Mylonites are rocks that have undergone modification of original textures by predominantly plastic flow due to dynamic recystallization. The formation of mylonites occur at depth, below brittle faults, in continental and oceanic crust. Formation of mylonites occur in strike-slip, extensional or compressional regimes, and are characteristic of moderately high temperature deformation. Microstructures that form during mylonitization vary according to original mineralogy and modal compositions, temperatures, confining pressure, strain rate, applied stresses, and the presence or absence of fluids. Mylonites are foliated rock with strong lineation resulting from shear in a ductile fault shear zone. Mylonitization, the process of forming a mylonite generally consists of crystal plastic strain by dislocation climb and recovery. Mylonitization reduces the grain size of a protolith and commonly produces a very fine- grained, well foliated rock with a pronounced linear fabric defined by elongate minerals. Mylonites can be found in places call mylonite zones. These zones are located along major fault zones, and can range in lengths of hundreds of kilometers, and can also be several kilometers thick.

A mylonitic rock which has unergone recrystallization and some melting as well with melt cooling down to glassy partices at many places.

An orthomylonite is an initial stage of formation C and S surfaces in a mylonite under lo strains. It can be seen generally developed in ductile shearing of granites in the initial stages. The photograph reproduced here is from the online book on Tectonic by P Rey.

Mylonite Zones are narrow planar regions in which deformation is intense relative to that of the adjacent rocks. The rocks in such zones are generally fine-grained equivalents of the adjacent rocks, the reduction in grainsize, being due to cataclasis, recrystallization, or a combination of both. Typical mylonites have a strongly layered appearance, the layering being due to variations in the amount of deformation or to variations in composition. A lineation is commonly developed within the foliation and is defined by elongate rods of mineral aggregates or ovoid patches of weakly deformed material surrounded by strongly deformed rock. Folding is invariably developed in the mylonite layering and commonly the axes of these folds are parallel to the lineation.

Mylonites are strongly foliated and lineated rocks and lack mesoscopic brittle fabrics. If predominantly planar fabric is present, they are called S tectnites or S mylonites. If predominantly a linear structure is present, then they are called L tectnites or L mylonites and if both then LS tectonites or LS mylonits. In contrast to cataclasites, the mylonites have a strong component of recrystallization in their making.

S-C Mylonite is a superb example of a type II S-C mylonite. C-planes are defined by thin mica-rich seams, with occasional mica "fish" which have been sheared to produce asymmetric shapes that indicate the sense of shear, which matches the sense of shear indicated by the S-planes defined by the quartz grain shape preferred orientation. The dynamic recrystallisation has reached an equilibrium with the straining such that the preferred orientation of grain shapes does not in any way reflect the finite strain.

Mylonites are zones of strong to extreme localisation of ductile deformation. It contains remnants of coarser protolith. If porphyroclasts are dominant with 10-50% matrix the rock is called a protomylonite; if 50-90% matrix, then (meso-) mylonite

andif >90% matrix then an ultramylonite.

MYLONITE ZONES

Mylonite zones are narrow planar regions in which deformation is inte relative to that of the adjacent rocks. The rocks in such zones are gener line-grained equivalents of the adjacent rocks, the reduction in grain:

being due to cataclasis, recrystallization, or a combination of both. Typical mylonites have a strongly layered appearance, the layering being due to variations in the amount of deformation or to variations in composition. A lineation is commonly developed within the foliation and is defined by elongate rods of mineral aggregates or ovoid patches of weakly deformed material surrounded by strongly deformed rock. Folding is invariably developed in the mylonite layering and commonly the axes of these folds are parallel to the lineation Although this parallelism of fold axes and lineation is common it is important to point out that the relationship is not always developed [for examples see Kvale (1941, ] 953)]. The folds vary from open through tight to isoclinal and are commonly intrafolial. Invariably where such folds are developed there appears to be a secondary mylonite layering developed parallel to their axial planes. The geometrical relationships between layering, lineation, and folding within mylonite zones have been poorly studied for the most part and there is a need for more detailed study.

Mylonite zones range in scale from the size ofthin sections up to zones that are hundreds of kilometers long and perhaps 30 km or so in width. The classical mylonite zone is associated with the outcrop ohhe Moine Thrust in the northwest highlands of Scotland . This zone is gently dipping and approximately 200 km in length. It ranges in thickness from 10 to 100 meters. The Moine Thrust itself though apparently postdates the formation of the mylonites . Mylonite zones occur in rocks of all metamorphic grades ranging from greenschist to granulite facies. Where mylonite zones occur in high grade metamorphic rocks they are commonly associated with retrogression and appear as zones of retrograde schists_ Particularly in such situations these zones are loosely referred to as shear zones. Recently the microstructure of quartz mylonites has been systematically studied experimentally by Tullis et al. (1973).

The term mylonite was introduced by Lapworth in 1885 to describe rocks

that appeared along the Moine Thrust in Scotland. Lapworth envisaged that these rocks had been produced by strong grinding or milling of the Moine schists; hence the term mylonite from the Greek mylon - a mill. He consid

ered these rocks to have been formed solely by crushing with no concurrent recrystallization. In the formation of these rocks a strong layering or banding had developed. To Lapworth then the important features of a mylonite were that it had been produced solely by cataclasis without any associated recrystallization of the constituent particles and that a well-developed layering had formed. The terminology of mylonites is discussed at length by Christie (1960) who points out that recrystallization is commonly very widespread in the Moine mylonites. However, he considered that recrystallization postdated the deformation. It is important to note that most of the classic microstructures described by older workers and interpreted by them as cataclastic structures are due to recrystallization. Such microstructures particularly include the classical mortar structure in which an aggregate of fine-grained, new recrystallized material is developed around the margins of older deformed grains (see Fig. 2.20b). The subject is discussed at length by Bell and Etheridge (1973). Christie employs the term blastomylonite to describe those rocks that

have been strongly recrystallized. The term was introduced by Sander in 1912. In nature there are presumably all gradations from strongly deformed rocks where the deformation is solely of a cataclastic nature through to these

where recrystallization has accompanied or postdated a ductile mechanism of deformation. Moreover, there are presumably many rocks that have been deformed in a cataclastic manner but that have been sintered or recrystallized at a later date. The lack of any systematic descriptive material in the literature on mylonites from different environments prohibits any definite statements.

The situation, however, is somewhat unsatisfactory if workers insist on using Lapworth's original term to signify that the deformation in mylonites has

been solely of a cataclastic nature with no accompanying recrystallization. This arises because within many mylonite zones there are rocks in which the shapes of original grains have been strongly distorted but where the deformation throughout has been of a

ductile nature produced by the 'propagation of dislocations through the crystalline structure of the grains. For these rocks the strict Lapworth term, mylonite, is not applicable nor is the term blastomylonite. We prefer to use the term mylonite in a general sense covering rocks that occur in these zones of relatively high deformations no matter if the deformation has been cataclastic or ductile on the scale of grains, and to use the term blastomylonite to refer specifically to rocks in these zones that have completely recrystallized, either during or after the deformation. This terminology is compatible with Bell and Etheridge (1973).

In many mylonite zones there is evidence that the rocks have melted, and extremely fine-grained material comprises dikes and sills that run out from the mylonite zone and intrude the neighboring rocks. These fine-grained rocks are called pseudot{}£hylites [Shand (1916)]. The term is also discussed by Christie (1960).

The strain associated with mylonite zones has been a matter of some discussion. A widespread interpretation is that such zones are the product of large shear strains. Where the displacement of the rocks either side of the zone can be established, this is commonly the case . However, in most very large mylonite zones such displacements cannot be demonstrated and then interpretation is not so clear. Johnson (1967) has proposed that in many mylonite zones and, particularly, the Moine Thrust, the strain is essentially a

flattening normal to the foliation and the mylonite layering has an origin much the same as slaty cleavage. Johnson bases this interpretation on the symmetrical patterns of preferred orientation of quartz that are developed in the Moine mylonites. This may, in fact, be

the case for the Moine mylonites but for many other zones such patterns of preferred orientation are strongly asymmetric It is important to also note that typical mylonite microstructures commonly thought of as the result of shear strain have been produced in axially symmetric straining by Tullis et al. (1973).

Another problem associated with the strain in mylonite zones is the common parallelism between lineation and fold axes. Many workers hold that the lineation is an axis of principal extension whereas others claim it to be a direction of shearing within the foliation plane. Generally, however, there is very little evidence to define the strain very clearly, but a problem exists in establishing the relationship of folds and lineation in mylonite zones to the distribution of strain within those zones and in correlating these features with the overall displacement associated with the zone.

(II) Crystallographic fabric of my/onites

The crystallographic fabrics have been studied for a number of decades; however. there has been a rapid advance in the interpretation of fabric patterns only very recently. Because of the prevalence of quartz-rich mylonites. most of this recent work is on quartz c-axis fabric. To prepare a fabric diagram. the orientations of quartz c-axes are determined from an oriented thin section mounted on a universal stage of the microscope. The c-axes are plotted in the equal area projection. On this pole figure the contours of concentration of the poles are then drawn. each point on a contour representing the percentage of poles lying within I per cent area of the equal area net. The fabric diagram usually shows the foliation as a plane (S) striking left to right and perpendicular to the plane of the diagram, with the lineation occurring at the periphery.