an oligocene palynoflora from the middle shoalhaven catchment n s w and the tertiary evolution of flora and climate in the southeast australian highlands abstract well preserved spores and pollen from a number of middle oligocene sedimentary formations in the middle shoalhaven catchment n s w are correlated with the upper nothofagidites asperus lower proteacidites tuberculatus zones of the gippsland basin victoria further confirmation of this correlation comes from three sources a the relative abundances of pollen component groups reveal the nothofagidites proteacidites and myrtaceidites groups within the shoalhaven assemblage to be typical of late eocene to middle oligocene sequences b comparisons with the ratios of phyllocladidites mawsonii to gymnosperms araucariacites to gymnosperms n flemingii to other nothofagidites and myrtaceae to nothofagidites in the murray basin indicate at least an upper n asperus age and c strong geomorphic and stratigraphic evidence reported elsewhere shows that the spore and pollen bearing sediments were deposited in a basalt dammed lake between 29 32 ma the mid oligocene shoalhaven assemblage fills a significant gap in the spore pollen record of the southeast australian highlands and as a consequence there is now a continuous record of development of flora and climate for this region for nearly the entire tertiary average annual temperature and thermal seasonality appear to have been the two most significant climatic parameters to have varied throughout the southeastern highlands from the palaeocene to the miocene after which progressively drier conditions ensued

population wide garnet growth zoning revealed by la icp ms mapping implications for trace element equilibration and syn kinematic deformation during crystallisation the strong partitioning of many trace elements into garnet and their slow diffusivities in both garnet and the rock matrix means that their distribution may record valuable petrogenetic information not documented by major elements in metamorphic rocks complex trace element growth zoning in garnet porphyroblasts from a garnet grade metapelite from the barrovian sequence of the sikkim himalaya is assessed using quantified la icp ms raster mapping coupled with x ray micro computed tomography the data document systematic changes in the zoning patterns from early to late nucleated crystals and also suggest that the ree y chemistry incorporated into garnet is dependent on persistent disequilibrium in the rock volume there is evidence for hree y diffusion haloes surrounding growing garnets and a heterogeneous hree y distribution in the rock matrix annuli superimposed on oscillatory zoning are not consistent with formation during some rock wide event but are dependent on the spatial disposition of the garnet annuli are interpreted to reflect an integrated history of varying growth rates and the incorporation of pre existing heterogeneities due to relatively slow matrix diffusivities conversely smooth zoning of many transition metals indicate that their distribution in garnet may be controlled by equilibrium partitioning between garnet and the rock matrix significant rotation of garnet porphyroblasts during growth is revealed due to immobility of cr over the duration of the crystallisation interval and overprinting of the heterogenous precursor cr distribution strain rate estimates derived from this zoning are on the order of 10 11 10 12 hbox s 1

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

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

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

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

the ancenis terrane an exotic duplex in the hercynian belt of armorica western france abstract the ancenis terrane bordering the loire france consists of precambrian brioverian basement and three paleozoic sedimentary sequences marine ordovician to devonian shelf sediments lower carboniferous synorogenic intermontane sediments the culm and coal bearing namurian deposits the terrane is completely bound by faults and its geology is in marked contrast with that of its neighbours new data and observations along the terrane s southern boundary show that the ordovician devonian sequence occupies a transpressional shear zone and can be described as a tectonic melange the culm outcrop previously interpreted as a sinistral pull apart structure is reinterpreted as a lower carboniferous dextral pull apart and it is shown that carboniferous transpression and thrusting remodelled the pull apart basin into its present mica fish shape with its remarkable concentric structure and within a plate tectonic setting of mega dextral displacements along the northern boundary of gondwana the ancenis terrane can be interpreted as an exotic duplex

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

garnet rotation and the development of axial plane crenulation cleavage rotated helicitic garnets syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold the garnet rotation is inconsistent with simple minded shear models for the development of the crenulation cleavage and it is useful to consider this problem in order to understand the development of crenulation cleavage better various models are considered in an attempt to explain both garnet rotation and cleavage morphology earlier models were based on the assumption that garnet rotation is driven by shear parallel to s2 they were mostly unsatisfactory but one involving migration of the crenulation hinges is viable when combined with other models described here the new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface s1 the new foliation s2 is considered to be a passive structure at least in the early stages of its development these models satisfactorily explain garnet rotation and development of the cleavage including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets comparison of observed and predicted helicitic structures strongly supports the proposed models evidence is presented to show that where a garnet has an se that is a schistosity and an si that is a crenulation cleavage the schistosity can have developed by unfolding of the crenulations

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

porphyroblast 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

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

a laser microprobe mass spectrometric study of ar kr k cl and br in an unconformity garnet associated fluid inclusions staurolite and micas from vermont u s a abstract garnet prophyroblasts in paragonite schist of the pinney hollow fm on the west limb of the athens mantled gneiss dome in vermont exhibit apparent angular unconformities cores of garnets contain inclusions of chloritoid rutile and ilmenite that define a foliation which is truncated at a high angle by the foliation in the rims electron microprobe traverses across a garnet indicate that the core rim boundary is the beginning of a steep compositional gradient in ca mg mn and fe that characterizes the rim following electron microprobe study a sample was irradiated in a nuclear pile and then studied by laser microprobe noble gas mass spectrometry ar isotopes were measured in garnet and in adjacent micas and staurolite and ar plus kr isotopes were measured in fluid inclusions fi in quartz in garnet and in fi trapped 40 ar 36 ar varies between 300 and 2000 with no simple correlations indicated between trapped ar isotope ratios and k cl and 84kr or petrographic features similar trapped ar occurs in the core and rim of garnet and in fi in quartz most fi indicate 40 ar e cl 10 3 and cl 36 ar 6 atom ratios br abundances in fi are usually too low to be measured indicating br cl 3 atom ratio 84 kr 36 ar in fi varies between 0 02 and 0 08 gases extracted from laser craters in white micas in schist outside the garnet indicate mean 40 ar 39 ar ages of 362 5 ma and in biotites 332 7 ma with k bearing inclusions in the garnet rim yielding ages close to 360 ma in the garnet core interpretation of 40 ar 39 ar ages is ambiguous due to the combination of low concentrations of in situ radiogenic 40ar and comparatively large amounts of trapped 40are poorly correlated with other parameters measured

development of a chevron cleavage pattern and porphyroblast rotation in graded metaturbidites slave structural province northwest territories canada abstract chevron cleavage pattern displays opposite cleavage vergence across a psammite pelite contact or within a single graded couplet in this example graded metaturbidites preserve an s2 cleavage with dextral vergence in their competent psammitic bases and an s3 crenulation cleavage with sinistral vergence in their less competent semi pelitic tops bedding parallel shear partitioned into tops of graded couplets resulted in buckle folding of s2 cleavage s3 is also a compositionally differentiated cleavage displaying alternating quartz rich microlithons q and muscovite rich cleavage domains m together q and m domains define s microfolds related to d3 biotite porphyroblasts developed during static metamorphism before d 2 s 2 contain quartz inclusion trails sii with three orientation patterns that are related to their location within graded beds porphyroblasts in psammitic bases of graded couplets are not affected by d 3 s 3 in comparison sii in porphyroblasts in semi pelitic tops of beds are on average about 5 clockwise in m domains and about 90 counter clockwise in q domains corresponding to flat and steep limbs of s shaped microfolds in s2 from these observations it is apparent that biotite porphyroblasts within q domains rotated about 90 counter clockwise during microfolding within m domains the sii data suggest that porphyroblasts rotated little relative to their orientation in the base of the beds

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

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

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

application of electron spin resonance esr dating to ductile shearing examples from the qinling orogenic belt china abstract shear zones are common structures in orogenic belts and elucidation of the tectonic evolution of these orogenic belts to a large degree depends on understanding the kinematics and timing of shear deformation however there is a lack of an accurate fast and convenient way to determine the timing of deformation in this paper we apply the esr electron spin resonance spectroscopy dating method to three syntectonic quartz veins from the funiushan tectonic belt in the qinling orogenic belt in central china the results agree well with the available ages of deformation in the area obtained through other dating methods this demonstrates the accuracy and feasibility of using the esr method to date quartz crystals formed during deformation the method is fast and convenient and satisfies the accuracy requirement it is an effective means for determining the timing of deformation especially in areas with intensive fluid activity during deformation

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

behaviour of spherical rigid objects and passive markers during bulk inhomogeneous shortening of a fluid abstract quantitative analysis of the kinematics of an experimental gravity driven inhomogeneous time dependent flow showed that the kinematics of the flow varied both temporally and spatially the variation in kinematics of the flow included vorticity rotation sense reversals and the results provide possible insight into the kinematics of gravity spreading flows the variation of the kinematics reflected the influence of the boundary conditions upon the flow spherical inclusions and thin columnar passive markers placed within the fluid prior to deformation also allowed the fluid flow to be used to examine some of the possible relationships that may form between porphyroblasts and passive foliations in inhomogeneous time dependent flows comparison of initial and final orientations of several spherical inclusions suggested an apparent lack of rotation during deformation conversely comparison of the initial and final orientation of passive strain markers placed in close proximity to these inclusions suggested a non coaxial deformation environment rather than a coaxial environment the apparent lack of rotation of these inclusions resulted from a reversal in their rotation sense during deformation of the system the reversal in rotation sense of these inclusions was the result of time varying levels of spin and shear induced vorticity during deformation fold like structures were produced in the passive markers this allowed the flow to be used to examine possible relationships that may form between porphyroblasts and layers passively folded during inhomogeneous time dependent flow the inclusion passive marker relationships across the folds suggest that relationships between porphyroblasts with inclusion trails and foliations in rock layers passively folded by an inhomogeneous time dependent flow may contain similar relationships to folds that formed by layer buckling knowledge of the level of passive behaviour of the layers and boundary conditions during this type of deformation is essential for utilizing porphyroblast foliation geometry to evaluate deformation history

evolution of quartz microstructure and c axis crystallographic preferred orientation within ductilely deformed granitoids arolla unit western alps abstract we have studied quartz microstructures and the c axis crystallographic preferred orientations cpos in four granitoid samples representative of increasing ductile shear deformation from a weakly deformed granitoid stage 1 to a mylonitic granitoid stage 4 the quartz c axis cpo measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area all the samples belong to the austroalpine arolla unit western alps and were deformed at greenschist facies conditions the quartz c axis cpo was analyzed using a u stage and the optical orientation imaging technique the magmatic plagioclase forming more than 50 of the volume of the granitoid is extensively replaced by a mica rich aggregate even in weakly deformed samples of stage 1 these aggregates flow to form an interconnected weak matrix with increasing deformation wrapping relatively less strained quartz grains that undergo dominantly coaxial strain recrystallization of quartz ranges from less than 1 in the weakly deformed granitoid to up to 85 in the mylonitic granitoid with average grain strain of 41 and 64 respectively with increasing strain and recrystallization quartz grains in the granitoids show a sequence of transient microstructures and cpos crystal plastic deformation is initially accomplished by dislocation glide with limited recovery and at 50 grain strain it results in a cpo consistent with dominantly basal a slip at 60 grain strain recrystallization is preferentially localized along shear bands which appear to develop along former intragranular cracks and the recrystallized grains develop a strong c axis cpo with maxima orthogonal to the shear band boundaries and independent of the host grain orientation within the granitoid mylonite at an average quartz grain strain of 64 recrystallization is extensive and the c axis cpo of new grains displays maxima overlapping the host c axis orientation and therefore unrelated to the bulk sense of shear the host controlled cpo is inferred to reflect pervasive recrystallization by progressive subgrain rotation the switch from shear band control to host control on c axis cpo occurred between 40 and 70 of recrystallization in the quartz mylonite the quartz c axis cpo develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions this study indicates that the cpo evolution of quartz may significantly differ in cases of polymineralic vs monomineralic rocks under the same deformation conditions if quartz in the polymineralic rock behaves as a strong phase

sequential growth of cordierite and andalusite porphyroblasts cooma complex australia microstructural evidence of a prograde reaction low pressure prograde metamorphism of pelitic rocks in the cooma complex south east australia has produced cordierite andalusite schists at intermediate grades the first foliation s1 is preserved largely as inclusion trails in cordierite porphyroblasts microstructural evidence indicates that the cordierite porphyroblasts grew during the early stages of development of a crenulation foliation s2 and that andalusite porphyroblasts grew during the development of a later crenulation foliation s3 microstructural evidence also indicates that the andalusite was a product of the prograde reaction cordierite muscovite andalusite biotite quartz the occurrence of the products of this reaction in beard structures between cordierite microboudins formed by extension in s3 confirms that the andalusite grew during the development of s3 the investigation shows that porphyroblast matrix relationships can preserve the orientation of an early s surface that has been largely obliterated from the matrix as well as providing relatively direct evidence of sequential mineral growth and metamorphic reactions

role of mixing corrosion in calcite aggressive h2o co2 caco3 solutions in the early evolution of karst aquifers in limestone two cave forming mechanisms in limestone are discussed currently first when two h2o co2 caco3 solutions saturated with respect to calcite but with different chemical compositions mix renewed aggressiveness to limestone dissolution occurs this process called mixing corrosion bogli 1964 1980 in combination with linear dissolution kinetics has been suggested as cave forming second it has been shown that solely the action of nonlinear dissolution kinetics can generate extended karst conduits this paper combines both mechanisms by digital modeling of the evolution of the aperture widths of a confluence of two fractures into a third one it is shown that the first mechanism does not create large cave conduits the combination of mixing corrosion and nonlinear kinetics however considerably intensifies karstification compared to that of nonlinear kinetics solely the times to terminate early evolution of karst are significantly reduced when the co2 concentrations of the inflowing solutions differ by no more than 30 we discuss the underlying mechanisms by inspection of the time dependence of the evolution of aperture widths flow rates through them and of the renewed undersaturation of the mixed solution at the confluence of two fractures finally the evolution of a karst aquifer on a two dimensional percolation network is modeled when mixing corrosion is present and compared to that on an identical net with identical nonlinear dissolution kinetics but mixing corrosion excluded large differences in the morphology of the net of cave conduits are found and also a reduction of the time of their evolution from these findings we conclude that climatic changes which influence the pco2 in the soil can divert the evolving cave patterns

microstructural evidence of rotation and non rotation of mica porphyroblasts abstract mica porphyroblasts in schists from several regions show nearly planar inclusion trails that are parallel over areas much larger than the wavelengths of later folds this indicates that the porphyroblasts have not rotated with respect to geographical co ordinates during deformation instead the matrix has rotated as suggested by ramsay 1962 even in zones of marked shortening in the matrix adjacent to large rigid porphyroblasts e g of cordierite or staurolite small biotite porphyroblasts have not rotated but have become thinned by solution as indicated by parallelism of inclusion trails in separate biotite grains and by evidence of truncation of inclusion trails by the matrix foliation less common are biotite porphyroblasts that have single asymmetrical microfolds in the matrix adjacent to the porphyroblasts and so appear to have rotated these porphyroblasts are characterized by kinking

late mesozoic compressional to extensional tectonics in the yiwulushan massif ne china and their bearing on the yinshan yanshan orogenic belt part ii anisotropy of magnetic susceptibility and gravity modeling abstract granitoids play an important role in deciphering both crustal growth and tectonic evolution of earth in the eastern end of the yinshan yanshan belt of north china craton the yiwulushan massif is a typical region that presents the tectonic evolution features of this belt our field work on the host rocks has demonstrated two phases of opposite tectonics compressional and extensional however the deformation is almost invisible in the intrusive rocks to improve the understanding of the tectonic evolution of the yiwulushan massif and the late mesozoic tectonics of east asia a multidisciplinary study has been carried out in this study anisotropy of magnetic susceptibility ams and gravity modeling have been applied on these jurassic plutons lushan jishilazi and guanyindong which intrude into the yiwulushan massif according to laboratory measurements and microscopic observations on thin sections the ams of the yiwulushan massif is characterized by secondary fabrics indicating that the intensive post solidus deformation has reset the primary magmatic magnetic fabrics a relatively gentle nw dipping magnetic foliation has been identified with two distinct groups of magnetic lineations of n34 e and n335 e orientations namely lm1 and lm2 relatively gravity modeling reveals a southward thinning of the massif with a possible feeding zone rooted in the northern part of the massif integrating all results from structural observation geochronological investigation ams measurement and gravity modeling two tectonic phases have been identified in the yiwulushan massif posterior to the jurassic 180 160 ma magmatism in the yinshan yanshan area the early one concerns a late jurassic early cretaceous 141 ma compressional event with a top to the south to southwest sense of shear the second one shows an early cretaceous 126 ma nw se ductile extensional shearing at that time sedimentary basins widened and jurassic plutons started to be deformed under post solidus conditions in fact the nw se trend of the maximum stretching direction is a general feature of east asian continent during late mesozoic

a post deformational post peak metamorphic timing for mineralization at the archaean big bell gold deposit western australia abstract previous workers were unable to find any suitable criteria to time gold mineralization at big bell mine an enigmatic low grade large tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the murchison province of the archaean yilgarn craton western australia earlier studies suggested that the gold mineralization occurred by hydrothermal alteration either synchronous with volcanism or prior to peak metamorphism the following conclusions have been drawn from a detailed meso and microstructural analysis of the mine sequence and greenstone belt hosting the orebody 1 1 prior to mineralizaton the big bell area underwent five phases of deformation and m1 regional metamorphism to the staurolite andalusite zone of the amphibolite facies for metasediments 2 2 granite emplacement post dated d5 folding causing local m2 thermal metamorphism this peaked at the ms qtz breakdown reaction for pelites on or close to the and sil transition but below the onset of partial melting characterized by the narrow non degenerate divariant field involving seven kfmash phases qtz ms bt crd kfs als h2o this hornfelsing consumed cross cut and post dated s5 cleavage prograde qtz and sil veins replaced this hornfels assemblage and s5 cleavage and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post deformational hornfelsing 3 3 retrograde m3 assemblages and two stages of gold related mineralization cross cut and replaced all previous structures the retrograde gold bearing assemblages are both underformed stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations py po ccp ilm mag au tur sch rt spn zrn ms bt ep czo qtz the second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre existing mineralogies including quartz are totally destroyed within sericitic domains the stage 2 mineral associations are apy sbt sb mo sp au py po ilm mag ccp sericite chl carbonate cal ep czo qtz 4 4 gold mineralization at big bell was post deformational and post peak metamorphic unlike many archaean gold deposits there are no obvious shear zones controlling the formation of the big bell gold deposit at the structural level exposed either on the surface or in drill core big bell mine was in a low strain domain during the formation of late stage sinistral sub vertical d6 shear zones that are typically concentrated at the granitoid greenstone contacts the retrograde character of the mineralization has been deduced from detailed meso and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the murchison province

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

a deformed polymictic conglomerate the influence of grain size and composition on the mechanism and rate of deformation abstract in a deformed polymictic conglomerate from the hill end area new south wales australia almost all the strain is confined to fine grained quartz rich rock fragments coarse quartz and feldspar clasts generally show little evidence of intracrystalline plasticity the most strongly elongated rock fragments are finer grained and richer in layer silicates relative to quartz and feldspar indicating that both grain size and composition have influenced finite strain and thus strain rate the relationship between grain size and strain rate is broadly consistent with a dominance of grain boundary deformation mechanisms over intracrystalline dislocation mechanisms at finer grain sizes however strong quartz c axis preferred orientations have been measured in the most highly strained clasts implying a significant component of dislocation glide close examination of the microfabric mineralogy and petrology of the fine grained clasts shows that other factors are likely to have influenced their mechanical behaviour and led to enhancement of strain rate these factors include the trace element chemistry of the phases especially quartz the role of a mobile fluid phase the presence of poorly bonded mica 001 interfaces and the evolutionary nature of microfabric and mineralogy all of which are relevant to the bulk of crustal metamorphic rocks and should be taken into account in developing mechanical models of metamorphic belts

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

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