a new indicator of movement direction during orogenesis measurement technique and application to the alps the orientation of axes of sigmoidal staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism a simple technique is presented in detail to find this axis in 3d by radially sectioning a horizontal slab cut from an oriented sample when viewed from one direction the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis this technique is independent of the model adopted for the formation of sigmoidal or spiral shaped inclusion trails and can be used to evaluate the mechanism by which they form it can also be used to evaluate theories of folding and orogenesis measurements of spiral and sigmoid axes in garnet porphyroblasts from the european alps show that they reflect the movement of the african plate relative to europe better than linear indicators preserved within the same rocks

role of horizontal thermal conduction and finite time thrust emplacement in simulation of pressure temperature time paths abstract comparison of metamorphic thermal histories produced by 1 one vs two dimensional conductive models and 2 instantaneous vs finite emplacement time thrusting indicates that the simple 1d or instantaneous models applied by many petrologists generally produce pressure temperature pt paths with topologies similiar to those generated by 2d time transitive models however maximum temperatures attained by footwall rocks are significantly lower in 1d than in 2d conductive models leading us to recommend that 2d thermal effects be incorporated into models whenever possible pt paths produced by instantaneous and finite duration thrust emplacement differ significantly during the synthrusting period but the peak metamorphic conditions t max and pressure at t max eventually attained by footwall rocks during erosional unroofing are the same within minimum analytical uncertainties 50 k 100 mpa on geothermobarometric data thus if erosion begins only after the end of the thrusting event instantaneous thrusting is a reasonable simplifying assumption that introduces relatively little inaccuracy into pt models devised to examine peak metamorphic conditions

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

x ray computed micro tomography of spiral garnets a new test of how they form abstract garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with x ray computed tomography for each garnet crystal the orientation of its internal foliation intersection inflexion axes fia was measured its volume and shape orientation the latter via calculation of best fit ellipsoids additionally image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to fia our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples this supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating the fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments our study exemplifies the strength of x ray computed micro tomography compared to previous techniques based on serial thin sections for distinguishing multiple fia sets present in single samples

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

progressive metamorphism in a crustal scale shear zone an example from the leon region north west brittany france this paper describes the progressive metamorphism and deformation of a series of metasediments le conquet schists and their higher grade equivalents which occur as tectonically emplaced screens within a sequence of foliated gneisses the gneiss de brest and gneiss de lesneven the sequence exhibits a steep south to north increase in metamorphic grade from garnet staurolite schist to sillimanite gneiss and sillimanite k feldspar migmatite the relationship of mineral growth to foliation development has been established for individual screens at least five phases of deformation d1 d5 are preserved analysis of porphyroblast inclusion trails is used to demonstrate sequential mineral growth during the successive development of orthogonal foliations s1 s4 porphyroblasts continued to grow during the subsequent development of c s mylonite fabrics and extensional crenulation cleavages which are genetically related to a series of high strain zones d5 mineral assemblages phase relations and mineral chemistry are consistent with porphyroblast growth being the result of continuous reactions microstructure porphyroblast relations are used to show that although mineral growth proceeded during continuous reactions these only operated episodically phase relations mineral chemistry and p t estimates are used to constrain p t trajectories and these are linked to the deformation histories within individual screens a comparison between the resulting pressure temperature deformation paths is used to demonstrate that the metamorphic peak occurred progressively later and at successively lower pressures with increasing metamorphic grade it is suggested that the early evolution of the belt is the result of crustal thickening by overthrusting the subsequent history is one of progressive heating and unroofing of the higher grade rocks in a dextral strike slip transtensional shear zone

crystallographic orientation chemical composition and three dimensional geometry of sigmoidal garnet evidence for rotation studies of the microstructure crystallographic orientation chemical composition and three dimensional shape of sigmoidal garnet reveal a number of features that can be used to discuss its origin and in particular to distinguish between rotational versus non rotational models crystallographic orientation mapping of sigmoidal garnet shows no evidence of subgrain formation or of being polycrystalline suggesting that neither ductile nor brittle deformation is significant chemical mapping shows that the garnet grew during a single metamorphic event arguing against ideas that sigmoidal garnet forms as the result of a series of independent growth events the chemical mapping also reveals anisotropic growth of garnet with the long axis of the ellipsoidal grain rotating in the same direction but to a lesser degree than the inclusion trails this is best explained as the result of syn growth rotation of the garnet with respect to the foliation and maximum growth direction high resolution x ray ct scanning shows that the inclusion trails have a complex three dimensional spiral geometry this type of geometry is predicted by rotational models of sigmoidal garnet formation

high resolution x ray computed microtomography a holistic approach to metamorphic fabric analyses an intrinsic limitation of studying microstructures in thin section is that their spatial three dimensional 3 d distribution shape and orientation have to be inferred by combining 2 d data from different sections this procedure always involves some degree of interpretation that in some cases can be ambiguous recent advances in high resolution x ray computed microtomography have made possible the direct imaging in 3 d of volumes of rock to centimeter scale this rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2 d methods spatial images can be generated through any part of the rock sample and used as virtual petrographic sections our application of this technique to an oriented drill core sample from the classic orijarvi metamorphic region of southern finland reveals a number of in situ 3 d aspects including 1 the spatial distribution and shape of andalusite porphyroblasts 2 the geometry of a matrix foliation anastomosing around the porphyroblasts 3 a millimeter scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization and 4 distinct inclusion trail patterns characterizing porphyroblast core versus rim zones the combined data indicate that the steeply dipping bedding subparallel foliation that characterizes the orijarvi area formed by bulk north south crustal shortening and associated vertical stretching

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

porphyroblast inclusion trails the key to orogenesis detailed microstructural analysis of inclusion trails in hundreds of garnet porphyroblasts from rocks where spiral shaped inclusion trails are common indicates that spiral shaped trails did not form by rotation of the growing porphyroblasts relative to geographic coordinates they formed instead by progressive growth by porphyroblasts over several sets of near orthogonal foliations that successively overprint one another the orientations of these near orthogonal foliations are alternately near vertical and near horizontal in all porphyroblasts examined this provides very strong evidence for lack of porphyroblast rotation the deformation path recorded by these porphyroblasts indicates that the process of orogenesis involves a multiply repeated two stage cycle of 1 crustal shortening and thickening with the development of a near vertical foliation with a steep stretching lineation followed by 2 gravitational instability and collapse of this uplifted pile with the development of a near horizontal foliation gravitational spreading near coaxial vertical shortening and consequent thrusting on the orogen margins correlation of inclusion trail overprinting relationships and asymmetry in porphyroblasts with foliation overprinting relationships observed in the field allows determination of where the rocks studied lie and have moved within an orogen this information combined with information about chemical zoning in porphyroblasts provides details about the structural metamorphic p t t paths the rocks have followed the ductile deformation environment in which a porphyroblast can rotate relative to geographic coordinates during orogenesis is spatially restricted in continental crust to vertical ductile tear transcurrent faults across which there is no component of bulk shortening or transpression

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

on the significance of foliation patterns preserved around folds by mineral overgrowth abstract simple inclusion trail geometries in porphyroblasts that vary symmetrically around folds suggest the porphyroblasts have rotated during folding however they do not provide proof that the porphyroblasts have rotated as such geometries may also form by overgrowth during folding this uncertainty results from the difficulty in establishing the relative ages of the fold and the porphyroblasts when the latter contain simple trails simple inclusion trail geometries in porphyroblasts that do not change around a fold remaining orthogonal to the axial plane from limb to limb suggest that the porphyroblasts have not rotated during folding however they can be rationalized as having rotated by arguing that synthetic rotation due to buckling has been exactly counterbalanced by antithetic rotation due to flexural flow if the folds vary in tightness from open to isoclinal and the trails still remain perpendicular to the axial plane this can still be rationalized by arguing for rotation during buckling balancing that due to flexural flow followed by coaxial deformation with no subsequent rotation tests of porphyroblast behaviour are only useful if they conclusively demonstrate that inclusion trail geometries around a fold are incompatible with rotation or alternatively non rotation of the porphyroblasts simple inclusion trail geometries in porphyroblasts that uniquely indicate a fold mechanism involving no rotation are those which remain parallel from limb to limb but lie oblique to the axial plane independent of limb angle particularly powerful indicators of folding mechanisms involving no porphyroblast rotation are folds containing two different porphyroblastic phases that preserve contrasting geometries for example rocks containing porphyroblasts preserving parallel trails from limb to limb in the earlier phase and symmetrically varying trails for the later phase such folds demonstrate that simple trails that vary symmetrically around a fold cannot be used as proof of porphyroblast rotation where the rotation axes of such simple inclusion trail geometries lie at a high angle to the axial plane and vary around the fold proof of porphyroblast rotation may be possible however such proof can be more readily provided using complex inclusion trails with an orthogonal and or truncational character that vary in orientation from limb to limb

sequential porphyroblast growth during progressive deformation and low p high t lpht metamorphism cooma complex australia the use of microstructural analysis to better understand deformation and metamorphic histories abstract detailed microstructural analysis has been conducted in the cooma complex which is one of the most thoroughly studied areas structurally and metamorphically in australia this was done to demonstrate the usefulness of microstructural analysis for better understanding deformation and metamorphic histories and the results add significant new information about deformation and metamorphism of this area microstructural relationships in the schists and gneisses indicate that four coaxially superposed foliations formed with alternating subhorizontal s 1 and s 3 and subvertical s 2 and s 4 orientations and the meso and macrostructural relationships are easily understood within this deformational framework three stages of sequential porphyroblast growth involve cordierite and andalusite in the schists and cordierite andalusite and k feldspar in the gneisses the porphyroblasts overgrew and preserved progressive stages in the structural development of these rocks with each period of growth occurring during a different deformation event d 2 d 3 and d 4 the sequential timing of these porphyroblasts is consistent with kfmash reaction curve topology and indicates a prograde p t t path convex to the temperature axis combining this with isobaric cooling inferred from previous studies gives an anti clockwise p t t path subvertical and subhorizontal foliations in porphyroblasts contrast with the variable orientations of the same foliations measured in the field however careful field work and microstructural analysis demonstrate that matrix foliation rotation rather than porphyroblast rotation relative to geographic coordinates is largely responsible for the contrasting observations even where bedding has been overturned after porphyroblast growth

structural evolution of a 1 6 ga orogeny related to the final assembly of the supercontinent nuna coupling of episodic and progressive deformation

episodic metamorphic reactions during orogenesis the control of deformation partitioning on reaction sites and reaction duration textural unconformities or truncations are common in porphyroblasts with complex inclusion trails they reflect cycles of successive foliations that develop against competent porphyroblasts during orogenesis and are preserved by successive growth increments their truncational character results from shear and dissolution along a particular foliation generating a differentiated crenulation cleavage the increment of porphyroblast growth that follows a textural unconformity may or may not mark a significant compositional change depending on the amount of movement of the rock through p t space between cleavage forming events although historically interpreted to result from a significant metamorphic hiatus most textural unconformities indicate that the reactions involved in the formation of these minerals are episodic during continuous prograde metamorphism starting and stopping as a function of the stage of crenulation of the matrix foliation and the pattern of deformation partitioning such episodic reaction behaviour can only occur for multivariant reactions or successive but different univariant reactions the reason why garnet is the most common porphyroblast to exhibit evidence for episodic reactions is probably the fact that it grows by multivariant reactions over a much wider p t range than most other common porphyroblast phases porphyroblast growth is micrometasomatic it is episodic because a significant reduction of strain occurs within domains of progressive shortening each time continuous progressive shearing domains form on their margins this stops microfracture development across the progressive shortening domains thereby preventing rapid access and interaction of fluid ions and complexes with porphyroblast boundaries shifting patterns of deformation partitioning and resulting small scale juxtaposition of different compositional layers spreads the duration and location of multivariant reactions and causes differential timing of porphyroblast growth along a particular stratigraphic horizon it may also locally preserve metastable metamorphic assemblages in regionally metamorphosing deforming pelites near simultaneous cessation of porphyroblast growth on all rims once continuous differentiated progressive shearing domains have formed nearby precludes fluid recirculation as a significant process for removal of material during cleavage development alternatively diffusion of simple molecules and dissociated ions along actively shearing and micro gaped phyllosilicates with recomplexing in fluid filled microfractures readily explains the control of deformation partitioning on reaction site and reaction duration

new insights into smectite illitization a zoned k bentonite revisited the illitization reaction in a thick k bentonite bed located in upper cretaceous marine shale in the montana disturbed belt was studied by x ray diffraction chemical analysis and thermal gravimetric analysis modeling of the experimental xrd patterns from oriented clay specimens in air dried and glycolated states shows that at each sample location in the bentonite bed a mixture of r0 illite smectite i s and r1 i s coexist each of these phases in all samples consists of the same or similar content of illite and expandable layers independent on location in the bed in particular the illite content in the r0 i s and the r1 i s from the 0 5 m fractions is equal to 30 and 62 respectively the main difference between the samples at different locations is the different weight concentrations of the coexisting i s phases the r1 i s content decreases progressively from the lower and upper contacts of the bed to its center the reverse trend was observed for the r0 i s the layer unit cell parameter b increases from samples located near the middle of the bed toward samples near the bed margins the dtg patterns of the samples contain two endothermic maxima at about 640 and 470 c corresponding to cis vacant cv illite and trans vacant tv smectite layers coexisting in the r1 i s and r0 i s analysis of the crystal chemical features of the r1 i s and r0 i s shows that in the middle of the bed both phases are characterized by the lowest octahedral mg and the highest tetrahedral al contents in the structural formula of the r1 i s the tetrahedral al content is significantly higher than the k na content independent of sample location in contrast tetrahedral al in the r0 i s located near the bed boundaries is lower compared with k na content to account for the crystal chemical features of the coexisting i s a first assumption is that the initial volcanic ash was altered into tv smectite having a homogeneous al rich composition throughout the bed second along with k the active role in illitization was controlled by mg mineralogical zonation of the k bentonite is explained by the progressive migration of k from the margins toward the bed center with the associated decrease of k cations in the pore fluids however the decrease in k concentration was accompanied by a successive increase in r0 i s content but not a progressive decrease in illite layer content in a single i s phase the main role of mg was to redistribute octahedral and tetrahedral al in the 2 1 layers of the r0 i s and r1 i s in such a way that the amount of al in the tetrahedral sheets increased at the expense of the substitution of mg for al in the octahedral sheet of the 2 1 layers in the initial smectite these results demonstrate a new insight into mineralogical sequences of intermediate members of smectite illitization instead of a statistically homogeneous and continuous reaction associated with the increase of illite layers in i s and the simultaneous increase of order of the layer stacking sequence the illitization reaction in the thick k bentonite consists of the formation of a physical mixture of two i s having a contrasting content of layer types and their distribution factors responsible for the formation of the coexisting r0 i s and r1 i s are discussed

unravelling the spirals a serial thin section study and three dimensional computer aided reconstruction of spiral shaped inclusion trails in garnet porphyroblasts seventy seven spatially orientated serial thin sections cut from a single rock reveal changes in the geometry of spiral shaped inclusion trails sslts in garnet porphyroblasts the observed sits are doubly curved non cylindrical surfaces with total inclusion trail curvature decreasing systematically from the cores to the rims of porphyroblasts the three dimensional geometry of the ssits reconstructed with the aid of computer graphics shows that the orientations of spiral axes defined by the ssits are not related in any expected nor predictable way to the main foliation in the matrix this suggests continued deformation after or during the latest stages of porphyroblast growth which has important implications for the use of ssits as shear sense indicators whether the formation of ssits involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data key wocds computer graphics garnet porphyroblasts serial thin sections spiral shaped inclusion trails three dimensional reconstruction

microstructural analysis of the classical spiral garnet porphyroblasts of south east vermont evidence for non rotation new data strongly suggest that the classical spiral garnet porphyroblasts of south east vermont usa generally did not rotate relative to geographical coordinates throughout several stages of non coaxial ductile deformation the continuity of inclusion trails si in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities consisting of truncations and differentiation zones where quartz graphite si bend sharply into more graphitic si discontinuous tight microfold hinges with relatively straight axial planes are also present these microstructures form part of a complete morphological gradation between near orthogonally arranged discontinuous inclusion segments and smoothly curving continuous si spirals some 2700 pitch measurements of well developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike from specimens collected at 28 different locations around the chester and athens domes the results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations irrespective of variations in the external foliation attitude macrostructural geometry and apparent porphyroblast matrix rotation angles combined with evidence for textural zoning this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation dimensionally elongate idioblasts also have a preferred shape orientation with long axes orientated normal to the mica folia within which epitaxial nucleation occurred truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins in association with progressive and selective strain induced dissolution of matrix minerals and locally also the porphyroblast margin non rotation of porphyroblasts relative to geographical coordinates suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed relatively large and rigid heterogeneities which cause the progressive shearing rotational component of deformation to partition around them the spiral garnet porphyroblasts therefore preserve the most complete record of the complex polyphase tectonic and metamorphic history experienced in this area most of which was destroyed in the matrix by progressive foliation rotation and reactivation together with recrystallization

porphyroblast microstructures a review of current and future trends many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies this paper reviews ten current applications of these microstructures 1 porphyroblast growth timing criteria 2 tracking progressive foliation development relative to changing metamorphic conditions 3 timing of pluton emplacement relative to deformation and metamorphism 4 finite longitudinal strain determinations 5 kinematics and porphyroblast rotation 6 use of linear fabrics preserved in porphyroblasts 7 porphyroblasts and folding mechanisms 8 inclusion trail orientations and orogenic processes 9 inferring shear strain rates from porphyroblast growth rates and 10 in situ age determinations although there is still no concensus on the interpretation of some porphyroblast microstructures a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings

fia trends along the precambrian rocky mountains a new approach to timing continental docking a similar succession of foliation inflection intersection axis fias trends preserved within porphyroblasts is present in two areas separated by 200 km along the rocky mountains the precambrian rocks in central colorado and northern new mexico were affected by deformation and metamorphism from 1506 to 1366 ma a succession of five fias trending w e ssw nne nnw sse nw se and wsw ene is distinguished in central colorado and dated at 1506 15 ma 1467 23 ma 1425 18 ma not dated and 1366 20 ma respectively to the south in northern new mexico a succession of five fias trending ssw nne wnw ese nnw sse nw se and wsw ene is distinguished and dated at 1482 48 ma 1448 12 ma 1422 35 ma not dated and 1394 22 ma the excellent correlation of the sequence of fia trends and their ages between regions reveals a sixfold fia succession across the region with the first developed fia set in central colorado not present in northern new mexico and the third fia set in the region not present in central colorado preferential partitioning of w e trending deformation into the central colorado region 1506 15 ma was followed by ssw nne trending deformation that affected both regions at 1470 20 ma however preferential partitioning of wnw ese trending deformation into northern new mexico at 1448 12 ma left central colorado unaffected both regions were then affected by the three remaining periods of orogenesis the first trending nnw sse at 1424 15 ma followed by one trending nw se that has not yet been dated and then one trending wsw ene at 1390 19 ma this suggests that the yavapai terrane was tectonized at 1506 ma prior to amalgamation with the mazatzal terrane 1470 ma subsequent orogenesis was initially partitioned preferentially into the mazatzal terrane but the following three periods of tectonism affected both terranes in a similar manner

radical changes in bulk shortening directions during orogenesis significance for progressive development of regional folds and thrusts abstract polyphase folding and metamorphism of rocks in the eastern mount isa inlier resulted from four distinct orogenic cycles these consist of ne sw shortening o00 from 1672 to 1648 ma involving a succession of five phases of deformation which was responsible for the development of the first generation nw se striking macroscopic folds plus cleavages that are mainly only preserved in porphyroblasts n s shortening o1 followed to 1600 ma involving at least three phases of deformation this period of n s directed orogenesis rotated large portions of the nw se trending regional folds into w e trends and the dominance of locally well preserved w e trending sub vertical matrix foliations as well as the intensification of foliation parallel to bedding subsequent w e shortening o2 from 1600 to 1515 ma was accompanied by the development of n s trending folds these dominate the mount isa inlier and where best developed involved the formation of at least three near orthogonal foliations where these large scale folds regionally are weakly developed or not visible only one sub vertical foliation has developed finally nw se shortening o3 produced locally developed sw ne trending folds and weak crenulations until 1500 ma the asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during o00 indicates thrusting to the sw similar inclusion trail asymmetries in garnet staurolite and andalusite porphyroblasts that developed in o1 indicate thrusting to the n confirming a change in thrust direction across the wonga duchess belt to the w formation of the meerenurker roof thrust near mount isa by thrusting to the sw during o00 with subsequent development of the kokkalukkanurker duplex below by thrusting to the s during o1 reduces the displacement required for the roof thrust from 234 to 65 kms foliations preserved in o00 garnets are truncated by foliations associated with the development of o1 in the matrix o1 garnet staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during o2 the inclusion trails entrapped within o00 and o1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case numerous phases of sub vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during o00 and o1

mechanism of emplacement of the cannibal creek granite with special reference to timing and deformation history of the aureole abstract the ovoid cannibal creek granite consists of a minor phase emplaced during the regional d 3 deformation and a major phase emplaced during the regional d 4 deformation it possesses an intense contact parallel foliation on its margin and in the adjacent porphyroblastic schist that formed through partitioning of the shearing component of deformation against the crystallized pluton margin during synchronous e w d 4 shortening during this process a pre existing shallow s 3 foliation was updomed around the pluton and re used reactivated by d 4 shear with associated dissolution of quartz resulting in the formation of an ovoid closed foliation around the pluton thus the granite is located within a domal shaped approximately n s trending pluton scale f 3 4 antiform that formed synchronous with emplacement three successive stages of porphyroblast growth record the history of processes operating during progressive emplacement and the structures that developed space problems were solved through upward movement of the rocks above the granite due to movement on the s 4 cleavage as well as on pre existing subvertical bedding and cleavages and by reactivation of steep d 2 faults

porphyroblast inclusion trail orientation data eppure non son girate extensive examination of large numbers of spatially orientated thin sections of orientated samples from orogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative to geographical coordinates during highly non coaxial ductile deformation of the matrix subsequent to their growth this has been demonstrated for all tectonic environments so far investigated the work also has provided new insights and data on metamorphic structural and tectonic processes including 1 the intimate control of deformation partitioning on metamorphic reactions 2 solutions to the lack of correlation between lineations that indicate the direction of movement within thrusts and shear zones and relative plate motion and 3 a possible technique for determining the direction of relative plate motion that caused orogenesis in ancient orogens

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

preferred orientation of planar microstructures determined via statistical best fit of measured intersection lines the fitpitch computer program the fitpitch computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection lines on different sections measured pitch or strike angles of such intersection lines are fitted to the theoretical intersection lines of a single model plane or a combination of two or three model planes the degree of fit is quantified in terms of the deviation between natural and theoretical intersection lines normalised by uniform data a first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the variscan iberian massif was measured analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes fia in porphyroblasts a second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a variscan granite in the rhine graben qualitatively established best fit planes derived originally from this data are compared with the numerically established planes using the fitpitch program and discussed in terms of the advantages and potential limitations of the method q 2002 published by elsevier science ltd

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

episodic gravitational collapse and migration of the mountain chain during orogenic roll on in the himalayas an w e belt of maximum bulk horizontal shortening the orogen core moved north relative to the overlying crust to form the himalayan syntaxes due to roll on of this portion of the indian plate this displacement occurred below a lengthy succession of gently dipping decollements that formed episodically at a depth of 30 km along the orogen core due to numerous periods of gravitational collapse and spreading of the overlying ductile crust successively developed basal decollements were deformed when continued bulk horizontal shortening of the orogen core below reasserted dominance over the effects of gravitational collapse above causing refolding about steeply dipping axial planes this resulted in northwards migration of the orogen core above depths of 30 km causing rocks metamorphosing at depths of 22 km on the north side of the orogen core to be moved to its south side with no change in depth as roll on progressed garnet porphyroblasts record this lengthy history of lateral migration across the orogen within their inclusion trails the 6 4 kbar average pressures accompanying it were obtained from the mn fe and ca contents of successive garnet cores garnet grew at depths of 22 km until movement towards the surface initiated on successively developed decollements that accommodated the volume constraints of gravitational collapse and spreading on both sides of the orogen the speed of extrusional displacement increased the further the rocks migrated from the orogen core developing mylonitic schists around the porphyroblasts this truncated inclusion trails against all matrix foliations as the porphyroblasts were carried towards the surface indeed these rocks were multiply deformed during at least four distinct periods of deformation after mylonitization began and prior to exposure above the main central thrust mct three or more sub vertical and sub horizontal foliations were formed during each of the five changes in fia trend foliation inflection intersection axes in porphyroblasts preserved in these rocks the inclusion trail asymmetries and p t of garnet core growth accompanying each fia reveal that the first four changes in fia trend which define periods of tectonism about one direction of horizontal bulk shortening relative plate motion occurred on the north side of the orogen core the fifth occurred on the south side of the orogen core and the switch in shear sense on gently dipping foliation planes that resulted from this shift to the south eventually led to the development of the mct when magnetic anomaly 22 that formed in the southern indian ocean ridge is taken into account these five changes in fia trend correlate markedly with changes in the motion of india relative to a constant eurasia from 50 to c 25 ma they reveal that eurasia moved nnw during fias 1 3 and 4 and sse during fia 5 when the shear sense on gently dipping foliations switched to top to the s they suggest collision of india with eurasia took place at 50 ma immediately prior to the development of fia 1

garnet porphyroblast timing and behaviour during fold evolution implications from a 3 d geometric analysis of a hand sample scale fold in a schist detailed 3 d analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand sample scale tight upright fold has revealed a complex deformation history the fold dominated by interlayered quartz mica schist and quartz rich veins preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica rich layers garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities garnet porphyroblast cores and rims pre date the macroscopic fold and preserve successive foliation inflection intersection axes fias which have the same trend but opposing plunges on each limb of the fold and trend nne ssw and ne sw respectively the fias are oblique to the main fold which plunges gently to the wsw inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32 whereas equivalent surfaces in tabular garnet adjacent to quartz rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17 this potentially could be explained by garnet porphyroblasts that grew over a pre existing gentle fold and did not rotate during fold formation but is more easily explained by rotation of the porphyroblasts during folding tabular porphyroblasts adjacent to quartz rich layers rotated more relative to the fold axis than those within mica rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz rich layers this work highlights the importance of 3 d geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix

porphyroblast rotation versus nonrotation conflict resolution comment fay et al 2008 1 present a few numerical simulations with unspecified code or boundary conditions as evidence for nonrotation of porphyroblasts during non coaxial flow however the large body of work on the behavior of porphyroblasts in deforming ductile rocks is significantly undervalued

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

lack of porphyroblast rotation in noncoaxially deformed schists from petrel cove south australia and its implications abstract inclusion trails representing an s 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non coaxial deformations the pelitic schists of the kanmantoo group at petrel cove south australia contain two generations of porphyroblasts the first one consists of cordierite porphyroblasts that formed early in d2 and contain straight to slightly sigmoidal inclusion trails of s 1 the second generation consists of andalusite porphyroblasts that overgrew crenulated s 2 late during d3 several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection hence s 1 was planar and horizontal prior to d2 s 1 measurements on limbs and hinges of a mesoscale d2 foldpair show that folding had little effect on porphyroblast orientation as the s 1 orientation remained constant and subhorizontal around the fold hence porphyroblasts have not rotated during any of the non coaxial deformations accompanying and following their growth this is interpreted as a result of the partitioning of the deformation around them s 1 in the matrix has been totally destroyed by the formation of s 2 as a fully differentiated crenulation cleavage what has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning the presence of a subhorizontal s 1 foliation suggests horizontal movements e g thrusting or detachment faulting during the earliest phase of the adelaidian orogeny

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