Mylonite
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Mylonite is a fine-grained, compact rock without cleavage or crystals. It is classified as a metamorphic rock and has a flow texture. Mylonites can have many different mineralogical compositions, it is a classification on the appearance of the rock.
[edit] Formation
Mylonite is a ductilely deformed rock formed by shear stress. It is generally attributed to be a metamorphic rock.
Mylonite forms by dynamic recrystallisation of rock as a response to shear stress, stress and deformation. Minerals in the rock recrystallise, and generally become smaller in grain size, a response which reduces the free energy state of grain boundaries. Some volume loss is experienced as a result of this recrystallisation. Mylonites may also form by some component of milling of fragments.
Mylonites generally develop in ductile shear zones where high rates of strain are focused into thin layers. They are the deep counterparts to cataclastic brittle faults that create breccias.
[edit] Classification
Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding. Ultramylonites have undergone extreme grainsize reduction. They are hard, dark, cherty to flinty in appearance and may be confused with pseudotachylite and obsidian. Phyllonites are phyllosilicate(e.g. chlorite or mica)-rich mylonites. They typically have a well-developed secondary shear (C') fabric.
[edit] Interpretation
Interpretation of shear sense in mylonites relies on identifying the S-plane, C-plane and the C-S intersection angle. Rotated porphyroblasts, rotated clasts and their mantles may aid in providing shear sense indicators. Other offset markers can include offset dikes and veins, crenulation cleavage, asymmetric folding, fragmented porphyroclasts, mica fish, and mantled porphyroclasts. Poikiloblasts and other relict structures may record a previous orientation, but due to their deformation, they can tell you nothing more about the origin of the protolith.
