Mylonite

Mylonite (from gr μύλη myle, mill ') is a metamorphic rock that is formed by the process of Dislokationsmetamorphose. The term was introduced in 1885 by Charles Lapworth and also adopted in the German literature in 1910. The term describes a mylonite rock type with a specific structure, it gives no information about the mineral constituents.

Formation

Mylonites occur at tectonic fault zones by shearing motion of two sliding past one another rock body. The essential feature of the mylonites ductile deformation of the rock at high temperatures. The vast majority of minerals in a mylonite must have been changed by plastic deformation. In contrast, in a cataclasite the minerals broken by mechanical friction, because the operation expired in Sprödbereich. Mylonites have a strong position texture and usually a clear linear stretching, indicating the direction of tectonic movement.

Dynamic crystallization

The plastic deformation is caused by the dynamic crystallization of the mineral. The adaptation of the minerals to the prevailing shear stress occurs continuously ( dynamically ), mainly due to misalignment at the interfaces of crystals by displacement of internal crystal lattice planes and by twinning. When two crystals of the same mineral species adjacent to each other, even a grain, by diffusion, the other " eat up "; This process is referred to as grain boundary migration.

Conditions at the Mylonitbildung

The Mylonitbildung one hand depends on the material properties of the parent rocks, on the other hand also upon the physical conditions during the deformation. In typical silicate rocks of the earth's crust, which contain large amounts of quartz and feldspar, the plastic deformation starts at about 280 ° C and 10 km depth. In salt rocks and marble, the temperatures are much lower. The deformation rates are between 10-13 and 10-15 s -1. The deformed minerals are usually smaller than in the parent rock. At very high temperatures and low strain rates, the minerals, however, tend to grow and there are coarse-grained mylonites ( Blastomylonite ).

Classification

Mylonites ( porphyroclasts ) are divided into Protomylonit, Orthomylonit and Ultramylonit according to the ratio of deformed minerals at unchanged components of the parent material. In addition, the name of the parent material ( granite Protomylonit ) or the most common mineral ( quartz Orthomylonit ) is prepended to name a rock detail.

• Protomylonit

Porphyroclasts, ie fragments of the original rock are adjusted with a volume fraction of more than 50 % of the whole rock as flat elements in a fine-grained environment, which acts streaky. The rock has a total of a lens-like parallel texture.

• Orthomylonit

In total rock fine-grained components reign ( less than 0.5 mm in diameter ), which are arranged in a clear parallel texture. Porphyroclasts constitute 50-10 % of the rock and flows around the fine-grained matrix. Mica rich mylonites are called Phyllonite.

• Ultramylonit

The whole rock consists of fine-grained components that generate by their arrangement a very significant position texture, the key feature of a Mylonits. Porphyroclasts are hardly or not available, its share is less than 10 %.