Decoding Archean-Paleoproterozoic carbonatite and syenite magmatism at the Dharwar Craton-Granulite Terrain boundary, southern India: Implication for petrogenesis, source characteristics and timing of terrane subduction-accretion

This study presents a detailed investigation of the field relationships, geochronology, mineral-whole rock geochemistry, and isotope systematics (Sr-Nd-Pb-Os-C-O) of a newly identified carbonatite-alkaline syenite intrusive system from the Gundlupete area, located near the tectonic boundary between the Western Dharwar Craton (WDC) and the Granulite Terrain of South India. The carbonatite intrudes the syenite and is exposed along an E-W to ENE-WSW trending splay of the Moyar shear zone near the southern margin of the WDC. In-situ U-Pb dating of titanite and monazite provides crystallization ages of 2590 ± 42 Ma and 2474 ± 27 Ma for the syenite and carbonatite, respectively, indicating two distinct magmatic episodes with independent petrogenetic histories. The syenite comprises alkali feldspar (Or_93.7-100), albite (Ab_98-99), clinopyroxene (Di_{22.08–65.68} Hd_{20.04–44.65} Aeg_{13.91–44.64}), biotite (X_mg: 0.54–0.58), titanite (Al: 0.03–0.06 apfu), and quartz. Geochemically, the syenite exhibits shoshonitic characteristics (K₂O/Na_₂O: 0.9–2.42), enrichment in LILEs and LREEs, depletion in Mg, Ni, Cr, and HFSEs (Nb, Ta, Ti, Zr, Hf), and crust-like ratios such as high Th/Nb_PM (avg. 79) and low Nb/U (avg. 2.17). Initial εNd values (-1.4 to 1.0) align with the Mesoarchean Dharwar TTG suite, suggesting a derivation from evolved partial melts of TTG sources, followed by clinopyroxene-biotite dominated fractional crystallization. The carbonatite is coarse-grained and composed predominantly of calcite, apatite, magnetite, monazite, amphibole, and phlogopite. Calcite and apatite are enriched in Sr and REEs, while phlogopite is Fe-Al-rich (Fe/(Fe+Mg)>0.22), and magnetite, containing 0.39–0.81 wt.% TiO₂, follows a typical titano-magnetite evolutionary trend. Geochemically, the carbonatite shows selective enrichment in LILEs (e.g., Ba and Sr) and Th, with lower HFSE concentrations (e.g., Zr, Hf, Ti, Nb, Ta). Isotopically, the carbonatite has a narrow range of Sr (⁸⁷Sr/⁸⁶Sr_i: 0.70307–0.70321), Nd (εNd_i: -3.7 to -2.1), and Pb (²⁰⁶Pb/²⁰⁴Pb_i: 13.49–13.85, ²⁰⁷Pb/²⁰⁴Pb_i: 14.70, ²⁰⁸Pb/²⁰⁴Pb_i: 33.32–34.96), while C-O isotopes range from -10.2‰ to -9.4‰ (δ¹³C) and 7.7‰ to 10.3‰ (δ¹⁸O). These characteristics suggest a primary carbonate melt derived from chondritic to slightly enriched mantle sources, with minor crustal assimilation and extensive crystal fractionation. The syenite’s geochemical signatures, εNdi values, and Nd model ages (2.8–3.0 Ga) support a derivation from Mesoarchean TTG sources. The carbonatite’s low δ¹³C values, higher time-integrated Rb/Sr ratios, and lower Sm/Nd and U/Pb ratios reflect the influence of recycled subducted components. Field, geochronological, geochemical, and isotopic evidence links the ca. 2.59–2.47 Ga magmatic events to the Neoarchean amalgamation of the Dharwar Craton and Granulite Terrain, driven by the northward subduction of the Dharwar Ocean lithosphere beneath the WDC. We propose a tectonic model where subduction-induced magma underplating triggered syenite emplacement at 2.59 Ga, coinciding with similar arc-related magmatism in the region. The carbonatite represents a later magmatic pulse in a post-collisional setting at 2.47 Ga, utilizing pre-existing conduits during the terminal accretion phase of the Dharwar Craton and Granulite Terrain at the Archean-Proterozoic boundary.