deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

microstructural investigation of the interaction and interdependence of cataclastic and plastic mechanisms in feldspar crystals deformed in the semi brittle field abstract plagioclase feldspar an 50 single crystals from the hogarth range nsw australia have been deformed in the brittle to semi brittle regime at 500 800 c 90 300 mpa effective pressure and strain rate between 10 5 and 10 6 this gem quality feldspar is both macroscopically and microscopically devoid of any microstructure other than very rare growth twins so all microstructures produced by experimental deformation can be easily identified the specimens were deformed to failure which occurred at strains of

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

a laser 40ar 39ar study of minerals across the grenville front investigation of reproducible excess ar patterns 40ar 39ar ages have been determined on single grains of biotite and amphibole from rocks on a 40 km traverse across the grenville front gf in the temagami area ontario minerals in the vicinity of the gf contain excess ar in varying amounts characterized by both anomalously high integrated and plateau ages the excess ar in biotite varies with distance from the gf such that the overall age distribution along the gf forms an asymmetric wave like pattern an argonami with two age maxima one on either side of the gf the integrated age maxima are 2 34 ga for biotite and 3 21 ga for amphibole located 6 and 0 2 km north of the gf respectively whereas maxima are 1 33 ga for biotite and 1 50 ga for amphibole 7 km south of the gf this wave like pattern of apparent ages shows a remarkable similarity with that seen in an earlier k ar study across the gf at chibougamau quebec and also with a small scale pattern of excess ar associated with the emplacement of a dyke variations in the magnitude of excess a

grain scale melt distribution in two contact aureole rocks implications for controls on melt localization and deformation the grain scale spatial arrangement of melt in layer parallel leucosomes in two anatectic rocks from two different contact aureoles located in central maine usa is documented and used to constrain the controls on grain scale melt localization the spatial distribution of grain scale melt is inferred from microstructural criteria for recognition of mineral pseudomorphs after melt and mineral grains of the solid matrix that hosted the melt in both rocks feldspar mimics the grain scale distribution of melt and quartz is the major constituent of the solid matrix the feldspar pockets consist of individual feldspar grains or aggregates of feldspar grains that show cuspate outlines they have low average width length ratios 0 54 and 0 55 respectively and are interstitial between more rounded and equant width length ratios 0 65 for both samples quartz grains in two dimensions the feldspar pockets extend over distances equivalent to multiple quartz grain diameters possibly forming a connected three dimensional intergranular network both samples show similar mesoscopic structural elements and in both samples the feldspar pockets have a shape preferred orientation in one sample feldspar inferred to replace melt is aligned subparallel to the shape preferred orientation of quartz indicating that pre or syn anatectic strain controlled the grain scale distribution of melt in the other sample the preferred orientation of feldspar inferred to replace melt is different from the orientations of all other mesoscopic or microscopic structures in the rock indicating that differential stress controlled grain scale melt localization this is probably facilitated by conditions of higher differential stress which may have promoted microfracturing grain scale melt distribution and inferred melt localization controls give insight into possible grain scale deformation mechanisms in melt bearing rocks application of these results to the interpretation of deep crustal anatectic rocks suggests that grain scale melt distribution should be controlled primarily by pre or syn anatectic deformation feedback relations between melt localization and deformation are to be expected with important implications for deformation and tectonic evolution of melt bearing rocks

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

the discovery and significance of a fossilized radiogenic argon wave argonami in the earth s crust we have discovered a curious fossilized asymmetric radiogenic argon wave trapped in single grains of biotite in the contact zones between a diabase dike 570 ma and the grenville country rock tonalitic gneiss 1000 ma near mattawa ontario canada the phenomenon is presumably the result of the interplay between the partial pressure of the ambient radiogenic argon and the opening and closing blocking of biotite to argon movement during the period of thermal and fluid perturbation following the dike intrusion the wavelength is roughly equal to one dike width 30 m the only previously reported example of such a wave which we call an argonami was one discovered in 1970 by wanless and his colleagues in a study of biotites from a traverse across the grenville front the two argonamis are remarkably similar even in their asymmetry but the wavelength scales differ by roughly two order of magnitude presumably reflecting the relative sizes of the mattawa dike and the continental scale grenville front the occurrence of the argonami phenomenon suggests that in some instances the analysis of excess argon may become a useful tool for the unraveling of complex tectono thermal and fluid migratory events taking place at plate boundaries and igneous contacts

effects of fluid rock interaction on 40ar 39ar geochronology in high pressure rocks sesia lanzo zone western alps abstract in situ uv laser spot 40ar 39ar analyses of distinct phengite types in eclogite facies rocks from the sesia lanzo zone western alps italy were combined with sims boron isotope analyses as well as boron b and lithium li concentration data to link geochronological information with constraints on fluid rock interaction in weakly deformed samples apparent 40ar 39ar ages of phengite cores span a range of 20 ma but inverse isochrons define two distinct main high pressure hp phengite core crystallization periods of 88 82 and 77 74 ma respectively the younger cores have on average lower b contents 36 g g than the older ones 43 48 g g suggesting that loss of b and resetting of the ar isotopic system were related phengite cores have variable 11b values 18 to 10 indicating the lack of km scale b homogenization during hp crystallization overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40ar 39ar ages than the respective cores they also show variable effects of heterogeneous excess 40ar incorporation and ar loss one acceptable inverse isochron age of 77 1 1 1 ma for rims surrounding older cores 82 6 0 6 ma overlaps with the second period of core crystallization compared to the phengite cores all rims have lower b and li abundances but similar 11b values 15 to 9 reflecting internal redistribution of b and li and internal fluid buffering of the b isotopic composition during rim growth the combined observation of younger 40ar 39ar ages and boron loss yielding comparable values of both parameters only in cores and rims of different samples is best explained by a selective metasomatic overprint in low permeability samples this overprint caused recrystallization of phengite rims whereas higher permeability in other samples led to complete recrystallization of phengite grains strongly deformed samples from a several km long blueschist facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40ar 39ar ages 64 7 68 8 ma yielding an inverse isochron age of 65 0 3 0 ma almost complete b and li removal in mylonitic phengite is due to leaching into a fluid the b isotopic composition is significantly heavier than in phengites from the weakly deformed samples indicating an external control by a high 11b fluid 11b 7 4 we interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone this event also caused partial resetting of the ar isotope system and further b loss in more permeable rocks of the adjacent unit we conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40ar 39ar data in partially metasomatized rocks

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

wide angle seismic data reveal extensive overpressures in the eastern black sea basin summary we present new data that explores the link between pore pressure and seismic velocity to estimate the magnitude of the overpressure within the deep sediments of the eastern black sea basin new wide angle seismic data combined with coincident reflection data have been modelled simultaneously using the seismic tomography code jive3d to provide a wellconstrained seismic velocity model of the sediments our models reveal a widespread lowvelocity zone at the depth of 5 5 8 5 km which is characterized by a velocity decrease from 3 5 to 2 5 km s 1 using two separate methods that relate changes in seismic velocity to changes in effective stress we estimate pore pressures of at least 160 mpa within the lowvelocity zone these pore pressures give values of 0 8 0 9 within the centre of the basin and above the mid black sea high the low velocity zone occurs within the maikop formation an organic rich mud layer identified as the source of mud volcanism in the black sea and south caspian sea

analogue models of melt flow networks in folding migmatites we have modelled the formation and the layer parallel shortening of layered stromatic migmatites the model consists of thin superposed layers of partially molten microcrystalline wax the melt 30 vol has a negative buoyancy and a high viscosity contrast with its solid matrix as soon as the shortening begins melt filled veins with high aspect ratios open along foliation the melt is segregated into the veins forming a stromatic layering during incipient folding crescent shaped saddle reefs open at the hinges of open sinusoidal folds further shortening and melt enhanced shear displacements on interlayer interfaces cause chevron folds to develop and the saddle reefs to become triangular in comparison a melt free experiment shows only a few layer parallel openings and no saddle reefs in chevron folds on the basis of our experimental results we propose that in migmatites 1 mesoscale melt migration is a combination of flow in immobile veins and movements of veins as a whole 2 the changes in the geometry of the mesoscale melt flow network create the pressure gradients that drive melt migration 3 the melt flow network does not need to be fully interconnected to allow local expulsion 4 melt expulsion is episodic because the temporal evolution of the network combines with the spatial heterogeneity of the deformation

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

the effect of melt distribution on the rheology of plagioclase rocks abstract synthetic plagioclase aggregates were fabricated by hot isostatic pressing glass powder with an60ab40 composition at a confining pressure of 300 mpa and a temperature of 1223 k the residual glass content 15 vol was reduced to 1 3 vol by annealing the specimens at temperatures between 1423 and 1523 k at 0 1 mpa high resolution electron microscopy showed that the residual glass was confined to grain triple junctions when these samples were annealed at 0 1 mpa and above the solidus 1620 k they also contained silica rich amorphous phases at grain boundaries other samples were hot pressed at 1423 k they often have grain boundaries containing thin amorphous films 5 50 nm wide the chemical composition of the films differs from pure labradorite with up to 80 wt sio2 the volume fraction of si rich glass increases up to 2 vol for samples that contain 1 wt water uniaxial creep experiments were performed at temperatures of 1373 1638 k and stresses between 2 and 50 mpa the plagioclase aggregates deformed by grain size sensitive creep with a stress exponent n 1 the activation energy for grain boundary diffusion controlled creep of the partially molten aggregates was 400 20 kj mol 1 it remained unchanged with varying glass content and glass topology the strength of samples containing glass along grain boundaries is 4 10 times less than without amorphous grain boundary films the transition between diffusion and dislocation creep is shifted towards higher stresses for samples that contain glass films at grain boundaries

direct observation of deformation processes in crystal mushes isothermal microstructural development in deforming melt bearing crystal mushes is studied using synkinematic microscopy and a polyphase thiocyanate system the aim of the work is to contribute to the basis for understanding the origin of microstructures in rocks which have undergone melt present deformation during high strain rate pure shearing experiments microstructures indicating crystal plasticity and dynamic recrystallization are observed during low strain rate pure shearing experiments a pressure solution like process called contact melting is active resulting in optically strain free crystals crystal boundary sliding is active in both regimes framework scale processes such as filter pressing grain flow and development of a micro shear zone are illustrated some of these processes are cryptic microstructurally and may be difficult or impossible to recognize in rocks the validity of the experimental system as an analog for magmatic systems is discussed we conclude that the experiments can serve as a source of ideas to test against the evidence from rocks but that no direct analogy to the behavior of natural crystal mushes has been or will readily be established

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

convergent phase equilibria at the massif anorthosite granulite interface near bolangir orissa india and thermal evolution of a part of the indian shield abstract this investigation is based on detailed geological mapping of the western part of the bolangir anorthosite massif of orissa india and its granulite borders detailed petrography whole rock chemistry mineralogy and an equilibrium thermodynamic analysis of the mineral phase relations structural analysis of the foliations of the granulites and the primary flow layers and the joint system of the anorthositic rocks strongly indicates that the pluton was forcefully intruded into the granulitic cover with considerable stretching and extension and approached the form and structure of a schlieren dome the anorthositic suite of rocks includes anorthositic norites median plagioclase composition an 75 noritic anorthosites median plagioclase composition an 70 and anorthosites median plagioclase composition an 52 while the bordering granulites include leptynites k feldspar plagioclase quartz orthopyroxene biotite garnet ilmenite khondalites k feldspar quartz sillimanite garnet graphite ilmenite biotite basic granulites plagioclase diopsidic clinopyroxene orthopyroxene garnet hornblende ilmenite k feldspar quartz magnetite and calc granulites diopside scapolite calcite garnet microline quartz sphene magnetite apatite the anorthositic rocks have a relatively high k 2 o sio 2 ratio with the mgo feo ratio mainly between 1 and 2 the mgo feo vs plagioclase mafics relations of the anorthositic suite indicate the fractionation trend anorthositic norite noritic anorthosite anorthosite the calculated p t curves for nine different mineral equilibria in the anorthosites and the granulites converge to a broad cluster within the region 3 7 kbar and 600 740 c the orthopyroxene garnet thermometer harley and orthopyroxene plagioclase garnet quartz barometer newton and perkins restrict the convergence to a slightly narrower p t region 4 7 7 kbar and 620 740 c the two pyroxene equilibria were possibly quenched at a somewhat higher temperature region and the temperature spread of the order of 200 c at pressures between 4 7 and 7 kbar may represent the p t path over which the pluton cooled in the final stage of its evolution the parent magma of the anorthosites believed to be coeval with the 1312 ma old chilka lake massif anorthosites of orissa may have formed under a proterozoic continental crust of well over 20 km thickness

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

origin of strongly reversed rims on plagioclase in cumulates abstract narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the kiglapait intrusion and may be a common feature of all such cumulates the rims occur at plag plag plag ol and less strongly at plag aug grain boundaries they are optically obvious at an and can reach an 32 mol or more in parallel k na drops sharply although ubiquitous from sample to sample the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact they are prominent in linear networks suggesting the last trace of intercumulus liquid a subsolidus origin is ruled out by the absence of reactants at plag plag and plag ol boundaries and by the local rather than pervasive development of rims the rims are required to grow from intercumulus liquid in which the partition of an component between crystals and liquid increases with the trapped augite component of the liquid calculations from published experimental data show that an 30 can easily be achieved by such a process it is also probable that the trapped liquid is part of an an rich boundary layer generated by solute rejection during adcumulus growth the ability of the rims to sustain steep k na gradients despite a long subsolidus cooling history proves that the k na exchange rate is vanishingly small over a geologic time scale in an rich feldspar suggesting that at low k content the potassium is site bound to the tetrahedral al si distribution reversed rims therefore provide important information on diffusion limits as well as on the late stage solidification history of plagioclase rich cumulates moreover they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored for example by augite content

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

melt assisted interior to margin switch from dislocation to diffusion creep in coarse grained plagioclase evidence from a deformed anorthosite pluton abstract at bolangir eastern india massive anorthosites in the pluton interior grade into anorthosites with outward dipping margin parallel foliation neighboring the pluton margin sheets and veins of ferrodiorites residual melts of anorthosite crystallization concordant and discordant to the margin parallel foliation suggest pluton deformation at near magmatic conditions t 950 c and p 6 12 kbar in the pluton interior the larger than centimeter sized magmatic plagioclase grains are replaced by aggregates of smaller 100 600 m dynamically recrystallized internally strained grains with un equilibrated boundaries neighboring the pluton margin poly sized 200 2500 m plagioclase grains in anorthosites are of two types strain free rectangular shaped plagioclase grains with high energy an richer margins indenting neighboring plagioclase grains formed by diffusion creep whereas unstrained end to end touching euhedral plagioclase grains showing tilling represent magmatic flow textures the pluton interior to margin switch in plagioclase deformation from grain boundary migration accommodated dislocation creep to grain boundary diffusion creep is attributed to the increasing melt fraction melt crystal ratio during syn deformation pluton emplacement plagioclase grains in the bolangir pluton are significantly coarser compared to plagioclase aggregates

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

the mortagne granite pluton france emplaced by pull apart along a shear zone structural and gravimetric arguments and regional implication the emplacement mechanism and kinematics and the 3 d geometry of the variscan granite pluton of mortagne are interpreted using systematic measurements of the granite structure allied with micro structural observations from a detailed gravimetric study it is proposed that magma infilled a pull apart structure which formed along a jog of a northwest southeast trending sinistral zone of shear a second episode of shearing under a dextral regime and independent of emplacement was responsible for perigranitic tectonites these successive transcurrent opposite movements already documented for variscan times in western france are discussed concerning the age of the mortagne pluton 313 15 m y

diffusion creep and partial melting in high temperature mylonitic gneisses hope valley shear zone new england appalachians usa field petrographic microstructural and isotopic studies of mylonitic gneisses and associated pegmatites along the hope valley shear zone in southern rhode island indicate that late palaeozoic deformation c 275 ma in this zone occurred at very high temperatures 650 c high energy cuspate lobate phase boundary microstructures a predominance of equant to sub equant grains with low internal lattice strain and mixed phase distributions indicate that diffusion creep was an important and possibly predominant deformation mechanism field and petrographic evidence are consistent with the presence of an intergranular melt phase during deformation some of which collected into syntectonic pegmatites rb sr isotopic analyses of tightly sampled pegmatites and wall rocks confirm that the pegmatites were derived as partial melts of the immediately adjacent isotopically heterogeneous mylonitic gneisses the presence of syntectonic interstitial melts is inferred to have permitted a switch from dislocation creep to melt enhanced diffusion creep as the dominant mechanism in these relatively coarse grained mylonitic gneisses 200 500 lm syn deformational grain size a switch to diffusion creep would lead to significant weakening and may explain why the hope valley shear zone evolved into a major regional tectonic boundary this work identifies conditions under which diffusion creep operates in naturally deformed granitic rocks and illuminates the deformation processes involved in the development of a tectonic boundary between two distinct late proterozoic avalonian basement terranes

diffusion creep in feldspar aggregates experimental evidence a combination of mechanical and microstructural results for experimentally deformed albite aggregates of 2 10 m grain size indicates a regime of grain boundary diffusion creep that depends upon the presence of water temperature and strain rate at 900 c and 10 5 s 1 deformation occurs dominantly by recrystallization accommodated dislocation creep when the aggregates contain 0 2 wt water but by grain boundary diffusion creep when the aggregates contain 0 9 wt water experiments at other temperatures and strain rates indicate that fine grained aggregates with added water may go directly from cataclastic flow to grain boundary diffusion creep with increasing temperature or decreasing strain rate bypassing the field of dislocation creep microstructural evidence for grain boundary diffusion creep in the wet fine grained aggregates includes the development of rectangular grains rather than polygonal grains grain growth a very low average dislocation density open grain boundaries including pores and channels and the presence of overgrowths of a different composition on grains in a two plagioclase an1 and an79 aggregate additional evidence for diffusion creep in the wet fine grained aggregates is indicated by their similar microstructures and strengths when deformed at 600 and 1500 mpa in contrast dry feldspar aggregates deforming by dislocation creep show an increase in strength and a transition to semi brittle behavior at lower confining pressures grain boundary diffusion creep of feldspars may be common in naturally deformed rocks due to the ease of grain size reduction by both cataclasis and dynamic recrystallization

grain size reduction and foliation development in a deformed granitoid batholith abstract the predominantly peraluminous wologorong batholith in eastern new south wales australia has undergone heterogeneous deformation it always has at least one gneissic foliation but in strongly deformed zones it has two foliations one of these is a gneissic foliation s 1 consisting of relatively discontinuous fine grained micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz the other foliation s 2 is more continuous than s 1 and the folia are fine grained and commonly laminated as in mylonites the two foliations could have formed siimultaneously or sequentially during a single deformation s 2 formed oblique up to 30 to s 1 in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith s 2 folia appear to have initiated on suitably oriented parts of anastomosing s 1 folia continued deformation locally reduced the angle between s 1 and s 2 and resulted in the development of mylonite zones grain size reduction by recrystallization and or neocrystallization occurred in all minerals during the deformation but quartz appears to have attained a steady state grain size of around 0 15 mm which was not reduced during more intense deformation deformation mechanisms are difficult to determine but microfracturing associated with offsetting and microboudinage intragranular slip and solution transfer of material may have operated in the earlier stages of deformation and grain boundary sliding and or solution transfer of material may have dominated the later stages the material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization

syn emplacement recrystallization and deformation microstructures in the poe mountain anorthosite wyoming plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed 1 43 ga poe mountain anorthosite wyoming are indicative of very high temperature deformation and recrystallization during the emplacement of the anorthosite body the poe mountain anorthosite consists of a core of recrystallized massive anorthosite transitional with a series of layered anorthositic cumulates at the margin of the intrusion irregular grain boundaries and dissected grain microstructures in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by fast grain boundary migration and possibly subgrain rotation recrystallization at very high temperatures 1050 c during emplacement of the intrusion in the mid crust 3 kbar the deformation and recrystallization of the poe mountain anorthosite was continuous from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion anorthosites with the lowest modal percentages of ferromagnesian minerals and fe ti oxides are always the most recrystallized this suggests that melt interstitial to the plagioclase crystal framework was removed during deformation and recrystallization of the intrusion bulging of plagioclase grain boundaries around fe ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the interstitial melt minerals

high temperature creep of partially molten plagioclase aggregates we have investigated the high temperature creep of synthetic labradorite an60ab40 between 1323 k and 1523 k at atmospheric pressure and low stresses 2 65 mpa average grain size varies from 12 to 16 m samples contained up to 12 vol melt an60ab40 glass fourier transform infrared measurements indicate that trace amounts of water 0 1 0 15 wt h2o were incorporated during crystallization this water was only partly released at high temperature and atmospheric pressure at low stresses and temperatures deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n 1 and an activation energy of 365 25 kj mol regime i with increasing stress and temperature mechanical data and microstructural observations indicate a transition to power law creep with n 3 regime ii this transition occurs in both partially molten and melt free aggregates in regime i the strain rate of samples containing as much as 12 vol melt is between 3 and 5 times higher than in fully crystalline specimens the activation energy is only slightly affected by the melt content between 0 and 10 2 vol transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets grain boundaries are not wetted except surrounding large melt pockets the melt distribution of undeformed and deformed samples is similar the strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water both plagioclase compositions are slightly weaker than synthetic diopside

partial melt topology in statically and dynamically recrystallized granite microstructures of the murray granite pluton central ontario canada show evidence of both static and dynamic crystallization subsequent to partial melting backscattered electron analyses reveal interstitial k feldspar and plagioclase at triple junctions of strain free isometric quartz grains the geometry of the quartz feldspar boundaries mimics the original topology of the quartz melt contacts during crystallization this conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains and by low 27 dihedral angles of quartz quartz feldspar boundaries similar to dihedral angles in experimentally crystallized quartz quartz silicic melt systems in contrast feldspar seams in deformed granites have high axial ratios are usually elongated perpendicular to the foliation plane and are located preferentially along individual grain boundaries quartz grains are dynamically recrystallized and occasionally transected by feldspar seams indicating that fracturing occurred in the presence of melt during crystal plastic deformation of quartz the subparallel orientation in quartz grains of intragranular feldspar bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures respectively partial melt topology was therefore controlled by intergranular and occasionally by intragranular fracturing

shape and microstructure of microgranitoid enclaves indicators of magma mingling and flow abstract at tarandore point 328 km south of sydney new south wales australia two tonalite phases are in contact with a gabbroic diorite all belonging to the moruya batholith lobate to crenulate contacts pillowing of the more mafic phase in the more felsic phase net veining of the more mafic phase by the more felsic local chilling of the more mafic phase against the more felsic and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement therefore all phases were emplaced before the others solidified and an order of intrusion cannot be determined the variety of microgranitoid enclaves suggests that repeated magma mingling has occurred linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the moruya suite of granitoids direct evidence of mixing is not present but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible restite unmixing is unlikely to be the prime process because near minimum melts which would occur in all granitoid phases according to that model should mix easily rather than mingling in fact mingling has occurred between tonalite phases of very similar composition which would not be expected if the liquid parts of enclaves were minimum melts available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at tarandore point owing to inadequately constrained parameters but mixing generally should be favoured by higher temperatures which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves in which the distortion has been accomplished by magmatic flow without solid state deformation as evidenced by strong dimensional preferred orientations coupled with a lack of microstructural evidence of intragranular plastic strain this suggests that elongate enclaves should not be used as indicators of solid state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

pore pressure terminology fluid pressures in the pore spaces of rocks are critical to several aspects of petroleum exploration and production however a general understanding of some basic concepts has been obscured by a lack of consistency in terminology it is the intent of this paper to clarify the meaning of certain terms so that the many disciplines involved with and affected by pore pressures can communicate effectively and clearly a most confusing aspect of pressure terminology arises from mixing the terms for pressure and pressure gradients the word gradient is often dropped when referring to pressure increases with depth even when the gradient distinction is made it can confuse because it can either mean pressure changes referenced from the surface or pressure changes measured over short depth ranges it is important to understand pressures in absolute terms before beginning to work with gradients please note that the terms pressure and stress are used interchangeably in the following they are not strictly the same but can be so considered for this discussion stress is a tensor while fluid pressure is isotropic figure 1 a very stylized diagram of pressure versus depth for a fictional well illustrates several concepts the hydrostatic line gives the pressure due to a column of water the slope would be 433 psi ft for pure water but is usually 45 465 for formation waters an important concept is that for a simple porous rock with pore spaces continuously connected to the surface i e an open system the pressure of the fluid in the pore space is just the pressure exerted by the weight of the overlying fluids this normal or hydrostatic pressure is simply the pressure due to a column of water figure 1 pressure plotted against depth in a fictional well overpressure is the amount of pore pressure in excess of

deformation high pressure metamorphism and exhumation of ultramafic rocks in a deep subduction collision setting cabo ortegal nw spain abstract new petrological and microstructural data from various metaperidotite boudins within large ductileshear zones in the cabo ortegal allochthonous complex in nw spain have important implications forthetectonicmodelsofthearea theperidotites myloniticgarnetharzburgite ti clinohumiteandmagnes ite olivine orthopyroxenite contain mineral assemblages that equilibrated at high to ultra high pressuremetamorphic conditions as well as microstructures of tectonic origin formed at temperatures well above800 c olivine and orthopyroxene fabrics resulted from ow at high temperature 1000 c and solid statenon coaxialplastic owatintermediatetemperature 800 900 c flowcauseddynamicrecrystalliz ationandformationofmoderatetostronglatticepreferredorientationsunderlowtomoderatediverentialstresses and strain rates characteristic of upper mantle and deep crustal deformation the microstructuresand textural relationships suggest that the mylonitic garnet harzburgite represents mantle fragments withlithospheric and asthenospheric imprints whereas the olivine orthopyroxenite resulted from serpentiniteburial to depths where it acquired a characteristic high ultra high pressure metamorphic signature bothtypes of ultrama tes converged to a common sitein a subduction zone that was later incorporated duringcontinental collision to the nw iberian massif as exotic allochthonous complexes that record structuraland metamorphic evidence of the earliest phases of the hercynian orogeny key words cabo ortegal metaperidotite petrofabric rheology ti clinohumite