphic belt the shear zones lie structurally below the uhp rocks and above the non uhp rocks suggesting the early exhumation of the uhp rocks by thrusting the large area distribution hp uhp nature high strain and structural association of the shear zones with the uhp rocks suggest that the shear zones are probably a regional detachment developed during the early stage of exhumation of the uhp rocks kinematic indicators suggest top to the n nw motion of the uhp slab during the exhumation which combined with isotope signature in mesozoic igneous rocks leads us to the interpretation that the subduction polarity is the north china plate down to the south rather than the yangtze plate down to the north in the sulu region

dynamic recrystallization of quartz under greenschist conditions abstract the sem electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions the degree of recrystallization has been determined at 10 20 50 and 100 respectively we demonstrate that dynamic recrystallization is a sequential process involving subgrain grain rotation and grainboundary migration it initiates via subgrain polygonization on a single crystal slip system and continues via subgrain and neoblast rotation on several slip systems in the early stages of recrystallization

micro xanes determination of ferric iron and its application in thermobarometry micro x ray absorption near edge structure xanes analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh pressure uhp eclogites it is observed that omphacite and phengite contain significant amounts of fe3 fetot 0 2 0 6 whereas only very low contents are present in garnet fe3 fetot 0 0 0 03 the latter being consistent with results from stoichiometric charge balance calculations furthermore considerable variations in the fe3 fetot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry eclogite and calc silicate matrix within one thin section the oxidation state of isofacial minerals is thus likely to depend on the local fluid composition which in the studied case is controlled by calcareous and meta basic mineral compositions these first in situ measurements of ferric iron in an eclogite sample from the dabie shan e china are used to recalculate geothermobarometric data calculations demonstrate that the temperature during uhp metamorphism was as high as 780 c about 80 100 c higher than previously estimated temperatures based on charge balance calculations often give erroneous results pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond 43 7 kbar at 750 c these p t data result in a geothermal gradient of ca 6 c km during uhp metamorphism in the dabie shan however accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for fe3 and the actual effect on mg fe2 partitioning is unknown the present study further shows that micro xanes is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample a clear advantage compared to bulk methods

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

geometry and structural evolution of ultra high pressure and high pressure rocks from the dora maira massif western alps italy abstract the crystalline nappes of the dora maira massif western alps essentially made of continental material from the upper crust show petrological relics of an ultra high pressure uhp to high pressure hp cold eclogite eoalpine metamorphism they also display relics of uhp hp structures preserved in boudins and or within large uhp porphyroclasts in a retrograde greenschist facies regional deformation fabric the greenschist facies overprint has the character of a shallow dipping mylonitic foliation s m bearing a penetrative stretching lineation l m which roughly parallels the axes of coeval isoclinal folds shear sense markers indicate a w verging overthrust mechanism the uhp and hp relic structures arc of variable nature the coexistence of equant and inequant either symmetric or asymmetric fabrics indicates that the deformation at uhp hp conditions was strongly heterogeneous and partitioned this is also supported by the local preservation of hercynian magmatic fabrics the uhp and hp deformation involved at least locally rotational components although less intensive than during the latter retrograde stage the regional structural evolution is envisaged as follows i the eoalpine subducted crust was subdivided into lenticular bodies surrounded by uhp hp shear zones the main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the uhp hp structural relics conflicting models are possible depending on the interpretation of the early sense of movement normal vs reverse along the faults that limit the lens shaped units and ii the late heterogeneous regional greenschist deformation can be attributed to the eocene collapse of the alpine orogenic wedge

discussion on coesite bearing eclogite breccia implication for coseismic ultrahigh pressure metamorphism and the rate of the process by yang et al contrib mineral petrol 2014a 167 1013 recently yang et al contrib mineral petrol 167 1013 2014a proposed that cataclasites breccias and pseudotachylytes of eclogites at yangkou in the chinese sulu uhp metamorphic belt are formed by a compression stress wave of earthquake they suggested that the intergranular coesite resulted from a rapid cooling from about 700 c to below 375 400 c and a sudden pressure release of seismic wave from 3 3 to 1 2 gpa in some hours however the earthquake induced uhp metamorphism under fluid deficient conditions proposed by these authors remains inconclusive and inconsistent with the available data from the uhp metamorphic belt the coesite inclusions within garnet zircon and omphacite and intergranular coesite grains between these minerals can be preserved by the presence of the pressure vessel effect the lack of fluid infiltration and the presence of a low temperature and nonhydrostatic deformation environment during rapid exhumation

ductility of garnet as an indicator of extremely high temperature deformation discoidal shaped garnets alm63 prp33 grs3 sps1 occur in the foliation plane of lineated quartzofeldspathic mylonites of the morin shear zone located along the eastern boundary of the morin anorthosite complex grenville province quebec the flattened shape of garnets records a component of coaxial strain x y z different from the dominant rotational shear strain x y z in this mylonite tem observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet extrapolation of available experimentally determined flow laws to natural strain rates shows an inversion of flow strength between quartz feldspar and garnet garnet much stronger than quartz and feldspar at temperatures below 700 c becomes weaker than quartz and feldspar above 900 c the coaxial ductile strain recorded by the garnets could be produced by a local uncharacterized deformation under extremely high temperature conditions at the roof of the rising anorthositic diapir 1155 ma subsequent lower temperature rotational deformation is related to strike slip movement along the morin shear zone 1020 ma

tectonics of the dabieshan eastern china and possible exhumation mechanism of ultra high pressure rocks the dabieshan is divided into three tectonic domains the southern dabieshan is a stack of allochtons from top to bottom i unmetamorphosed cambrian early triassic series unconformably covered by jurassic sandstone ii weakly metamorphosed proterozoic slate iii hp rocks mostly retrogressed into greenschist facies iv uhp allochton v uhp free gneisses these units exhibit a nw se lineation and top to the nw shearing reworked by s verging folds the central dabieshan is a migmatitic dome superimposed on uhp metamorphism and bounded by a detachement fault responsible for the exhumation of the uhp unit during its retrogression into amphibolite facies in the northern dabieshan early foliation and n s trending compressional lineation are deformed by n verging folds coeval to the syn exhumation ductile structures of the central dabieshan a geodynamic model involving coeval thrusting and normal faulting is discussed

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 estimating the strength of calcite rocks under natural conditions abstract field studies of calcite mylonites often document microstructures produced by dislocation creep in contrast flow laws derived from experiments predict that calcite rocks should deform mostly by diffusion creep during tectonic processes to investigate this apparent discrepancy we compare stresses estimated by microstructural piezometers to those obtained by extrapolation of experimentally derived flow laws considering shear zones from different geological settings a clear trend is observed of increasing recrystallized grain size with increasing temperature however there is a large spread in grain size and associated stress because separate flow laws have been defined for various different marbles and limestones the strengths predicted for a given set of conditions differ significantly the stress estimates based on the piezometers and strength extrapolated from the various experimentally derived dislocation creep flow laws agree qualitatively but no single flow law predicts all the palaeostress estimates even if experimental data are disregarded the field observations are not consistent with a hypothetical law for coble creep they are consistent with a power law for dislocation creep but only if the material constants are different from those currently determined in laboratory experiments

occurrence and field relationships of ultrahigh pressure metagranitoid and coesite eclogite in the su lu terrane eastern china coesite eclogite is associated with metagranitoid in a 50 100 m 2 outcrop within the regionally developed amphibolite facies su lu orthogneiss primary intrusive relationships between the metagranitoid and basic rocks and bulk chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed the presence of rutile sodie pyroxene corona garnet and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid this is the first documented occurrence of ultrahigh pressure uhp metagranitoid outside of the european alps the existence of uhp metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths even at peak metamorphic conditions the uhp composite igneous body reported here would have a bulk density less than the mantle buoyancy forces may therefore have been important in the early exhumation of this unit other outcrops of coesite eclogite in the su lu region may also have been originally metamorphosed along with low density granitoid rocks

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

preservation of primary volcanic textures in the ultrahigh pressure terrain of dabie shan delicate primary volcanic features such as ash layers volcaniclastic breccias and pillow lavas with amygdaloidal rims have been identified in a terrigenous volcanosedimentary sequence forming part of the ultrahigh pressure unit of the dabie shan china the presence of coesite relics in crosscutting dikes confirms an ultrahigh pressure evolution for the entire sequence evaluation of the probable reaction history for zeolite compositions as supposed for the amygdaloidal rims reveals a scarcity of reactions at very high pressures such special conditions of low reactivity and fluid conserving reactions combined with a lack of directed stress have allowed the remarkable preservation of these primary volcanic features despite deep subduction

faulting induced by precipitation of water at grain boundaries in hot subducting oceanic crust dehydration embrittlement has been proposed to explain both intermediate and deep focus earthquakes in subduction zones1 2 3 4 5 because such earthquakes primarily occur at shallow depths or within the core of the subducting plate dehydration at relatively low temperatures has been emphasized6 7 8 however recent careful relocation of subduction zone earthquakes9 10 shows that at depths of 100 250 km earthquakes continue in the uppermost part of the slab probably the former oceanic crust that has been converted to eclogite where temperatures are higher here we show that at such pressures and temperatures eclogite lacking hydrous phases but with significant hydroxyl incorporated as defects in pyroxene and garnet develops a faulting instability associated with precipitation of water at grain boundaries and the production of very small amounts of melt this new faulting mechanism satisfactorily explains high temperature earthquakes in subducting oceanic crust and could potentially be involved in much deeper earthquakes in connection with similar precipitation of water in the mantle transition zone 400 700 km depth of potential importance for all proposed high pressure earthquake mechanisms is the very small amount of fluid required to trigger this instability

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

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

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

piplia kalan eucrite fall petrography and chemical characteristics two stones that fell near piplia kalan on 1996 june 20 are identified as ca rich achondrites belonging to the eucrite group the conditions of their fall are described the petrographic description texture and chemical composition the elemental ratios particularly k la and yb sc and relatively flat rare earth element ree patterns 6 8 chondrite of bulk samples without any significant eu anomaly all agree with the characteristic values found in eucrites a feldspar rich fraction shows a significant positive eu anomaly the presence of several different lithologies and clasts indicates that it is a texturally diverse breccia and the chemical analysis suggests that it belongs to the noncumulate group of eucrites falling in the nuevo laredo trend

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

synexhumation anatexis of ultrahigh pressure metamorphic rocks petrological evidence from granitic gneiss in the sulu orogen abstract petrological evidence is provided for partial melting of ultrahigh pressure uhp metamorphic granitic gneiss in the sulu orogen petrographic observations show the occurrence of elongated highly cuspate feldspars in grain boundaries interstitial cuspate feldspars in triple junctions felsic veinlets mainly consisting of k feldspar quartz and feldspar crystal faces against quartz these features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss zircon domains grown from these melts were identified based on cl images mineral inclusions and ree patterns some large zircon grains 100 m contain small relict domains of magmatic origin suggesting nearly complete dissolution of the protolith zircon during the anatexis all newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions one group of domains contain no coesite inclusion and exhibit high u contents but low th u ratios pb ages of 217 2 to 224 2 ma the other group of domains contain coesite inclusions and exhibit low u contents and very low th u ratios pb ages of 221 5 to 226 3 ma the two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative eu anomalies that are absent for metamorphic zircons grown from aqueous fluids the zircon u pb ages of 217 2 to 226 3 ma are close to but slightly younger than known ages for major uhp metamorphism in the sulu orogen therefore the uhp gneiss experienced incipient melting during the initial exhumation subsequent to the peak uhp metamorphism and extensive anatexis later at lower pressures muscovite relicts coexist with cuspate feldspars suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the uhp granitic gneiss as the degree of partial melting increases with temperature significant fractions of the anatectic melts would be produced in the regional gneiss provided that the uhp rocks experienced hot exhumation such melts can be gathered together being eventually emplaced as synexhumation granitic intrusions

thermobarometry of phengite bearing eclogites in the dabie mountains of central china pressure temperature conditions for formation of the peak metamorphic mineral assemblages in phengite bearing eclogites from dabieshan have been assessed through a consideration of fe2 mg2 partitioning between garnet omphacite and garnet phengite pairs and of the reaction equilibrium celadonite pyrope grossular muscovite diopside which incorporates an evaluation of the extent of the strongly pressure dependent inverse tschermak s molecule substitution in the phengites for the latter equilibrium the calibration and recommended activity composition models indicated by waters martin 1993 have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others confirmation that in some phengite eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite and in some cases even diamond rather negates previous suggestions that coesite may have been stabilized in only very localized possibly just intracrystalline domains inherent difficulties in the evaluation of peak metamorphic temperatures from fe2 mg2 partitioning between mineral phases due to uncertainties over fe3 fe2 ratios in the minerals especially omphacites and to re equilibration during extensive retrograde overprinting in some samples are also assessed and discussed our results indicate the existence in south central dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes thus our data do not support earlier contentions that south central dabieshan comprises a structurally coherent continental crust terrane with a regional p t gradient signalling previous deepest level subduction in the north instead we recognize the central dabie ultra high pressure coesite eclogite bearing terrane to be structurally overlain by a southern dabie high pressure quartz eclogite bearing terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section

dehydration of dioctahedral aluminous phyllosilicates thermodynamic modelling and implications for thermobarometric estimates we propose a solid solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations including hydration observed in natural smectites interlayered illite smectite illites and phengites from diagenetic to high grade metamorphic conditions the suggested formalism involves dehydrated micas and hydrated pyrophyllite like thermodynamic end members with these end members the equilibrium conditions of quartz water k bearing mica like phyllosilicates of fixed 2 1 composition are represented by a line in p t space along which the interlayer water content varies the relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions which covers a range between 25 c and few mpa to 800 c and 5 gpa the temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data at low temperature and pressure the amount of interlayer water in k deficient phengite and illite is predicted to reach 100 of the apparent vacancies which is consistent with previous values reported in the literature although the amount of interlayer water is predicted to decrease with pressure and temperature it is calculated to be significant in k deficient phengite from lt hp pelites metamorphosed at about 350 c 10 kbar the presence of molecular water in the interlayer site of such phengites has been confirmed by ftir mapping its implications for p t estimates are discussed

sr isotopes in peridotite xenoliths and their basaltic host rocks from the northern hessian depression nw germany 87sr 86sr ratios of alkali olivine basalts nepheline basanites and olivine nephelinites of miocene age from the northern hessian depression vary between 0 7032 and 0 7036 tholeiitic rocks from this area which are possibly affected by crustal contamination have more radiogenic sr 0 7035 to 0 7042 peridotite xenoliths with coarse protogranular 10 samples and with porphyroclastic textures 2 samples contain k and na rich glasses which are products of reaction of metasomatic fluids with depleted peridotite the sr abundance in xenoliths is related to the amount of glass and phlogopite

what brought them up exhumation of the dabie shan ultrahigh pressure rocks metamorphic coesite and diamond in the dabie shan eastern china testify to subduction of continental crust to 100 km depth exhumation of these ultrahigh pressure rocks through the crust encompassed two stages 1 south dipping foliation southeast plunging stretching lineation lineation parallel isoclinal folds and boudins indicate extreme subhorizontal shortening and subvertical extension during top to northwest shearing at 200 180 ma syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability 2 northwest southeast subhorizontal extension from 133 to 122 ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the dabie shan pluton cores at deep structural levels have weak hypersolidus fabrics and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions deformation is concentrated in greenschist facies mylonites and ultramylonites along the xiaotian mozitang detachment fault at the northern topographic limit of the dabie shan our preferred exhumation model involves two stages triassic indentation vertical extrusion and erosion followed by cretaceous plate margin transtension

plastic flow of coesite eclogite in a deep continent subduction regime microstructures deformation mechanisms and rheologic implications abstract ultrahigh pressure uhp metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts despite the long distance transport of the uhp rocks in a deep subduction channel and the fast strain rate required for rapid exhumation little evidence for uhp deformation is found in the exhumed uhp terrains so far identified some uhp rocks have even preserved primary igneous and volcanic structures these observations have led to the postulation that the stress level is low a few mpa in the deep subduction zones deformation microstructures of some eclogite mylonites from a ductile shear complex in the sulu uhp metamorphic belt eastern china provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the uhp rocks in a highly strain localized shear zone at the uhp conditions omphacite phengite and kyanite are the main strain accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration coesite is either fractured with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure garnet does not show optically discernable plastic deformation p t conditions for the deformation are estimated to be p 3 3 gpa and t 624 c using garnet omphacite kyanite phengite coesite quartz thermobarometry suggesting that deeply subducted continental crust remains cold at the upper mantle depths large part of the deeply subducted crust remains relatively undeformed strain is strongly localized into narrow shear zones stress level in the deep subduction channel is extrapolated on the order of tens of mpa much higher than it was previously thought

kyanite deformation in whiteschist of the ultrahigh pressure metamorphic kokchetav massif kazakhstan microstructures in minerals from ultrahigh pressure metamorphic uhpm terranes are keys to understanding the rheological properties and the exhumation mechanisms of rocks from subduction zones kyanite bearing whiteschist associated with eclogite lenses is part of uhpm unit ii located south west of lake zheltau in the kulet region of the kokchetav massif the equilibrium assemblage is kyanite garnet talc phengite coesite quartz previously reported peak pressure temperature p t conditions are 3 5 gpa at 750 c a strong foliation is defined by the talc and phengite with a corresponding weak shape preferred alignment of kyanite crystallographic orientation maps and analysis of kyanite blades were performed using electron backscatter diffraction methods the data are consistent with a 100 001 slip system for the formation of undulose extinction and kink bands in kyanite rotations measured across individual kink bands are 10 50 about and rotations along kyanite with undulose extinction are up to 50 about with variations between adjacent points typically 2 the undulose extinction is interpreted to have developed through crystal plastic deformation by dislocation creep kink bands mark the development of high angle grain boundaries by dislocation climb the deformation of kyanite occurred in the fault bounded terrane during the exhumation of the kokchetav massif

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

alha 81005 meteorite chemical evidence for lunar highland origin based on the well established characteristic lunar and meteoritic ratios of feo mno cr2o3 v and k la and ree patterns alha 81005 meteorite is undoubtedly of lunar highland origin the alha 81005 meteorite is an anorthositic gabbro 72 pl and matches closely in chemical composition to apollo 15 15418 and luna 16 21013 highland rocks the rock is a non kreepy and vlt type the siderophiles au ni and ir content is high 1 5 cl equivalent and similar to other lunar highland breccias the clast has a different ancient meteorite component than the bulk and matrix the meteorite has a young terrestrial age implying a recent lunar cratering event therefore during major lunar basins formation in early eons the earth must have received a significant amount of lunar material which is now mixed in the crust the finding of a lunar meteorite lends credibility to the suggestion that snc achondrites may come from mars

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

deformation enhanced hp uhp metamorphic transformation of two pyroxene granite in ductile shear zones at yangkou sulu uhp metamorphic belt china abstract we report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure hp uhp metamorphic transformation of a two pyroxene granite in deep continental subduction zone integrated structural and petrologic study of rocks from the two pyroxene granite block and shear zones in the block in the uhp shear complex at yangkou sulu uhp metamorphic belt reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene augite plagioclase k feldspar quartz and biotite metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics in contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to hp uhp phase minerals dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite which increases reaction surface area and enhances metamorphic transformation fluid released by biotite transformation to hp uhp minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone deformation conditions are estimated ca 546 c and pressure 2 5 gpa

mineral chemistry and the petrogenesis of eucrites ii cumulate eucrites abstract we carried out an ion microprobe study of rees and other selected trace and minor element concentrations in orthopyroxene augite lamellae and plagioclase of two cumulate eucrites moama and moore county in order to reevaluate the relationships between cumulate and noncumulate eucrites and among cumulate eucrites pyroxene and plagioclase in these two meteorites are more homogeneous in major minor and trace elements than in noncumulate eucrites and trace and minor element concentrations in moore county are higher than in moama although subsolidus equilibration erased and original igneous zoning it did not affect the ree patterns of these minerals both pigeonite and plagioclase of moama and moore county have higher lree hree ratios than those phases in noncumulate eucrites this indicates that cumulate eucrites derived from melts with fractionated ree patterns and thus are not related to noncumulate eucrites in addition the inconsistency between degrees of fractional crystallization deduced from major and trace elements and the differences of lree hree ratios recorded in pigeonite and plagioclase also suggest that moama and moore county did not form in the same fractional crystallization sequence this study confirms earlier observations and conclusions of ma and schmitt 1979 and poses problems for prevalent models

an experimental study into the rheology of synthetic polycrystalline coesite aggregates coesite has been found as a relic in ultrahigh pressure metamorphic uhpm crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at uhpm conditions we performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3 1 to 3 7 gpa temperatures of 700 to 1160 c and strain rates betweeen 6 10 7 and 1 10 3 s 1 the problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress the correlation between the mechanical data and the microstructural record of the deformed samples as a function of temperature and imposed strain rate is consistent with deformation by dislocation creep we deduced parameters of a power law as n 3 1 and q 275 50 kj mol 1 extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures due to the lack of preserved coesite other than as single crystal inclusions nevertheless the extrapolation indicates a low strength of order 10 mpa for natural strain rates at typical uhpm conditions absent deformation of the uhpm brossasco granite dora maira massif western alps thus implies low stresses deformation must have been localized in very weak shear zones during burial and exhumation

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed

origin of howardites diogenites and eucrites a mass balance constraint abstract howardite meteorites are regolith breccias impact produced mixtures that consist of two main rock types basalts that strongly resemble eucrite meteorites and pyroxene cumulates that strongly resemble diogenite meteorites there are basically two petrogenetic models for the noncumulate basaltic parts of howardites 1 1 genesis as residual liquids of the same magma s that had earlier crystallized the pyroxene cumulates and 2 2 genesis as primary unmodified partial melts from the interior of the parent asteroid a mass balance constraint based on al indicates that unless the ratio degree of melting during genesis of melts parental to pyroxene cumulates degree of melting during genesis of primary basalts was on average greater than about 2 8 a scenario that implies an implausible bimodality of degree of melting on the howardite parent body an average howardite must contain more residual liquid basalt than primary basalt if as seems likely eucrites per se are identical to the howardite basaltic components and diogenites per se are the pyroxene cumulate components then more than half of the eucrites probably formed from residual liquids of magmas that had earlier produced the diogenites considering all the potential complexities of petrpgenesis on the howardite parent body neither the partial melting model nor the fractional crystallization residual liquid model is likely correct for all noncumulate eucrites