Mylonite

Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock. Mylonites are ductilely deformed rocks formed by the accumulation of large shear strain, in ductile fault zones. There are many different views on the formation of mylonites, but it is generally agreed that crystal-plastic deformation must have occurred, and that fracturing and cataclastic flow are secondary processes in the formation of mylonites. Mechanical abrasion of grains by milling does not occur, although this was originally thought to be the process that formed mylonites, which were named from the Greek μύλος mylos, meaning mill. Mylonites form at depths of no less than 4 km. There are many different mechanisms that accommodate crystal-plastic deformation. In crustal rocks the most important processes are dislocation creep and diffusion creep. Dislocation generation acts to increase the internal energy of crystals. This effect is compensated through grain-boundary-migration recrystallization which reduces the internal energy by increasing the grain boundary area and reducing the grain volume, storing energy at the mineral grain surface. This process tends to organize dislocations into subgrain boundaries. As more dislocations are added to subgrain boundaries, the misorientation across that subgrain boundary will increase until the boundary becomes a high-angle boundary and the subgrain effectively becomes a new grain. This process, sometimes referred to as subgrain rotation recrystallization, acts to reduce the mean grain size. Volume and grain-boundary diffusion, the critical mechanisms in diffusion creep, become important at high temperatures and small grain sizes. Thus some researchers have argued that as mylonites are formed by dislocation creep and dynamic recrystallization, a transition to diffusion creep can occur once the grain size is reduced sufficiently.