Interfacial friction-induced pressure and implications for the formation and preservation of intergranular coesite in metamorphic rocks

The finding of intergranular coesite and coesite pseudomorphs has been taken as critical evidence to support that the formation of coesite was related to regional ultrahigh pressure (UHP) metamorphism rather than the pressure-vessel effect. Previous laboratory deformation experiments found that coesite occurs in practically undeformed strain-forbidden zones immediately adjacent to the piston–sample interfaces while the mean stress applied to the specimen is remarkably lower than the quartz–coesite equilibrium boundary. The formation and preservation of intergranular coesite in eclogites from UHP metamorphic terranes and the occurrence of coesite in strain-forbidden zones within experimentally deformed rocks can be more satisfactorily explained by the theory of interfacial friction-induced pressure, which is a well-known in metal forging. During certain episodes of fast tectonic deformation under high transient differential stresses, the interfacial friction can induce very significant deformation pressures in thin layers of weak materials (e.g., SiO2) between large garnet crystals that are refractory and mechanically strong, thus act as excellent anvils. The local deformation pressure, which deviates from the lithostatic value, is likely an intrinsic property of highly constrained flows of weak materials between strong walls at least on a microstructural scale.