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Foreword Note From Coordinator Acknowledgement Textures in Igneous Rocks Textures in Pyroclastic Rocks Textures in Sedimentary Rocks Textures in Metamorphic Rocks Deformation related Microstructures Vugs,Veins and Pseudotachylite Textures of Shock Metamorphism Textures in Chondritic meteorites Fluid Inclusion microstructures Gemstones

Deformation related Microstructures

A study of deformation related microstructures is essential for proper evaluation of a terrain. In this section, short descriptions of different microstructural features have been produced as per present status of knowledge in this subject as worked out by many workers over time. In this section, some Indian examples of these microstructures have been presented. The material has been contributed by several workers both from within and outside GSI.

Brittle Deformation

Brittle deformation tends to be very localized. Deformation at low temperatures results in fracturing and loss of cohesion with or without frictional sliding along the fracture surfaces and relative rotation of the angular blocks. Such deformation is known as brittle deformation. Brittle deformation mechanism are cataclasis (grain fracture), rigid grain rotation and grain translation through frictional grain boundary sliding (grain reorganization). Grain scale faults and formation of angular fragments are common responses to stress at this stage. Although subgrains sometimes form in the brittle regime, TEM studies prove that such subgrains are not due to dislocation tangles but are caused through very small scale brittle failures (Passchier and Trouw, 1998). Pressure solution is an important deformation mechanism at this stage

Breccia

Breccia is a term used to denote rocks that are composed of large angular poorly sorted rock fragments (over two millimeters in diameter) bound by finer grained matrix or cement. If the rock fragments are all of same type, it is called a monomictic breccias. If the fragments of different rocks are involved, the breccias is polymictic. Breccia can be of several types depending on their mode of origin:

Sedimentary Breccia: The breccias that are formed dominantly by sedimentary processes are classified as sedimentary breccias. These rocks may include cemented scree, sediments that have been transported only little distance from alluvial fans and then redeposited and lithified, collapse breccia and intraformational collapse breccias. The collapse-breccias may form as a result of karst processes in evaporitic successions (Friedman, 1997). Chert breccia is a well known form of sedimentary breccia. The brecciation in chert generally occurs as a result of collapsing or crushing of chert nodules (or layers) after the dissolution of underlying soluble bedrock or due to tectonic disturbances (Middleton, 1961; Chatellier, 1988; Kolodny et al., 2005). The clasts in chert breccia are penecontemporaneous with the matrix very much like any other intraformational conglomerate or breccia. Lthified debris flow may also lead to development of sedimentary breccias.
Igneous Breccia: These rocks are associated with igneous processes. Volcanic-breccia is a rock composed predominantly of angular volcanic fragments (>2 mm to >64 mm in diameter) set in a subordinate matrix of any composition and texture (Reynolds, 1928; Anderson, 1933; Macdonald, 1953; Fisher, 1958; 1960). It is further subdivided into flow breccia, which is formed by the fragmentation of lava during its flow and tuff-breccia (discussed under ‘Pyroclastic Rocks’ section).
Hydrothermal Breccia: Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150°C and 350 °C, when seismic or volcanic activity causes a void to open along a blind fault deep underground. The void draws in hot water, and as pressure in the cavity drops, the water violently boils. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilise and implode inwards, the broken rock gets caught up in a churning mixture of rock, steam and boiling fluid. Rock fragments collide with each other and the sides of the void, and the angular fragments become more rounded. Minerals are precipitated in between the angular breccias fragments and cements the later into a compact rock. Thus hydrothermal breccias are constituted of wall rock fragments of all possible sizes in log-normal distribution. The fragments are normally highly angular. They are cemented together by low to medium temperature crystallized hydrothermal minerals. Breccia-hosted ore deposits are quite common where the space between angular host rock fragments are filled by ore minerals.
Fault Breccia: Fault breccia results from brittle deformation and fragmentation of wall rocks during faulting. Subsequent seepage of groundwater or hydrothermal solutions and precipitation of secondary minerals may lead to cementation of these angular rock fragments.
Impact Breccia: A deposit of angular rock debris produced by the impact of an asteroid or other cosmic body.

Cataclasis

Fracturing processes along with frictional grain boundary sliding is known as cataclasis. Cataclasis may be in dry state which generates high localized heat flow or it may be associated with hydrofracturing. Sibson (1977, 1990) noted the differences between (1) incohesive fault zone rocks like breccia and fault gauge and (2) cohesive fault zone rocks like cataclasite. Mylonites are also cohesive and associated with relative displacement of crustal blocks on either side of a narrow plane. But mylonites involve intense crystal-plastic ductile deformation and shall be discussed in a subsequent section.

Microstructures indicative of cataclastic flow include microfractures, displacement along cleavages and relative displacement and/or rotation of rock or mineral fragments as rigid bodies without internal crystal plastic deformation. Breccia are, by definition, non-cohesive rocks. However, subsequent or syndeformational infiltration of fluids and precipitation of minerals from such fluids often leads to cementation of the breccia. Silicified, calcified or ferruginized breccias are thus common. In contrast, cataclasite are cohesive products of high strain brittle deformation. The fragmented grains lack internal (crystal-plastic) strain. Foliations, indicative of cataclastic flow, if present, are defined by preferred orientation of clay minerals and long axes of the fragments. Presence of water in the system and its action on the increased total surface area of the fine crushed rock leads to profuse alteration and development of clay minerals in fault zone

Ductile Deformation

Crystal plasticity:Dislocation Creep

Crystal plasticity is permanent deformation in grain scale through ductile flow. It involves movement of defects or dislocations through crystal lattice and resultant change in grain shape without loss of cohesion in the grain scale. Such flow involves dislocation glide and dislocation climb and deformation twinning. Microstructural evidences of dislocation creep include deformation twins, kink bands, undulose extinction and deformation lamellae.

1.Deformation Twins:

At lower temperature range and faster strain rate, some minerals accommodate strain by twinning in preference to slip. Such twins are called deformation twins. Twinning can accommodate limited amount of strain and always operates in specific crystallographic directions (Passchier and Trouw, 1998). Twinning produces a change in orientation which shows up, under microscope, as a change in colour and/or birefringence. Deformation twins can commonly be distinguished from growth twins by their shape – deformation twins are commonly tapered while growth twins are straight. Also deformation twins are produced in sets – they are always multiple twins and never simple Often deformation twins are patchy and concentrated in zones of high stress. In plagioclase, both growth and deformation twins can occur, the later tapering towards crystal center. Deformation twins are common in calcite and cordierite. The cross hatch twins in microcline also result from grain scale deformation

2.Microkinks:

Kinking occurs when a grain sharply bends or kinks and the deformation localizes into the kink bands. Therefore a kink band may be defined as part of the grain that undergoes rotation with respect to the unkinked parts of the grain (Vernon, 2004). Kink bands can affect the entire width of the grain or may be lenticular or wedge shaped affecting only part of the grain. Kinking is more common in minerals with strongly anisotropic crystal structure and consequently only one slip plane that proves insufficient to accommodate the strain. Thus kinks are common in biotite, kyanite etc. However grains with more than one slip systems like quartz and olivine can also develop kinks

3.Undulose Extinction:

A crystal lattice containing a large number of similar dislocations that are yet unarrayed or unorganized can be effectively slightly bent. As a result, the crystal does not extinguish homogeneously under crossed polars. This is known as undulose extinction

4.Deformation Lamellae:

Deformation lamellae are very narrow (0.5-10 µm wide or less), planar, crystallographically oriented zones with a refractive index slightly different from that of the adjacent grain (Vernon, 2004). Deformation lamellae are most common in quartz, but also reported from olivine, calcite and plagioclase (Vernon, 2004). They are more common in low temperature deformation and are associated with arrays of dislocation tangles, fluid inclusions and at times very small elongate subgrains. Deformation lamellae always develop in particular crystallographic orientation. Superficially similar features in several planes are characteristic of quartz subjected to shock deformation at meteorite impact sites. These are known as Planar Deformation features (PDFs). They are very narrow (1µm wide or less) and are formed 1-10 µm apart. They vary from twins, to dislocation concentrations to fluid inclusions and even glass bands (Vernon, 2004).

Diffusion Creep:

Diffusion creep or diffusive mass transfer involves change in grain shape by dissusion of chemical components through either (1) aqueous solution (dissolution-precipitation creep) or by (2) solid state diffusion along grain boundaries (Coble creep) or (3) diffusion through crystal at high temperature ( Nabarro-Herring creep).

Dissolution-precipitation creep or Stress-induced-solution-transfer or pressure-solution typically occurs at low temperature and involved solution of materials in intergranular fluid from points of stress concentration, transfer of materials to low stress site and reprecipitation at these sites in optical continuity with the crystal structure of the grain concerned. In quartz such solution-precipitation can often be identified by remnant line of dusty iron oxide that coated the original grain (fig 7.3.2b). In carbonate dominated rocks, truncated detrital grains or truncated ooids and fossil fragments or stylolitic seams are suggestive of pressure-solution. Solution and mass transfer however continues to play an important role in deformation and successive generations of cleavage development as will be discussed in subsequent sections. Also solution and fluid mediated material transfer plays an important role in new softer mineral formation and thus reaction softening of rock. Development of crystallographic preferred orientation in rock has commonly been attributed to dislocation creep. However, Bons and den Brok (2000) has suggested that dissolution-precipitation creep may also be important in development of crystallographic preferred orientation in a rock. Thus the presence of crystallographic preferred orientation alone cannot be used as evidence for dislocation creep (Vernon, 2004).

Recovery and Recrystallization

Recovery includes all processes that minimizes that total energy of the system by attempting to return the deformed grains to strain-free conditions. Recovery may be static or dynamic, depending on whether or not it occurs during or after deformation respectively.

One way to remove dislocation tangles from a deformed grain is dislocation climb which includes movement of edge-dislocations out of their slip-planes by addition of vacancies and movement of screw dislocations from one slip plane to another. The dislocations tend to migrate along certain planes to form sub-grain boundaries. The disorientation between a grain lattice and its sub-grain is less than 10ᵒ. Slightly misaligned fragments formed by microfracturing can also resemble subgrains but can easily be separated from the former through TEM studies.

There are three kinds of recovery-recrystallization: (i) Bulging recrystallization; (ii) Subgrain rotation recrystallization, and (iii) Grain boundary migration recrystalization. Recrystallization involves formation of new strain free grains from deformed grains so as to minimize the total energy of the system. One form of recrystallization occurs through subgrain rotation. This happens with addition of more and more dislocations to subgrain boundary. Progressive addition of dislocations leads to rotation of lattice on either side of the sub-grain boundary until the two sides form two distinct grains due to high angular mismatch of lattices. Recrystallization in a grain is generally zonal and occurs along the grain boundaries, kink bands or fracture planes that act as higher strain locales. Preferential recrystallization along the grain boundaries leaving a coarser core in between gives rise to the core-mantle structure. Often the recrystallized smaller grains favour progressive deformation through grain boundary sliding.

The other method of recrystallization involves strain-induced grain boundary migration. Atoms along a grain boundary with high dislocation density can be displaced/ diffused slightly to fit them to the lattice of an adjoining grain with low dislocation density. Such diffusion is favoured at higher temperatures and results in bulging of the grain boundary. A sutured grain boundary is formed with markedly smaller strain-free grains along the boundary. Grain boundary migration (GBM) can also occur along the boundaries of deformation twins or deformation bands

Sub-grain rotation recrystallization and grain boundary migration recrystallization are both important processes in formation of new smaller strain-free grains in the system. If such recrystallization occurs after the deformation, the recrystallized grains remain strain-free. This is known as static recrystallization. If however, recrystallization occurs during progressive deformation, an aggregate of smaller grains of roughly same size forms, each of which tend to show evidences of internal strain. This is called dynamic recrystallization.

There are several mechanisms of strain softening. Recovery and recrystallization and development of strain free smaller grains lead to strain softening during dynamic recrystallization and thereby help in progressive ductile deformation. This is known as strain softening. Another method of effective softening of rock during deformation is neo-mineralization through chemical reactions – reaction or chemical softening. This is particularly effective in low to medium temperature in presence of an aqueous fluid. The strain hardened minerals react to form grains of softer hydrous phases that are easy to deform or that orient in accordance with the strain in the system. Fig 7.3.3.3a shows field photograph of garnet grains in mafic granulite that has been subjected to shearing subsequent to granulite facies metamorphism. While the garnet porphyroclasts have underwent deformation themselves and developed σ-type porphyroclast shapes, the retrogression of the assemblage and development of hornblende and plagioclase at the cost of garnet have further fascilitated the deformation. Another example is fig 7.3.3.3b where beard structure of muscovite have developed on feldspar porphyroclasts. Muscovite being a much softer mineral compared to the parent feldspar has easily oriented perpendicular to the compression direction.

Deformation in polymineralic rocks :

The nature of deformation of a mineral in a polymineralic rock depends on several factors including rheology, bulk strain rate, geothermal gradient, orientation of stress field, presence of any preexisting fabric and fluid pressure in the system. The behavior of a rock under stress may vary from grain to grain depending on such factors. Thus within the same microdomain, while quartz can undergo ductile deformation, the adjoining feldspar may respond to the stress through brittle failure

Quartz, under dry conditions, respond to stress through brittle fracturing at temperatures below 300ᵒC. Pressure solution and solution transfer of materials are dominant deformation mechanisms at this stage. At low grade conditions (300-400ᵒC) dislocation creep and glide becomes important while at medium to high grade conditions (400-700ᵒC) dislocation creep is dominant (Passchier and Trouw, 1998, Trouw et.al.2010).

In feldspars, at temperatures below 300ᵒC, grain scale faults, formation of angular fragments, bent cleavage plains and deformation twins are common responses to stress. Although subgrains sometimes form at this stage, TEM studies prove that these are not due to dislocation tangles but are caused through very small scale brittle failures. At low temperatures (300-400ᵒC) feldspar deform by internal microfracturing with minor dislocation glide. Tapering deformation twins, bent twin lamellae, undulose extinction, deformation bands, kink band with sharp boundaries are common features at this stage (Passchier and Trouw, 1998). Flame perthites develop through a process involving microfracturing at high stress point and selective replacement of K-feldspar by albite along these lines. Dislocation climb becomes important in feldspar in the temperature range of 400-500ᵒC. Fracturing becomes less common but cataclastic failure is still abundant at the sites of dislocation tangles. Microkinking, kink band formation as well as recrystallization through nucleation into smaller strain free grains characterize this phase. Core-mantle structures are common in mylonites. At higher temperatures deformation twinning becomes less important while myrmekite growth at the contact of plagioclase and K-feldspar is more abundant. Myrmekite mainly occurs along crystal faces parallel to foliation. At temperatures above 500ᵒC recovery through dislocation climb occurs easily and recrystallization occurs by subgrain rotation and grain boundary migration. Core-mantle structures still form but boundaries between core and mantle becomes more gradational (Passchier and Trouw, 1998).

Deformation partitioning:

Deformation partitioning is common in rocks due to inherent heterogeneity both in micro- and macro-scales. In inequigranular rocks, foliation tends to swerve round porphyroblasts or porphyroclasts leading to development of cleavage domains and protected strain shadow. Even in absence of such initial difference in rheology of constituent minerals, the process of cleavage formation invariably involves development of anastomosing cleavage domains around less deformed microlithons.

Mylonite (Ductile Shear Zone) :

The term mylonite is now being used for strongly deformed rocks that have been exposed to grain size reduction due to plastic deformation, while the related term cataclasite is used where cataclastic flow dominates. In the central parts of some plastic shear zones, strain can get so high that preexisting textures and structures are totally flattened and transposed. The rock becomes strongly banded ( and may also be lineated) and is called a mylonite. The characteristics of mylonites vary with temperature, pressure, mineralogy, grain size, presence of fluids and strain rate. Therefore, the metamorphic/ metasomatic condition prevailing during mylonite formation plays an important role of its texture and structure.

Mylonite forms discrete zone of intense ductile non-coaxial deformation and simultaneous grain-size refinement that separates relatively undeformed blocks on either side of the deformed zone. While ductile deformation of minerals in mylonite zone leads to asymmetricity of grain shapes and generation of lattice defects and dislocation tangles, simultaneous grain-size refinement through recrystallization and neocrystallization causes strain softening and aids in progressive deformation. Permeation of fluid into the mylonite zone helps in chemical softening of the rock further aiding and intensifying the deformation process. The coarser grained minerals in a mylonite zone are simultaneously recrystallized along the grain boundaries and rotated due to non-coaxial nature of the strain. Marginal recrystallization leads to development of porphyroclasts with core-mantle structure. Depending upon the proportions of porphyroclasts to recrystallized matrix, the mylonites can be classified as (1) protomylonites (10-50% matrix), (2) mylonites (50-905 matrix) and (3) ultramylonite (>90% matrix). However, such terminologies are easier to apply only in coarse to medium grained rock and can prove extremely difficult for inherently fine grained rocks.

Mylonitization involves development of anastomosing domains of very high non-coaxial strain around lenses that are dominated by co-axial shortening. The lenses can be of every possible dimension. As a result, multiple sets of foliation evolve together in a mylonite zone. Mutual relationship between these simultaneously developed foliations is a good indicator of shear sense. Aggregates of small grains in mylonite (usually formed by dynamic recrystallization of deformed coarser grains) can be characterized by sharply elongate shape of most grains. These elongate grains commonly define a shape fabric oblique to C-planes (Passchier and Trouw, 1998). The acute angular relation between such oblique foliation and C-plane is a shear sense indicator (Passchier and Trouw, 1998).

The S-C fabric (Berthe et.al.1979; Lister and Snoke, 1984) in mylonite can easily be identified both in outcrop and under microscope. The S-folia forms at a high angle to the local shortening direction and the C-folia form approximately parallel to the shear zone. Once formed, the S-folia are dragged parallel to local C-folia and thus it curves into and becomes the C-folia. This suggests that S-folia forms first and are subsequently rotated by shearing. In course of progressive mylonitization, the two folia appear to form simultaneously. As the deformation proceeds, the angle between the two folia decreases and ultimately coincides in the intensely deformed and recrystallized mylonites. The curvature of S-folia into C-planes reflects the sense of shear in properly oriented section. The stretched mineral ribbons are often recrystallized statically or dynamically.

Another type of shear band cleavage in C’ folia. These are spaced folia that develop late in deformation history and are oblique to the main foliation. Their late development is suggested by bending or curving of C-folia at C’-planes. The shear sense suggested by C-C’ relation is sympathetic to S-C folial relation. At times a set of conjugate shear planes C”-folia develop in the shear zone, the sense of non-coaxial movement along these later planes being antipathic to the dominant shear sense.

Asymmetrically deformed porphyroclast and porphyroclast tails are shear sense indicators. The rigid porphyroclasts rotate in a non-coaxial strain field and simultaneously undergo internal deformation and marginal recrystallization thereby developing into mantled porphyroclast. The recrystallized grains can flow through grain boundary sliding. The angular velocity of a rigid non-spherical object can fluctuate even if the strain rate and vorticity of the flow remains constant (Ghosh and Ramberg, 1976; Passchier, 1987). Elongate objects will accelerate and decelerate with changing orientation. If the vorticity number is between pure and simple shear, such elongate objects may even become stationary in the flow when they exceed a critical aspect ratio (Ghosh and Ramberg, 1976; Passchier, 1987; Passchier and Trouw, 1998). Rigid objects may also remain stationary if the flow is strongly partitioned around the object in the form of shear bands.

A porphyroclast in a fine-grained flowing matrix causes a perturbation in the flow field. The demarcation surface separating the flow of material only under the influence of simple shear and the flow of material under the effects of porphyroclast related flow-perturbation is known as separatrix. Experimental data suggests that a seperatrix will have either an eye-shape or a bow-tie-shape around a spherical porphyroclast (Passchier et. al, 1993) and may have a much more complicated shape in case of porphyroclasts with different aspect ratios. The shape of the deformed mantle around a porphyroclast depends on the geometry of the mantle vis-à-vis the geometry of the porphyroclast. Thus, for a spherical porphyroclast or rather an equiaxial one, the porphyroclast-tail will not develop significantly if the entire mantle lies within the separatrix. Such a porphyroclast is known as ϴ-type porphyroclast. It lacks well developed wings or tails and cannot indicate shear sense. Similarly, if the separatrix is fully within the mantle and shape of separatrix is that of an “eye”, then porphyroclast tails with orthorhombic symmetry develops (Ф-type porphyroclasts) and once again determination of shear sense becomes difficult. However, if the separatrix is fully within the mantle and shape of separatrix is that of a “bow-tie”, the flow results in asymmetrically disposed porphyroclast tails with development of σ-type porphyroclasts. These are useful indicators of shear sense. Similarly intersection of porphyroclast mantle with either “eye”-shaped or “bow-tie” shaped separatrix leads to δ-type porphyroclasts that can also readily suggest shear sense in an oriented section. Interested readers are recommended to consult Passchier (1994) or Passchier and Trouw (1998) for detailed discussions on the subject.

Snowball garnets and their possible development in domains of non-coaxial strain have been subjects of much discussion. The subject has been covered in greater details in the Chapter – “Textures in Metamorphic Rocks”. Single crystals of mica or hornblende may have lozenge-shape and are called mica-fish. Many mica fish have monoclinic shape symmetry with one curved and one planar side that can be used as shear sense indicator (Passchier and Trouw, 1998). Often trails of small mica fragments extend into the matrix from tips of individual fish and further helps in shear sense determination. Amphibole-fish can also develop under suitable conditions. Asymmetric microfolds can also be used as an indicator of shear sense. Another shear sense indicator is quarter folds developed close to porphyroclasts without mantles. These structures are possibly developed through rotation of layering around porphyroclasts during progressive deformation.

Bookshelf glide microstructure or grain-scale microfaults in shear zones can be both synthetic and antithetic to the bulk shear sense. It depends on bulk shear strength, shape and orientation of the porphyroclast in the shear zone and on initial orientation of the microfaults which on its turn is dependant on crystallographic directions in the porphyroclast. In absence of all these data, it is difficult to use book-shelf glide structure as shear sense indicator.

Flame perthite and myrmekite, subjects that have already been discussed under Chapters 5 and 6, are generally associated with stress. Flame perthites seem to originate from points of stress concentration within feldspar grains while myrmekitic replacement lobes generally proceed from boundaries of porphyroclasts that are in contact with the surrounding foliations.

Foliation and Lineation:

Foliation is a planar fabric developed in the rock due to deformation with or without simultaneous growth of minerals. Slaty cleavage develops in low grade pelitic bulk while greenschist to amphibolites facies rocks are characterized by schistosity. The grain size of schistose rocks increases progressively with rise in metamorphic grade.

A continuous foliation consists of a non-layered homogeneous distribution of platy mineral grains with a strong preferred orientation. However, cleavage or foliation in rock is intrinsically domainal / layered in character. Thus a continuous foliation or slaty cleavage in thin section will possibly, when viewed at higher magnification under SEM, show very fine lensoidal microlithon domains over which the cleavage domains anastomose. In contrast, in spaced cleavage, the alternating cleavage domains and microlithon domains are observable in thin section and often in mesoscopic exposure. The cleavage domains may be planar in thin section scale but they anastomose around lensoidal microlithon domains of varying dimensions. Within the cleavage domains, the fabric elements are subparallel to the regional trend with refractions at lithocontacts. The grains in microlithon domains are more equant. For mica schists, the microlithon domains are mostly constituted of quartz and at times quartz and feldspar. They are therefore described as Q-domain or QF-domain. The cleavage domains are dominated by mica or phyllosilicate and are therefore called M-domain or P-domain.

The development of foliation during deformation may be due to several reasons. Some workers have proposed mechanical rotation of grains. Originally rosatte aggregate of platy minerals have often, in field, been found to maintain their randomly oriented habit after deformation and respond to the deforming strain by kinking of suitably oriented grains. Thus rotation does not occur in every case. However rotation of the limbs of folds and crenulations are common with progressive deformation and leads to the development and tightening of crenulation cleavage.

Pressure solution is one of the most important processes for foliation development and is widely observed in both mesoscopic and microscopic scales. Pressure solution involves solution of grains from points of strain concentration and reprecipitation of the material in low strain domains. Under diagenetic conditions, such processes can lead to …………

overgrowth on detrital grains (fig 7.3.2b),
indented ooids or fossil fragment in calcareous rocks (fig 7.3.2c),
development of grain scale stylolites, partly missing detrital grains or ooids or fossils against stylolites or
outcrop scale stylolites with concentration of insoluble opaque phases (fig 7.3.7.2a).

In greenschist to amphibolites facies conditions, the effects of pressure solution is generally evident from……

relative displacement of foliation plains or veins (fig 7.3.7.2.b),
concentrations of phyllosilicates at high strain contacts caused through deformation partitioning by porphyroblasts or porphyroclasts and preferential solution and removal of quartzofeldspathic minerals from the points of stress concentration (fig 7.3.7.2c) or
relatively higher concentration of mica and insoluble oxide phases in the apressed limbs of zonal crenulation cleavage (fig 7.3.7.3h-j).

Presence of fluid in the system is an essential prerequisite for pressure solution to operate. At lower grade to amphibolites facies, interstitial fluids or fluids liberated through dehydration or decarbonation reactions during metamorphism can provide the required fluid in the system. Fluid can also permeate from outside as is proved by development of hydrous retrograde assemblages from higher grade dry rocks. Redistribution of primarily disseminated sulphides along cleavage plains is one example of movement of fluids along the foliation plains (fig 7.3.7.2e).

Crystallographic control can also help in determining the shape of cleavage and microlithon domains. Figures 7.3.7.2f & g show abundant evidences where the quartz grains in the microlithon domains have GBM texture but are ‘pinned’ against mica concentrates along cleavage domains, thereby develoing straight grain boundaries against the mica.

Crystal plastic deformation of grains can contribute to development of foliated fabric. The best possible examples are granitic mylonites with yet unrecrystallized quartz and feldspar ribbons. Recrysrallization along the deformed boundaries of porphyroclasts and grain boundary sliding of the small recrystallized grains under the influence of shear related drag leads to development of compositional banding in the rock. Fig 7.3.6.3f is one such example where the δ-type porphyroclast tails have formed bandings parallel to shear bands.

In course of prograde metamorphism simultaneous with deformation, new platy or flaky minerals tend to grow parallel to the foliation plains. This may be due to a preferential growth in the narrow phyllosilicate-rich domains where the major reactants for formation of the new minerals are concentrated. Fig 7.3.7.3i is a zonal crenulation cleavage where the cleavage domain has a much higher concentration of mica and oxide phases. Prograde metamorphism has lead to the development of staurolite in the rock. Since the reactant phases for staurolite growth are generally concentrated in the cleavage domain, the newly formed prismatic staurolite are more abundant in the cleavage domains. A few staurolite grains follow the mica and oxide rich crenulations into the microlithons (in central and south eastern parts of the photograph).

In polydeformed rock more than one set of cleavages will develop in a rock. If the constituent minerals are more or less isotropic (as in quartz, dolomite or calcite), superposed deformation leads to development of intersecting sets of cleavages and lozenge shaped domains in between them (fig 7.3.7.3a). In phyllosilicate dominant assemblages, crenulations (fig 7.3.7.3b-c) and crenulation cleavages develop during superposed deformation. Crenulation cleavage can be discrete (fig 7.3.7.3d-g) or zonal (fig 7.3.7.3h-j). There is no sharp demarcation between discrete and zonal crenulation cleavage and one may laterally grade into the other.

The planar fabric coarsens with increase in grade of metamorphism and gneissosity is developed in granulite facies rocks. The gneissic bands are thicker and easily observable in mesoscopic scale (fig 7.3.7.4a). Partial melting during deformation leads to development of migmatitic banding with melt veins parallel to gneissic fabric. Such rocks contain melanosomes (dark restitic portions after extraction of melt), leucosomes (pale coloured melt rich fraction) and mesosome (portion of rock from which melt has not been extrcated, fig 7.3.7.4b). Gneissosity is often developed in coarse grained rocks like granite, not because these rocks have undergone high grade metamorphism but rather due to the fact that their coarse grain size allows for development of a spaced foliation which is sufficiently coarse to be observed in mesoscopic scale (fig 7.3.7.4c).

Porphyroblast-matrix relation:

The geometric relation between porphyroblast and matrix can provide large quantities of data on relative timings of deformation and metamorphism. The subject has been elaborated under section 6.6, Chapter-6: Metamorphism.

Deformed Conglomerate:

Conglomerate are coarse grained sedimentary rocks with clasts that are too coarse to be studied under microscope. Chapter 4 features a range of clast supported and matrix supported conglomerates from different parts of India. Conglomerates coarse grained, are also subjected to deformation. As it is impossible to show such features in thin section scale, this section includes a few field photographs of deformed clasts in conglomerates.

Syndeformational granite emplacement and textures developed in partially molten rocks

Granites are coarse grained rocks and it is often impossible to represent the different textural features of coarse porphyritic granite in the scale of thin section. Viscosity of magma is controlled by the melt: crystal ration. Increasing with increase in proportion of crystals growth in the system. So in a syntectonically emplaced granitis body rocks may show gradation from a PFC fabric to CP (crystal plastic) fabric. This section shows textures developed in some coarse phenocrysts bearing granitic melts that were subjected to syn-emplacement deformation. At higher melt percentages, in case of lamellar flow, the phenocrysts often show a strong preferred orientation (pre-full crystallization or PFC fabric) without any visible internal deformation (fig 7.4a-c). In particular, fig 7.4b and 7.4c are from the same sheet of granite emplaced along the Son Narmada South Fault near Singrauli. This is a zone of ductile shear marking the southern margin of Mahakoshal Belt. In each of the two photos, no internal strain is discernible in the aligned feldspar megacrysts. In fact, even at low melt percentages, a large amount of strain can be accommodated in rock without being recorded by the final microstructure (Vernon, 2004). These include:

melt-assisted grain-boundary sliding
melt-assisted grain-boundary migration
deformation twinning (for example cross hatch twinning in microcline) and
transfer of melt to sites of low mean stress (Vernon, 2004)
As a critical limit when the magma is sufficiently rich in solid crystals and melt percentage is low, the grains take internal crystal plastic strain at high temperature.

Figs 7.4d-h are all mesoscopic scale field photos from the same granite that shows different degrees of shearing and internal deformation based on local concentration of shear bands and state of crystallization of the rock. Thus in fig 7.4d, the megacrysts still retain their euhedral shape although the finer grained matrix show clear evidences of deformation and foliation development. The megacrysts have been boudinaged perpendicular to tectonic foliation - a case of brittle-ductile deformation. The single feldspar phenocryst awthwart to foliation have not been subjected to c-axis parallel stretching and, therefore, is deformed to produce only simple twinning without the boudinage. Much greater deformation of the phenocrysts and development of augen gneiss can be observed in fig 7.4e. Brittle-ductile deformation in the augen is evident in fig 7.4f where ductile deformation preceded brittle fracturing during the same deformation. Strong ductile deformation and mylonitic foliation along with conjugate shear bands can be observed in fig 7.4g and finally ultramylonite development and refolding of ultramylonite bands in fig 7.4h.

Compiled by Dr. Kasturi Chakraborty, Director