Dynamically recrystallised quartz c-axis fabrics in greenschist facies quartzites, Singhbhum shear zone and its footwall, eastern India—influence of high fluid activity

Abstract

Oriented specimens of quartz tectonites from the Singhbhum shear zone (SSZ), an important tectonic element in the Precambrian eastern Indian shield, and its footwall have been analysed for c-axis fabrics of dynamically recrystallised quartz grains. In individual specimens the fabric pattern is either an asymmetric type I cross-girdle or an asymmetric, kinked single-girdle, irrespective of whether the fabric is from the SSZ or its footwall. The asymmetry of the fabric confirms a southward thrust movement of the northern block on a northerly dipping shear zone. The measured fabrics are characterised by a concentration near the Y-axis of the finite strain ellipsoid with an equally or subordinately developed concentration near the Z-axis. Although some earlier works suggest the Y-concentration of the quartz c-axis fabric as due to the influence of higher temperature (obtained in amphibolite facies), the occurrence of a similar concentration in the quartz c-axis fabric from quartz tectonites of the SSZ and its footwall, interpreted to have formed under greenschist facies condition, is explained by formation of the fabric under high fluid activity during deformation.