Microstructure and cleavage development in selected slates

A detailed microstructural study of three slates by high voltage transmission electron microscopy is reported. The slates are mineralogically similar, come from minor fold cores and exhibit differing degrees of cleavage intensity. All three slates have domains of orientated phyllosilicates (cleavage lamellae) which contain only a low percentage of quartz and carbonate. Between these lamellae are lenticular domains which contain deformed phyllosilicates and which are enriched in secondary minerals. The initiation of cleavage lamellae can be clearly observed in electron micrographs from one of the slates studied. It occurs along zones of intense deformation, viz. along kinks and microfolds, which form from initial crenulations that are difficult to detect in a petrological microscope.The important observed re-orientation mechanisms of the phyllosilicates during cleavage development in the three slates are strain induced crystallization and the growth of metamorphically stable phyllosilicates together with mechanical rotation. Microstructural evidence suggests that the cleavage lamellae once initiated can extend laterally into the lenticular domains as deformation proceeds. Interference between adjacent phyllosilicates during deformation is commonly observed and resultant extension sites are often enriched in secondary minerals. Chlorite rich pods occur in all of the slates studied and have complex microstructures consisting of both deformed and undeformed phyllosilicates. It is concluded that these pods may form after cleavage initiation.     

The nature of grain boundaries in slates implications for mass transport processes during low temperature metamorphism

The structure of grain boundaries in some very low-grade slates has been studied with transmission electron microscopy. All phyllosilicate boundaries have structural widths of less than 1 nm. A range of structural types have been observed from apparently coherent basal layer chlorite-muscovite boundaries, semi-coherent chlorite-chlorite boundaries and incoherent boundaries which are commonly defined by a thin layer, 7–10 nm thick, of crystalline second phase. Remnants of isolated fluid inclusions are only found at quartz-quartz boundaries. The cleavage microstructures suggest that a large amount of volume loss occurred during cleavage development at low temperatures. This is most likely to have been achieved by diffusion and/or advection through a fluid-filled network present along grain boundaries or grain edges. The phyllosilicate grain boundaries in their present state could not have acted as the pathways for extensive fluid-assisted mass transport. This suggests that the grain boundary structure during cleavage formation was different from the present state. An interconnected fluid network may be maintained along grain boundaries during deformation by hydrofracturing or by grain boundary migration during dehydration reactions, but as deformation and reactions cease the grain boundaries develop an equilibrium structure with very narrow structural widths and restricted fluid distribution.     

Dissolution, solution transfer, diffusion versus fluid flow and volume loss during deformation/metamorphism

ABSTRACT Dissolution and solution transfer during deformation/metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components. Progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries. Progressive shortening is accommodated by the cores of most other minerals (including sulphides). These minerals develop strain, and hence dislocation density gradients, on their rims due to progressive shearing along grain boundaries. These gradients are particularly large when the mineral abuts phyllosilicate or graphite. The resulting chemical potential gradients between the core and rim drive dissolution, causing removal of the highly strained grain margins. Removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation. Interlayers and interfaces on boundaries lying at a low angle to the direction of shearing, and oriented relative to the sense of shear such that they can open, gape by small amounts. Water present in these interlayer spaces becomes destructured, considerably enhancing diffusion rates along the foliation. Penetrative volume loss, especially in deforming/metamorphosing pelitic rocks, is large at all metamorphic grades, increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies. Transference of material by fluid flow from deep to high levels in the earth's crust is precluded because thousands to tens of thousands of rock volumes of fluid are required, necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells, unless some extremely large-scale form of fluid channelling is possible. Reassessment of diffusion mechanisms, and hence rates, during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid, indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required. The amount of fluid can be reduced sufficiently to allow large volume losses by a one-way flow of fluid to the earth's surface, in deforming/metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure. Deformation partitioning-controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation/metamorphism because matrix minerals, other than phyllosilicates and graphite, are preferentially removed. The large size of porphyroblasts, if present, tends to preserve them from dissolution. Hence, the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew, in rocks in which bedding is still well preserved.     

The structure and the phyllosilicates (chemistry, crystallinity and texture) of Talas Ala-Tau (Tien Shan, Kyrgyz Republic): comparison with more recent subduction complexes

Geological mapping and structural analysis of the Talas Ala Tau (Tien Shan, Kyrgyz Republic) have revealed a complex structure composed of folds with axial-plane cleavage and thrust faults verging towards the NE. The main structures of the range correspond to minor Tertiary and Carboniferous–Permian deformation superimposed on the main deformation event that took place during the Baikalian orogeny. The pervasive axial-plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the Uzunakhmat and Karagoin sheets. The main thrusts developed phyllonitic shear-related rocks on the hangingwall immediately above the thrust planes. A crystal-chemical study of the phyllosilicates growth during the Baikalian deformation event along a cross-section revealed changes in the crystallinity, composition and lattice parameters of them. The phyllosilicates present in the Talas Ala Tau rocks were crystallized in very low-grade metamorphic conditions, that is below 300 °C, as indicated by their Kübler Index (KI), which decreases from SW towards the NE. Detailed TEM study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower KI parameters. There is also a clear difference in crystal-chemical parameters (KI and b) and composition between the phyllosilicates growth in relation to the axial-plane cleavage and the ones belonging to the thrust-related phyllonites. The first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites, closer to the theoretical phengitic component. Huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar. We propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates, as they are similar to those obtained in more recent accretionary complexes.     

Phyllosilicates in hydrothermally altered basalts from DSDP Hole 504B,Leg 83 — a TEM and AEM study

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 Å to a few hundred Å. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of Å thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.Contribution No. 488 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan     

Episodic metamorphic reactions during orogenesis: the control of deformation partitioning on reaction sites and reaction duration

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

低级变质沉积岩区压溶作用在褶皱变形中的意义

拉卡兰褶皱带中,发育于Ballarat-Bandigo冲断带中的低级变质砂、泥岩的宏观构造以间离劈理和人字形褶皱为特征,而且劈理在褶皱中呈扇形发育。劈理和褶皱的几何关系分析显示:劈理和褶皱的形成为压溶作用、压扁作用、弯曲作用和被动旋转共同作用的结果,而褶皱砂、泥岩中变形构造则以与压溶作用和再沉淀过程有关的显微构造为其典型特征。Fry法进行的全岩应变测量显示,褶皱砂岩的内部应变相当低(X/Z=1.40—1.83),褶皱应变格局给出变形机制的信息包括:缩短过程中的压扁作用和压溶作用、褶皱过程中由弯滑导致的层平行剪应变、以及褶皱后期发育阶段内弧区强烈的压溶作用。宏观构造、显散构造以及应变特征多方面信息证明:低级变质的沉积岩在褶皱变形过程中,压溶作用为一重要的变形机制。应变分解显示在30％—50％的总地壳水平缩短量下,弯曲导致的缩短最为14％—36％,压扁导致的缩短量为3％—14％,压溶导致的缩短量为8％—26％,而且压溶作用主要发生在褶皱内弧区。     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

Stratabound phyllosilicate zones associated with syntectonic copper orebodies at Mt Isa,Queensland

Three distinctive stratabound phyllosilicate zones are present at the margins of the syntectonic Mt Isa Cu orebodies and their host rock, the ‘silica‐dolomite’, in northwestern Queensland. The zones show close spatial relationships with adjacent stratiform Pb‐Zn ores within the overall host, the dolomitic Urquhart Shale. The Pb‐Zn orebodies may be either sedimentary‐exhalative or, as suggested recently, late diagenetic in origin, whereas the Cu ores were formed during the third regional deformation event. Talc‐stilpnomelane layers within the footwall of the silica‐dolomite, and biotite‐rich layers at the margins and adjacent to the silica‐dolomite are present in sideritic intervals within the dolomitic sequence. These Fe‐carbonate‐rich layers correlate with the Pb‐Zn orebodies, but have a much greater lateral extent. Chloritic layers occur along the hanging wall contacts of several Pb‐Zn orebodies with overlying silica‐dolomite lobes. Microstructural and petrographic studies suggest that the phyllosilicates grew at the silicification stage during early syn‐D3 alteration that also formed the silica‐dolomite and its Cu orebodies. The stratabound distribution of talc, stilpnomelane and biotite is explained by the chemical control of the sideritic beds on silicification. The origin of the Fe‐carbonates is not known and may be either sedimentary or diagenetic. The chlorite zones may have developed because of more extensive fluid‐rock interactions controlled by higher permeabilities along the brittle shale and ductile galena layering within the Pb‐Zn orebodies.     

A range of fault slip styles on progressively misoriented planes during flexural-slip folding,Cape Fold Belt,South Africa

Flexural slip folds are distinctive of mixed continuous-discontinuous deformation in the upper crust, as folding is accommodated by continuous bending of layers and localized, discontinuous slip along layer interfaces. The mechanism of localized, layer-parallel slip and the stress and fluid pressure conditions at which flexural slip occurs are therefore distinctive of shear localization during distributed deformation. In the Prince Albert Formation mudstone sequence of the Karoo Basin, the foreland basin to the Cape Fold Belt, folds are well developed and associated with incrementally developed bedding-parallel quartz veins with slickenfibers oriented perpendicular to fold hinge lines, locally cross-cutting axial planar cleavage, and showing hanging wall motion toward the fold hinge. Bedding-parallel slickenfiber-coated veins dip at angles from 18° to 83°, implying that late increments of bedding-parallel shear occurred along unfavorably oriented planes. The local presence of tensile veins, in mutually cross-cutting relationship with bedding-parallel, slickenfiber-coated veins, indicate local fluid pressures in excess of the least compressive stress.Slickenfiber vein microstructures include a range of quartz morphologies, dominantly blocky to elongate-blocky, but in places euhedral to subhedral; the veins are commonly laminated, with layers of quartz separated by bedding-parallel slip surfaces characterized by a quartz-phyllosilicate cataclasite. Crack-seal bands imply incremental slickenfiber growth, in increments from tens of micrometers to a few millimeters, in some places, whereas other vein layers lack evidence for incremental growth and likely formed in single slip events. Single slip events, however, also involved quartz growth into open space, and are inferred to have formed by stick-slip faulting. Overall, therefore, flexural slip in this location involved bedding-parallel faulting, along progressively misoriented weak planes, with a range of slip increments.     

Microtextural investigation (SEM and TEM study) of phyllosilicates in a major thrust fault zone (Monte Perdido, southern Pyrenees): impact on fault reactivation

Thrusting fault zone in foreland basins are characterized by highly foliated zones generally enriched in phyllosilicates which can play a major role on the mechanical behaviour of the fault. In this context, investigations of synkinematic clay minerals permit to determine the origin of the fluid from which they precipitated as well as the mechanisms of deformation. Our study is focused on clay mineral assemblages (illite and chlorite) in a major thrust fault located in the Monte Perdido massif (southern Pyrenees), a shallow thrust that affects upper cretaceous-paleocene platform carbonates and lower Eocene marls and turbidites. It implied 3?km of displacement of the Monte Perdido thrust unit with respect to the underlying Gavarnie unit. In this area the cleavage development by pressure-solution is linked to the Monte Perdido and Gavarnie thrust activity. The core zone of the fault, about 6?m thick, consists of an interval of intensely deformed clay-bearing rocks bounded by major shear surfaces. The deformed sediment is markedly darker than the protolith. Calcite-quartz shear veins along the shear planes are abundant. Detailed SEM and TEM observations of highly deformed fault zone samples indicate that clay mineral enrichment in the core zone of the fault is not only related to passive increase by pressure-solution mechanism but that dissolution?Crecrystallization of phyllosilicates occurs during deformation. A mineral segregation is observed in the highly deformed zone. Newly formed 2M ₁ muscovite is present along the cleavage whereas IIb chlorite crystals fill S_V2 shear veins suggesting syntectonic growth of phyllosilicates in the presence of fluids in low-grade metamorphic conditions. These mineralogical reactions act as weakening processes and would favour Monte Perdido fault creeping.     

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.     

The microstructure and internal architecture of shear bands in sand–clay sequences

The microstructures of cm-scale displacement faults offsetting unlithified sequences of finely interbedded sands, silts and clays from outcrops in Denmark have been examined. A variety of shear band types are recognised based on their grain-scale deformation mechanism and internal structure. Shear bands in a Jurassic sequence exposed along the coastline of Bornholm are characterised by intense cataclasis of both sand and clay layers. This deformation mechanism is accompanied by extensive grain scale mixing along discrete shear bands to give a fault rock composition that reflects the relative amount of sand and clay within the faulted sequence. In contrast, shear bands at Nr. Lyngby and Jensgaard, both on the Jutland coast, are characterised by granular flow within the sand units. Grain scale mixing is subdued at these locations so that layers maintain their integrity across the shear band to form a layered internal structure of sand, silt and clay smears. In some instances, particularly at Nr. Lyngby, clays have deformed in a brittle manner so that they do not contribute material to the shear band, which is then comprised exclusively of coarser-grained components. The different deformation mechanisms and internal structures of shear bands are thought to be controlled by burial depth at the time of faulting.     

Semi-brittle deformation within the Alpine fault zone, New Zealand

This paper reports the results of an attempt to identify the deformation micromechanisms in the brittle-ductile transition zone of the Alpine fault, New Zealand. Characterization of sequentially developed microstructures was carried out using optical microscopy, HVEM and STEM analysis. It was found that the transition zone corresponds to a broad zone of retrogression produced initially by microcracking and fluid infiltration. The non-catastropic nature of the fracturing indicates that fluid-enhanced sub-critical cracking is a significant crustal deformation mechanism. After the initial phase of retrogression, both ductile and brittle processes coexist and cyclically alternate in response to stress cycling. The relative importance of macroscopic brittle processes may decrease with time as more ductile minerals, especially phyllosilicates, develop during continued retrogression.     

Differentiation of diamictons (glacigenic – non-glacigenic) using microstructure abundancies and types

As mobile diamicton sediments move across already deposited sediments whether on land or into oceans generated by either glaciers/ice sheets or landslides, a series of soft-sediment deformation processes occur. These sediments carry signatures of processes at both macro- and micro scales. The processes occur across thin layers of sediment, it is at the microsedimentological scale that deformation structures are detected and is the subject of this paper. Examination of numerous diamicton thin sections of both known glacigenic and non-glacigenic sediments illustrate a myriad of microstructures. Microstructures can be subdivided into brittle, ductile, porewater induced and plasmic fabrics. These fabrics are part of a spectrum of development from grain stacks, to microshear to rotations and, in addition, domains are likely to occur due to scavenging and are all part of a repeating cycle of deformation as sediment is added, eroded, re-transported into the accumulating sediment pile. Diamictons can be subdivided into several levels of abundance of microstructure types from very common to rare to being absent. In general, other than ‘tile’ structures, all diamictons have all types of microstructures. A table is presented illustrating the differentiation of various type of diamictons in terms of microstructure type abundancies in relation to individual environments in which diamictons occur around the past margins of glaciated continents. A distinction can be successfully drawn that allows diamictons to be distinguished and differentiated.     

Phyllosilicate injection along extensional carbonate-hosted faults and implications for co-seismic slip propagation: Case studies from the central Apennines,Italy

We document phyllosilicates occurrence along five shallow (exhumed from depths < 3 km) carbonate-hosted extensional faults from the seismically-active domain of the central Apennines, Italy. The shallow portion of this domain is characterized by a sedimentary succession consisting of ∼5–6 km thick massive carbonate deposits overlain by ∼2 km thick phyllosilicate-rich deposits (marls and siliciclastic sandstones). We show that the phyllosilicates observed within the studied carbonate-hosted faults derived from the overlying phyllosilicate-rich sedimentary deposits and were involved in the faulting processes. We infer that, during fault zone evolution, the phyllosilicates downward injected into pull-aparts (i.e., dilational jogs) that were generated along staircase extensional faults. With further displacement accumulation, the clayey material was smeared and concentrated into localized layers along the carbonate-hosted fault surfaces. These layers are usually thin (a few centimeters to decimeters thick), but can reach also a few meters in thickness. We suggest that, even in tectonic settings dominated by high frictional strength rocks (e.g., carbonates), localized layers enriched in weak phyllosilicates can occur along shallow fault surfaces thus reducing the expected fault strength during earthquakes, possibly promoting co-seismic slip propagation up to the Earth's surface.     

Serpentinization of phlogopite phenocrysts from a micaceous kimberlite

Phenocrysts of phlogopite from a micaceous kimberlite contain finely interlayered serpentine. These phenocrysts occur in the kimberlite groundmass and are altered along the mica layers and are slightly deformed. Lizardite is the predominant serpentine mineral, but chrysotile and mixed structures also occur. The lizardite occurs as lamellae within phlogopite, oriented such that (001) layers of the two minerals are parallel and the [100] direction of lizardite is parallel to the [100] or 110 directions of phlogopite. The serpentinized regions of phlogopite are localized and extensive along the (001) layers. Chrysotile fibers and chrysotile-like curled serpentine occur within regions of disrupted material, and polygonal structures occur in folded lizardite lamellae. Textural relations suggest three events: 1) replacement of phlogopite by lizardite, 2) deformation of the phenocrysts, and 3) partial transformation of the lizardite to chrysotile-like structures. Deformation features include openings along (001), folds, and regions of disrupted or broken material. The folded and broken material consists of lamellar lizardite and phlogopite, indicating that lamellar replacement preceded deformation. Intergrowths of lizardite and curled serpentine are associated with cleavage openings and voids in disrupted material, suggesting that a partial transformation of lizardite to chrysotile occurred within openings created by deformation. Clay minerals also occur within phlogopite as either a minor product of serpentinization or of late-stage alteration.     

Low-Temperature Plasticity of Naturally Deformed Calcite Rocks

Optical, cathodoluminescence and transmission electron microscope (TEM) analyses were conducted on four groups of calcite fault rocks, a cataclastic limestone, cataclastic coarse-grained marbles from two fault zones, and a fractured mylonite. These fault rocks show similar microstructural characteristics and give clues to similar processes of rock deformation. They are characterized by the structural contrast between macroscopic cataclastic (brittle) and microscopic mylonitic (ductile) microstructures. Intragranular deformation microstructures (i.e. deformation twins, kink bands and microfractures) are well preserved in the deformed grains in clasts or in primary rocks. The matrix materials are of extremely fine grains with diffusive features. Dislocation microstructures for co-existing brittle deformation and crystalline plasticity were revealed using TEM. Tangled dislocations are often preserved at the cores of highly deformed clasts, while dislocation walls form in the transitions to the fine-grained     

Strain refraction in layered systems

Strain refraction across competence contrasts is presented as a simple model consisting of two components, a homogeneous strain and a heterogeneous simple shear. For Newtonian materials, the ratio of the layer-parallel simple shear component in adjacent layers is the inverse of their viscosity ratio. Strong changes in ellipsoid size, shape and orientation are predicted across viscosity contrasts.The geological implications of strain refraction theory are considered within the context of the ‘cleavage/strain debate’. The particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies (and kinematic histories) should appear to be subparallel to the XY planes of measured strain ellipsoids. Competent rocks should develop dominantly layer-orthogonal strain, and incompetent layers shear-dominated deformation. A variety of structural features ranging from cleavage refraction, changing lineation orientations, folds transected by cleavage, changes from coaxial to non-coaxial deformation, and ramp-flat fault geometry may be the result of stress and strain refraction in rocks.     

流体作用下石榴石溶解-沉淀蠕变过程——以红河-哀牢山剪切带内石榴夕线片麻岩研究为例

石榴夕线片麻岩是中、下地壳主要组成岩石之一,岩石内石榴石和夕线石的结晶学优选方位会显著影响地壳深部流变性质,因此探讨特征变质矿物的变形机制和主要受控因素对构造带深部演化过程有深远意义。本文选取红河-哀牢山韧性剪切带内石榴夕线片麻岩为研究对象,通过定向切片内显微构造、电子探针、X-ray成分扫描、电子背散射衍射（EBSD）和相平衡模拟综合研究,揭示出石榴石在溶解沉淀反应过程中具有明显的粒度敏感性,不同粒径石榴石表现出截然不同的长宽比、成分环带、包裹体排列方式和压力影发育情况。石榴石表面流体活动明显截切早期生长环带。EBSD分析揭示石榴石破碎颗粒以绕〈112〉轴机械旋转为主,溶解过程主要集中于颗粒表面和裂隙内高曲率位置。夕线石的EBSD结果表明基质内夕线石以绕〈010〉轴旋转为主,而流体作用明显区域夕线石则以（100）[001]滑移为主。岩石相平衡模拟限定岩石变质峰期P-T条件达高压麻粒岩相,退变过程中同剪切变形导致大量流体渗入而形成降温降压退变轨迹,由~9.5kbar、760℃演化至~6.0kbar、500~600℃,并在粗粒石榴石内保存早期进变质环带,剪切抬升过程中石榴石内普遍发育垂直剪切方向的裂隙,并在流体作用下进一步改造其形态。此研究揭示红河-哀牢山剪切带内除前人报道的石榴石高温韧性变形外,还存在大量中-上地壳层次同变质反应下的溶解-沉淀蠕变作用。因此,石榴石变质-变形的综合研究有助于揭示变质杂岩带挤压-剪切-伸展多阶段构造演化过程。