Mylonite
Mylonite is a fine-grained, compact
Formation
Mylonites are
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,[3] 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.
Mylonites generally develop in ductile shear zones where high rates of
Classification
- Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding.
- Ultramylonites usually have undergone extreme grainsize reduction. In
- Mesomylonites have undergone an appreciable amount of grainsize reduction, and are defined by their modal percentage of matrix grains being between 50 and 90%.[9][10]
- Protomylonites are mylonites which have experienced limited grainsize reduction, and are defined by their modal percentage of matrix grains being less than 50%. Because mylonitisation is incomplete in these rocks, relict grains and textures are apparent, and some protomylonites can resemble foliated cataclasite or even some schists.
- Phyllonites are .
Interpretation
Determining the displacements that occur in mylonite zones depends on correctly determining the orientations of the finite
Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. Most kinematic indicators are based on deformation in simple shear and infer sense of rotation of the finite strain axes with respect to the incremental strain axes. Because of the constraints imposed by simple shear, displacement is assumed to occur in the foliation plane in a direction parallel to the mineral stretching lineation. Therefore, a plane parallel to the lineation and perpendicular to the foliation is viewed to determine the shear sense.
The most common shear sense indicators are C/S fabrics, asymmetric porphyroclasts, vein and dike arrays, mantled porphyroclasts and mineral fibers. All of these indicators have a monoclinic symmetry which is directly related to the orientations of the finite strain axes. Although structures like asymmetric folds and boudinages are also related to the orientations of the finite strain axes, these structures can form from distinct strain paths and are not reliable kinematic indicators.
References
- ^ Lapworth, C. (1885). "The highland controversy in British geology; its causes, course and consequence". Nature. 32: 558–559.
- ^ Mylonitic marble, alexstreckeisen.it
- ^ Urai J.L.; Means W.D.; Lister G.S. "Dynamic recrystallization of minerals". Archived from the original on 5 September 2019. Retrieved 9 July 2016.
- ^ S2CID 131446805.
- .
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- ISBN 978-3-662-08734-3.
- ISBN 978-3-642-03607-1.
External links
- Mylonite photo gallery Archived 2020-11-08 at the Wayback Machine