mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

strain rates from snowball garnet spiral inclusion trails in garnet porphyroblasts are likely to have formed due to simultaneous growth and rotation of the crystals during syn metamorphic deformation thus they contain information on the strain rate of the rock strain rates may be interpreted from such inclusion trails if two functions are known 1 the relationship between rotation rate and shear strain rate 2 the growth rate of the crystal we have investigated details of both functions using a garnetiferous mica schist from the eastern european alps as an example the rotation rate of garnet porphyroblasts was determined using finite element modelling of the geometrical arrangement of the crystals in the rock the growth rate of the porphyroblasts was determined by using the major and trace element distributions in garnet crystals thermodynamic pseudosections and information on the grain size distribution for the largest porphyroblast size fraction size l 12 mm we constrain a growth interval between 540 and 590 c during the prograde evolution of the rock assuming a reasonable heating rate and using the angular geometry of the spiral inclusion trails we are able to suggest that the mean strain rate during crystal growth was of the order of 6 6 10 14 s 1 these estimates are consistent with independent estimates for the strain rates during the evolution of this part of the alpine orogen

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

porphyroblast inclusion trail geometries in the grand canyon evidence for non rotation and rotation abstract porphyroblasts in paleoproterozoic supracrustal rocks of the grand canyon grew during a progressive deformation that involved shortening and heterogeneous development of ne striking subvertical foliation s2 that crenulates and transposes an earlier foliation s1 inclusion trails in syn s2 porphyroblast cores show a remarkably consistent nw strike across the regional transect suggesting a lack of appreciable porphyroblast rotation relative to the trace of the ne striking s2 foliation these apparently non rotated porphyroblast cores thus preserve a regional nw striking s1 orientation porphyroblast rims and younger porphyroblasts locally overgrow ne striking s2 indicating that porphyroblast growth and continued non rotation spanned s2 apparent non rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening in contrast single thin sections show domains of these non rotated garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45 this documents local rotation of garnets relative to s2 during a heterogeneous late or post s2 reactivation of the foliation thus these types of studies should continue to look for both rotational and non rotational behaviors at all scales and continue to specify the frame of reference

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

interpretation of porphyroblast inclusion trails limitations imposed by growth kinetics and strain rates porphyroblast inclusion trails provide important information about the tectonometamorphic evolution of a metamorphic rock however there remains considerable controversy over whether porphyroblasts rotate during bulk non coaxial deformation with reference to an area of the scandinavian caledonides and utilizing existing data from theoretical and experimental modelling this study demonstrates that both straight and s shaped inclusion trails are consistent with an interpretation in terms of syndeformational porphyroblast growth in a regime approximating to newtonian simple shear at crustal strain rates of 10 14 s 1 and porphyroblast growth times of 0 1 1 0 ma it is shown that a maximum of 5d 9d angular rotation would occur during growth at faster strain rates of 10 12 s 1 e g those in a shear zone porphyroblast angular rotations of 90d are shown to occur in 0 1 0 25 ma i e times comparable with or faster than porphyroblastesis in view of this s shaped inclusion trails are to be expected in porphyroblasts growing in active shear zones or other situations of high shear strain whereas straight inclusion trails can be interpreted as static overgrowth of an existing fabric or as syndeformational porphyroblastesis at low strain rates

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

albite porphyroblasts with sigmoidal inclusion trails and their kinematic implications an example from the taconic allochthon west central vermont microstructural analyses of albite porphyroblasts in phyllite of the taconic allochthon were conducted in order to determine the origin of the sigmoidal inclusion trails within the porphyroblasts and to evaluate if and when the porphyroblasts rotated during the deformation history three cleavages s2 s3 and s4 with similar strike are present in the phyllite the albite porphyroblasts are characterized by inclusion rich cores and inclusion free rims and the boundary between the cores and overgrowths is characterized by relatively straight and parallel zones of concentrated rutile the sigmoidal inclusion trails within the cores are inferred to be trapped microfolds the zones of concentrated rutile are interpreted to be truncation planes that formed during the development of s3 in contrast to the tight orientation distribution of s3 the truncation planes show a wide distribution pattern the mean orientations of s3 and the truncation planes also differ these orientation data indicate that the porphyroblasts have not only rotated with respect to s3 but also to the geographic reference frame in addition the growth of the porphyroblasts was influenced by differentiation associated with s3 crenulation cleavage formation the porphyroblasts grew preferentially within microlithon domains where chlorite a reactant mineral for the porphyroblasts is abundant

a 3d study of aligned porphyroblast inclusion trails across shear zones and folds inclusion trails si within 741 equant garnet porphyroblasts show a consistent orientation across a 100 m2 outcrop of precambrian metapelite with later d2 folds and shear zones in posio finland variation in si orientations is not systematic with respect to these later folds and shear zones but instead represents microfolding and warping of s1 before the formation of d2 structures the development of the d2 folds and shear zones is investigated by comparing the pattern of si in the outcrop to those predicted for porphyroblasts in tangential longitudinal strain axial slip flexural flow pure shear partitioned flexural flow and partitioned pure shear folds si patterns are also compared with models for non coaxial and coaxial shear zones along with corroborating evidence from structures in the outcrop and matrix the comparisons indicate that d2 folds and shear zones in the outcrop formed in response to nw se axially symmetric shortening during which garnet porphyroblasts did not rotate relative to each other the axial plane of f2 folds and d2 shear zone boundaries

deformation partitioning during transpression in response to early devonian oblique convergence northern appalachian orogen usa abstract transpressive deformation was distributed heterogeneously within the central maine belt shear zone system which formed in response to early devonian oblique convergence during the acadian orogeny in the northern appalachians straight belts are characterized by tight folds s l fabrics and sub parallel form lines and asymmetric structures that together indicate dextral se side up kinematics in contrast intervening zones between straight belts are characterized by open folds and l s fabrics within both types of zone metasedimentary rocks have fabrics defined by the same minerals at the same metamorphic grade including a penetrative moderately to steeply ne plunging mineral lineation thus we interpret accumulation of plastic deformation and regional metamorphic re crystallization to have been synchronous across the central maine belt shear zone system discordance between inclusion trails in regionally developed porphyroblasts of garnet and staurolite and matrix fabrics in straight belt rocks records shortening by tightening of folds and greater reorientation of matrix fabrics with respect to porphyroblasts kinematic partitioning of flow was responsible for the contrasting states of finite deformation recorded in the central maine belt shear zone system perturbations in the flow were caused by serially developed thrust ramp anticlines in the stratigraphic succession immediately above the avalon like basement at which decollement of the shear zone system was initially rooted general shear deformation at the ramps involved strain softening with an enhanced component of noncoaxial flow in contrast deformation during extrusion in the intervening zones involved strain hardening with a greater component of coaxial flow part of the thickening stratigraphic succession exceeded tsolidus reflected by the occurrence of migmatites and granites the latter were partly sourced from the underlying avalon like basement that was involved in the deformation and melting

fold mechanisms in the canton schist constraints on the contribution of flexural flow abstract fold mechanisms operating in the canton schist have been resolved using the geometric relationship between folds and spiral inclusion trail geometries of the four end member fold mechanisms tangential longitudinal strain folding and slip folding are unable to produce the observed inclusion trail and fold geometries but a combination of flexural flow and pure shear folding is consistent with the geometric constraints the maximum flexural flow component during each fold event was determined from the geometric data during f 3 and f 4 flexural flow produced 27 and 37 of measured limb rotation respectively which corresponds to a maximum of 24 limb rotation by flexural flow in each fold event to satisfy the geometric constraints the remainder of limb rotation must be a product of pure shear folding the maximum possible component of flexural flow folding during f 3 and f 4 increases with increased variation in vorticity between layers in the rock mass in the flexural flow pure shear model a maximum of 28 of inclusion trail curvature is produced by rotation of porphyroblasts relative to irrotational fold limbs with a minimum 72 curvature due to rotation of fold limbs relative to irrotational porphyroblasts all model solutions produce less than 8 syn folding porphyroblast rotation relative to geographic coordinates

rotated staurolite porphyroblasts in the littleton schist at bolton connecticut usa staurolite porphyroblasts 1 5 8cm in length and 0 3 2cm in width in the littleton schist at bolton connecticut contain curved quartz inclusion trails which document synkinematic rotations of at least 135 the orientations of long axes of these staurolite crystals define a weak preferred orientation in a plane approximately parallel to the external foliation serial sections of four differently orientated crystals and u stage measurements of the orientations of their inclusion trails demonstrate that the inflection hinge line and the statistical symmetry axis characterizing the foliation within a porphyroblast are unrelated to the orientations of external crenulations and are in all cases parallel to the long axis of the porphyroblast the cumulative rotation reflected in the curvature of the inclusion trails is a maximum in a c axis section through the initial core of a crystal the amount of rotation about the c axis decreases linearly along the length of the crystal away from the nucleation site the sense and amount of rotation recorded by a porphyroblast is related to its orientation a tightly constrained transition from clockwise to anticlockwise rotation defines a slip direction that coincides with the preferred orientation of the staurolite c axes the total rotation reflected by the inclusion trails increases as a function of the angle between the c axes of the staurolite crystals and the slip direction initially random staurolite porphyroblasts rotated during growth as a consequence of laminar shear in the surrounding viscous matrix this interpretation is quantitatively consistent with the staurolite preferred orientation its coincidence with the apparent slip direction the correlation between both the sense and the amount of rotation and the orientation of the long axis of the porphyroblast and the twisted conical shape of the family of surfaces defined by the inclusion trails

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part i methods and techniques macroscopic textures resulting from different atomic scale mechanisms for metamorphic crystallization display different degrees of order clustering intergrowth and relative isolation of porphyroblasts data on the sizes and locations of thousands of crystals in a three dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features these data can now be acquired by means of high resolution computed x ray tomography numerical models that simulate porphyroblast formation governed by either interface controlled or diffusion controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities these numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples from which inferences can be drawn concerning the relative importance of these mechanisms in nature an independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius rate relations

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

the ti saturation surface for low to medium pressure metapelitic biotites implications for geothermometry and ti substitution mechanisms the ti content of biotite can serve as a geothermometer for graphitic peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4 6 kbar the relationship between ti content temperature and mg mg fe value was calibrated empirically using an extensive natural biotite data set 529 samples from western maine and south central massachusetts in combination with the petrogenetic grid of spear et al 1999 the calculated ti saturation surface is curved such that for a given mg mg fe value ti concentration increases as a function of temperature in a nonlinear fashion and for a given temperature ti concentrations decrease with an increase in mg mg fe the fit to the ti saturation surface can be reformulated as the geothermometric expression t ln ti a c x mg 3 b 0 333 in which t is temperature in degrees celsius ti is the number of atoms per formula unit apfu normalized on the basis of 22 o atoms x mg is mg mg fe a 2 3594 b 4 6482 10 9 and c 1 7283 the calibration range for this expression is x mg 0 275 1 000 ti 0 04 0 60 apfu and t 480 800 c precision of the ti in biotite geothermometer is estimated to be 24 c at the lower temperature range and improves to 12 c at higher temperatures application of the ti in biotite geothermometer to ilmenite or rutile bearing graphitic peraluminous metapelites equilibrated at 3 6 kbar is generally consistent with independent temperature determinations but with some deviations that represent local reequilibration consequently the ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium or lack thereof application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor to significant errors in estimated temperatures biotite ti substitution mechanisms are controlled by several factors based on the biotite calibration data set magnesian biotites x mg 0 65 incorporate ti in accordance with the exchange vector tial2r 1si 2 where r is the sum of the divalent cations mg fe mn this substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites intermediate biotites x mg 0 65 particularly at higher temperatures exhibit enhanced ti concentrations most consistent with the ti deprotonation tio2r 1 oh 2 exchange vector dominance of ti deprotonation substitution is largely a function of reduction of h2o activity at higher metamorphic grades supplementary biotite data from metaluminous amphibolites and mafic granulites metamorphosed isothermally with variable h2o activities reveal that low al biotite incorporates significantly higher concentrations of ti relative to peraluminous biotite as a result of a combination of the exchange vectors tio2r 1 oh 2 and rsial 2 substituting in roughly an 8 1 ratio

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

stepping stones and pitfalls in the determination of an anticlockwise p t t deformation path the low p high t cooma complex australia low pressure high temperature low p high t metamorphic rocks of the cooma complex southeastern australia show evidence of an anticlockwise pressure temperature time deformation p t t d path similar to those of some other low p high t metamorphic areas of australia prograde paths are reasonably well constrained in cordierite andalusite schists cordierite k feldspar gneisses and andalusite k feldspar gneisses these paths are inferred to be convex to the temperature axis involving increase in pressure with increase in temperature evidence of the retrograde path is inconclusive but is consistent with approximately isobaric cooling as are available isotopic data on the cooma granodiorite which indicate initially rapid cooling following attainment of peak temperatures the retrograde path is inconsistent with either a clockwise p t t d path involving rapid or even moderate decompression immediately post dating the peak of metamorphism or a path in which the retrograde component simply reverses the prograde component because both these paths should cross reactions forming cordierite from aluminosilicate for which no evidence has been observed determination of the deformational metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples evidence necessary for successful elucidation of the prograde and part of the retrograde deformational metamorphic history in the cooma complex includes 1 sequentially grown porphyroblasts that can be timed relative to surrounding foliations 2 partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development 3 a tectonic marker foliation s4 at cooma that allows correlation of foliations from one location to another and 4 single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone the latest two or three foliations involve low strain accumulation allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined sequential porphyroblast growth and foliation development in the cordierite andalusite schists is examined for situations involving rotation and non rotation of porphyroblasts relative to geographically fixed coordinates although the number of foliations developed varies in the rotational situation depending on the deformation history proposed the sequential order of porphyroblast growths does not differ from the non rotational situation thus whether or not porphyroblasts rotated in the cooma rocks the sequence of reactions and therefore p t t paths inferred from the relative timing of porphyroblast growths remain the same for the deformational histories evaluated

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

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

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

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

kinematics of orogenic collapse in the variscan pyrenees deduced from microstructures in porphyroblastic rocks from the lys caillaouas massif in the pyrenees a phase of bulk vertical shortening d3 is indicated by a horizontal crenulation cleavage s3 which is developed in a pre existing schistosity s2 porphyroblasts of the lys caillaouas massif regionally preserve 1 1 the initial pre d3 orientation of s2 in the form of a vertical and e w trending preferred orientation of their inclusion trails 2 2 a vertical l2 stretching lineation defined by the elongation of the inclusions these data are consistent with n s compression and crustal thickening during d2 the angle between the inclusion trails s2 i and the external schistosity s2 e records the subsequent rotation of s2 from vertical to gently dipping orientations at higher crustal levels s2 remained steep suprastructure and little d3 deformation is evidenced with depth d3 deformation progressively intensifies and zones can be distinguished with opposite dip directions of s2 opposite apparent porphyroblast rotation senses and opposite f3 crenulation asymmetries these micro structures on a n s section through the lys caillaouas and the garonne dome massifs record a regional partitioning of bulk coaxial vertical shortening this deformation partitioning was probably controlled by a not exposed gneiss dome which behaved relatively rigid as compared to its metasedimentary cover in zones of non coaxial d3 deformation s2 was extended reactivated and forms the dominant schistosity where d3 was locally more coaxial s2 was shortened isoclinally crenulated and s3 forms the macroscopic cleavage the fact that d3 extension in the lys caillaouas massif immediately followed d2 compression and that similar extension is recorded in large parts of the variscan foldbelt suggests gravitational collapse of a thickened orogen this is consistent with the flattening type of d3 strain recognized in a number of pyrenean variscan massifs peak high t low p metamorphic conditions coincide with the onset of orogenic extension when very little crustal thinning could have been accomplished yet this suggests that heat induced strain softening within the orogen may have triggered its collapse

the rotation of garnet porphyroblasts around a single fold lukmanier pass central alps abstract the orientation of the straight internal foliation s i within large 5 mm garnet porphyroblasts has been measured relative to the orientation of the external foliation s e around a single antiform of 0 5 m wavelength which folds the dominant regional foliation the internal foliation is not constant in orientation but varies consistently both with position around the fold and with the porphyroblast ellipticity the dip of s i hinge dip taken as zero is consistently less than the dip of s e it increases with increasing dip of s e and with increasing ellipticity of the porphyroblasts s i effectively defines a fold with an opening angle greater than that in the external foliation the opening angle of this fold in s i decreases with increasing porphyroblast ellipticity the observed variation in the orientation of s i can be explained qualitatively by a flattened flexural flow model for fold development as could be expected for folding of a pre existing strongly anisotropic foliation the measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate

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

porphyroblast non rotation during crustal extension in the variscan lys caillaouas massif pyrenees abstract the dominant foliation s 2 in the variscan lys caillaouas massif central pyrenees france spain formed in a subvertical orientation during crustal thickening d 2 subsequent non coaxial crustal extension produced a subhorizontal crenulation cleavage s 3 whereby s 2 was folded and rotated andalusite staurolite cordierite and biotite porphyroblasts grew very early during this extension d 3 and included s 2 as straight curved and weakly crenulated inclusion trails these inclusion trails exhibit a strong subvertical preferred orientation which is independent of the local magnitude of d 3 strain the dip angle of the external main foliation s 2 or the aspect ratio and shape orientation of porphyroblasts this indicates that porphyroblasts did not rotate in the d 3 deformation field and hence preserve the orientation of s 2 at the time it was trapped in the porphyroblast relative to the bulk d 3 flow plane porphyroblasts could maintain a stable orientation in the d 3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain the lack of vorticity of local volumes of zero shear strain such as porphyroblasts was balanced by concentration of shear strain in planar zones anastomosing around them it is likely that s 2 maintained a subhorizontal orientation throughout the d 3 crustal extension which would imply that porphyroblasts also remained stationary relative to the earth s surface during d 3 whereas s 2 in the deforming matrix experienced shear induced rotation a late brittle ductile folding event d 4 involving differential rigid block rotations dispersed the orientation of inclusion trails to some degree these late rotations could be corrected for by artificial unfolding of the f 4 folds

inclusion trail patterns in porphyroblasts from the foothills terrane california a record of orogenesis or local strain heterogeneity a major problem with the current use of porphyroblast matrix microstructural relationships to infer orogenic histories such as multiple orthogonal orogenic events is that other evidence for these events is typically lacking for example a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the foothills terrane sierra nevada california shows that 1 except in shear zones contact aureoles and local zones along lithological contacts the foothills terrane has a single regional cleavage although locally formed by multiple processes 2 the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen 3 structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts 4 complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles indicating that local complexity in porphyroblasts does not imply regional complexity because of these conclusions multiple datasets rather than data acquired only from porphyroblasts should be considered when attempting to understand the evolution of orogens furthermore using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

environmental change over the last millennium recorded in two contrasting crater lakes in western uganda eastern africa lakes kasenda and wandakara the last millennium is a key period for understanding environmental change in eastern africa as there is clear evidence of marked fluctuations in climate effective moisture that place modern concern with future climate change in a proper context both in terms of environmental and societal impacts and responses here we compare sediment records from two small nearby closed crater lakes in western uganda lake kasenda and lake wandakara spanning the last 700 wandakara and 1200 years kasenda respectively multiproxy analyses of chemical sedimentary parameters including c n ratios 13c of bulk organic matter and 13c and 18o of authigenic carbonates and biotic remains diatoms aquatic macrofossils chironomids suggest that kasenda has been sensitive to climate over much of this period and has shown substantial fluctuations in conductivity while wandakara has a more muted response likely due to the increasing dominance of human activity as a driver of change within the lake and catchment over the length of our record evidence from both records however supports the idea that lake levels were low from ad 700 1000 ad with increasing aridity from ad 1100 1600 and brief wet phases around ad 1000 and 1400 wetter conditions are recorded in the 1700s but drought returned by the end of the century and into the early 1800s becoming wetter again from the mid 1800s comparison with other records across eastern africa suggests that while some events are widespread e g aridity beginning ad 1100 at other times there is a more complex spatial signature e g in the 1200s to 1300s and from the 1400s to 1600s this study highlights the important role of catchment specific factors e g lakemorphometry catchment size and human impact in modulating the sensitivity of proxies and lake records as indicators of environmental change and potential hazards when regional inference is based on a single site or proxy

structure and emplacement history of a multiple center cone sheet bearing ring complex the zarza intrusive complex baja california mexico the cretaceous zarza intrusive complex located in the peninsular ranges of baja california norte mexico is perhaps the bestpreserved multiple center cone sheet bearing ring complex documented in north america the 7 km 2 elliptical complex hosts three nested non concentric intrusive centers that are successively younger to the south the northern and central centers show the same evolutionary sequence of 1 intrusion of concentric gabbroic cone sheets 2 intrusion of massive core gabbros and 3 development of subvertical ductile ring faults ring fault kinematics indicate that both centers moved down relative to the surrounding country rocks suggesting collapse into an underlying magma chamber the southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets aluminumin hornblende barometry on the tonalite indicates a maximum emplacement depth of 2 3 0 6 kbar the zarza intrusive complex is surrounded by a ductile deformation aureole and bedding is inward dipping and inward younging around the entire complex excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole and the three most applicable models are 1 collapse of the complex into its underlying magma chamber 2 sinking of the complex and its chamber after solidification and 3 formation of the aureole prior to emplacement of the complex the preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers and suggest that the complex may have been overlain by a caldera

analogue modelling of the influence of shape and particle matrix interface lubrication on the rotational behaviour of rigid particles in simple shear abstract the rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear couette flow was studied in 2d rock analogue experiments the influence of particle shape elliptical vs monoclinic aspect ratio and the nature of the matrix particle interface lubricated vs unlubricated was investigated both matrix pdms and lubricant liquid soap were linear viscous with a viscosity ratio of ca 10 4 without lubricant the rotational behaviour of all particles closely approximates the jeffery theory lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation in general for elliptical particles rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle for thick mantles initially 10 of the volume of the particle rotation rates are faster than jeffery theory for very thin mantles they are markedly slower compared with thick mantles particularly when the long axis is nearly parallel to the shear direction rotation rates are never strictly zero so true stabilization does not occur however for more elongate particles axial ratio 6 rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population in experiments the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain in natural examples of porphyroclast systems the weak mantle continually develops by recrystallization and or cataclasis of the rigid clast core and a steady state between production and thinning could be attained potentially leading to true stabilization for particles with a high axial ratio

three dimensional grain fabric measurements using high resolution x ray computed tomography abstract high resolution x ray computed tomography hrxct provides detailed imagery of the interiors of rocks up to hand sample size non destructively and in three dimensions new tools described here allow these data to be used for analysis of grain fabrics two separate sets of measurement techniques have been developed the first concentrates on quantifying individual crystals phenocrysts porphyroblasts or other discrete objects or void spaces within a sample quantifiable properties include location size shape orientation and contact relationships with adjacent objects the second set of techniques performs a more general fabric analysis on any distinguishable component in a sample a fabric tensor can be computed based on a number of metrics including the star volume distribution svd star length distribution sld and mean intercept length mil methods the fabric tensors provide principal component directions and magnitudes which in turn provide a measure of degree of anisotropy and shape indices because the svd and sld measure only the material of interest whereas the mil is also influenced by spatial distribution results can be divergent for sparse phases also introduced is a three dimensional rose diagram that can be viewed interactively and inspected to reveal further details about non orthogonal directional components these techniques are demonstrated through analysis of a garnet kyanite schist from mica dam british columbia

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

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

a sensitive vorticity gauge using rotated porphyroblasts and its application to rocks adjacent to the alpine fault new zealand variable aspect ratio porphyroblasts deformed in non coaxial flow and internally containing rotated relicts of an external foliation can be used to characterise plane strain flow regimes the distribution obtained by plotting the orientation of the long axis of such grains classified by aspect ratio against the orientation of the internal foliation is potentially a sensitive gauge of both the bulk shear strain as previously suggested and kinematic vorticity number we illustrate the method using rotated biotite porphyroblasts in the alpine schist a sequence of mid crustal rocks that have been ramped to the surface along the alpine fault a major transpressional plate boundary results indicate that at distances greater than or equal to similar to1 km from the fault the rocks have undergone a combination of irrotational fattening and dextral oblique normal sense shear with a bulk shear strain of similar to0 6 and kinematic vorticity number of similar to0 2 the vorticity analysis is compatible with estimates of strongly oblate bulk strain of similar to 75 maximum shortening dextral reverse transpressional flow characterises higher strain s tectonite mylonite within similar to1 km of the alpine fault these relationships provide insight into the kinematics of flow and distribution of strain in the hangingwall of the alpine fault and place constraints on numerical mechanical models for the exhumation of these mid crustal rocks c 2001 elsevier science ltd all rights reserved

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 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

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

determining finite longitudinal strains from oppositely concave microfolds in and around porphyroblasts a new quantitative method abstract this paper describes a precise new method for determining finite longitudinal strains in porphyroblastic metamorphic rocks which makes use of oppositely concave microfolds ocms formed by heterogeneous strain of the matrix around porphyroblasts the initial spacing between two foliation surfaces is measured inside a porphyroblast and compared to the spacing between the same two surfaces in the matrix which results in a measure of extension e experienced by the rock during and or after porphyroblast nucleation a natural example is provided by the well known millipede plagioclase porphyroblasts from the robertson river metamorphics in queensland australia twenty four measurements were made from 22 serial thin sections cut parallel to both the x z and x y planes of finite strain giving an average extension of 1 72 parallel to the x direction of finite strain the least squares best fit line to a plot of initial length vs change in length gives an r 2 value of 0 998 a minimum estimate of maximum shortening negative e was also made by measuring the total lengths of s 1 folia that had been crenulated during ocm formation giving a value of 0 54 which falls short of the 0 63 expected for constant volume plane strain deformation because the ocm method is particularly suited to metapelites results may provide new insight into mechanisms of folding and crenulation cleavage development pressure temperature time deformation histories mass transport during deformation and metamorphism and kinematic studies of porphyroblast behavior rotation vs non rotation during ductile deformation

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

resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains the spring hill synform vermont determining the timing of macroscopic folds of bedding in multiply deformed terrains is difficult especially for rocks that have undergone a succession of overprinting near orthogonal deformations the spring hill synform in southeast vermont is an example of such a fold the origin and timing of this structure has been the subject of several previous studies understanding its development is crucial to unraveling orogenesis in the vermont appalachians the fold formed during a deformation path that involved a succession of overprinting near orthogonal deformations that produced matrix fabrics s3 s4 and s5 these foliations developed with subvertical subhorizontal and subvertical orientations respectively before being rotated by the effects of younger deformations the spring hill synform is generally thought to have formed as a recumbent structure during regional nappe development s4 developing as an axial planar foliation however we demonstrate that the spring hill synform developed as a fold with a steeply dipping axial plane that was overprinted by s4 and s5 although this geometry and overprinting history are consistent with a d3 time of formation we can find no change in the asymmetry of pre s3 foliations across the fold we suggest that the synform may have formed at a much earlier stage in the orogen s history and was subsequently modified and rotated to its present geometry by the long history of west to east shortening that dominated the later stages of acadian orogenesis in southeast vermont

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

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

the case for simultaneous deformation metamorphism and plutonism an example from proterozoic rocks in central arizona abstract the syntectonic 1 70 ga crazy basin monzogranite provides an example of the complex spatial and temporal interactions between metamorphism deformation and plutonism synchronous plutonism and deformation is indicated by syn shortening dikes sills and veins parallel magmatic and solid state fabrics fabrics in xenoliths and a foliation triple point synchronous plutonism and metamorphism is indicated by a systematic increase from 400 c to 630 c towards the pluton at a constant pressure of 300 mpa 3 kb temperatures are consistent with a conductive cooling model in which a 700 c pluton was emplaced into country rocks undergoing greenschist facies regional metamorphism synchronous deformation and metamorphism is indicated by porphyroblast inclusion geometries that document the synmetamorphic development of the s2 cleavage the pluton was emplaced adjacent to the shylock shear zone during progressive shortening emplacement of granite as ne trending sheets was facilitated by temporal partitioning of transpressional convergence into strike slip and dip slip components at the scale of the pluton s aureole and on the relatively rapid time scale of 103 106 y regional deformation and metamorphism were punctuated by thermal softening and increased diffusion rates data suggests that accretion of proterozoic arcs in arizona involved diachronous pluton enhanced deformation and associated high temperature low pressure regional metamorphism