Mineralogy of Pakkanadu carbonatites and associated rocks, South India: constraints on evolution and evidences for REE enrichment.

The Pakkanadu carbonatite-alkaline complex (PCAC) is one of the nine carbonatite-alkaline complexes, situated along the NE-SW trending Dhramapuri Rift Zone in Southern Granulite Terrain in South India. This oval-shaped intrusion comprises of syenite–carbonatite–dunite–pyroxenite association. The carbonatites of this deformed complex are dominated by calcite carbonatite over dolomite carbonatite and ankerite. The present study mainly focuses on identifying evidence of rare-earth element (REE) remobilization and enrichment based on petrography, mineral chemistry and whole-rock geochemistry of the PCAC carbonatites and associated rocks. Chief rare-earth and rare metal bearing minerals in these rocks include monazite-(Ce), allanite-(Ce), chevkinite-(Ce), pyrochlore, besides strontianite and barite. Geochemically, the studied carbonatites and fenites have enriched, but variable Ba (5300–236,900 ppm), Sr (4360–98,700 ppm), La (84–11,500 ppm), Ce (155–17,200 ppm) and Nd (55–3000 ppm), and Sr/Ba (0.06–3.23) molar ratio. The ΣREE range from 3508 to 32,300 ppm, dominantly contributed by light rare-earth elements (LREE). Presence of a component geochemically similar to benstonite carbonatite is reported during the present study. Geochemically the studied rocks are characterised by enrichment of LREE and elevated concentrations of incompatible, with positive spikes Ba, Sr and strong depletion of Nb, Ta, Ti, Zr and P in primitive mantle-normalized spider diagram. Besides these characters, the presence of phlogopite, amphibole, early magmatic monazite-group minerals and fluorapatite in the Pakkanadu carbonatites indicate that the initial carbonatite magma was enriched in REE, and generated by a low degree of partial melting of a metasomatised fertile mantle in intra-plate rift setting. In addition, low Nb/Y ratio (≤ 1.0), Zr/Sm (up to 6.4) and Hf/Sm (up to 0.12), very high Ba content is indicative of fluid-related metasomatism. The textural relations and mineral chemistry of REE-bearing mineral phases indicate that carbo-hydrothermal processes resulted in remobilization and enrichment of REE. All these factors point towards REE fertility in the PCAC carbonatites and associated fenites. [ABSTRACT FROM AUTHOR]