A Comparative Study of Tectonic Fabrics and Deformation Mechanisms in Dharwar Grits and Phyllites and Sargur Quartzites on the West of Chitradurga Supracrustal Belt, Karnataka

Late Archaean t·ectonic LS fabrics in quartz-pebble conglomerates and grits in the lower part of the Dharwar Supergroup are defined by micas and the preferred planar orientation of long (X) and intermediate (Y) axes of ellipsoidal clasts of vein quartz. Long axes of clasts define the linear component ofthe fabric. With increasing strain the grits become L tectonites. Pressure solution and intragranular processes were important at lower strains, but dislocation creep was dominant at higher strains related to the LS fabric development.

Dharwar phyllites and polymict conglomerates have a bedding-parallel S fabric of phyllosilicates and detrital grains of quartz. This fabric is attributed to neocrystallization and recrystallization of phyllosilicates coplanar with a primary bedding-parallel fabric during diagenesis and subsequent burial and regional heating deep within the Dharwar volcanosedimentary pile. Coeval, domainal and penetrative tectonic LS fabrics, with transitions from domainal to penetrative types, which formed contemporaneously with LS fabrics in the grits were superimposed on the bedding-parallel fabrics. Microbuckling and pressure solution dominated early stages of the cleavage development, but they were overtaken by coplanar growth of biotite in the later stages. Discordant prisms of tourmaline and blue-green amphibole, radiating intergrowths of chlorite and quartz and octahedra of magnetite and their pseudomorphs of quartz indicate that grain growth took place during the low-grade metamorphism that outlasted the late Archaean deformation.

Transitional crenulation to penetrative S fabrics, locally with a linear component coaxial with folds, were superimposed on an older tectonic S fabric broadly parallel to bedding in the Sargur quartzites which form part of the basement immediately below the Dharwar Supergroup. The younger fabrics are correlated with the late Archaean tectonic fabrics in the Dharwar grits and phyllites. The older fabric, which includes prisms of kyanite, is believed to be related to pre-Dharwar deformation and medium- to high-grade metamorphism. Whilst microbuckling, pressure solution and dislocation deformation processes were important in the development of the younger fabric, their role in the development of the older fabric is not clear because of the superimposed effects of the later processes. Discordant chloritoid grown across pre-Dharwar garnet. staurolite and kyanite and their low-grade alteration products in the Sargur quartzites suggests grain growth took place during low-grade metamorphism that outlasted the development of the younger fabric, a relation identical to that in the Dharwar grits and phyllites.