Kadavur Layered Anorthosite-Gabbro Complex, Tamil Nadu: A Case Study of Magmatic Differentiation from the Southern Granulite Terrane of the Indian Shield

The Kadavur anorthosite-gabbro complex (10°35'N: 78°11'E), Tamil Nadu, consists of the three distinct rock units which are layered anorthosite-gabbro, schistose anorthosite-gabbro, and sporadic pegmatoidal variant. Based on texture, the layered-type can again be divided into a mesocumulate unit (MCU) and a non-cumulate unit (NCU). Several major oxides and oxide ratios indicate, from Harker variation diagrams that the mesocumulate unit (MCU) and schistose unit (SU) are geochemically similar. The plots for non-cumulate unit (NCU) show a compositional spread in those variation diagrams, suggesting fractionation (involving clinopyroxene and plagioclase) from the supernatant magma on separation of the MCU. Variations in the compositions of TiO₂ vs P₂ O₅, Ba vs SiO₂, La vs Zr, and Yb vs Zr also indicate that NCU has a compositional spectrum, whereas plots for MCU and SU are distinctly different. The geochemical behaviour of the pegmatoidal unit (PU) shows affinity for both the NCU and MCU. The normalized rare earth element (REE) ratios as well as the Zr and Ni concentrations, strongly favour an “Archean basalt” parent magma having tholeiitic or calc-alkaline/ (or transitional) affinity. The present study strongly indicates that the Kadavur anorthosite-gabbro complex was influenced by deformation during intrusion.