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Fine grained, strongly layered fabric

forms by crystal plastic deformation.

Mylonites are strongly foliated and lineated rocks and lack mesoscopic brittle fabrics.

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.


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 considered 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.