k feldspar megacrysts in granites phenocrysts not porphyroblasts abstract k feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts most of the microstructural mineralogical and chemical evidence e g shape alignment concentration ba content zoning inclusions and twinning favours a phenocryst origin the main features that have been used to support a porphyroblast origin are occurrence of megacrysts 1 across aplite vein boundaries 2 in country rocks and 3 in or across boundaries of microgranitoid enclaves mafic inclusions however these features can be explained by the phenocryst hypothesis in particular megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave all available evidence favours or is consistent with a phenocryst origin for k feldspar megacrysts in granitoid rocks and their enclaves the large size of the megacrysts is evidently due to nucleation difficulties for k feldspar in granitic melts though k feldspar is commonly the last mineral to begin crystallizing in granitic magmas abundant melt is still present at that stage allowing sufficient space for the megacrysts to grow the reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called megacrysts until the magma contains such a high proportion of crystals that it cannot erupt

preservation of chemical residue melt equilibria in natural anatexite the effects of deformation and rapid cooling a combined petrologic textural and chemical approach is used to interpret the relationship between chemical exchange and deformation during partial melting of a granodioritic gneiss in the contact aureole of the bergell pluton central alps in contrast to most regional metamorphic anatexites chemical equilibrium between residue and segregated melt was attained and preserved by several elements as shown by their present distribution equilibrium involved both the major and the trace elements hosted in the rock forming minerals and the light rare earth elements lree hosted primarily in allanite equilibrium with respect to lrees was achieved because allanite had a reactive behaviour during water present melting in contrast to the refractive behaviour of xenotime which mostly controls the distribution of the hree s the attainment and preservation of equilibrium between leucosome and residue requires peculiar boundary conditions we propose that the achievement of equilibrium between melt and residue was promoted by deformation operating via a mechanism of melt present granular flow i e dissolution accommodated grain boundary sliding active during partial melting microstructural observations indicate that melt present granular flow was the dominating deformation mechanism in these anatexites the preservation of residue melt equilibrium is inferred to result from rapid cooling under contact metamorphic conditions and from the lack of deformation below the solidus

how late are k feldspar megacrysts in granites abstract various petrologists have suggested that k feldspar megacrysts grow in granites that are extensively crystallized even at subsolidus conditions however experimental evidence indicates that though k feldspar nucleates relatively late in the crystallization history abundant liquid is available for development of large crystals a great deal of evidence involving many different factors favours a magmatic phenocrystic origin for k feldspar megacrysts in granites namely simple twinning oscillatory zoning euhedral plagioclase inclusions and concentric crystallographically controlled arrangements of inclusions in addition abundant evidence has been presented of 1 mechanical accumulation of k feldspar megacrysts in granites 2 alignment of megacrysts and megacryst concentrations in magmatic flow foliations 3 involvement of megacrysts in zones of magma mixing in granite plutons and 4 occurrence of megacrysts in some volcanic rocks implying that the megacrysts were suspended in enough liquid to be moved without fracturing or plastic deformation detailed trace element and isotopic data also indicate that megacrysts can move between coexisting felsic and more mafic magmas irregular overgrowths on megacrysts are consistent with continued magmatic growth after euhedral megacrystic growth ceased the overgrowths being impeded by simultaneously crystallizing quartz and feldspar grains

shakeout d ground motion estimates using an ensemble of large earthquakes on the southern san andreas fault with spontaneous rupture propagation 1 we simulate ground motion in southern california from an ensemble of 7 spontaneous rupture models of large mw7 8 northwest propagating earthquakes on the southern san andreas fault shakeout d compared to long period spectral accelerations from the next generation attenuation nga empirical relations shakeout d predicts similar average rock site values i e within roughly their epistemic uncertainty but significantly larger values in los angeles and ventura basins due to wave guide focusing effects the shakeout d ground motion predictions differ from those of a kinematically parameterized geometrically similar scenario rupture 1 the kinematic rock site predictions depart significantly from the common distance attenuation trend of the nga and shakeout d results and 2 shakeout d predictions of long period spectral acceleration within the basins of the greater los angeles area are lower by factors of 2 3 than the corresponding kinematic predictions we attribute these differences to a less coherent wavefield excited by the complex rupture paths of the shakeout d sources

characteristics of internal contacts in the tuolumne batholith central sierra nevada california usa implications for episodic emplacement and physical processes in a continental arc magma chamber internal contacts along the eastern margin of the tuolumne batholith central sierra nevada california and structures preserved along these contacts are highly variable the contacts range from relatively sharp to gradational boundaries to sheeted zones to very complex boundaries formed by multiple processes fractional crystallization kilometer scale mixing within broad transition zones voluminous magmatic stoping along sharp contacts and downward return flow and or margin collapse of older magma units were important large scale processes along these contacts during chamber construction in contrast sheeting extensional cracking and diking represent only second order small scale complexities formation of the most complex zones along internal contacts resulted from the interaction of the sequential emplacement of different units with the irregular geometry of these contacts which often resulted from removal of earlier phases by stoping our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved we also argue that fairly large magma chambers existed in this batholith and thus that large accumulations of magma may exist in upper crustal chambers for significant periods of time

fluid enhanced deformation transformation of granitoids to banded mylonites western sierra nevada california and southeastern australia abstract deformation of tonalite and adamellite from the western sierra nevada california and southeastern australia has produced mylonite zones that show a broad range of microstructural development varying from strongly banded and or foliated to locally non foliated accompanying these fabrics are new mineral assemblages commonly rich in quartz and epidote whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host they also occur in close proximity cm m to domains of non banded and or weakly to non foliated fabrics of similar mineralogy suggesting a complex timing and partitioning of deformation fluid flow and metamorphism in the mylonite zone whole rock major and trace element analyses of the tonalites show large enrichments of sio 2 cao fe 2 o 3 total fe and sr and strong depletions in na 2 o k 2 o feo mgo ba and rb with increasing stages of deformation although substantial chemical change appears to occur under static conditions locally chemical changes in the adamellite are much less dramatic all suites show systematic decrease in 18 o and convergence of oxygen isotopic compositions of whole rock and quartz with increasing deformation minimum fluid rock is estimated to lie between 0 1 and 10 increasing with intensity of deformation and depending on host composition isotopically exchanged seawater or connate water are the most plausible fluids chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior gain loss of major and minor elements eleven factors control which and how much elements are gained lost during mylonitization of the granitoids the most influential of which are primary composition mineralogy and primary texture of the host as well as several aspects of fluid interaction active during deformation these influences may be a function of local rather than regional geological conditions

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

assembly of a dike fed magma chamber the jackass lakes pluton central sierra nevada california the mechanisms of ascent assembly and emplacement of granitic magma in the crust are critical to understanding the dynamics of continental margin growth yet these mechanisms remain controversial and poorly understood detailed study of structural and petrologic fabrics in the middle cretaceous jackass lakes pluton wall rock system central sierra nevada california coupled with u pb geochronology indicates that the pluton formed via sheet like assembly of a dike fed magma chamber final emplacement of the pluton was facilitated by multiple brittle and ductile mechanisms that were active at different times and places within the system this supports hybrid viscoelastic emplacement models as realistic alternatives to end member models i e dike versus diapir fracture propagation which initiated 40 of the space required for emplacement may have been facilitated by a small component of arc parallel dextral shear that produced north northwest striking tension gashes a combination of ductile wall rock shortening during lateral expansion of sheets and return flow of elongate strongly deformed wall rock septa produced an additional 25 of the space required other mechanisms including coeval formation of the overlying minarets caldera and stoping in the subvolcanic part of the magma chamber must account for the remaining 35 space implying that vertical transfer of material is an important emplacement mechanism at shallow crustal levels

is stoping a volumetrically significant pluton emplacement process magmatic stoping is widely called upon to explain discordant pluton contacts but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons first xenoliths rarely make up more than a few percent of most plutons this sparseness even on well exposed pluton floors argues that few xenoliths make it into plutons second piles of rock fragments have sufficiently high porosities 50 that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much if not all of the available magma producing pluton scale magmatic breccias that are not observed third although settling calculations show that large meter scale or larger xenoliths could sink rapidly in magma natural fragmentation processes are likely to produce a fractal distribution of particle sizes with small particles far more abundant than large ones the absence of small xenoliths at pluton contacts where magma cools most rapidly thus argues against stoping fourth although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks finally there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally stoping cannot be volumetrically important in incrementally emplaced plutons because blocks can only fall through the volume that is molten at any given time where xenoliths are abundant in plutons they may commonly represent isolation of wall rock bodies between successive intrusive increments rather than wall rock fragments that were engulfed by magma however even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber although stoping undoubtedly occurs during the emplacement of some plutons it is unlikely to be a volumetrically significant process

strain pattern in the aar granite central alps orthogneiss developed by bulk inhomogeneous flattening the central part of the aar granite aar valley shows lens shaped domains of low strain separated by anastomosing domains of high strain a comparable pattern is found on all scales of observation deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites ultramylonites are concentrated within conjugate ductile shear zones where moderate the strain is concentrated in very narrow small scale faults which affect relatively undeformed granite within foliated domains finite strain measurements on deformed xenoliths indicate that i strain intensity increases gradually with grain size reduction and ii finite strain ellipsoids are of flattening type irrespective of the degree of mylonitization the pattern of small scale faulting the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation

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

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

ultra high temperature and subsolidus shear zones examples from the poe mountain anorthosite wyoming abstract ultra high temperature shear zones formed with interstitial melts present during the emplacement of the poe mountain anorthosite wyoming the shear zones are characterized by coarse grained plagioclase with flat extinction and rare deformation microstructures dissected grain microstructures suggest that plagioclase underwent recrystallization by fast grain boundary migration orthopyroxene crystallized from interstitial melts as strain free foliation parallel tablet grains plagioclase has a magmatic petrofabric characterized by poles to 010 normal to the shear foliation and 100 parallel to the foliation and movement direction poles to 001 have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation 100 defines a point maximum perpendicular to the shear foliation and 010 and 001 have great circle distributions approximately parallel to the shear foliation microstructures of the ultra high temperature shear zones contrast with those of granulite grade shear zones cutting across the intrusion the granulite grade shear zones initiated as fractures which evolved into shear zones by dynamic recrystallization deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains

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

four magmatic fabrics in the tuolumne batholith central sierra nevada california usa implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust for the first time four distinct magmatic fabrics are documented in a composite plutonic body the tuolumne batholith central sierra nevada california usa one type of fabric was formed by strain caused by highly localized magma flow type 1 whereas the other three chamber wide fabrics recorded strain increments during boundary processes along batholith margins type 2 superimposed by increments of heterogeneous regional tectonic strain types 3 and 4 our present work indicates that in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static actively deforming and rheologically complex crystal mushes at lower melt percentages we demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem placement regional tectonic strain however it may be commonly very problematic to infer the exact nature of flow or even fabric forming process from the preserved rock record our study also exemplifies how examination of magmatic fabric patterns in plutons complemented with geochronology may provide crucial constraints on the interplay among successive magma emplacement fabric preservation temporal evolution of strain fields in a crystallizing magma chamber and the development of crystal mush zones

fractional crystallization in granites of the sierra nevada how important is it although compositional variation in zoned calc alkalic plutons is often ascribed to crystal fractionation diagnostic large scale field evidence of crystal accumulation in these slowly cooled bodies is generally missing in many plutons however small scale crystal cumulates have been preserved as layered schlieren and in microcosm may allow an assessment of the importance of crystal fractionation in their host pluton9s development small widely separated patches of schlieren in the tuolumne intrusive series yosemite national park california formed as cumulates their darkest layers show high concentrations of magnetite sphene biotite horn blende and zircon and have unusually fractionated major and trace element compositions feo 33 al 2 o 3 in chondrites 750 zr 2000 ppm the layers define smooth trends on major and trace element al 2 o 3 variation diagrams that diverge strongly from patterns for the main sequence rocks of the tuolumne series and granitoids throughout the sierra nevada removal of such cumulates from any main sequence magma would produce al rich evolved rocks not the al poor felsic rocks of the pluton the findings suggest that fractional crystallization did not produce the dominant chemical patterns seen in the tuolumne and similar sierra nevada granites

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

how extensive are subsolidus grain shape changes in cooling granites abstract the hypothesis that granites sensu lato commonly undergo extensive grain shape changes recrystallization during cooling is not supported by microstructural evidence especially the abundance of non truncated zoning patterns in mineral grains regional metamorphic felsic gneisses that have undergone extensive neocrystallization recrystallization at amphibolite facies conditions which some have suggested are equivalent to those in slowly cooling granites typically lose igneous microstructures including oscillatory zoning and are characterized by polygonal to irregularly xenoblastic grains of quartz and feldspar as well as by rounded inclusions of quartz in feldspar and vice versa in contrast granites typically show oscillatory zoning especially in plagioclase elongate euhedral to subhedral feldspar grains and euhedral plagioclase inclusions in quartz and k feldspar the contrast between these two situations is probably the result of felsic gneisses undergoing chemical reactions and regional deformation especially in the presence of water released pervasively from dehydration reactions which are strong driving forces for the production of new grains whereas the minerals in cooling granites are chemically compatible without strong deformation and so do not react with each other or recrystallize therefore massive granites without solid state foliations generally do not undergo grain shape changes owing to inadequate driving forces as well as to decreasing temperature an important implication of this conclusion is that internal structures formed during chamber construction and magmatic flow in granite plutons are not likely to be removed by subsolidus intergranular microstructural changes

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

on curved laminae graded layers convection currents and dynamic crystal sorting in the ploumanac h brittany subalkaline granite curved layered objects in a subalkaline potassic intrusive granite are analyzed biotite laminae and cyclic reverse size and modal layers are encountered in an annular zone both cyclic and intermittent layers are observed in rhythmic arrangement truncation the bifurcation curvature of layers with ladle shape represents the action of magmatic currents as would convection cells or fluid dynamic fingers emphasis is put on the bagnold effect related to such currents which would explain the reverse size grading of layers this kind of size sorting hydrodynamic occurs in highly viscous granitic magma simple convection seems more likely than diffusive convection

high temperature microstructures and rheology of deformed granite erzgebirge bohemian massif abstract three stages of granite deformation were recognized in porphyric granitoids sheared under amphibolite facies conditions in the erzgebirge mts 1 weakly deformed granite in lozenge shaped pods 2 s c orthogneiss and 3 thin zones of banded mylonites weakly deformed granite forms a load bearing framework lbf structure in which quartz and feldspar aggregates exhibit similar strain intensities and shapes k feldspar deforms by fracturing and onset of dynamic recrystallization along clast margins plagioclase recrystallizes completely and quartz shows effects of grain boundary migration recrystallization and activity of basal a slip s c orthogneiss is interpreted as an interconnected weak layer iwl structure with high viscosity contrast between quartz and both feldspars concentration of deformation into thin quartz layers is indicated by intense strain of quartz grain size variations of recrystallized quartz and dominant prism a slip feldspars show well annealed structures and slip on 010 001 banded mylonite is marked by similar strain intensity of quartz and feldspar aggregates and low viscosity contrast between constituent phases quartz shows evidence for grain size increase and rhomb a c slip fully recrystallized feldspars show evidence for 010 100 slip the whole microstructural sequence and the transitions from lbf structure to iwl structures show the dependence of microstructural evolution of different materials on the bulk strain intensity

microstructure and geochemistry of plagioclase and microcline in naturally deformed granite abstract naturally deformed feldspars from foliated granites in a shear zone in newfoundland exhibit transitional brittle ductile behaviour brittle failure is subordinate to dynamic recrystallization microcracking strain enhanced diffusion and reaction enhanced ductility during the deformation both plagioclase an 28 and k feldspar are transformed to albite with increasing strain interaction of metamorphic and structural processes at the grain scale is emphasised this is illustrated with examples of quartz filled veins segregation bands in plagioclase and recrystallized polycrystalline aggregates in plagioclase and k feldspar the role of microcracking in plagioclase and of pre existing internal growth structures in the formation of initially coarse grained recrystallized aggregates from large single crystals is suggested