Are crenulation cleavage zones mylonites on the microscale?

Abstract

Mylonites commonly show characteristic structures such as S– C fabric and C′ type shear bands. In the present paper, the presence of similar structures on the microscale is reported from the cleavage zones of differentiated crenulation cleavage in garnet biotite schists belonging to the Lunavada Group of Proterozoic metasedimentary rocks, India. These rocks have experienced three episodes of deformation. A differentiated crenulation cleavage ( S 2), characterized by alternating cleavage zones and microlithons developed during D 2 by microfolding of the S 1 foliation. Although the schists under investigation do not show any macroscopic- or mesoscopic-scale evidence of mylonitization, they show the presence of shear structures within the cleavage zones. The fabric resembling S– C and C′ shear bands within these zones indicates shearing within them during D 2 deformation. A model incorporating shearing along the cleavage zones is proposed to explain the genesis of shear structures within them. Accordingly, it is invoked that solution transfer and grain rotation are important deformation mechanisms during the early stages of crenulation and this results in the migration of quartz from the limbs to the hinges of the microfolds. At the later stages of crenulation the phyllosilicates (micas) forming the limbs of the microfolds are at an oblique angle to the direction of shortening and most of the mobile material like quartz has already been removed from the limbs by solution transfer. Therefore, the stress conditions are ideal for shearing and intracrystalline crystal–plastic deformation to occur along the limbs during the later stages of crenulation. It is proposed that the fabric resembling S– C, embryonic C′ type shear bands and well developed C′ (in that order) develop with increasing strain and shearing within the cleavage zones. At still higher strains, the shear bands may rotate into parallelism with the domain boundary between the cleavage zones and the microlithons. Composition of muscovite constituting cleavage zones and microlithons is discussed and it is concluded that the deformation mechanisms that operate during the later stages of crenulation, especially under upper greenschist to lower amphibolite conditions, are similar to those during mylonitization.