mylonite

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mylonite A mylonite is a strongly foliated rock that has undergone intense ductile deformation, with accompanying reduction in grain size. Mylonites occur in zones of high strain, or shear zones, which may vary in thickness from a few millimetres to several kilometres. They were originally interpreted as forming by brittle break-up of original grains (cataclasis), but it is now recognized that mylonite textures result mainly from syntectonic recrystallization, in which large original grains become strained, leading to the nucleation of small unstrained grains which then grow at the expense of the parent grain. Quartz and carbonate are particularly susceptible to syntectonic recrystallization in this way; less readily recrystallized minerals such as feldspar or garnet often crack and break up as the ductile matrix deforms around them, but remain as relatively coarse porphyroclasts, albeit smaller than their original grain size. Many minerals will undergo some syntectonic recrystallization if subjected to sufficient strain at appropriate temperatures. For example, the quartz in granite mylonites may be fully recrystallized into long-drawn-out fine-grained ‘ribbons’ that wrap round porphyroclasts of feldspar whose outermost part has itself recrystallized to a fine-grained mortar texture of polygonal grains.

A number of distinct terms are used for mylonites that have experienced various amounts of strain and recrystallization, as indicated by the proportion of porphyroclasts that remain. Protomylonites retain more than 50 per cent of original grain fragments as porphyroclasts, whereas mylonites in the strict sense have 10–50 per cent of porphyroclasts only. In cases of extreme deformation, ultramylonite may be produced, in which less than 10 per cent of porphyroclasts remain. Ultramylonites are usually restricted to narrow zones within a mylonite. The term phyllonite is sometimes used for a mylonite dominated by sheet silicates.

The fabric that is produced as a mylonite is deformed and recrystallizes usually contains characteristic elements that reflect the movement during deformation. Thus there is commonly a stretching lineation visible on fabric planes that indicates the movement direction, at least during the final stages of fabric development. The progressive development of the mylonite fabric also commonly leads to shearing around porphyroclasts with a distinct asymmetry that reflects relative movement.

Mylonites can form in most rock types over a range of metamorphic conditions. Although some mylonites, especially those in igneous bodies, are associated with prograde metamorphism, there are also many examples of mylonites formed during retrograde metamorphism, typically under greenschist facies conditions, of higher-grade rocks. In such instances, the ductile deformation leading to the development of the mylonite is coupled to infiltration of water from an external source that both enables syntectonic crystallization to occur and leads to hydration of the original assemblage.

Bruce W. D. Yardley

Bibliography

Passchier, C. W. and and Trouw, R. A. J. (1996) Micro-tectonics. Springer-Verlag, Berlin.
Sibson, R. H. (1977) Fault rocks and fault mechanisms. Journal of the Geological Society of London, 133, 191–213.

mylonite Rock produced in zones of tectonic dislocation, e.g. fault and shear zones. Mechanical crushing and grinding or cataclasis produces a rock that has a foliation which is often crude but which is sometimes very well developed, and that has a much finer grain size than its precursor. Mylonite is well laminated and often hard and splintery. The name literally means ‘milled rock’ from the Greek mylon, a grinding mill. See DYNAMIC METAMORPHISM.