Mylonitic rhyolite

Mylonite: a mylonite is a foliated and usually lineated rock that shows evidence for strong ductile deformation and normally contains rounded porphyroclasts and lithic fragments of similar composition to minerals in the matrix.

The word mylonite derives from the Greek "μυλων" (a mill) since the original opinion on these rocks was that they formed by brittle milling of the rock (Lapworth 1885). However, present use of the word mylonite refers to rocks dominantly deformed by ductile flow, while brittle deformation may play a minor role. Mylonite is a strictly structural term that refers only to the fabric of the rock and does not give information on the mineral composition. Mylonite should therefore not be used as a rock name in a stratigraphic sequence.

Shear Zones

In general, deformation in rocks is not homogeneously distributed. One of the most common patterns of heterogeneous deformation is the concentration of deformation in planar zones that accommodate movement of relatively rigid wall-rock blocks. Deformation in such high-strain zones usually contains a rotation component, reflecting lateral displacement of wall rock segments with respect to each other; this type of high-strain zone is known as a shear zone. Deformation in a shear zone causes development of characteristic fabrics and mineral assemblages that reflect P-T conditions, flow type, movement sense and deformation history in the shear zone. As such, shear zones are an important source of geological information.

Shear zones can be subdivided into brittle zones or faults, and ductile zones. Ductile shear zones are usually active at higher metamorphic conditions than brittle shear zones. Ductile shear zones generally record a non-coaxial deformation and may range from the grain scale to the scale of a few hundreds of kilometers in length and a few kilometers in width. Major shear zones which transect the crust or upper mantle have both brittle and ductile segments (Fig.1). The depth of the transition between dominantly brittle and ductile behavior depends on many factors such as bulk strain rate, geothermal gradient, grain size, lithotype, fluid pressure, orientation of the stress field and pre-existing fabrics.

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Fig.1: Simplfied model of the connection between faults, which normally form in the upper crust, and classic ductile shear zones. The transition is gradual and known as the brittle-plastic transition. The depth depends on the temperature gradient and the mineralogy of the crust. For granitic rocks it normally occurs in the range of 10-15 km. From Fossen, H. (2016).



Ideally, a ductile shear zone is contained between two parallel and imaginary boundaries, the shear zone walls outside of which the rock is unstrained (Fig.2). Ideal shear zones are produced by plane strain, simple shear deformation. Accordingly, there is no stretch along the intermediate, Y axis of finite strain, perpendicular to the plane of strain. The structural study of shear zones is carried out in the XZ plane of finite strain. Sections cut perpendicular to the mineral stretching lineation commonly appear to be less deformed and show symmetrical structures (e.g. symmetrically mantled porphyroclasts), whereas sections cut parallel to the lineation appear to be much more deformed and show asymmetrical structures that can be used as shear-sense indicators (Fig.3).

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Fig.2: Ideal shear zone. Note how the planar, and circular markers change orientation and thickness across the zone. The strain is at its maximum in the central part of the shear zone. From Jean-Pierre Burg.



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Fig.3: Diagrams showing asymmetrical porphyroclasts and other microstructures indicating sense of shear in mylonites. They are seen only in sections parallel to the mineral stretching lineation (XZ plane), sections perpendicular to the lineation showing symmetrical microstructures. From Passchier (2005).



Strain localization and strain softening

The relatively high finite strain values reached in mylonites imply that strain rate in the mylonite zone must have exceeded that in the wall rock for some time, and that the material in the zone must have been softer than the wall rock. Nevertheless, many mylonites have the same chemical and mineral composition as the wall rock. Apparently, changes occur in the rheology of material in a ductile shear zone after its nucleation. This effect is known as softening or strain-softening. The most important mechanisms that contribute to softening are:

Geometric softening: Ductile deformation changes the texture (grain shape and lattice orientation) of the rock. Grain lattices progressively rotate with grain shapes towards parallelism with the shear plane. The process is envisioned as follows: Intra-granular slip systems are micro-faults (Fig.4). The resolved shear stress required on a given crystallographic slip plane decreases as this plane rotates into parallelism with the shear plane and as the intra-crystalline slip direction rotates into parallelism with the stretching lineation. Geometric softening is most pronounced in minerals with few slip systems. The extent of softening depends on the starting fabric and its orientation to the shear plane and shear direction, but any amount of softening significantly enhances strain localization.

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Fig.4: Geometric softening due to grain shape and lattice rotation into shear plane. From Jean-Pierre Burg.



Reaction softening: Reaction softening is a complex process involving micro-mechanical and chemical processes. Syn-kinematic metamorphic reactions generate new minerals, usually growing in the local flow plane and direction, hence under easiest-slip conditions. This is particularly true for new mica on foliation planes. Hard mineral phases may be converted to softer assemblages (e.g. feldspar replaced by quartz and sericite).

Fluid-related softening: As in brittle deformation, pore fluids can significantly reduce the effective normal stress, hence the cohesive strength of grain boundaries and assist micro-fracturing. Besides this pore pressure contribution, infiltration of aqueous fluids into the shear zone may lead to retrograde hydration reactions. Fluid circulation is also efficient at dissolving grains that resist ductile deformation and changing dislocation or diffusion flow to dominantly diffusive mass transfer deformation mechanisms.

Deformation of polymineral aggregates

Ductile behaviour of minerals and rocks is generally dfined as the capacity to deform without fracturing on the grain scale However, ductility has also been defined as the capacity for substantial change of shape without gross fracturing. (Paterson,1978). This definition refers to megascopic or macroscopic flow, and is independent of the microscopic mechanisms of deformation, which can include not only crystal plasticity and diffusional flow (which maintain cohesion at the microscopic scale), but also cataclastic (microscopically brittle) mechanisms. In other words, on the basis of this definition, a rock can be ductile on the scale of a hand specimen or outcrop, but partly brittle on the microscope scale.

A major factor governing ductility of minerals is the number of slip systems available for deformation to occur without producing holes or cracks. However, many minerals have strongly anisotropic structures (e.g. mica, kyanite) and so have fewer slip systems than those with more less-isotropic structures (e.g. quartz, olivine). The result is that some minerals are able to change their shapes in response to general local stress fields more readily than others. This situation produces deformation contrasts between different minerals, which occur when stronger and weaker minerals coexist.

Natural rocks typically have several minerals with different deformation properties that can vary with external conditions (e.g. temperature, pressure, water activity). For example, strong feldspar and weak quartz typically coexist in granite deforming at relatively low temperatures (<500┬░C). The feldspar deforms plastically a little before it fractures (brittle deformation), whereas the quartz flows and recrystallizes in a ductile manner, commonly forming ribbons of fine-grained recrystallized aggregates. Evidence of both ductile and brittle behaviour is seen in many felsic mylonites (Fig.5), in which feldspar deforms cataclastically, whereas quartz and mica deform mainly by dislocation creep, commonly assisted by neocrystallization.

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Fig.5: Mylonite from Cap de Creus, Spain, showing marked contrast between quartz, which has flowed plastically and recrystallized to fine-grained aggregates with "ribbon microstructure", and orthose, which shows evidence of much less plastic deformation and extensive fracturing and fragmentation. XPL image, 2x (Field of view = 7mm).



Mylonite Classification

Mylonites are classified according to the metamorphic grade at which deformation took place (e.g. high-grade mylonite) or according to the lithotype or mineralogy in which they are developed (e.g. quartzite-mylonite, granodiorite-mylonite, quartz-feldspar mylonite). Another commonly used classification of mylonites is based on the percentage of matrix as compared to porphyroclasts. Three stages are distinguished on the relative proportion of porphyroclasts to fine-grain, syn-kinematically recrystallized matrix:

Protomylonite is a rock in the early stages of mylonitisation, containing more than 50% porphyroclasts. With the onset of deformation, a protomylonite shows a mortar texture with a very fine-grained matrix surrounding large residual grains of the parent rock (mantled grains). The rock has a coarse foliation and rather weak lineation that mould the porphyroclasts (Augen for feldspars in deformed granites).
Mylonite is a foliated and lineated rock which has undergone a drastic reduction in grain size by dominantly crystal-plastic processes. A mylonite contains 10-50% porphyroclasts, i.e. 50 to 90% of matrix.
Ultramylonite is hard, flint-like, and dark, which is the visual result of extreme grain size reduction and dynamic recrystallization. The surviving, commonly tiny porphyroclasts constitute less than 10% of the rock.

Other commonly used terminology is blastomylonite for a mylonite with significant static recrystallisation and phyllonite for a fine-grained mica-rich mylonite (resembling a phyllite).

The problem with this classification is that an arbitrary limit must be defined between matrix grain size and porphyroclast grain size. Another problem is that mylonites developed at high metamorphic grade or in fine-grained or monomineralic parent rocks do not normally develop porphyroclasts; for this reason, ultramylonite does not necessarily represent a higher strain than mylonite or protomylonite.

Bibliography



• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.


Photo
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. PPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)
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Fragmented quartz porphyroclasts in a fine grained foliated matrix. XPL image, 2x (Field of view = 7mm)