trace element zoning in garnet as a monitor of crustal melting x ray composition maps of sc y p and cr in garnet have been measured using the electron microprobe zoning is discontinuous and is correlated with fluid absent crustal melting reactions the magnitude of the discontinuity is a function of the amount of muscovite that melts and the partition coefficients of solids versus melt trace element zoning is apparently not modified by diffusive processes so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis

onset timing of significant unroofing around qaidam basin northern tibet china constraints from 40ar 39ar and ft thermochronology on granitoids 40ar 39ar and fission track thermochronological results from two granitoid suites in the mountain ranges along the northern and southern edges of qaidam basin constrain the timing of a tertiary denudation event and their tectonic association in northern tibet the cooling histories based upon 40ar 39ar and fission track ages suggest a rapid cooling event 7 5 10 7 c ma during oligocene early miocene time this cooling event is thought to reflect an increasing denudation rate associated with the rapid uplift and denudation of the north qaidam and kunlun shan mountain ranges with consideration to the field evidence sedimentation rate of the qaidam basin and the propagation rate along the altyn tagh fault the rapid uplifting and denudation are probably due to crustal thickening associated with cenozoic propagation of the altyn tagh fault in accommodating the collision of india with asia

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

prolonged acadian orogenesis revelations from foliation intersection axis fia controlled monazite dating of foliations in porphyroblasts and matrix numerous phases of garnet growth are revealed by detailed studies of foliation inflection intersection axes preserved in porphyroblasts fias in acadian metamorphic rocks in the chester dome region of vermont u s a a regionally consistent succession of four different fia tends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe these monazite inclusions which lie within the various foliations that define the fias provide absolute ages for multiple periods of deformation and episodic phases of garnet growth the monazite inclusions reveal a progression in foliation ages from 431 2 to 349 3 million years within porphyroblasts and from 366 3 to 327 5 million years in pervasive matrix foliations three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts through the medians to the rims and then into the matrix ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the fias the four fia sets began forming prior to 424 3 405 6 386 6 and 366 4 million years ago respectively thus multiple stages of garnet growth occurred throughout acadian deformation and metamorphism in vermont and orogenesis was far more prolonged than previously thought invariably garnet growth occurred early in the accompanying deformation event dating of monazite inclusions without careful separation of phases of garnet growth by fia studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history analysis of monazite grains in the matrix alone will likely only present ages for the youngest events these ages can be amalgamated from grain to grain to yield apparently precise ages but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis or the overall continuity of the deformation and metamorphism that accompanies plate motion fia and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed this in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories

deformation volume and cleavage development in metasedimentary rocks from the ballarat slate belt psammites from the ballarat slate belt in se australia exhibit well developed differentiated or spaced cleavage defined by alternating phyllosilicate and quartz rich domains termed p and q domains respectively strain estimates derived from independent microstructural and chemical observations suggest that the p q fabrics developed in response to plane strain deformation dominated by solution transfer with the principal finite shortening in the p domains approximately twice that in the adjacent q domains significant finite extensions are indicated by ubiquitous quartz albite chlorite overgrowths in both p and q domains while pressure shadow development around syntectonic pyrite porphyroblasts suggest finite extension of at least 100 this estimate is comparable with the extension predicted for constant volume deformation and consequently there appears to have been no significant bulk material loss or gain on the hand specimen scale these observations are consistent with solute transfer scales as little as a few centimetres via diffusion in a stationary fluid and do not require although do not necessarily preclude large scale advective fluid transport through the slate belt during cleavage formation as suggested in previous studies

sigmoidal inclusion trails punctuated fabric development and interactions between metamorphism and deformation porphyroblast inclusion fabrics are consistent in style and geometry across three proterozoic metamorphic field gradients comprising two pluton related gradients in central arizona and one regional gradient in northern new mexico garnet crystals contain curved sigmoidal inclusion trails in low grade chlorite schists these trails can be correlated directly with matrix crenulations of an older schistosity s1 the garnet crystals preferentially grew in crenulation hinges but some late crenulations nucleated on existing garnet porphyroblasts at higher grade biotite staurolite and andalusite porphyroblasts occur in a homogeneous s2 foliation primarily defined by matrix biotite and ilmenite biotite porphyroblasts have straight to sigmoidal inclusion trails that also represent the weakly folded s1 schistosity staurolite and andalusite contain distinctive inclusion rich and inclusion poor domains that represent a relict s2 differentiated crenulation cleavage together the inclusion relationships document the progressive development of the s2 fabric through six stages garnet and biotite porphyroblasts contain stage 2 or 3 crenulations staurolite and andalusite generally contain stage 4 crenulations and the matrix typically contains a homogeneous stage 6 cleavage the similarity of inclusion relationships across spatially and temporally distinct metamorphic field gradients of widely differing scales suggests a fundamental link between metamorphism and deformation three end member relationships may be involved 1 tectonic linkages where similar p t time histories and similar bulk compositions combine to produce similar metamorphic and structural signatures 2 deformation controlled linkages where certain microstructures particularly crenulation hinges are favourable environments for the nucleation and or growth of porphyroblasts and 3 reaction controlled linkages where metamorphic reactions particularly dehydration reactions are associated with an increase in the rate of fabric development a general model is proposed in which 1 garnet and biotite porphyroblasts preferentially grow in stage 2 or 3 crenulation hinges and 2 chlorite consuming metamorphic reactions lead to pulses in the rate of fabric evolution the data suggest that fabric development and porphyroblast growth may have been quite rapid of the order of several hundreds of thousands of years in these rocks these microstructures and processes may be characteristic of low pressure first cycle metamorphic belts

p t d histories from quartz a case study of the application of the titaniq thermobarometer to progressive fabric development in metapelites 1 this study investigates the ability of quartz to record segments of the pressure temperature deformation p t d path in poly deformed metamorphic terranes and the associated application of the ti in quartz thermobarometer titaniq metapelites from the strafford dome eastern vermont were selected for this study because they record progressive fabric development during prograde metamorphism and have well constrained p t d histories from previous work results of this investigation are in agreement with previous studies and demonstrate that quartz in these samples records additional distinct intervals of the p t d path six preserved quartz equilibration events include 1 early prograde growth during burial 2 kyanite in quartz producing reactions 3 prenappe emplacement equilibration during isobaric heating 4 precipitation of quartz in quartz feldspar domains during crenulation cleavage development due to solution transfer 5 quartz producing chlorite out reactions during heating postnappe emplacement and 6 retrograde quartz overgrowths from an influx of siliceous fluids modeling quartz volume allows for the identification of quartz producing reactions in p t space the same may be done for ti phases to better constrain activities during reequilibration events an integration of cathodoluminescence imaging microstructural and petrographic investigation isochemical forward stability modeling and thermobarometry allows identification and interpretation of zoning patterns in quartz to unravel histories that would otherwise not be obtainable

structure and tectonics of the hudson valley fold thrust belt eastern new york state analysis of structural features displayed in silurian and devonian strata of the hudson valley region indicate that these rocks form the western edge of a now largely eroded west verging fold thrust belt the hudson valley fold thrust belt or hvb west of the hudson river remnants of this fold thrust belt define a miniature valley and ridge province faults of the hvb die out to the west as blind thrusts in the basal units of the catskill clastic wedge structural geometries displayed in the hvb vary rapidly along strike and are locally complicated by internal deformation of thrust sheets and by movement on out of the syncline faults it is proposed that exposed faults of the hvb west of the river arise from a basal detachment here called the rondout detachment that lies above the taconic unconformity the rondout detachment and the overlying thrust system have been bent into large amplitude folds ordovician units beneath the rondout detachment must have shortened during development of these folds area balancing of cross sectional models suggests that the shortening affecting ordovician units could be restricted to rock above a lower blind detachment the austin glen detachment positioned within ordovician strata at a depth of 400 m below ground surface the age of deformation and the tectonic setting of the hvb is enigmatic its spatial relation to the catskill clastic wedge and to acadian metamorphic terranes suggests that the belt represents foreland shortening developed during the acadian orogeny

evidence for mesozoic shear along the western kunlun and altyn tagh fault northern tibet china the strike slip faults of north tibet accommodate part of the cenozoic convergence between india and asia along the karakax valley south of yecheng and near the xidatan trough south of golmud the active traces of the altyn tagh and kunlun faults follow narrow belts of metamorphic rocks the deformation recorded in those mylonites is sinistral strike slip rb sr and 40ar 39ar ages of deformation from syntectonic fabrics formed at 350 400 c 120 ma argon loss suggests that deformation was associated to a 250 300 c thermal pulse that lasted 5 20 ma after the onset of movement unroofing occurred much later around 25 ma when sudden cooling suggests a component of thrusting or more likely normal faulting the cretaceous shear may be related to collision between the qiantang and the lhasa blocks the karakax and xidatan shear zones may have formed a unique continuous boundary in the cretaceous which was later reused by the tertiary strike slip faults leading to potentially calculable offsets along the altyn tagh fault

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

flame foliation evidence for a schistosity formed normal to the extension direction foliations are normally thought to develop approximately parallel to the xy plane of the finite strain ellipsoid i e perpendicular to the main shortening direction we present a new type of schistosity named flame foliation that forms orthogonal to the main extension direction approximately parallel to the yz plane of finite strain flame foliation consists of anastomosing biotite rich selvedges overprinting s1 in pelitic layers of metaturbitites in nw namibia the biotite crystals in the selvedges are peculiar because they are oriented oblique or orthogonal to the flame foliation itself and parallel to the previous s1 cleavage a feature no other foliation shows in some cases biotite flames flank minor quartz veinlets implying that the flame foliation developed by infiltration of fluids and biotite growth along extensional fractures perpendicular to the main d1 extension direction where overprinting ductile d2 deformation is strong the flame foliation is transformed into a normal crenulation cleavage by rotation of biotite in the flames flame foliation is found preferentially close to syntectonic syenite granite plutons and their formation in metapelitic rocks may be enhanced by fluid overpressure due to devolatilisation reactions 2006 elsevier ltd all rights reserved

structural evidence for the permo triassic tectonic evolution of the yidun arc eastern tibetan plateau abstract deformation across the yidun arc occurred as a two phase process i early triassic deformation of paleozoic sequences resulting from closure of the jinsha jiang suture and ii late triassic deformation of middle to late triassic rocks as a result of final basin closure deformation in the western yidun arc is dominantly west vergent a north south striking west directed thrust the jinsha thrust forms the boundary between the western yidun arc and the eastern qiangtang block the jinsha thrust incorporates within it ophiolitic material of the jinsha jiang suture and the structural geometry across the jinsha thrust suggests that early triassic subduction was east dipping along the jinsha jiang suture deformation within middle to late triassic sequences of the eastern yidun arc eastern qiangtang block and western songpan garze fold belt is thin skinned being restricted largely to a single generation of upright folds and associated thrusts late triassic westwards subduction underneath the combined western yidun arc qiangtang block resulted in arc volcanism in the eastern yidun arc coeval arc volcanics in the northern margin of the qiangtang block suggest that the suture along the northern margin of the qiangtang block may be correlated with the garze litang suture rather than the jinsha jiang suture

potential minerals for determining u th pb chemical age using electron microprobe abstract after a decade of studies and development it is now accepted that reliable u th total pb isochron ages can be calculated for monazite using an electron microprobe at m scale either directly on thin sections or on separated grains mounted in polished section the potential for determining u th pb chemical ages from other u and th enriched phases has been investigated compared to chemical monazite dating results for which individual spot age precisions of 20 to 100 ma can be achieved from individual spot analyses using isochron plots for monazite the age homogeneity of a given population of data can be assessed and depending upon the number of analyses n 50 a precision of 5 to 10 ma can be obtained the u content in xenotime widely varies from less than 0 1 wt up to 3 wt but th rarely exceeds 1 wt as a consequence the amount of radiogenic pb produced during a given period remains significantly lower for xenotime than for monazite leading to a lower precision 20 ma on the mean ages xenotime however appears to remain as a closed system but common pb must be carefully checked furthermore the electron microprobe technique epma allows controlling any age discrepancy on xenotime grains as small as 10 20 m that cannot be dated by other isotopic methods such xenotime ages can be useful when studying the monazite xenotime equilibrium the electron microprobe is not the most reliable method for dating zircon since u and th concentrations are generally low and common pb is not negligible nevertheless the spatial resolution of epma coupled with isotope methods allows conclusive in situ studies about radiogenic pb mobility and metamictization thorite does not seem suitable for dating with either isotope methods or epma because of continuous radiogenic pb loss conversely the oxide phases thorianite and baddeleyite are robust minerals with closed systems they are rather rare and seem to incorporate negligible common pb making epma a method of choice for dating them for thorianite the precision on the mean age can be similar as that obtained for monazite or even better while the precision for baddeleyite cannot be significantly better than 20 to 50 ma due to the limited amount of u 0 1 and the lack of th

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

exhumation history of the gangdese batholith southern tibetan plateau evidence from apatite and zircon u th he thermochronology abstractto test previously suggested exhumation histories of the gangdese batholith in the central part of the transhimalayan plutonic belt we conducted paired apatite and zircon u th he thermochronological investigations of the yarlung zangbo gorge in the central part of the batholith age elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation 46 48 22 18 and 11 8 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history the cretaceous early paleogene exhumation of the gangdese batholith was probably caused by both the northward subduction of the neo tethys and the collision between the lhasa and qiangtang blocks the early miocene rapid exhumation might be a response to shortening caused by the gangdese thrust or erosion driven by dynamic uplift following lithospheric delamination in contrast the late miocene exhumation is coi

large volume loss during cleavage formation hamburg sequence pennsylvania green reduction spots in red slate of the hamburg sequence exposed near shartlesville pennsylvania have axial ratios of 1 42 1 0 0 28 on the limbs of near isoclinal folds and 1 0 0 79 0 41 in fold hinge zones conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44 preceded the tectonic strain associated with cleavage formation this strain having identical maximum extensions but greater shortening in fold hinges as compared to limbs was characterized by 41 extension in x no change in y 50 to 59 shortening in z and 29 to 42 tectonic volume loss the general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates where deformation was an almost constant volume process and extension in x compensated for shortening in z

local volume change vs overall volume constancy during crenulation cleavage development in low grade rocks abstract the role of volume change in crenulation cleavage development involving possible pressure solution of quartz and mica is investigated taking into account two end member crenulation cleavage evolutionary models involving 1 mica loss from microlithons and 2 immobile mica mathematical relationships derived for both models are used to generate data sets for comparison with measured mineralogical proportions for two samples of crenulation cleavage from the singhbhum district eastern india the development of crenulation cleavage is considered to be under overall volume constancy if the average volume loss from cleavage domains is counterbalanced by average volume gain in adjoining microlithons a test based on 2 statistics obtained from comparison of expected values from model calculations and observed proportions of mineral phases in microlithon and cleavage domains is proposed to accept or reject the null hypothesis of overall volume constancy during crenulation cleavage development an application of the test to the two examples of crenulation cleavage from india suggests that mica was relatively immobile local volume loss in cleavage domains was about 6 19 and there was an overall volume constancy on a decimetre scale during crenulation cleavage development

quaternary and active faulting observed on the 1985 academia sinica royal society geotraverse of tibet active and recent faulting along the main north south road in tibet is dominated by normal faulting occurring on northerly trending planes and by strike slip faulting both of which reflect an east west extension of the plateau normal faulting is prevalent in the southern half of the plateau but we saw no evidence for any major graben in the northern half strike slip faulting on roughly easterly trending structures is m ore prevalent in the northern half but conjugate faulting with right lateral slip on northwesterly trending planes and left lateral slip on northeasterly trending planes is common in the southern half in two areas we also observed components of thrust faulting apparently in association with young strikeslip faulting our most important results are bounds on the rates of slip on the two main strands of the kunlun strike slip fault system which trends east w est through the kunlun range ground moraine containing boulders of pyroxenite is separated by 30 km from the nearest outcrop of such rock implying that amount of displacement in the last 1 5 to 3 m a therefore the average rate of slip during the quaternary period has been between 10 and 20 mm a with a likely value of 13 mm a abundant fresh tension cracks and mole tracks imply continued slip on the main strand the xidatan tuosuohu maqu fault and the likely occurrence of a major earthquake in the last few hundred years consistent offsets of gullies and dry stream channels of about 10 m may reflect slip of that amount during such an earthquake and possible multiple offsets at one site suggest that slip may occur by large displacements of 10 m during infrequent great earthquakes along the other strand the kunlun pass fault offsets of roughly 50 to 150 m of apparently post glacial valleys and of one glacier and its terminal moraine suggest a holocene rate of slip between 5 and 20 mm a and most likely about 10 mm a on this fault these rapid rates of displacement imply that tibet is being extruded rapidly eastward at a rate com parable to the rate at which india is penetrating into eurasia and therefore that at present a substantial fraction of this penetration is being absorbed by the eastward extrusion of tibet

coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex the 370 377 ma mooselookmeguntic igneous complex was emplaced at 14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca 400 405 ma during the acadian orog eny gravity and drill hole data isograd geometry thermal modeling and struc tural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east dipping tabular shaped intrusion this roof preserves a classic example of low pressure high temperature metamorphism caused by the underlying intrusion ther mal modeling indicates that the roof is no more than 1000 m thick over an area exceed ing 100 km2 unlike other roofs described in the literature the mooselookmeguntic igneous complex roof preserves a thick 600 m emplacement related strain gradi ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense nondifferentiated folia tion approximately parallel to the intrusive contact and perpendicular to the steeply dip ping regional acadian age foliation fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution precipitation creep which is a linear viscous deformation mechanism that can occur at lower differ ential stress than for example dislocation creep as the crenulation cleavage evolves a stage is reached where further deformation requires a change in deformation mecha nism which may lead to hardening of the rock development of crenulation cleavage requires a mica rich foliation at an initially high angle to the emplacement related fl at tening plane and this confi guration may be the primary reason why the strain aureole in the mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs

numerical simulations of spiral shaped inclusion trails can 3d geometry distinguish between end member models of spiral formation numerical 3d simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that a 3d spiral geometry differs between the rotation and nonrotation end member models of spiral formation proposed in the literature and b 3d spiral geometry can be used as a criterion to distinguish between the two end member models in rocks four principal differences are identified between the two sets of simulations smoothness of spiral curvature spacing of foliation planes alignment of individual foliation planes either side of the sphere representing the porphyroblast and spiral asymmetry with respect to matrix shear sense of these differences only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end member models in the absence of a readily applied test to distinguish the end member models interpretation of spiral inclusion trails is problematic it is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation

finite strain heterogeneity and volume loss in slates of the taconic allochthon vermont u s a abstract slates of the taconic allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites the strain is highly heterogeneous at all scales ranging from several centimeters to many kilometers variations in x y ratios is small but variations in y z ratios is large and this pattern is seen at all scales the heterogeneity can be used to determine a strain path which is nearly horizontal on a flinn diagram we propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss and that the most highly strained sites have experienced an average 55 volume loss at a minimum explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems consideration of the strain history of these rocks suggests that folding related strains have been modified by later cleavage related events the last of which was uniaxial flattening accommodated by volume loss the finite strains are reflective almost exclusively of the latest cleavage producing strains

volume loss during cleavage formation in the meguma group nova scotia canada abstract diffusive mass transfer pressure solution is a widely recognized cleavage forming mechanism under low grade conditions the amount of accompanying volume change during cleavage development however is difficult to quantify and remains controversial the distance between sites of mineral dissolution and reprecipitation and transport mechanisms also are controversial the structural history of the cambrian ordovician meguma group nova scotia canada is well known and abundant strain markers provide an opportunity to assess geometrically volume changes accompanying cleavage development during diagenesis about 70 compaction in muds and 15 in silts and sands occurred the pelitic to psammitic meguma group was cleaved prior to being folded pressure solution was the dominant cleavage forming deformation mechanism under chlorite grade metamorphic conditions during the early devonian acadian orogeny measurement of tectonic strain in principal directions x y z shows that bedding parallel coaxial shortening z is 60 and there is no evidence of fold hinge parallel extension y estimation of tectonic vertical extension x in the cleavage plane is complicated by diagenetic compaction corrections to primary strain markers but is insufficient to compensate for the observed 60 shortening in z indicating 40 60 volume loss during cleavage formation because cleavage formed at least 50 ma after deposition and under approximately 6 km of silurian to early devonian strata porosity of the sediments upon onset of cleavage was likely less than 15 pore space reduction therefore cannot account for the apparent volume loss during cleavage formation cleavage related volume loss applies regionally and to a stratigraphic thickness of several thousand meters implying that a large amount of material was removed and transported large distances when combined with 50 shortening by folding a bulk tectonic shortening of 80 is estimated for the meguma group during the acadian orogeny

on volume change and mass transport during the development of crenulation cleavage abstract a comparison of the bulk chemistry between isolated zones of crenulation cleavage and the surrounding relatively homogeneous slates from three localities boscastle sw england moselle valley germany lac de roselend french alps demonstrates that marked bulk volume change is not a prerequisite for the development of crenulation cleavage x ray powder diffraction results also demonstrate that there is no significant mineralogical change between crenulated and uncrenulated zones for the samples studied mineralogical differentiation and mass transfer takes place on a local scale between the limb and hinge domains of individual crenulation folds and of larger scale folds in layering but can occur without significant overall volume change on the scale of crenulated zones containing many microlithon and cleavage domains relative volume changes between crenulated and uncrenulated regions do not exceed ca 5 at boscastle and the majority of samples from the moselle valley at one locality in the moselle valley however a volume loss of around 35 40 occurred from the crenulated zone produced almost entirely by loss of sio2 as quartz from the system isolated parallel sided bands of crenulation cleavage such as the examples from boscastle and the moselle valley are geometrically constrained to represent some combination of volume change and heterogeneous simple shear superimposed upon a background homogeneous strain in examples with minimal volume changes the heterogeneous strain component must be one of simple shear

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

alvin seabeam studies of the sumisu rift izu bonin arc abstract bimodal volcanism normal faulting rapid sedimentation and hydrothermal circulation characterize the rifting of the izu bonin arc at 31 n analysis of the zigzag pattern in plan view of the normal faults that bound sumisu rift indicates that the extension direction 080 10 is orthogonal to the regional trend of the volcanic front normal faults divide the rift into an inner rift on the arc side which is the locus for maximum subsidence and sedimentation and an outer rift further west transfer zones that link opposing master faults and or rift flank uplifts further subdivide the rift into three segments along strike volcanism is concentrated along the ene trending transfer zone which separates the northern and central rift segments the differential motion across the zone is accommodated by interdigitating north trending normal faults rather than by ene trending oblique slip faults volcanism in the outer rift has built 50 700 m high edifices without summit craters whereas in the inner rift it has formed two multi vent en echelon ridges the largest is 600 m high and 16 km long the volcanism is dominantly basaltic with compositions reflecting mantle sources little influenced by arc components an elongate rhyolite dome and low temperature hydrothermal deposits occur at the en echelon step in the larger ridge which is located at the intersection of the transfer zone with the inner rift the chimneys veins and crusts are composed of silica barite and iron oxide and are of similar composition to the ferruginous chert that mantles the kuroko deposits a 1 2 km transect of seven alvin heat flow measurements at 30 48 5 n showed that the inner rift bounding faults may serve as water recharge zones but that they are not necessarily areas of focussed hydrothermal outflow which instead occurs through the thick basin sediments the rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as sumisu and south sumisu volcanoes

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite grade phyllites se vermont detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south eastern vermont show that grain scale zoning is common and sympathetic zoning in adjacent minerals is nearly universal we interpret this to reflect a pressure solution mechanism for cleavage development where precipitation from a very small fluid reservoir fractionated that fluid multiple analyses along single muscovite biotite and chlorite grains 30 200 m in length show zoning patterns indicating tschermakitic substitutions in muscovite and both tschermakitic and di trioctahedral substitutions in biotite and chlorite using cross cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite grade chlorite grade overprinted by biotite grade and biotite grade zoning patterns are comparable in all three settings requiring a similar cleavage forming mechanism independent of metamorphic grade moreover the use of 40ar 39ar geochronology demonstrates this is true regardless of age furthermore samples with chlorite grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of al si mg and fe ions are greater than the temperature of the biotite isograd 400 c parallel and smoothly fanning tie lines produced by coexisting muscovite chlorite and muscovite biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization these results do not support theories suggesting cleavages form in fluid dominated systems if crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir then the compositions of individual grains defining these cleavages would be uniform on the contrary the fine scale chemical zoning observed probably reflects a grain scale process consistent with a pressure solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation thus the role of fluids was probably limited to one of catalysing pressure solution and fluids apparently did not drive cleavage development

age geochemistry and tectonic setting of buqingshan ophiolites north qinghai tibet plateau china abstract the buqingshan ophiolite complex is a sector of the a nyemaqen ophiolite belt in the east kunlun southern marginal suture zone in the northern part of the qinghai tibet plateau the wnw trending ophiolite complex consists of metaperidotite gabbro diabase pillow basalt massive basalt and pelagic sedimentary rocks including radiolarian chert we have determined that early paleozoic and early carboniferous early permian ophiolites are present in this ophiolite complex that were previously thought of permian middle triassic age a zircon u pb age of 467 2 0 9 ma for gabbro and a rb sr isochron age of 481 130 ma for diabase and gabbro have been obtained from the early paleozoic ophiolite slice and middle late ordovician acritarchs have been found in the melange matrix that envelopes the sliver we interpret the first two ages to represent the formation age of the ophiolite granodiorite tonalite plutons with a zircon u pb age of 402 24 ma intrude the slice early carboniferous early permian radiolarians have been discovered in cherts and argillaceous cherts in the second ophiolite slice geochemistry indicates that metaperidotites in the first slice represent a depleted ocean lithosphere mantle most mafic rocks in the early paleozoic ophiolite are of n morb type with small amount of t morb and they resulted from the magma from the depleted oceanic lithosphere mantle with little crustal contamination the basalts in the early carboniferous early permian ophiolite have similar characteristics to the mafic rocks and also show dupal anomaly and a mixed source of dmm and em ii the original formation environments of the two stage ophiolites are both mid ocean ridges these lines of evidence suggest that a mature early paleozoic kunlun qilian qinling ocean basin and a mature paleotethyan ocean basin once existed in the study area

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

limited localized nonvolatile element flux and volume change in appalachian slates geometric evidence for major nonvolatile volume losses at several appalachian slate localities contradicts geochemical evidence for minimal compositional changes during low grade metamorphism of shales relative to adjoining microlithons spaced cleavage zones are depleted in si quartz na albite and for carbonate rich slates ca mn and mg calcite and dolomite zr ti al p and fe are enriched in cleavage zones the greatest enrichments are in zr proportional k gains and na and si losses suggest that cleavage fractionation combines stoichiometric reactions e g k albite chlorite muscovite sio 2 na with pressure solution of quartz and carbonate minerals cleavage and sorting pelite to psammite fractionation patterns are similar but can be distinguished by higher zr in al rich cleavage zones relative to zr in al rich pelitic rocks the compositional similarities of homogeneous shales and slates do not indicate large volume losses during slate metamorphism for paleozoic slates from localities where large volume losses have been hypothesized on geometric grounds martinsburg slate belt of pennsylvania and new jersey meguma group of nova scotia and taconic slate belt of vermont and new york slate compositions show no evidence of large nonvolatile volume losses relative to likely shale protoliths detailed sampling of slates folded at hand specimen to quarry scales indicate that substantial element and volume fluxes are limited to the submeter scale in small scale folds volume losses in tight inner arcs appear to be balanced by volume gains in adjoining areas the complications of metamorphic recrystallization and heterogeneous strain are probably responsible for erroneous interpretations of large nonvolatile volume losses in these appalachian slate localities

zoning and recrystallization of phengitic micas implications for metamorphic equilibration white micas phengites in the metasediments of the scottish dalradian display a large range of compositions within single samples the variations in the composition of these phengites are strongly controlled by their structural age with early fabrics containing a paragonite poor celadonite rich phengite whereas in later fabrics the micas are generally paragonite rich and celadonite poor retrograde phengite growth identified using back scattered electron imaging occurs as celadonite rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica plagioclase grain boundaries the presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient it is proposed that diffusion controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible this closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation recrystallization controlled equilibria

gravity inversion ams and geochronological investigations of syntectonic granitic plutons in the southern part of the variscan french massif central magnetic fabric analyses gravity inversion associated with chemical u th pb dating and structural observations are carried out to elucidate relationships between faulting and magmatic processes this multidisciplinary study has been undertaken on late carboniferous plutons situated in the southern part of the variscan french massif central fmc the glenat omps and boisset plutons crop out on both sides of the crustal scale sillon houiller fault shf the anisotropy of magnetic susceptibility ams measurements and structural observations show that i the plutons acquired their final structure during the magma crystallization and record a nw se maximum stretching trend ii in the boisset pluton post magmatic fabrics predominate with a nnw sse trending lineation the structural pattern deduced from the ams study is thus consistent with the nw se late orogenic extensional tectonic regime that has been documented in other parts of the fmc during late carboniferous the 3d geological modelling refined by 3d gravity inversion does not show any evidence of rooting of the granites along the shf therefore despite the apparent cartographic relationship between the shf and the three plutons our study does not support a genetic link between fault and plutons it also questions the existence of the shf in this part of the massif central at the time of pluton emplacement and emphasises the dominant role of the regional tectonic framework rather than local faulting as a factor controlling pluton emplacement

electron microprobe age mapping of monazite high resolution x ray maps of th u pb and y in monazite can be used to construct age maps which reveal the continuous spatial distribution of ages in a single grain of monazite the age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique the insights it can provide into monazite geochronology in general and its limitations compared to electron microprobe emp quantitative dating and other in situ geochronologic techniques age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques and they are a valuable aid for rigorously locating in situ analytical points age mapping also can be used to better understand the behavior of the u th pb system in monazite during metamorphism deformation and s uid circulation events age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale like a now healed fracture not visible in back scattered electron bse images or young domains less than 5 m in width located inside an older core in both cases using age maps as a template for locating in situ analysis points will minimize the peril of age mixing and erroneous geological interpretations in addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization and thus even more powerful applications of the monazite geochronometer the agemap program is available in a windows version and can be downloaded from the internet at the following address http www geo umass edu probe agemap

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

tectonics of the northern himalaya since the india asia collision abstract the india asia collision resulted in the construction of the vast himalayan orogen the northern himalayan domain began to experience extensional tectonics since eocene oligocene transition eot when the himalayan orogeny was still in progress major structures generated by the extension in the northern himalaya include the south tibet detachment system stds the north himalayan gneiss domes nhgd and the north south trending rifts nstr the earliest emplacement of syn deformational leucogranite at 36 32 ma along the stds marks the initial transformation from thickening to thinning in the northern himalayan domain at eot the thickening before eot caused crustal partial melting which formed the weak root of the thickened orogen or the so called channel flow this channel flow triggered the collapse of the orogen extrusion of the greater himalayan crystalline complex ghc and the onset of extensional tectonics of the stds the similarities in tectono thermal history and geochemistry of rocks between the stds and nhgd suggest that the formation of the nhgd has a direct relationship to the activity of the stds the extension of stds and the resultant thinning caused further partial melting of the crust leading to the larger scale emplacement of leucogranite during miocene 27 13 ma diapirism of these plutons shaped the domes in nhgd exposing the ghc and the shear zones of the stds in the northern himalaya in gyirong another tectonic transform from extension to shortening occurred after 18 7 ma in dinggye region the stds was offset by the nstr culminating the deformation at 13 ma when the nstr began to be active this indicates another tectonic transform in the northern himalaya at 13 ma when the n s extension of the stds ceased and gave way to e w extension of the nstr marking the end of the peak granitic magmatism in northern himalaya multiple episodes of deformation in the major structures such as the stds mct and nstr and superposition of different structures indicate a multiphase orogenic process in the himalayas in which mountain building and collapse occurred alternately with the formation of nstr and conjugate shear zones in tibet this tectonic scenario was possibly controlled by the india asia convergent rate we propose that the extension was an integral part of the orogenic process rather than a simple marker of the culmination of the orogeny

evidence for tectonic emplacement of ultramafic and associated rocks in the pre silurian eugeoclinal belt of western new england vestiges of an ancient accretionary wedge in northern vermont detailed 1 10 000 mapping of the hazens notch ottauquechee stowe and moretown formations in the 60 km 2 jay area has shown that metasedimentary rocks and serpentinites are highly faulted to produce a tectonic stratigraphy in which serpentinites and talc carbonate rocks occur as slivers along faults that separate contrasting lithic assemblages remnants of the ophiolite sequences of quebec in western massachusetts the ultramafic bearing section of the rowe schist and the western part of the moretown formation are largely equivalent to the rocks in northern vermont mapping at 1 24 000 scale and compilation work for the new bedrock map of massachusetts have shown that the rowe schist consists of numerous lenses of greenschist carbonaceous schist gray schist amphibolite and serpentinite although the geometric arrangement of the lenses and serpentinites is strikingly similar to that seen in the jay area higher grade taconian and acadian metamorphism has partially obscured the fault fabrics seen in vermont modified journal abstract

high resolution compositional mapping of matrix phases implications for mass transfer during crenulation cleavage development in the moretown formation western massachusetts abstract high resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation the moretown formation of western massachusetts contains a well developed crenulation cleavage with alternating mica rich crenulation limbs and mica poor crenulation hinges compositional mapping shows two generations of plagioclase the second of which was synchronous with the crenulation cleavage a significant amount of the syn crenulation plagioclase 10 20 modally grew in hinge domains a small amount of syn crenulation plagioclase 1 and a large amount of phengitic muscovite grew in limb domains the maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages and thus cannot be used as undeformed reference domains for comparison with crenulated regions compositional mapping facilitates a new degree of integration between petrologic and structural analysis knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation in the moretown formation plagioclase and phengite producing reactions play a large role in controlling the nature and magnitude of mass transfer but microstructures control the location of reactants and products within the evolving fabric

tectonic synthesis of the taconian orogeny in western new england a tectonic synthesis based on stratigraphic and structural analysis of western new england is proposed for the ordovician taconian orogeny it emphasizes arc continental collision in which ocean floor continental margin and ensialic rift rocks were imbricated westward in a repeatedly deformed accretionary wedge continued compression displaced segments of the north american sialic crust to the west and deformed the earlier emplaced slices of the taconic allochthons which were derived from the continental margin critical arguments for this synthesis are 1 the west to east stratigraphic relations among the basal rift clastic rocks of the dalton pinnacle and hoosac formations of late precambrian to early cambrian age 2 the stratigraphic and sedimentological similarities between the rocks of the lower taconic sequence and rocks in the pinney hollow and underhill slices to the east and north of the green mountain massif 3 the environmental similarities between the cambrian and lower ordovician section of the giddings brook slice and the age equivalent section in the st albans synclinorium 4 the presence of carbonate platform rocks as slivers between each of the successively higher and younger premetamorphic slices groups 1 and 2 of the taconic allochthons 5 the presence of synmetamorphic fault related structures in the youngest and highest slices group 3 of the taconic allochthons 6 the recognition of extensive thrust zones in the pre silurian eugeoclinal sequence east of the middle proterozoic basement of the housatonic berkshire green mountain and lincoln massifs 7 the location of the taconic root zone within the pre silurian eugeoclinal sequence 8 the recognition of numerous faults in the serpentinite belt 9 the similarity between the rocks of the moretown formation and modern fore arc basin sequences 10 the recognition that the volcanic arc continental complexes of the ascot weedon and bronson hill have been displaced westward over the moretown and or hawley formations along such faults as the bristol and coburn hill thrusts 11 the allochthonous and internally imbricated nature of the north american basement in the berkshire massif 12 the proposition that the housatonic green mountain and lincoln massifs as well as the middle proterozoic cored domes of southeastern vermont are also thick sialic slices of north american basement 13 the recognition of medium high to high pressure metamorphic mineral assemblages in the pre silurian eugeoclinal rocks of vermont and 14 the recent synthesis of isotopic age data by sutter and others 1985 on the basis of an analysis of the foregoing arguments and relationships a chronological sequence of seven structural sections between albany new york and the bronson hill anticlinorium in central massachusetts is used to depict the evolution of the taconian orogeny retrodeformed distances are based on structural overlap and restoration of the taconic slices to their depositional setting along the ancient north american continental margin these easterly younging diverticulated slices formed as a result of horizontal compression rather than gravity sliding this palinspastic analysis implies the following 1 approximately 1 000 km of shortening has occurred during the emplacement of the taconic allochthons and the subsequent imbrication of north american basement as thick sialic slices approximately 330 km of this shortening is attributed to multiple cleavage generations 2 repeated movement along such major surfaces as the cameron9s line whitcomb summit belvidere mountain thrust zone has buried the taconic root zone we suggest that the northern extension of this root zone is exposed to the east of the lincoln massif in vermont where the underhill pinney hollow and hazens notch formations are exposed these formations here considered thrust slices disappear along the belvidere mountain whitcomb summit thrust zone as it is traced southward into western massachusetts and western connecticut 3 taconian metamorphic rocks particularly the older medium high pressure rocks in northern vermont have been transported westward on such reactivated surfaces as the belvidere mountain thrust 4 the anticlinorial form of the middle proterozoic basement in the green mountain and lincoln massifs may have resulted from fault bend folding on deep mantle involved thrusts that developed late in the taconian orogeny

triassic granitoids in the eastern songpan ganzi fold belt sw china magmatic response to geodynamics of the deep lithosphere the songpan ganzi fold belt sgfb sw china was developed from a passive continental margin into an orogenic belt with the consumption of the paleo tethys during the evolution of the sgfb numerous late triassic granitic plutons formed and exhibited a progressive development from adakite i type granite high ba sr granite a type granite and monzonite representative late triassic plutons were studied to unravel the bewildering evolution of the eastern sgfb the menggu pluton 224 3 ma consists of granites with high alkali k2o na2o 7 85 10 4 wt and adakitic characteristics sr y 19 38 the endt values 2 77 to 5 03 initial 87sr 86sr ratios 0 7050 0 7063 and low nb ta ratios 8 10 are indicative of an origin by partial melting of amphibolitic lower crust rocks from the niuxingou pluton 215 3 ma are richer in k than na k2o na2o 1 1 1 5 and contain high sr 1006 1662 ppm and ba 1277 2009 ppm typical of shoshonite and high ba sr granite they have less enriched endt values 0 08 to 2 04 and less radiogenic 87sr 86sri ratios 0 7047 0 7048 and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle the taiyanghe pluton 205 3 ma consists of monzonites with high al2o3 20 wt but low mgo 0 94 1 39 wt the rocks are richer in na than k k2o na2o 0 7 contain high large ion lithophile element lile 681 834 ppm sr and 2142 9453 ppm ba and display strongly fractionated ree patterns la yb n 35 63 these features together with their enriched nd sr isotopic compositions endt 4 78 to 6 50 87sr 86sri 0 7074 0 7090 suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle although a continuous compressional regime during the mid and late triassic has been invoked for the sgfb the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the late triassic the 205 ma taiyanghe pluton was emplaced simultaneously with a rapid uplift of the lithosphere when surface deposits changed from deep water turbidite to tidal flat sediments it was therefore generated during decompression probably related to the rapid removal of the overthickened lithospheric mantle the triassic magmatism in the eastern sgfb is therefore important for probing geodynamic processes in the deep lithosphere

numerical modeling of crenulation cleavage development a polymineralic approach abstract the finite element method was used to investigate how the elastic interactions of quartz and muscovite minerals affect grain scale stress and strain distributions at different stages of crenulation cleavage development the polymineralic structure comprises individual grains that were each assigned their own 3d stiffness tensor and orientation gradients in mean stress and volumetric strain within quartz grains develop between the limbs and hinges of microfolds at the earliest stages of crenulation development with higher values in the microfold limbs these gradients decrease with development of the crenulation cleavage as the microfold limbs become phyllosilicate rich p domains and the hinges become quartz and feldspar rich qf domains crystallographic orientations of the quartz grains have a relatively minor effect on the mean stress and volumetric strain distributions our findings are broadly consistent with both pressure solution and strain driven dissolution models for crenulation cleavage development however because crenulation cleavage development typically involves metamorphic reactions we favor a model in which dissolution is driven by those reactions and mass transfer leading to development of the mineralogically segregated fabric is driven by pore fluid pressure gradients that follow gradients in volumetric strain local concentrations of stress and strain across mineral interfaces may identify sites of enhanced reaction

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

multi stage tectono magmatic events of the eastern kunlun range northern tibet insights from u pb geochronology and u th he thermochronology article i nfo to understand the magmatic and tectonic events in the eastern kunlun range we conducted systematic zircon u pb geochronology and paired zircon zhe and apatite ahe u th he themchronology investigations in the central and eastern segments of this range zircon u pb ages show two stage magmatic events of late silurian and latest permian early jurassic zhe and ahe ages and multi system thermochronometers reveal multi stage rapid cooling events from a combination of these data with previously published geochronological and thermochronological ages and regional geological setting we confirm several important phanerozoic tectono magmaticeventsinthisregion whereasthesiluriangranitesmightberelatedtoasmallpaleo oceansub duction and subsequent collision the latest permian early triassic granitoids were produced by northward sub duction of the songpan ganzi paleo tethyan ocean the late triassic earliest jurassic granitoids and late triassic early jurassic exhumation are interpreted as response to the collision between kunlun qaidam and qiangtang samples close to the kunlun fault show rapid late oligocene early miocene exhumation 1 3 1 6 km ma whereas those from the northern part of central segment display extremely low exhumation rates 0 02 0 05 km ma this heterogeneous denudation during 30 20 ma between northern and southern part of the central segment requires that crustal thickening was completed by lower crustal underthrusting with littleex humation or crustal thickening in pre cenozoic times the activity of the kunlun fault and associated normal faulting component were probably responsible for this stage of rapid exhumation in the southern part the com plex tectono magmatic evolution of the eastern kunlun range is controlled by pre cenozoic paleo ocean subduc tions and subsequent continent collisions and paleogene india asia collision

volume change accompanying cleavage development in graptolitic shales from gisborne victoria australia studies on shape changes in deformed oncograptus upsilon combined with strain analysis of pressure shadows around framboidal pyrite revealed that a small volume change of the order of 5 percent accompanied the cleavage development in the shales from gisborne victoria starting from the proximal end oncograptus upsilon exhibits progressive increase in the spacing between successive thecae along the stipes a new technique utilizing the distance between a specific number of thecae is used for computing the change in length along stipes of oncograptus upsilon the low volume change accompanying a more than 70 percent shortening deformation indicates that there is a less significant mass loss during cleavage development than has been inferred from some studies on volume change from thecal spacing in deformed graptolites

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

reaction softening by dissolution precipitation creep in a retrograde greenschist facies ductile shear zone new hampshire usa we describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host littleton schist near claremont new hampshire here na poor muscovite and chlorite progressively replace first staurolite then garnet and finally biotite porphyroblasts as the core of the fault zone in approached across the transect higher grade fabric forming na rich muscovite is also progressively replaced by fabric forming na poor muscovite the mineralogy of the new phyllonitic fault rock produced is dominated by na poor muscovite and chlorite together with late albite porphyroblasts the replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement in contrast element mapping shows that fabric forming na rich muscovite is selectively replaced at high strain microstructural sites indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite the replacement of strong high grade porphyroblasts by weaker na poor muscovite and chlorite constitutes reaction softening the crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated na rich muscovite defined foliation destroys not only the metastable high grade mineralogy but also its stronger geometry this process constitutes both reaction and microstructural softening the deformation mechanism here was thus one of dissolution precipitation creep activated at considerably lower stresses than might be predicted in quartzo feldspathic rocks at the same lower greenschist facies conditions this article is protected by copyright all rights reserved

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

porphyroblast rotation during crenulation cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa in the low pressure high temperature metamorphic rocks of western maine usa staurolite porphyroblasts grew at c 400 ma very late during the regional orogenesis these porphyroblasts which preserve straight inclusion trails with small thin section scale variation in pitch were subsequently involved in the strain and metamorphic aureole of the c 370 ma mooselookmeguntic pluton the aureole shows a progressive fabric intensity gradient from effectively zero emplacement related deformation at the outer edge of the aureole 2900 m map distance from the pluton margin to the development of a pervasive emplacement related foliation adjacent to the pluton the development of this pervasive foliation spanned all stages of crenulation cleavage development which are preserved at different distances from the pluton the spread of inclusion trail pitches in the staurolite porphyroblasts as measured in two dimensional 2 d thin sections increases nonlinearly from 16 to 75 with increasing strain in the aureole these data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage the data spread is qualitatively modelled for both pure and simple shear and both solutions match the data reasonably well the spread of inclusion trail orientations 40 75 in the moderately to highly strained rocks is similar to the spread reported in several previous studies we consider it likely that the sample scale spread in these previous studies is also the result of porphyroblast rotation relative to one another however the average inclusion trail orientation for a single sample may in at least some instances reflect the original orientation of the overgrown foliation

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

interpreting data dispersion and inverted dates in apatite u th he and fission track datasets an example from the us midcontinent abstract new apatite u th he ahe and apatite fission track aft data were acquired for cratonic basement samples from an 80 m span of drillcore in northeastern kansas the short depth interval over which the samples were collected indicates that they should have undergone thermal histories that would be indistinguishable using low temperature thermochronometry techniques individual ahe dates from four samples range from 99 to 464 ma three samples yield dates 300 ma higher eu 39 113 ppm and substantial data dispersion uncorrelated with eu and grain size these apatites have strong and variable eu zonation aft dates for five samples range from 242 to 291 ma the sample with the highest eu apatites and oldest ahe dates yields the youngest aft results these results are inverted with ahe dates distinctly older than the corresponding aft date we explore both the causes of data dispersion and the overall compatibility of this cratonic dataset we find that geologically reasonable thermal histories can 1 explain the distribution of the moderate eu ahe data when accounting for the influence of radiation damage grain size and eu zonation on apatite he diffusivity 2 reproduce the observed dispersion in the high eu ahe data when using a viable range of eu zonation and grain size and 3 explain the aft data for the same samples the ahe and aft data are mutually consistent and viable thermal histories successfully predict the observed pattern of older ahe than aft dates for the high eu apatites together these results suggest that appropriately accounting for the known controls on apatite he diffusivity can explain the observed dispersion and inverted ahe and aft results in some thermochronometry datasets a range of ahe dates should be especially common in cratonic data because small differences in apatite he diffusivity are amplified by the thermal histories that typify cratonic settings we use these results to develop some guidelines for interpreting dispersed ahe datasets first date eu and date grain size correlations should be evaluated and if these patterns occur they can be used to better resolve the thermal history second for samples that yield inexplicably large dispersion of ahe dates uncorrelated with eu and crystal size the appropriate strategy is either to reject these samples from the suite used for thermal history interpretation or to acquire additional data to help decipher the significance of the age distribution

from static alteration to mylonitization a nano to micrometric study of chloritization in granitoids with implications for equilibrium and percolation length scales strain accommodation in upper crustal rocks is often accompanied by fluid mediated crystallization of phyllosilicates which influence rock strength and shear zone formation the composition of these phyllosilicates is frequently used for pressure temperature time constraints of deformation events although it is often highly heterogeneous even in mylonites this study investigates the reactions producing a phyllosilicate chlorite in and below grenschist facies conditions and the variations in chlorite composition along a strain gradient in the variscan bielsa granitoid axial zone pyrenees compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly strained samples and related to mechanisms of fluid percolation in the bielsa granitoid altered at the late variscan alpine age shear zones are found with high strain gradients the undeformed granitoid exhibits local equilibria pseudomorphic replacement and high compositional heterogeneities in chlorite this is attributed to variable reaction mechanisms at nanoscale and element supply little interconnected intra and inter grain nanoporosity and isolation of fluid evolving in local reservoirs in samples with discrete and mm sized fractures channelized fluid triggered the precipitation of homogeneous alpine chlorite in fractures preserving late variscan chlorite within the matrix in low grade mylonites where brittle ductile deformation is observed micro and nanocracks and defects allows the fluid percolating into the matrix at the scale of hundreds of m this results in a more pervasive but incomplete replacement of late variscan chlorite by alpine chlorite despite the high strain in studied granitoids deformed under greenschist facies conditions local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according reaction mechanisms and element mobility controlled by i matrix fracture porosity contrasts at nanoscale and ii the location and interconnection of nanoporosity between crystallites of phyllosilicates this preservation influences our ability to reconstruct the pre and syn kinematic metamorphic history of granitic rocks in low grade units of orogens

lack of porphyroblast rotation in the otago schists new zealand implications for crenulation cleavage development folding and deformation partitioning porphyroblasts of garnet and plagioclase in the otago schists have not rotated relative to geographic coordinates during non coaxial deformation that post dates their growth inclusion trails in most of the porphyroblasts are oriented near vertical and near horizontal and the strike of near vertical inclusion trails is consistent over 3000 km2 microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures this is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding several crenulation cleavage fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding in the light of accumulating evidence that porphyroblasts do not generally rotate the applicability of these models to deformed rocks is questionable whether or not porphyroblasts rotate depends on how deformation is partitioned lack of rotation requires that progressive shearing strain rotational deformation be partitioned around rigid heterogeneities such as porphyroblasts which occupy zones of progressive shortening or no strain non rotational deformation therefore processes operating at the porphyroblast matrix boundary are important considerations five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast a a thin layer of fluid at the porphyroblast boundary 2 grain boundary sliding 3 a locked porphyroblast matrix boundary 4 dissolution at the porphyroblast matrix boundary and 5 an ellipsoidal porphyroblast shadow unit