Micro-drilling ID-TIMS U-Pb dating of single monazites: A new method to unravel complex poly-metamorphic evolutions. Application to the UHT granulites of Andriamena (North-Central Madagascar).

In terranes subject to complex metamorphic evolution, zircon and monazite U-Pb system can record successive growth and/or resetting episodes. Conventional isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) dating of fractions or single grains from conventionally separated zircon and monazite populations may produce inaccurate ages. A new technique combining textural analysis and ID-TIMS dating on single monazite crystals is proposed. This method is applied directly on thin sections with three successive steps comprising (1) a characterisation of textural relationships using electron microprobe analyses and images, (2) extraction by micro-drilling of selected monazites and (3) U-Pb dating of each extracted grain by ID-TIMS. The potential for this in situ dating technique are tested on Ultra High Temperature (UHT) granulites from North-Central Madagascar. Four distinct events on a single thin section were distinguished. The UHT metamorphism was dated at 2.5 Ga, while two retrogression events were dated at 790 and 500 Ma. Finally, a single crystal shielded in quartz seems to preserve a pre-metamorphic age around 2.7 Ga.Editorial responsibility: J. Hoefs     

碎石颗粒形状测量与评定的初步研究

颗粒形状是影响碎石料密实特性及力学、渗流特性的因素之一。选取粒径为2～5 mm和5～10 mm的两组灰岩碎石颗粒样本作为研究对象,采用影像测量仪和特制夹具,获取不同旋转角度下的颗粒轮廓影像;使用图形处理软件获得颗粒几何尺寸测值;计算获得各旋转角度下常用颗粒形状评定参数值,运用其平均值进行统计分析,避免了依据单一角度测值评定伴随的人为因素影响。结果表明,灰岩碎石颗粒与标准圆有较大差异, 且粒径大者差异性更明显;两组样本颗粒形状参数均服从偏态分布;长宽比、扁平度和球形度能够更敏感地反映颗粒偏离球形颗粒的程度,而长宽比和球形度便于获取,因而更具优势。     

Does AMS data from micaceous quartzite provide information about shape of the strain ellipsoid?

Anisotropy of magnetic susceptibility (AMS) in micaceous quartzites with mean susceptibility (K _m) >50 × 10⁻⁶ SI units is known to be on account of the orientation distribution of the para/ferromagnetic minerals (e.g. micas, magnetite), which comprise the minor phase in the rocks. However, the strain in such deformed micaceous quartzites is dominantly accommodated by the quartz grains, which are the major phase in them. The objective of this paper is to explore the extent to which AMS data from micaceous quartzites provide information about the shape of the strain ellipsoid. AMS analysis of 3 quartzite blocks is performed, and the shape of the AMS ellipsoid is recorded to be oblate. From AMS data, the three principal planes of the AMS ellipsoid are identified in each block and thin sections are prepared along them. Quartz grain shape (aspect ratio, R _q), intensity of quartz and mica shape preferred orientation (κ_q and κ_mi, respectively) and 2D strain (E) recorded by quartz are measured in each section. R _q, κ_q, κ_mi and E are all noted to be minimum in the section parallel to the magnetic foliation plane as compared to the other two sections. This indicates that the quartz grains have oblate shapes in 3D and accommodated flattening strain, which is similar to the shape of the AMS ellipsoid. The role of mica in causing Zener drag and pinning of quartz grain boundaries is discussed. It is concluded that during progressive deformation, migration of pinned grain boundaries is inhibited. This causes enhanced recrystallization at the grain boundaries adjacent to the pinned ones, thus guiding the shape modification of quartz grains. A strong correlation is demonstrated between κ_q and κ_mi as well as κ_mi and E. It is inferred that fabric evolution of quartz was controlled by mica. Hence, the shape of the AMS ellipsoid, which is on account of mica, provides information about shape of the strain ellipsoid.     

Transferable wavelet method for grain‐size distribution from images of sediment surfaces and thin sections,and other natural granular patterns

In images of sedimentary or granular material, or simulations of binary (two‐phase) granular media, in which the individual grains are resolved, the complete size distribution of apparent grain axes is well‐approximated by the global power spectral density function derived using a Morlet wavelet. This approach overcomes many limitations of previous automated methods for estimating the grain‐size distribution from images, all of which rely on either: identification and segmentation of individual grains; calibration and/or relatively large sample sizes. The new method presented here is tested using: (i) various types of simulations of two‐phase media with a size distribution, with and without preferred orientation; (ii) 300 sample images drawn from 46 populations of sands and gravels from around the world, displaying a wide variability in origin (biogenic and mineralogical), size, surface texture and shape; (iii) petrographic thin section samples from nine populations of sedimentary rock; (iv) high‐resolution scans of marine sediment cores; and (v) non‐sedimentary natural granular patterns including sea ice and patterned ground. The grain‐size distribution obtained is equivalent to the distribution of apparent intermediate grain diameters, grid by number style. For images containing sufficient well‐resolved grains, root mean square errors are within tens of percent for percentiles across the entire grain‐size distribution. As such, this method is the first of its type which is completely transferable, unmodified, without calibration, for both consolidated and unconsolidated sediment, isotropic and anisotropic two‐phase media, and even non‐sedimentary granular patterns. The success of the wavelet approach is due, in part, to it quantifying both spectral and spatial information from the sediment image simultaneously, something which no previously developed technique is able to do.     

Shock deformation of quartz influenced by grain size and shock direction: Observations on quartz-plagioclase rocks from the basement of the Ries Crater,Germany

Planar elements in quartz, produced by shock induced plastic deformation, have been investigated in four quartz-plagioclase veins contained in an amphibolite from the crystalline basement of the Ries Crater from the drill hole Nördlingen 1973.The crystallographic orientation of planar elements in quartz grains is similar in all four rocks ({10¯13} predominant, {0001} less frequent, {10¯12} and others still rarer), indicating an average shock pressure in the range between 150 and 200 kbar.The spatial density of planar elements as measured by the number of systems per shocked grain, the number of individual elements per shocked grain, or as ratio shocked: unshocked grains increases with increasing grain size. This grain size effect is supposed to be primarily a consequence of the heterogeneity of the stress field which produced a random distribution of local stress maxima and locally restricted areas of plastic quartz deformation in the rock. The probability that planar elements develop within one individual grain increases, therefore, with increasing grain size.In one leucosome in which the quartz grains were randomly oriented planar elements parallel to {10¯13} cluster in a stereographic projection within one belt. It is supposed that the pole of this belt indicates the direction in which the shock front passed through the rock.     

Secondary Ion Mass Spectrometry Methodology for Isotopic Ratio Measurement of Micro‐Grains in Thin Sections: True Grain Size Estimation and Deconvolution of Inter‐Grain Size Gradients and Intra‐Grain Radial Gradients

In a thin section, grains that were approximately spherical in situ appear circular in cross section, and the distribution of apparent diameters frequently assumed to be their size distribution. Scanning ion imaging by secondary ion mass spectrometry (SIMS) is capable of providing precise (< 1‰) stable isotope ratio measurements of such grains, but, importantly, also registers their rate of evolution in apparent size as they are ablated by the primary beam. By assessing rates of radius change with depth, the described methodology enables the ‘true’ size of grains to be estimated, as well as the distance of the sectioned surface from the original grain centre. Transects in three dimensions are made possible, and this capability enables better identification (and thus separation) of both inter‐grain chemical signatures as a function of grain size, and intra‐grain radial trends. In this example, we highlight the specific application to pyrite (FeS₂) minerals, which are frequently analysed by SIMS to determine their inter‐grain and intra‐grain geochemical variations, particularly in their sulfur stable isotopic ratios (δ³⁴S). Benefits of the new methodology over the Faraday cup ‘spot mode’ are described. Data correction algorithms and precision considerations are discussed.     

The initiation and development of metamorphic foliation in the Otago Schist, Part 1: competitive oriented growth of white mica

The 3D shape, size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand, reveal that metamorphic foliation, as defined by mica shape‐preferred orientation (SPO), developed rapidly at sub‐greenschist facies conditions early in the deformation history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition, higher aspect ratios and a strong preferred orientation. The metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation. Once initiated, foliation progressively intensified by an increase in the aspect ratio, size and alignment of grains, although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in SPO strength despite continued increase in grain size. These trends are interpreted in terms of progressive competitive anisotropic growth of blade‐shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher grade samples. Reversal of SPO intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field.     

Laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current

Important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel-sized spheres (ball-bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement. The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand-sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand-size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel. The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross-section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity. Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.     

Glauconitization of detrital silica substrates in the Barton Formation (upper Eocene) of the Hampshire Basin, southern England

Upper Eocene detrital silica grains (chert and quartz) of the Hampshire Basin display alteration and replacement fabrics by glauconite. Silica grains have etched surfaces due to glauconitization which appear green in reflected light and thin section. Quartz grains were glauconitized by surface nucleation and replacement, which spreads from the margin with progressive glauconitization, replacing the quartz grain interior. Chert grains were glauconitized by surface replacement and nucleation internally along cracks and in pores. Different forms of glauconite are associated with the two minerals; glauconite associated with quartz is generally highly-evolved whereas glauconite associated with chert is of the evolved variety. This is interpreted as being due to different surface-reaction control mechanisms associated with the two forms of silica. There is no evidence to suggest that glauconite evolved in stages from a nascent form. Two crystalline morphological forms of glauconite are found associated with both quartz and chert. Glauconite growing within a confined space has a laminated morphology whilst glauconite occurring on the surface has a rosette morphology.     

A mechanism of shape-transformation of quartz inclusions in albite of regional metamorphic rocks

The process of shape-transformation of quartz inclusions from polyhedral to spherical grains in albite single crystals during metamorphism is mainly controlled by the grain boundary diffusion of oxygen along the quartz/albite interface to reduce the interfacial free energy. The rate of the process, which is represented by the growth rate of the curvature of the edge surface of the grain, depends significantly on temperature and on the grain size of the quartz inclusion. The relations between temperature, T, the time, t_r, and the critical radius, R_c, which is equal to the radius of maximum spherical grains, are given by log R_c = −0.11E_b/RT + 0.25log t_r + C, in which E_b is the activation energy of the grain boundary diffusion of oxygen along the quartz/albite interface and C is a material constant.

The mean critical radius of spherical quartz inclusions in albite is 5 μm for the upper chlorite zone and garnet zone, 10 μm for the lower biotite zone, and 20 μm for the upper biotite zone in the Sambagawa metamorphic terrain. The mean values of the critical radii of spherical quartz inclusions in oligoclase of the Ryoke metamorphic rocks is about 5 μm for the chlorite zone and about 10–20 μm for the sillimanite zone.

Assuming temperatures of about 350°C for the upper chlorite and garnet zones, 400°C for the lower biotite zone, 550°C for the upper biotite zone, and 700°C for the sillimanite zone, the activation energy for the grain boundary diffusion of oxygen along the quartz/plagioclase interfase is estimated to be about 30 kcal/mol.     

基于正交偏光序列消光特征的岩石薄片颗粒分割与孔隙提取

在岩石薄片正交偏光显微镜下角度域序列图像采集的基础上,分析了不同岩石组分在消光角度域上的光学特征及其差异性,并据此提出一种新的岩石颗粒分割和孔隙分析的方法。充分利用岩石颗粒赋存状态及其晶体光轴产状的复杂性、岩石颗粒空间排列及其接触关系的多样性,对岩石薄片在消光角维度上进行像素尺度的相关分析,并提出相关系数均值、相关系数标准差以及相关系数均差比等敏感参数,实现了岩石颗粒的分割和孔隙的提取。研究表明岩石颗粒内部的像素点灰度和RGB值在角度域上的相关性较强,在颗粒边缘及孔隙内部填隙物分布区域,其相关系数较低,且相关系数标准差要明显高于岩石颗粒内部。该方法从消光特征出发论证了角度域信息完整性的意义,提取的颗粒边缘较为清晰,孔隙结构骨架得以刻画,颗粒分割的效果好于Sobel和Canny等方法。     

Gold paragenesis and chemistry at Batu Hijau, Indoneisa: implications for gold-rich porphyry copper deposits

Gold is an important by-product in many porphyry-type deposits but the distribution and chemistry of gold in such systems remains poorly understood. Here we report the results of petrographic, electron microprobe, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and flotation test studies of gold and associated copper sulfides within a paragenetic framework from the world-class Batu Hijau (914 mt @ 0.53% Cu, 0.40 g/t Au) porphyry copper–gold deposit, Indonesia. Unlike many other porphyry copper–gold deposits, early copper minerals (bornite–digenite–chalcocite) are well preserved at Batu Hijau and the chalcopyrite–pyrite overprint is less developed. Hence, it provides an excellent opportunity to study the entire gold paragenesis of the porphyry system. In 105 polished thin sections, 699 native gold grains were identified. Almost all of the native gold grains occurred either within quartz veins, attached to sulfide, or as free gold along quartz or silicate grain boundaries. The native gold grains are dominantly round in shape and mostly 1–12 m in size. The majority of gold was deposited during the formation of early A veins and is dominantly associated with bornite rather than chalcopyrite. The petrographic and LA-ICP-MS study results indicate that in bornite-rich ores gold mostly occurs within copper sulfide grains as invisible gold (i.e., within the sulfide structure) or as native gold grains. In chalcopyrite-rich ores gold mostly occurs as native gold grains with lesser invisible gold. Petrographic observations also indicate a higher proportion of free gold (native gold not attached to any sulfide) in chalcopyrite-rich ores compared to bornite rich ores. The pattern of free gold distribution appears to correlate with the flotation test data, where the average gold recovery value from chalcopyrite-rich ores is consistently lower than bornite-rich ores. Our data suggest that porphyry copper-gold deposits with chalcopyrite-rich ores are more likely to have a higher proportion of free gold and may require different ore processing strategies.Editorial handling: R. P. Richards     

Estimation of soil internal erosion potential

Probability that the grains of cohesionless soil in an investigated range of grain diameters can be eroded from a volume of soil by the groundwater flow force is estimated using a two-person zero-sum game theory. The estimation of probability of erosion of soil grains is based on the two dimensional model of a soil structure, which is derived from the soil grain-size distribution and measured soil porosity, as well as on the assumption that soil grains are circular in their cross sections. A comparison of the results obtained by proposed method and by experiments shows good agreement.     

Quartz <Emphasis Type="Italic">c</Emphasis>-axis texture mapping of mylonitic metapelite with rod structures (Calabria,southern Italy): Clues for hidden shear flow direction

This quantitative microstructural study deals with textures of quartz domains within a mylonitized metapelite collected near a thrust surface corresponding to the tectonic contact between two metamorphic units, which crop out in the Aspromonte Massif, southern Calabria (Italy). The sample investigated lacks a mesoscopic stretching lineation. Therefore, quartz c-axis fabrics were investigated in two mutually orthogonal thin sections (a) parallel to the quartz rod lineation and perpendicular to the foliation (YZ plane) and (b) perpendicular to the quartz rods and perpendicular to the foliation (XZ plane); the data were generated using classical (manual measurements of quartz c-axis using U-stage) and modern methods (Computer Integrated Polarization microscopy). Both these sections show oblique foliations at ca. 40° from the main shear plane, implying that the actual X direction (stretching lineation that is absent on the mesoscopic scale) must lie between these two sections. Quartz c-axis data from the YZ section when rotated by 90° are similar with those from the XZ section. Hence, the data from the two sections are merged. These data when rotated by an angle of 50° from the direction of quartz rod lineation, gives an asymmetrical pattern indicating top-to-the-North sense of shear. This was confirmed by investigating quartz c-axis patterns in a section striking NS and perpendicular to the foliation. Based on the study it is thus concluded that this method can be used to do kinematic analysis in rocks that are devoid of stretching lineations. Apart from the above, the advantages and disadvantages of the classical and modern methods of quartz c-axis analysis are discussed.     

Domainal petrofabric analysis of micaceous quartzite using EBSD data: Role of muscovite in LPO evolution of quartz

Lattice Preferred Orientation (LPO) of quartz and muscovite are measured in a micaceous quartzite using SEM based Electron Backscatter Diffraction (EBSD) analysis. The measurements were done in a thin section prepared parallel to the K₁K₃ plane of the anisotropy of magnetic susceptibility (AMS) ellipsoid, which is equivalent to the XZ plane of the strain ellipsoid. Bulk data from the sample reveal that quartz c-axis develop an oblique single girdle LPO pattern indicating a dextral sense of shear. A similar pattern is produced by poles to the basal planes (001) of muscovite, which implies that muscovite controls the LPO of quartz in the rock. Petrofabric analysis of quartz is performed in domains viz. (I) not containing muscovite, (II) between two muscovite grains, and (III) rim of quartz grains around a muscovite grain. Quartz LPO pattern in Domain-III is noted to be similar to the bulk LPO recorded from the whole sample, thus confirming that muscovite was critical in textural evolution of the rock. The study thus establishes that a minor mineral phase in a rock can control the LPO of the major mineral phase, thus playing a significant role in development of bulk texture of the rock. The advantages of domainal petrofabric analysis in evaluation of slip systems in minerals in different parts of a rock, and vorticity quantification are discussed.     

Till kinematics in the Stargard drumlin field,NW Poland constrained by microstructural proxies

Deciphering the relationships between streamlined subglacial landforms and deposits therein helps to constrain the dynamics of past ice sheets. Here we present microstructural analyses of till from the largest drumlin field in the North European Lowland generated by a major palaeo-ice stream of the Scandinavian Ice Sheet. Data derived from thin sections and X-ray microtomography (μCT) reveal multiple microstructures including microshears, circular structures, grain stacks and crushed grains. Long axes of micro-clasts measured in thin sections are directionally clustered and have low, down-ice-oriented dip angles. Some samples reveal subordinate clustering oriented at high angles to the main direction. The μCT data show low clustering strengths of clasts and no systematic fabric pattern in the entire set of samples. There is no correspondence between the clast orientations and the directions of drumlin long axes, suggesting that the till is antecedent in relation to the drumlin-shaping process. The observations indicate spatially and temporally variable, shallow-seated brittle and ductile deformation controlled by porewater pressure fluctuations and generated time-transgressively parallel to till accretion. Consistently with previous studies, the data suggest that the Stargard drumlins are erosional remnants of a pre-existing terrain shaped by material removal from the inter-drumlin areas.     

川西北杨坝剖面埃迪卡拉系灯影组核形石组构特征

埃迪卡拉系灯影组核形石研究对揭示该期古环境特征及演化具有重要意义。在杨坝剖面埃迪卡拉系灯影组2段核形石发育段的宏观及微观描述（564块薄片）的基础上,分析了核形石组构、类型及垂向分布。该剖面核形石发育段共分为上、下两个亚段,厚度分别为109.53 m、126.2 m,核形石累积厚度分别为22.6 m、49.5 m。核形石核心及壳层的成分、组构、形态多样;核形石类型包括不规则状核形石、椭圆状核形石、次圆状—圆状核形石、帽状核形石等4种,每种类型的粒径、形态、层位分布、形成环境不同;粒度在垂向上表现为10个反旋回。整体上核形石发育不够完善,以薄皮核形石、弥散粒为主,代表着核形石发育的最初阶段。不规则状、次圆状、帽状核形石形成于弱搅动的浅水低能环境,包括潮下低能带、潮间带等;椭球状、圆状核形石形成于连续搅动的潮下高能带。核形石粒度差异大、形态多样、垂向多旋回变化的特征反映了埃迪卡拉纪灯影期海水受限、水体相对深浅及能量频繁动荡,同时受一定物源影响的特征。     

Fractal Modelling of the Microstructure Property of Quartz Mylonite During Deformation Process

Mylonite is the result of the dynamic metamorphism and minerals in mylonite are deformed gradually with an increase in the degree of metamorphism. Quantifying the degree of deformation including the irregularities of shapes and the frequency distribution of the minerals becomes one of the most challenging efforts in mylonite analysis. Fractal modelling has been demonstrated in this paper to be an effective mean to achieve the above goal. Perimeter-Area fractal model was used to quantify the irregularities in the geometries and Cumulative Number-Area model is used to characterize the irregularities of distribution of quartzs in mylonites, respectively. Examples of quartz from five types of mylonites with different degree of deformation within the foreland of the Moine Thrust Zone in NW Scotland are chosen to study the evolution processes of deformation. As the main mineral component of quartzite mylonite, patterns are extracted from digital photomicrographics of the multiscale-grey image grid data to show quartz grains with different degree of deformation, The areas and perimeters of the quartz grains were calculated by GIS-based image processing technologies. From type one to type five, with an increase in degree of deformation, the corresponding Perimeter-Area exponent increases from 1.20, 1.28, 1.38, 1.46, to 1.60, respectively, the fractal dimension of the perimeter from 1.07, 1.08, 1.17, 1.23, to 1.44, as well as the exponent of Cumulative Number- Area from 0.50, 0.51, 0.58, 0.82, to 0.85, respectively. The result has shown that as increase of the intensity of deformation, the shape of quartz grains tends to be more irregular, grain size tends to be smaller, and the number of grains increases. The results obtained using GSI model has indicated that as an increase in the intensity of deformation, the patterns of quartz grains tends to be more stratified and randomness increases.     

The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain-shape data

Shape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.