an analysis of the microstructures developed in experimentally deformed polycrystalline pyrite and minor sulphide phases using electron backscatter diffraction abstract samples of polycrystalline pyrite previously deformed in tri axial compression tests at a confining pressure of 300 mpa and a strain rate of 2 10 4 s 1 but varying temperature were analysed in this study five samples including the original starting material have undergone analysis using forescatter orientation contrast oc imaging coupled with electron back scatter diffraction ebsd to determine how the micro structures in pyrite and to an extent the surrounding minor phases have changed with increasing temperature between 550 c and 650 c the dominant deformation mechanism in pyrite is that of dislocation creep above 650 c this mechanism becomes less apparent and few if any dislocation walls are present within pyrite grains by 700 c this is a result of either increased dynamic recrystallisation with temperature or a change in the dominant deformation mechanism dislocation creep occurs via lattice rotation principally about a single 100 axis but also about two separate 100 axes and more rarely about a single 110 axis deformation has not resulted in a crystallographic preferred orientation cpo within any of the samples shape change of pyrite grains has occurred and this can account for most if not all of the shortening applied but dynamic recrystallisation of pyrite and grain boundary sliding accommodated along the minor phases are also likely to be important mechanisms in all of the samples

grain size sensitive creep of plagioclase accompanied by solution precipitation and mass transfer under mid crustal conditions abstract we report here on a study of three deformed granitoids two mylonites and an ultramylonite from the inner ductile shear zone of the ryoke metamorphic belt sw japan monophase layers composed of quartz plagioclase or k feldspar are present in all samples the plagioclase rich layers consist of grains 6 10 m in size and sometimes include patchy k feldspar and quartz indicating solution precipitation in the mylonite the fine grained plagioclase is mainly an23 25 and the composition of plagioclase porphyroclast is an21 39 without any significant maximum the an compositions together with textural observations indicate that fine grained plagioclase nucleated from solution with mass transfer during deformation in the ultramylonite fine grained plagioclase is widely changed to be an15 37 indicating that the grain size reduction process includes fracturing of original plagioclase porphyroclasts in addition to the solution precipitation process which results in the composition concentrated around an30 in all samples the crystallographic orientations of fine grained plagioclases are almost random and do not correlate with neighbouring porphyroclasts grain size sensitive creep occurred during rock deformation subsequent to the process of solution precipitation that involved mass transfer via fluids

calcite textures microstructures and rheological properties of marble mylonites in the bancroft shear zone ontario canada calcite textures in several microstructural domains from the bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions the coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms the textures are consistent with high temperature deformation experiments where r f and c slip systems were active complete dynamic recrystallization produced equant medium sized grains 50 m with a texture that is nearly random secondary calcite growth observed under cathodoluminescence and variation in 18o value with microstructurc indicate large fluid fluxes during mylonitization fluid assisted grain boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms finer grained s c mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great circle girdle attributed to attainment of an easy slip orientation for most grains ultramylonites have homogeneous textures with finc elongate grains 20 30 m mantled by disseminated secondary phases textures in ultramylonites are characterized by single point maxima and great circle girdles the most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction texture microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size strain and secondary phases in addition to temperature strain rate and differential stress the varied deformation mechanisms and application of experimentally derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains these differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain boundary sliding to dislocation creep

eoarchean within plate basalts from southwest greenland the majority of 3 ga metabasalts have chemical features such as high field strength element hfse depletions that are characteristic of modern island arc basalts these compositions have been interpreted as evidence for subduction of oceanic crust early in earth s history alternatively the apparent absence of archean mafic rocks with mid oceanic ridge basalt morb and ocean island basalt oib compositions and the ubiquitous occurrence of metabasalts with hfse anomalies suggest that these chemical features may instead be a widespread characteristic of the archean mantle related to early chemical differentiation and unrelated to modern style recycling of crust here we present major and trace element data for a suite of metabasalts from innersuartuut island southwest greenland which have a minimum age constraint of 3 75 ga and are likely as old as 3 85 ga samples from innersuartuut show no evidence for crustal contamination or subduction related magmatism and have a petrogenesis comparable to modern oib the new data demonstrate that a compositional range for volcanic rocks comparable to that seen in the phanerozoic existed in the eoarchean therefore rather than a global anomaly subduction related processes are the likely origin for the compositions of the most commonly preserved archean mafic rocks with island arc basalt characteristics

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane abstract in situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure and simple sheared octachloropropane at 75 85 of its absolute melting temperature and at strain rates of 10 5 10 6 s 1 are discussed discontinuities in the strain rotation and or translation components of deformation across the grain boundary induce grain boundary sliding the influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding grain boundary diffusion also causes grain boundary migration types ii and iii that is different from conventional grain boundary migration type i in being non conservative and in the details of boundary movement with respect to material points within grains grain boundary openings filled with fluid octachloropropane vapor are strongly associated with grain boundary sliding openings develop preferentially along grain boundaries at low angles to the shortening direction once openings grow they are closed by thrusting of sliding grains and by diffusion in faster strain rate experiments and entirely by diffusion in slower strain rate experiments an approximately steady openings ratio of 0 5 3 of the sample volume persists without the development of any large scale fracture all grain boundary openings disappear during static readjustment of the microstructure after deformation

misorientation analysis and the formation and orientation of subgrain and grain boundaries in contrast to the behaviour of individual grains both inter and intra granular boundaries within rocks have received much less attention however many geological processes particularly during deformation e g yielding dislocation creep recrystallisation superplasticity and various fracture mechanisms and petrophysical properties depend to some extent on the nature of boundaries present in a rock in this contribution we consider the role of intergranular and intragranular crystal boundaries a precise characterisation of such boundaries depends on defining the crystallographic and dimensional orientations of the boundary and the misorientation between the adjacent regions i e grains subgrains etc separated by the boundary although several theoretical descriptions of boundary configuration are available practical precision is lacking and approximations are necessary we describe two specific approximations for boundary formation and orientation obtained using the sem electron channelling technique the first is a geometrical interpretation of electron channelling patterns ecp in terms of the likely formation and orientation of the intervening boundary the second considers the misorientation between adjacent regions across a boundary this involves a model which assumes a simple geometrical relationship between crystal slip systems responsible for the rotation and misorientation between adjacent regions and the formation and orientation of the resulting boundary these approximations are capable of a identifying trends in the dispersion of crystallographic directions during deformation b identifying active slip systems c calculating the relative schmid factors for each crystal slip system and therefore the most likely system to be activated d modelling synthetic misorientations and predicting the crystal slip systems and boundary configurations to be expected and e comparing real data with synthetic models our analyses are illustrated via natural examples of dynamic recrystallisation in quartzite and a theoretical simulation of the behaviour of an individual quartz grain during deformation

the determination of true shear senses from the deflection of passive markers in shear zones the sense of movement in shear zones can often be determined from the reorientation of pre existing planar or linear features within the zone this cannot always be done simply by examining the change in strike or trend of these features sometimes these will show the opposite sense of rotation to the true sense of shear in the zone this is illustrated for a generalized oblique slip zone using the deflection method9 described here the true movement sense can be deduced from the apparent rotation the emphasis being on a three dimensional approach consideration of the movement on a large scale shear zone the laxford front underlines the importance of being aware of this aspect of shear zone geometry the method may be applied to brittle fault offsets as well as to deflections in ductile shear zones

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

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

natural deformation and recrystallization of some intermediate plagioclase feldspars abstract intermediate an25 an48 plagioclase feldspars from the nordre stromfjord shear zone central west greenland have undergone extensive ductile deformation by intracrystalline slip and by twinning the plagioclases were deformed under low granulite facies conditions optical substructures include undulatory extinction deformation bands subgrains and deformational twins mainly on the albite law dynamic recrystallization occurred primarily by subgrain rotation with bulge nucleation migration of a high angle boundary as a secondary mechanism in some cases recrystallization is accompanied by a small decrease in the an content which is thought to indicate prograde metamorphic conditions the plagioclases exhibit a good crystallographic preferred orientation which is compatible with 010 001 being the principal slip system quartz has in the same rocks undergone more plastic deformation and more extensive recovery than plagioclase this is at least in part due to the fact that most dislocations in plagioclase are dissociated

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

plagioclase microfabrics in a ductile shear zone from the jotun nappe norway abstract gabbroic and anorthositic rocks from the jotun nappe are transected by small ductile shear zones in which a high grade paragenesis and a new foliation are formed most plagioclase grains show evidence of plastic deformation and have recrystallized by subgrain rotation and bulge processes to fine grained mylonite during these processes a plagioclase grain shape fabric was destroyed and with increasing strain a pre existing plagioclase crystal fabric was successively modified enhanced and finally obliterated this could be evidence of superplastic flow inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that 021 a could be an operative slip system and planes close to 132 could be possible slip planes

the effect of grain size distribution on the rheological behavior of polycrystalline materials abstract this paper investigates the possible effect of grain size distribution on the rheological behavior of polycrystalline materials specifically on the stress exponent n activation energy q and grain size exponent p in the steady state deformation all grains in a specimen are assumed to deform simultaneously by power law creep and grain boundary diffusion coble creep it is shown that the overall values of n q and p are affected by the sizes of grains and their volume fractions in the specimen in the case that grains of different sizes deform at the same stress but at different rates a small number of fine grains in a coarse polycrystal may initiate an overall newtonian creep behavior in the case that grains of different sizes deform at the same strain rate but support different stresses a small number of coarse grains in a fine polycrystal may induce an overall power law creep behavior when a diverse grain size distribution exists but only the average grain size is considered the conclusion about the dominant mechanism operating is misleading care should be taken when inferring the overall deformational behavior of rocks containing grains of widely differing sizes based on the microstructures preserved

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

grain boundary hierarchy development in a quartz mylonite abstract orientation contrast imaging using forescatter detectors and backscattered electron diffraction techniques in the scanning electron microscope have been used to investigate the boundary hierarchy characteristics of a quartz mylonite shear zone from torridon nw scotland the boundary hierarchy is defined as the relationship between the boundary misorientations their enclosed domain size and their frequency distribution by measuring the misorientation across every grain and subgrain boundary the characteristics of the boundary hierarchy can be found two microstructural domains were studied one partially recrystallized low strain domain with large relict grains and one fully recrystallized high strain mylonitic domain our results indicate that the processes of recovery subgrain rotation and grain boundary migration recrystallization each produce identifiable boundary hierarchy signatures in the relict quartz grains we have identified the processes of recovery and subgrain rotation in the recrystallized quartz a cyclic steady state exists between these and other processes coupling these data with traditional microstructural observations allows a more rigorous investigation into the development of a high strain fine grained mylonite from a coarse grained undeformed protolith we suggest that this type of detailed crystallographic microstructural analysis can greatly further our understanding of microstructural development in shear zones and may have implications for the effective use of sub grain size palaeopiezometers

temperature constraints on microfabric patterns in quartzofeldsphatic mylonites ribeira belt se brazil abstract based on samples from the major high temperature tres rios alem paraiba padua transpressive shear zone in the ribeira orogenic belt brazil we discuss the applicability of titaniq geothermometry to constrain peak temperature conditions during high temperature mylonitization of quartzofeldspatic rocks and explore the microfabrics formed at these conditions we discuss various aspects of the titaniq method and conclude that deformation occurred at temperatures ranging from 612 to 740 20 c in the studied segment of the shear zone this high temperature deformation resulted in relatively large grain size quartz ribbons and abundant intracrystalline deformation however the cpo fabrics are weak and microstructures suggest that quartz deformation was accommodated by dynamic recrystallization involving grain boundary migration with subsequent grain growth and later some subgrain rotation during exhumation we relate the weak fabrics to diffusion processes during or immediately after dynamic recrystallization and dislocation creep and to the effect of competing slip systems during deformation in terms of rheology evidence for dauphine twinning in our samples suggest strain softening during mylonitization and we suggest that such twinning may add to the rheologically weak nature of quartzo feldspatic portions of hot middle to lower crust

microstructural features of high temperature shear zones in gabbros of the northern apennine ophiolites abstract this paper presents preliminary data on the microstructural evolution of upper amphibolite facies mylonitic metagabbros from the bracco ophiolite complex northern apennines italy high temperature deformation possibly related to the initial stages of breakup and oceanization in the ligurian tethys led to the development of three different types of fabrics in the coarse grained mg gabbros the most widespread rock type in this area with progressive deformations coarse grained porphyroclastic mylonites fabric type 1 banded fabric type 2 and flinty ultramylonites fabric type 3 were formed in plagioclase and clinopyroxene the two main minerals in the studied rocks optical microstructures and scanning electron microscopy analyses indicate crystal plasticity as the main deformation mechanism a possible change from rotation recrystallization regime through grain boundary migration to diffusion assisted high temperature grain boundary sliding is suggested for the plagioclase grains in high temperature shear zones

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

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

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

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

plagioclase fabric development in a high grade shear zone jotunheimen norway abstract shear zones of precambrian age transform meta anorthositic rocks to mylonites of high metamorphic grade in jotunheimen norway a plagioclase microfabric study is presented for one of these shear zones the shear zone is heterogeneously and asymmetrically developed regarding the degree of recrystallization and this is inferred as reflecting strain variations pre shear zone plagioclase is preserved in the deformed rocks as porphyroclasts which display large variations in grain elongation in the plane of mylonitic foliation with aspect ratios long dimension average short dimension as large as 25 1 the preferred orientation of lattice and optical directions in the plagioclase porphyroclasts indicates that they are preferentially oriented with the pole to 010 at a moderate angle to the foliation pole in such a way that they are unfavorably oriented for mechanical twinning on the albite and pericline laws in a flattening strain across the plane of the mylonitic foliation this is taken to indicate that the porphyroclasts represent those pre shear zone plagioclase grains which were oriented in order to accommodate such a flattening strain by mechanical twinning while grains otherwise oriented accommodated the strain by kinking with an axis of external rotation at 100 and glide which accumulated lattice strain and eventually led to recrystallization the plagioclase porphyroclasts with large aspect ratios are preferentially oriented with 100 at a large angle to the inferred translation direction of simple shear strain they are the products of preferential recrystallization of zones of large lattice strain in grains which were strongly kinked due to their special orientation this implies that grain elongation is not applicable as a measure of strain the microstructures indicate that the recrystallization was dynamic the produced neoblasts display a preferred orientation which is weak although with a preference for 010 to have a low angle to the foliation plane which mimics the preferred orientation of the porphyroclasts there is a slight indication that the orientation of neoblasts is controlled by the plagioclase host such that 100 of the neoblasts prefer intermediate rather than large angles against 100 of the porphyroclast these features and microstructural observations indicate that rotation recrystallization is a basic recrystallization mechanism under high grade metamorphic conditions

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

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

plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust thus the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust to this end we have measured p and s wave velocities and anisotropy in 17 plagioclase rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 mpa anorthosites and gabbroic anorthosites containing 80 vol plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns vp z vp y 1 and vp z vp x 1 for anorthosites while 0 8 vp y than the gabbros laminated anorthosites with vp x vp y vp z display a strong crystal preferred orientation cpo of plagioclase whose 010 planes and 100 and 001 directions parallel to the foliation for the gabbros and amphibolites characterized by vp x vp y vp z and vp x vp y vp z respectively pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy the poisson s ratio calculated using the average p and s wave velocities from the three principal propagation polarization directions x y and z of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent the cpo induced anisotropy enhances and decreases the foliation normal incidence reflectivity at gabbro peridotite and anorthosite peridotite interfaces respectively

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

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

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

three stages to form a large batholith after terrane accretion an example from the svecofennian orogen abstract the magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse chemical composition age and structure give information about the magma source and emplacement depths the target of this study is the central finland batholith cfgc that provides a magmatic record after terrane accretion during the svecofennian orogeny followed by orogenic collapse the crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion and when the stress regime has changed from contraction to extension here we present a three stage magmatic evolution model for accretional batholith in the svecofennian orogen in the first stage the lower crust of the accreted crust melts partially and produces calcic tonalitic melts that are emplaced above the upper middle crustal detachment zone group 1 1887 3 ma in the second stage the middle crust begins to melt producing calc alkalic granodioritic melts that are emplaced close to the surface group 2 1890 1886 ma and at both sides of the upper middle crustal detachment zone group 2 1884 1883 ma in the third stage the lower crust remelts and produces bimodal calc alkalic gabbroic and alkali calcic to alkalic granitic melts group 3 1881 1880 ma which are emplaced above the upper middle crustal detachment zone this kind of crustal differentiation produces a mafic lower an intermediate middle and a felsic upper crust the third stage represents the last magmatic event of the svecofennian accretional orogeny where the high velocity lowermost crust is formed during this stage crustal thickening and widespread mid crustal partial melting have inititated lateral flow after or concurrently with the emplacement of the group 2 granodiorite the lateral flow has reactivated large scale shear zones allowing the transportation of the group 3 rocks from the lower crust to the upper middle crustal detachment zone the change in the degree of deformation at the exposed parts from undeformed through local to pervasive is attributed to differences in the exhumation level

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

microstructures deformation mechanisms and seismic properties of a palaeoproterozoic shear zone the mertz shear zone east antarctica abstract the mertz shear zone msz is a lithospheric scale structure that recorded mid crustal deformation during the 1 7 ga orogeny we performed a microstructural and crystallographic preferred orientation cpo study of samples from both mylonites and tectonic boudins that constitute relics of the terre adelie craton tac the deformation is highly accommodated in the msz by anastomosed shear bands which become more scattered elsewhere in the tac most of the msz amphibolite facies mylonites display similar cpo thermal conditions intensity of deformation and dominant shear strain preserved granulite facies boudins show both coaxial and non coaxial strains related to the previous 2 45 ga event this former deformation is more penetrative and less localized and shows a deformation gradient later affected by a major phase of recrystallization during retrogression at 2 42 ga both msz samples and granulite facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted cpo of minerals quartz feldspar biotite amphibole and orthopyroxene in particular we highlight the development of an uncommon cpo in orthopyroxene from weakly deformed samples characterized by 010 planes oriented parallel to the foliation plane 001 axes parallel to the stretching lineation and clustering of 100 axes near the y structural direction lastly we computed the seismic properties of the amphibolite and granulite facies rocks in the msz area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the msz our results reveal that i the low content of amphibole and biotite in the rock formations of the tac and ii the interactions between the cpo of the different mineralogical phases generate a seismically isotropic crust thus the seismic anisotropy recorded by the seismic stations of the tac including the msz must be due to mantle rather than crustal structures

p m and g tectonites a classification based on origin of mineral preferred orientations abstract the mechanisms responsible for mineral preferred orientations can be used as the basis for a classification of tectonites p tectonites develop mineral preferred orientations by plastic deformation m tectonites by mechanical reorientation of inequidimensional grains and g tectonites by various growth processes intermediate tectonite types can be defined with the aid of a triangular diagram examples discussed include marble and quartzite p types slates m or mg types certain hornblende or feldspar rich schists g types granite derived gneisses and mylonites pm types and common schists mg or pmg types the classification is designed to promote a holistic approach to fabric studies in addition the pmg triangle provides a framework for tracking the sequential fabric development of tectonites

preferred orientation of anorthite deformed experimentally in newtonian creep abstract synthetic anorthite aggregates were deformed in a paterson gas deformation apparatus at confining pressures up to 400 mpa in torsion and axial compression at temperatures between 950 c and 1200 c samples deformed in torsion under newtonian creep display development of texture or crystallographic preferred orientation as documented with synchrotron x ray diffraction measurements complex diffraction patterns were deconvoluted with the rietveld method to obtain quantitative texture information torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0 3 develop clear textures texture and shape preferred orientation spo of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide these results show that a material deforming in linear viscous creep can develop a strong texture in striking contrast to the paradigm that the presence of a texture precludes low stress newtonian behavior our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological mechanical regime as sometimes applied for interpretation of seismic anisotropy in the earth

deformation mechanisms in some cover thrust sheets from the external french alps abstract rocks from two parts of the ultradauphinois zone of the external french alps have been examined and the mechanisms by which they were deformed have been assessed from petrographic data jurassic and triassic limestones deformed by pressure solution dissolving non ferroan calcite and precipitating ferroan calcite calcite pressure shadows are usually less elliptical in shape than pyrite pressure shadows and grain boundary sliding is therefore thought to have played a significant role eocene rocks deformed by a variety of mechanisms limestones show mylonitic textures whereas limestone conglomerates with a quartz sandstone matrix deformed by pressure solution of calcite and grain boundary sliding of quartz a model for enhanced diffusion of silica along mica seams is proposed to account for planar quartz mica boundaries greywackes deformed by incongruent pressure solution involving the metamorphic reaction of feldspar to mica and quartz coupled with the replacement of feldspar by calcite

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

the influence of three dimensional grain size distributions on the rheology of polyphase rocks abstract in this study the relation between the deformational behaviour of two phase rocks and the grain size distributions of the constituent phases is investigated samples of hot isostatically pressed 50 50 volume percent mixtures of calcite and anhydrite were experimentally deformed in a triaxial apparatus at a temperature of 500 c a confining pressure of 300 mpa and a strain rate of 2 10 5 s 1 for calcite and anhydrite these are the deformational conditions for dislocation and diffusion creep respectively and for a grain size of 8 m these are the conditions of the equiviscous point it was found that samples from different batches of the synthetic material while having an identical average two dimensional grain size displayed significantly different rheological behaviour the ensuing full analysis revealed that the volumetric three dimensional grain size distributions of the different phases and the different batches varied significantly since the anhydrite is in the transition between the dislocation and the grain size sensitive diffusion creep regime the deformational behaviour of the bulk depends critically on the grain size distribution of the anhydrite phase the usefulness of the concept of an average grain size is discussed

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

orientation related deformation mechanisms of naturally deformed amphibole in amphibolite mylonites from the diancang shan sw yunnan china abstract sheared amphibolite rocks from diancang shan high grade metamorphic complex along the ailao shan red river shear zone southwestern yunnan china show typical mylonitic microstructures the mylonites are characterized by porphyroclastic microstructures and the ultramylonites are highly lineated with alternating amphibole and quartzofeldspathic domains microstructural analysis and p t estimation suggest that the amphibole grains in the mylonitic rocks are deformed and dynamically recrystallized at amphibolite facies in the mylonitic amphibolites there are two types of amphibole porphyroclasts i e type i hard and type ii soft porphyroclasts they have their 001 crystallographic orientations subnormal and sub parallel to the stretching lineation of the rocks respectively the two types of porphyroclasts show distinct deformation microstructures and sub microstructures formed by various deformation mechanisms which contribute in different ways to the generation of the fine grained matrix shape preferred orientation analysis misorientation analysis of the two types of porphyroclasts and new fine grains around them further prove the generation of the fine grains in matrix from the type ii porphyroclasts the type i hard porphyroclasts are deformed mainly by mechanical rotation work hardening and intragranular microfracturing in contrast the deformation of the type ii soft porphyroclasts is mainly attributed to crystalline plasticity i e twinning dislocation creep and dynamic recrystallization during the deformation of the type ii porphyroclasts the 100 001 slip system plays a dominant role during deformation and grain size reduction of amphibole twinning along the active 100 slip system in combination with dislocation creep gliding and climbing governs the nucleation of subgrains and formation of dynamically recrystallized fine grains a process here named twinning nucleation recrystallization