Pseudotachylite

Pseudotachylite or Pseudotachylyte (original spelling) is a cohesive glassy or very fine-grained rock that occurs as veins and often contains inclusions of wall-rock fragments. Pseudotachylite is typically dark in color; and is glassy in appearance. It was named after its appearance resembling the basaltic glass, tachylyte. Pseudotachylyte formed via frictional melting of faults during seismic movement attracts much attention of researchers, so many researchers often define the rock as one formed via the melting (c.f. fault rock). However, the original description/definition (Shand, 1916) did not include interpretation about its generation, and it is suggested that there are pseudotachylytes formed via comminution without melting (e.g. Wenk, 1978) for ones without evidence of melting. Typically, glass has been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It occasionally contains crystals with quench textures that began to crystallize from the melt. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry.