development of p t prograde and p retrograde t prograde isogradic surfaces during blueschist to eclogite regional deformation metamorphism in new caledonia as indicated by progressively developed porphyroblast microstructures the mid tertiary blueschists eclogites and eclogitic gneisses of northern new caledonia are the products of four phases of regional meta morphism and deformation d d4 omphacite lawsonite and mn rich garnet isogradic surfaces were developed during the second deformation d2 under prograde pressure and temperature conditions subsequent deformations d3 d folded these d isogradic surfaces however within the p retrograde t prograde metamor phic environment of the d phase omphacite altered to albite and chlorite as a result a late stage sub horizontal isogradic surface developed for omphacite out where this mineral is preserved as relics within syn d albite porphyroblasts other minerals that crystallized for the first time epidote or had rim additions almandine phengite during d also form nearly horizontal isogradic surfaces porphyroblastic garnet and albite contain inclusion trails which allow their microstructural development and crystallization of the matrix to be traced from d to d late syn d the temperature increased mark edly in association with an extensive exothermic decarbonation even though the rocks were in a state of pressure retrogression this caused considerable neocrystallization recrystallization and growth of matrix and porphyroblasts such that although s foliation crenulated by d and d is readily observable almost all signs of stored strain due to d and d have been removed and the deeper schists and eclogitic gneisses superficially appear to have undergone a drastic annealing recrystallization post dating deformation

the properties and geological environments of helicoids axially symmetric surfaces in torsional and non torsional deformations abstract a helicoid is a non developable surface swept out by a line the generator which rotates with constant sense about another non parallel line the heliaxis whilst moving parallel to that line in a constant direction torsion may produce helicoids though this is not a necessity previous references to helicoids and helices in structural geology are reviewed and the basic properties of helicoids are compared with those of cylindrical and conical surfaces a map and stereogram technique is developed for the recognition of nested initially parallel equally rotated coaxial macroscopic helicoids the typically sigmoidal outcropping traces of nested helicoids show dip direction reversals along trend on stereograms linear loci each representing the poles from a single helicoid trace diverge from a common pole a nested set of macroscopic helocoidal cleavage surfaces from the hill end trough southeastern australia are modelled and interpreted to have formed in two stages of torsion associated with accommodation of an adjacent granitoid intrusion the volume affected by torsion is bound by curviplanar scissor faults helicoids may be found associated with rotational porphyroblasts deformed cylindrical bodies or strip like surfaces e g intrafolial fold axial planes cylindrical diapirs with helical flow patterns cylindrical scissor faults ductile shear zones zones of constrictional strain and en echelon segmented dykes and fracture planes

rapid growth and strain rates inferred from synkinematic garnets penokean orogeny minnesota recent reinterpretations of the early proterozoic penokean orogeny in minnesota invoke a plate tectonic model with characteristics similar to phanerozoic collisional belts deformation during collision produced large flattening strains 89 and the development of a tectonic foliation and progressive metamorphism up to amphibolite facies as indicated by the presence of synkinematic s1 garnets biotite garnet thermometry and plagioclase garnet biotite muscovite barometry give southward increasing temperatures and pressures and indicate burial to depths of 21 25 km similar to medial and internal zones of collisional orogens worldwide gibbs method modeling on zoned synkinematic garnets indicates virtually no change in pressure p or temperature t during garnet growth growth and rotation of garnets at a point in p t space in an actively evolving deformational and thermal regime are surprising and suggest rapid garnet growth rates and high rates of strain during orogeny this is consistent with previous studies that indicate more rapid rates of tectonometamorphic processes in the precambrian relative to younger orogenic belts

raft model of crustal extension evidence from seismic reflection data in southeast arizona an archlike zone of seismic reflectivity interpreted as an uplifted zone of ductilely deformed middle and lower crust is imaged below the pinaleno mountains core complex in southeast arizona the top of the reflective zone coincides with the base of an inferred mid tertiary detachment fault beneath the safford basin but diverges from the detachment fault as an apparent mylonite front to form a culmination at 1 9 s 4 km beneath the pinalenlo mountains from this culmination the zone of reflectivity dips to the southwest below the eagle pass detachment fault and flattens at 4 8 s 13 5 km beneath the relatively unextended upper crust of the galiuro mountains most of the reflective fabric probably formed during mid tertiary extension although some of it may be older these data suggest that mylonite zones form not only as the continuation of detachment faults into the brittle ductile transition but also along a regional zone of decoupling between the middle and upper crust highly extended and relatively unextended domains in the basin and range may be separated by zones of discrete simple shear in the upper crust but both are rafted above regional bulk pure shear in the middle and lower crust

shear zone vs folding origin for spiral inclusion trails in the canton schist the three dimensional geometry of spiral inclusion trails from the canton schist were measured to determine whether the spirals were a product of porphyroblast rotation within a shear zone or porphyroblast growth during a series of overprinting fold events the spiral inclusion trails are composed of three separate sub planar inclusion trail surfaces occupying texturally distinct parts of the porphyroblasts these surfaces are correlated across a 10 km2 area using textural criteria and relative timing measured patterns of inclusion trail orientation and asymmetry suggest they did not form by porphyroblast rotation within a non coaxial shear zone rather the porphyroblasts grew during three successive overprinting fold events f2 f4 and the spiral inclusion trails represent the accumulated curvature associated with folding of successive axial plane foliations the data show that spiral garnets are not peculiar to shear zones and can form by overprinting crenulations and folds this is consistent with the common occurrence of spiral garnets in multiply deformed regionally metamorphosed fold belts

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

deformation history of the otago schists new zealand from progressively developed porphyroblast matrix microstructures uplift collapse orogenesis and its implications abstract foliation overprinting relationships preserved in porphyroblasts and the surrounding matrix in the otago schists indicate development of up to six penetrative foliations during the late paleozoic mesozoic rangitata orogeny s1 s3 and s5 formed with steep dips and s2 s4 and s6 formed with gentle dips s1 and s2 are preserved only as inclusion trails in porphyroblasts because of matrix foliation reworking during progressive deformation s3 s6 are preserved in the matrix as well as in porphyroblasts and are generally correlatable from one to the other inclusion trail asymmetries define the location of the orogenic uplift core during the rangitata orogeny when the porphyroblasts grew the location of this core is consistent with macroscale geological and structural observations alternating steep and gently dipping foliations are inferred to indicate alternating horizontal compression and extension predictions made by an orogenic model compatible with continually alternating horizontal compression and extension compare favorably with geometries observed in the schists reconstruction of the gross geometry of the otago schists during the rangitata orogeny based on the three dimensional geometry of porphyroblast inclusion trails is similar to reconstructions based on regional geological considerations and plate tectonic calculations

ductile thrusting within subduction complex rocks on signy island south orkney islands abstract a model involving repeated nnw directed ductile thrusting in subduction complex rocks on signy island south orkney islands accounts for the consistent nnw trending alignment of fold axes stretching and shearing directions in the absence of major discrete thrust planes stratigraphic control or younging criteria the evidence for consistent ductile thrusting is more cryptic and is found in folding stretching relations porphyroblast rotations and mylonite fabrics early fold axes parallel to stretching syn tectonic garnet rotations augen lineations and sheath folds all support nnw directed thrusting as do a common late shear band foliation and minor shear zones development of late open folds with axes parallel to stretching may be due to constrictional deformation a lateral ramp or differential movement during thrusting the structures described from signy island may characterize the type of deformation in deep level ductile thrust zones at the base of an accretionary wedge or in a zone of subcretion the thrusting direction on signy island is inferred to be parallel to the direction of subduction in the south orkney islands in the early jurassic plate reconstructions suggest that this direction was at a high angle to the arcuate trend of the antarctic peninsula continental margin

an estimate of kinematic vorticity from rotated elongate porphyroblasts internal fabric in strain shadows around elongate rutile porphyroblasts indicates rotation of porphyroblasts relative to the incremental stretching direction during deformation within a regional scale shear zone in south central alaska for porphyroblasts of various aspect ratios and initial orientations this fabric is used to assess the sense of rotation relative to the direction of maximum incremental stretching the total rotation and the vorticity the total rotation of porphyroblasts relative to the external fabric based on these parameters varies as a function of porphyroblast orientation all 111 porphyroblasts regardless of orientation have rotated in a clockwise sense relative to the external fabric comparison of these results with previous models suggests that the deformation which affected the porphyroblasts took place in the field of continuous forward rotation with a maximum ratio of the rate of pure shearing to the rate of simple shearing sr of 0 21 or a kinematic vorticity number wk 0 92 the model curves showing rotation vs final orientation do not vary significantly over the range of sr and r that is relevant to this study the data have significant scatter yet there is remarkable agreement between the data and the model for a of 1 5 these results illustrate how strain shadows around porphyroblasts with large aspect ratios and variable initial orientations provide a more complete assessment of kinematics than inclusion trails within more equant porphyroblasts

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

non rotation of garnet porphyroblasts and 3 d inclusion trail data an example from the imjingang belt south korea abstract we describe non rotation of garnet porphyroblasts in non coaxially deformed metapelites of the imjingang orogenic belt in the middle korean peninsula the three dimensional geometry of inclusion trails within 1 2 mm garnet porphyroblasts was analyzed by serial polishing and reflected light microscopy most of the garnet porphyroblasts have straight inclusion trails of quartz and some plagioclase s i oblique to an external foliation s e serial polishing of two samples normal to mineral elongation lineation a plane and normal to s e and parallel to the lineation b plane with twenty intervals of about 0 1 mm reveals that the straight s i is a planar structure measurement of the angles between s i and s e on both planes from samples of sixteen outcrops shows that s i orientation of individual garnets is fairly constant in each sample the attitude of s i in geographic space determined by using its pitch angles on the a and b planes is more or less constant throughout the area over 6 5 km traversing normal to the strike of regional foliation the sense of asymmetry of strain shadows at the margin of garnet porphyroblasts and the s i s e relationship indicate rotation of s e rather than garnet porphyroblasts with respect to the geographic reference frame rotation of s e is also supported by a sense of shear inferred from other shear criteria all indicating a top up to the south shearing

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

microtextures of opaque inclusions their use as indicators for hiatuses during garnet porphyroblast growth within an analysed garnet porphyroblast opaque inclusions imaged with the backscatter facility of a scanning electron microscope show different microtextures depending on their position within the porphyroblast three different zones can been distinguished zone 1 contains a ti rich magnetite that has decomposed to a symplectite of fine and narrowly spaced exsolution lamellae of ilmenite and magnetite zone 2 shows a ti rich magnetite symplectite with thicker and more widely spaced exsolution lamellae of ilmenite and magnetite within zone 3 ti rich magnetite symplectite has totally been replaced by recrystallized magnetite crystals bordered by a thin ilmenite rim similar microtextures within ulvospinel rich magnetite have elsewhere shown to be the result of an increase in oxidation and rate of diffusion during metamorphism of metapelites such an increase can be reasonably envisaged because of dehydration reactions progressing during rising temperatures and this has occurred during the overgrowth of the three different microtextures by the garnet porphyroblast because the microtextures are homogeneous within the three different zones it is deduced that the oxidation reaction rate of the opaque inclusions was substantially lower than the garnet growth rate as a consequence hiatuses in the garnet growth history must have occurred between the evolution from one microtexture to the next a comparison between the inclusion trail geometry and the microtextural zone boundaries shows a perfect coincidence between these and the sites where inclusion trails become strongly deflected and truncated this correlation confirms that in the studied case sharp microstructural boundaries as truncation zones or deflection zones coincide with growth hiatuses the study therefore highlights the potential use of opaque inclusions to confirm or reject the occurrence of growth hiatuses within garnet porphyroblasts especially in cases where discontinuities in the inclusion trail patterns are otherwise arbitrarily associated with growth hiatuses

rotated garnets a mechanism to explain the high frequency of inclusion trail curvature angles around 90 and 180 abstract this paper presents numerical data from garnets with inclusion trail curvature angles of up to 260 three hundred and twenty five garnets were studied from an outcrop of greenschist facies phyllite in southern minas gerais state brazil apart from the inclusion trail curvature angle also the aspect ratio r and the angle between the long axis of the garnets and the foliation were measured the results show a remarkable concentration of at 180 and a minor one at 90 r varies between 1 and 2 showing that the garnets deviate from sphericity and shows that all garnets have their long axis in the forward rotated quadrant supporting the rotational interpretation a model is proposed to explain the concentrations of based on preferential growth of the garnets into the mica rich strain caps orthogonal to the foliation causing elongated crystals that because of their shape and position would experience accelerated rotation until relatively stable positions with their long axes parallel to the foliation would be attained renewed growth again into the mica rich strain caps orthogonal to the foliation would first restore the spherical shape and then produce an elongated shape again perpendicular to the foliation forcing a repetition of the process it is concluded that this model is capable of explaining the concentration of in multiples of 90 in a rotational model where this concentration was considered earlier as an argument in favor of the non rotational model

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

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

how useful are millipede and other similar porphyroblast microstructures for determining synmetamorphic deformation histories oppositely concave microfolds ocms in and adjacent to porphyroblasts can be classified into five nongenetic types type 1 ocms are found in sections through porphyroblasts with spiral shaped inclusion trails cut parallel to the spiral axes and commonly show closed foliation loops type 2 ocms commonly referred to as millipede microstructure are highly symmetrical the foliation folded into ocms being approximately perpendicular to the overprinting foliation type 3 ocms are similar to type 2 but are asymmetrical the foliation folded into ocms being variably oblique to the overprinting foliation type 4 ocms are highly asymmctrical only one foliation is present and this foliation is parallel to the local shear plane type 5 ocms result from porphyroblast growth over a microfold interference pattern types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths respectively during porphyroblast growth however theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing given particular initial foliation orientation and timing of porphyroblast growth type 3 ocms can also form during these intermcdiatc deformation paths and are commonly found in the same rocks as type 2 ocms type 4 ocms may indicate highly noncoaxial deformation during porphyroblast growth but may be difficult to distinguish from type 3 ocms thus types 1 3 and possibly 4 reflect the finite strain state giving no information about the rotational component of the deformation s responsible for their formation furthermore there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation s during the growth of porphyroblasts containing ocms type 2 ocms that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening but there is a lack of supporting physical or computer modelling it is possible that microstructures in the matrix around ocms formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another however determining degrees of noncoaxiality from rock fabrics is a major long standing problem in structural geology

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

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

testing models for the development of spiral shaped inclusion trails in garnet porphyroblasts to rotate or not to rotate that is the question the formation of spiral shaped inclusion trails ssits is problematical and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing one model argues for porphyroblast rotation with respect to a geographically fixed reference frame whereas the other argues for no such porphyroblast rotation but instead rotation of the matrix foliation around the porphyroblast thus porphyroblasts with ssits cannot be used as shear sense indicators until it is conclusively determined which model best explains them any successful model must explain features associated with ssits including 1 foliation truncation zones 2 smoothly curving ssits 3 millipede microstructure 4 total inclusion trail curvature in median sections 5 porphyroblasts with ssits that have grown together 6 evidence for relative porphyroblast displacements 7 shear sense indicators inside and outside porphyroblasts 8 crenulations associated with porphyroblasts and 9 geometries in sections subparallel to spiral axes axes of rotation a detailed study of these features suggests that most if not all can be explained by both the rotational and non rotational models in spite of these models involving diametrically opposed movement senses therefore geometrical analysis of individual porphyroblast microstructures may not determine which model best explains ssits until the kinematics required to form these microstructures are better understood in particular the sense of shear along a developing crenulation cleavage specific tests for determining the shear sense along crenulation cleavages are proposed and results of such tests may conclusively resolve the debate over how ssits form

the internal inclusion trail geometries preserved within a first phase of porphyroblast growth the inclusion trail geometry within a first phase of porphyroblast growth can differ significantly from that preserved by further enlargement because the porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events the geometry of the first overgrown inclusion trails is affected by their primary orientation including any pre existing curvature combined with any heterogeneous rotation of this foliation about the developing stretching lineation this can impact the apparent timing of foliation intersection inflection axes preserved within porphyroblasts fias that nucleated during the development of a sub horizontal foliation but is readily resolved 3 d computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for fia determinations is unaffected by the cut location relative to the porphyroblast core significantly perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30 away from the fia therefore since fias in most porphyroblasts bear no relation to matrix structures there is a 17 chance that thin sections cut relative to the foliation lie within 30 of a fia and could contain such an apparent spiral fias maintain consistent trends for the first phase of porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge fias have sub horizontal plunges for porphyroblasts nucleating during gravitational collapse but may vary in trend for all periods of porphyroblast regrowth the data available indicates that fias remain consistently trending and sub horizontal until the relative direction of plate motion causing orogenesis changes

complex remanent magnetization in the kizilkaya ignimbrite central anatolia implication for paleomagnetic directions abstract pyroclastic flow deposits known as ash flow tuffs or ignimbrites are invaluable materials for paleomagnetic studies with many applications for geological and tectonic purposes however little attention has been paid to evaluating the consistency and reliability of the paleomagnetic data when results are obtained on a single volcanic unit with uneven magnetic mineralogy in this work we investigate this issue by concentrating on the kizilkaya ignimbrite the youngest large volume unit of the neogene ignimbrite sequence of the central anatolian volcanic province in turkey bringing evidence of significant magnetic heterogeneities in ignimbrite deposits magnetic mineralogy susceptibility magnetic remanence coercivity etc and emphasizing the importance of a stratigraphic sampling strategy for this type of volcanic rocks in order to obtain reliable paleomagnetic data six sections were sampled at different stratigraphic heights within the devitrified portion of the ignimbrite isothermal remanence measurements point to low ti titanomagnetite as the main magnetic carrier at all sites at some sites the occurrence of oxidized ti magnetite and hematite is disclosed the bulk susceptibility k m decreases vertically at two out of six sections its value for the topmost samples is commonly one order of magnitude lower than that of the samples at the base in most cases low k m values relate to high coercivity of remanence b cr values which range from 25 to 400 mt and to low s ratios measured at 0 3 t between 0 28 and 0 99 these data point to the occurrence of oxidized magnetic phases we therefore consider the k m parameter as a reliable proxy to check the ignimbrite oxidation stage and to detect the presence of oxidized ti magnetite and hematite within the deposit the characteristic remanent magnetization is determined after stepwise thermal and af demagnetization and clearly isolated by principal component analysis at most sites for these sites the site mean paleomagnetic direction is consistent with data from the literature at a few other sites the remanence is more complex the direction moves along a great circle during demagnetization and no stable end point is reached the occurrence of oxidized ti magnetite or hematite as well as two remanence components with overlapping coercivity and blocking temperature spectra suggest that the kizilkaya ignimbrite acquired first a thermal remanent magnetization and then during the final cooling or a short time later a secondary remanent magnetization component which is interpreted as a crm acquired during post emplacement devitrification processes notwithstanding the kizilkaya ignimbrite is a single cooling unit its magnetic properties suffered substantial variations laterally and vertically within the deposit the kizilkaya case shows that thick pyroclastic deposits should be sampled using a stratigraphic approach at different sites and different stratigraphic heights at each individual sampling location otherwise under sampling may significantly affect the paleomagnetic results when sampling is performed on a short duration or on very poorly preserved deposits we recommend drilling the lower central portion in the most strongly welded and devitrified facies such sampling strategy avoids complications arising from the potential presence of a pervasive secondary crm masking the original chrm