Stable isotopic study of carbonatites from the Pakkanadu Alkaline Complex, southern India: Constraints on carbonatite melt evolution and sub-solidus, high temperature fluid-rock interaction

Abstract

Carbonatites of the Pakkanadu Alkaline Complex (PAC) of the Dharmapuri Rift Zone (DRZ), in the Northern Granulite Terrain (NGT) of southern India, were intruded as small concordant and discordant bodies into pyroxenites and syenites during the Neoproterozoic (∼759‐–750 Ma). Carbonatite samples from the PAC were grouped into three major groups for oxygen and carbon isotope analysis: (A) calcite-dominated (88–100%), (B) dolomite-dominated (83–97%) and (C) admixtures of calcite and dolomite. End-member isotopic compositions were ẟ 13 C = −5.7 ± 0.1‰ and ẟ 18 O = +8.4 ± 0.2‰ for calcite and ẟ 13 C = −6.0‰ and ẟ 18 O = +6.9‰ for dolomite. The carbonatite isotopic compositions lie within the range known for primary igneous carbonatites (PIC), indicating their origin from a mantle source. The higher ẟ 18 O of the calcite end-member than the dolomite end-member indicates oxygen isotopic disequilibrium in the carbonatites. This disequilibrium was the product of sub-solidus oxygen isotope exchange with fluids derived from carbonatite melt during the brine-melt stage (600–400 °C). The narrow range of ẟ 13 C of the carbonatites and its similarity to the ẟ 13 C of depleted upper mantle (−6 ± 2‰) suggests that very little CO 2 was associated with the carbonatite melt. We infer that this type of carbonatite melt evolved from the upper asthenosphere or the base of the lithosphere (> 3 GPa, i.e., >100 km). Similar observations were made based on compiled ẟ 13 C and ẟ 18 O data for carbonatites from Salem-Attur fault, Sevathur, and Hogenakkal regions of the NGT. The ẟ 13 C and ẟ 18 O compositions of carbonatites from Samalpatti, India, however, are distinctly different and suggest two sources for carbonatite melts, one containing a significant amount of CO 2 and possibly evolved from somewhere in the middle of the lithospheric mantle (∼ 2–3 GPa, i.e., 60–100 km) and the other associated with crustal sedimentary or metasedimentary carbonates, which were possibly incorporated during emplacement of carbonatite melts originating from the lithospheric mantle. This study shows that carbonatite melts in the NGT mostly evolved from different levels of the upper mantle and were emplaced to crustal levels, perhaps via a rifting-induced fracture network. • Carbonatites of the Pakkanadu Alkaline Complex from southern India were investigated. • ẟ 13 C and ẟ 18 O of the carbonatites indicate their origin from a mantle source. • O isotope exchange occurred between the carbonatite and carbonatite-derived fluids. • CO 2 production was insignificant during partial melting of the mantle source. • The carbonatite was emplaced to crustal levels via a rift-related fracture system.