Mylonite

Mylonite: "A fault rock which is cohesive and characterised by a well-developed schistosity resulting from tectonic reduction of grain size, and commonly containing rounded porphyroclasts and lithic fragments of similar composition to minerals in the matrix ". (definition accepted by the SCMR)

The characteristic feature of mylonites is grain-size reduction through crystal-plastic deformation that results in a rock with a strong foliation produced by ribbon structures. Larger crystals within mylonites are porphyroclasts that are relict grains that have experienced less grain-size reduction than the surrounding matrix. Porphyroclasts in mylonites often form eye-shaped augen due to the development of pressure shadows during rotation of the crystal. The sense of shear within mylonites can often be determined using kinematic indicators. These include S and C shears, riedel shears, fibre orientation, drag folding of pressure shadows, porphyroclast mantles and fragmentation and shear folds. Mylonites form in ductile shear zones where significant rates of strain occur. They generally occur at depths > 4 km in the crust, whilst cataclasites and fault breccias from at shallower depths.

Shear zones are zones of very high ductile deformation, formed by localisation of deformation. Strain localisation is a common phenomenon in materials, especially rocks. There are several known causes, though the exact process is still poorly understood:

• Strain softening: Many materials become softer with increasing strain, e.g. by recrystallization that reduces the grain size or the formation of a foliation by alignment of micas
• Softening by fluids: Fluid flow tends to localise in shear zones with an elevated permeability and wet rocks tend to be softer than dry ones
• Reaction softening: Deformation (and fluids) may cause reactions that produce softer minerals

The reduction of grain size affects all particles in the mylonite. Large strong crystals, like feldspars in a quartzo-feldspathic rock (e.g. a granite) also fall apart by ductile and brittle processes. These hard crystals, called porphyroclasts, float in the more ductile, very finegrained matrix. Foliation usually bends around these porphyroclasts. Ongoing diminution (making smaller) of porphyroclasts slowly converts them to matrix. Mylonites can therefore be classified according to their porphyroclast content:

♦ Protomylonite: A mylonite in which less than 50% of the rock volume has undergone grain size reduction.
♦ Mesomylonite: A mylonite in which more than 50% and less than 90% of the rock volume has undergone grain size reduction.
♦ Ultramylonite: A mylonite in which more than 90% of the rock volume has undergone grain size reduction.
♦ Augen Mylonite: A mylonite containing distinctive large crystals or lithic fragments around which the foliated fine-grained matrix is wrapped, often forming symmetric or asymmetric trails.
♦ Blastomylonite: A mylonite that displays a significant degree of grain growth related to or following deformation.
♦ Phyllonite: A phyllosilicate-rich mylonite that has the lustrous sheen of a phyllite.

Mylonites can be recognised in the field by their small grain size and strongly developed, unusually regular and planar foliations and straight lineations. Lenses and layers of fine-grained material that are common in mylonites are thought to derive from a more coarse-grained parent rock by intracrystalline deformation and recrystallisation. Such deformed lenses usually have a "surf-board" shape defining both a planar and linear fabric element. This shape may be explained by common development of mylonites in an approximately plane strain regime such as in simple shear. Many mylonites contain porphyroclasts which are remnants of resistant mineral grains of a size larger than grains in the matrix. The foliation in the matrix wraps around porphyroclasts.

Porphyroclasts develop because of a difference in rheology between constituent minerals; relatively "hard" minerals will form porphyroclasts,while relatively soft ones form part of the matrix. However, porphyroclasts do not always form in the same minerals, since rheological properties of minerals depend on metamorphic conditions and initial grain size.

The planar fabric element of mylonites is known as planar shape fabric, shape preferred orientation or more specifically as a mylonitic foliation; the linear fabric element is known as a linear shape fabric or aggregate lineation. Lowstrain lenses around which the shape fabric anastomoses are common in mylonites , from lozenge-shaped single feldspar crystals to km-scale lenses. In originally fine-grained rocks, especially if they were monomineralic and no grain size reduction took place, aggregate lineations may be absent even if strain is high. At high-grade deformation conditions, grain lineations dominate.

milonite.jpg

Distribution of the main types of fault rocks with depth in the crust. a) Schematic cross-section through a transcurrent shear zone. The zone may widen, and changes in geometry and dominant type of fault rock occur with increasing depth and metamorphic grade. b) Schematic representation of four typical fault rocks (out of scale) and the local geometry of the shear zone in a 1-m-wide block, such as would develop from a phenocryst granite. Inclined (normal or reverse) shear zones show a similar distribution of fault rocks and shear zone geometry with depth. From Microtectonics (2005).



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Mylonite sample. From AGU (American Gephysical union).



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S-C Fabric in a Mylonite, France. From www.geosci.usyd.edu.au/users/prey/





Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• Anthony R. Phillpotts & Jay J. Ague: Principles of igneous and metamorphic petrology. Cambridge editore.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005)

Photo
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Mylonitic band in a Marble. PPL image, 2x (Field of view = 7mm)
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Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
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Mylonitic band in a Marble. PPL image, 2x (Field of view = 7mm)
marmomilonitico0084(5).jpg

Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
marmomilonitico0084(6).jpg

Mylonitic band in a Marble. PPL image, 2x (Field of view = 7mm)
marmomilonitico0084(7).jpg

Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
marmomilonitico0084(8).jpg

Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
marmomilonitico0084(9).jpg

Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
marmomilonitico0084(10).jpg

Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)
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Mylonitic band in a Marble. XPL image, 2x (Field of view = 7mm)