Analysis of deformation fabric in an Alkaline Complex (Koraput): Implications for time relationship between emplacement, fabric development and regional tectonics

Abstract

The Koraput Alkaline Complex (KAC) lies on the NE-SW trending Sileru Shear Zone (SSZ) separating the Proterozoic Eastern Ghats Province from the Archaean Indian craton. The core of the KAC is made of hornblende gabbro, which is rimmed by a band of nepheline syenite in the east and syenodiorite in the west. The timing of magmatism with respect to the SSZ is disputed. The KAC was deformed during emplacement, and a magmatic foliation related to the syn-emplacement deformation, D1, is present in the gabbroic core. The dominant D2-related field fabric strikes NESW and is penetrative in parts of the gabbro and marginal lithologies. E-W trending D3 shear zones cut across the complex. Distinct textural domains resulted from strain partitioning during deformation. Parts of the complex with magmatic textures constitute Domain-1, while D2 and D3 fabric zones comprise Domains-2 and 3 respectively. Temperatures in the KAC initially decreased following D1, but increased through D2 and D3. Anisotropy of magnetic susceptibility (AMS) studies show that the magnetic fabric generally follows S1 in Domain-1. While the magnetic fabric in Domain-2 is dominantly parallel to S2, some of it parallels S1. The latter is a relict D1 fabric that is recognized from AMS analysis but is obliterated in the field, which confirms that the KAC pre-dates the SSZ. The response of magnetic fabrics to temperature and implications of the study for Indo-Antarctica amalgamation are discussed.