Geochemical and Geochronological Data from Charnockites and Anorthosites from India’s Kodaikanal–Palani Massif, Southern Granulite Terrain, India

Abstract

The Kodaikanal–Palani Massif is an important component of India’s Southern Granulite Terrain; understanding the tectonic history of its rocks lends considerable insight into its role within South India. The massif is located south of the Palghat Cauvery Shear Zone (PCSZ). Compilations of available geochronologic and geochemical information from charnockites north and south of the PCSZ show these rocks largely differ in age, with northern samples recording Archaean crystallization events, whereas those to the south yielding Cambro-Ordovician and Neoproterozoic ages. The Kodaikanal–Palani charnockitic rocks contain monazite grains that fall within the Cambro-Ordovician timescale. The Oddanchatram anorthosite, located along the northern boundary of the Kodaikanal–Palani Massif, contains zircon grains that record mid-Neoproterozoic to Cambro-Ordovician crystallization ages. This anorthosite differs in texture and composition depending on location, that may be the result of its multi-stage metamorphic and/or intrusion history. Charnockitic rocks north and south of the PCSZ also differ geochemically. For example, north of the PCSZ, these rocks become more calcic with increasing SiO2 contents, whereas those to the south become alkali-calcic. Southern charnockitic rocks tend to have higher K2O/TiO2, Zr/SiO2, Rb/Sr, Ba and Rb contents, but lower Sr/Ba ratios. Using available geochemical data, we find more charnockitic rocks south of the PCSZ record zircon saturation temperatures between 800°C and 900°C than those to the north. Although samples of charnockitic rocks within the Kodaikanal–Palani Massif yield similar monazite ages, the rocks differ in their whole rock geochemistry and zircon and monazite saturation temperatures depending on location. The geochemical data from these rocks suggest that charnockitic rocks within the Kodaikanal–Palani Massif possibly experienced different mechanisms of generation and/or metamorphic histories.