Phyllonite

Phyllonite is a type of fault rock formed by the intense shearing and grinding of protoliths along thrust faults or strike-slip faults. It is characterized by its fine grain size and foliated texture, which is a result of the mineral grains aligning parallel to the fault plane during deformation. The term "phyllonite" is derived from the Greek word "phyllon," meaning leaf, referring to its platy structure.

The formation of phyllonite involves significant cataclasis, a process where rocks are broken down into smaller fragments. This comminution, combined with recrystallization and some degree of plastic flow, leads to the development of the characteristic phyllonitic fabric. The protoliths can vary widely, including igneous, sedimentary, or metamorphic rocks, but the intense shearing process tends to homogenize the resulting phyllonite, making it difficult to determine the original rock type in highly deformed zones.

Phyllonites are typically found in narrow shear zones and are often associated with other fault rocks such as mylonites and pseudotachylytes. The presence of phyllonite can indicate significant tectonic activity and large-scale crustal movements. Geologists study phyllonites to understand the kinematics and dynamics of fault zones, including the amount of displacement and the conditions of temperature and pressure under which deformation occurred. The mineralogy of phyllonite can include fine-grained quartz, feldspar, micas, and chlorite, depending on the original rock composition and metamorphic conditions.