Mylonite is a metamorphic rock that forms when rocks are subjected to intense pressure and heat during tectonic events, like mountain building. This process, known as ductile deformation, causes the rocks to deform and flow like a viscous fluid.
Characteristics of Mylonite:
- Fine-grained texture: Mylonite is characterized by its extremely fine-grained texture, often with a foliated appearance. This fine-grained texture is a result of the intense crushing and grinding that occurs during ductile deformation.
- Strong foliation: The intense pressure and shear stress create a distinct foliation in the rock, known as mylonitic foliation. This foliation is often parallel to the direction of the tectonic forces that created the rock.
- Presence of mineral lineations: Mylonite often contains mineral lineations, which are elongated minerals that are aligned parallel to the direction of the tectonic forces.
- Presence of porphyroclasts: Mylonite may also contain larger, pre-existing mineral grains called porphyroclasts, which have been deformed and stretched by the tectonic forces. These porphyroclasts can be useful for determining the direction of the tectonic forces that created the rock.
Formation of Mylonite:
Mylonite forms in a zone called the mylonite zone, which is located near the fault plane where rocks are subjected to intense deformation. The rocks in the mylonite zone undergo a series of transformations, starting with the original rock and progressing through various stages of deformation, including:
- Protomylonite: This is the initial stage of deformation, where the rock begins to show signs of ductile deformation.
- Mylonite: This is the intermediate stage of deformation, where the rock has undergone significant deformation and has a fine-grained texture and strong foliation.
- Ultramylonite: This is the final stage of deformation, where the rock has been completely deformed and has a very fine-grained texture and a strong foliation.
Examples of Mylonite:
Mylonite is found in many tectonic settings, including:
- Fault zones: Mylonite is commonly found in fault zones, where rocks are subjected to intense shear stress.
- Mountain belts: Mylonite is also found in mountain belts, where rocks are subjected to intense pressure and heat during mountain building.
- Subduction zones: Mylonite can also form in subduction zones, where one tectonic plate is forced beneath another.
Practical Insights:
- Understanding tectonic history: Mylonite can provide valuable information about the tectonic history of a region. The orientation of the mylonitic foliation and the presence of mineral lineations can be used to determine the direction and magnitude of the tectonic forces that created the rock.
- Exploration for mineral deposits: Mylonite can be a good indicator of mineral deposits. Some minerals, such as gold and copper, are often found in association with mylonite.
