Very Low-Grade MetamorphismVis-a-vis Penetrative Deformation in the Southern Nallamalai Fold-Fault Belt, Cuddapah Basin, Andhra Pradesh

The grade of metamorphism and its time relationship with penetrative deformation has been investigated in the western half of the southern Nallamalai fold-fault belt in the Cuddapah basin using: ( 1 ) illite crystallinity (1C) index and (2) microstructure of chlorite-mica aggregates. IC index values ranging between 0.30°Ε2θ and 0.40°Ε2θ (in a set of six samples) indicate lower anchizone (very low grade) metamorphic conditions within a temperature range of 200- 250°C. From morphology, orientation and development of chlorite-mica aggregates, an initial pre-deformational but post diagenetic (burial metamorphic) origin of one of the existing two groups of aggregates is favoured. This group later underwent shape modification and growth during deformation and associated progressive metamorphism. The other group of aggregates appears to be entirely synkinematic with the cleavage forming penetrative deformation. Therefore, in the southern Nallamalai fold-fault belt, an early very low-grade burial metamorphism was overprinted by a relatively later low-grade metamorphism accompanying penetrative deformation.