Petrogenesis and tectonic reworking of nepheline syenites from the Khariar Alkaline Complex, Eastern Indian Shield

The Khariar Alkaline Complex (KAC), emplaced within the Terrane Boundary Shear Zone (TBSZ) along the boundary between the Eastern Ghats Mobile Belt (EGMB) and the Bastar Craton, represents an important Mesoproterozoic alkaline magmatic centre developed during intracontinental extension along a long-lived craton–mobile belt interface. Nepheline syenite constitutes the dominant lithology and is composed of nepheline, K-feldspar (orthoclase–perthite), aegirine, amphibole, biotite, apatite, titanite, and opaque phases, with garnet-bearing variants containing calcic garnet enriched in andradite–grossular components. Field and petrographic evidence indicate two distinct emplacement phases: an early deformed and metamorphosed suite exhibiting syn-tectonic fabrics, and a younger, massive, undeformed suite indicative of post-tectonic intrusion. Geochemically, the nepheline syenites are silica-undersaturated, miaskitic foid syenites with high-K alkaline affinity. They are characterized by low concentrations of compatible elements (Ni, Cr and Co), enrichment in large-ion lithophile elements (Sr, Ba and Rb) and high-field-strength elements (particularly Zr and Nb), and strongly fractionated chondrite-normalized REE patterns marked by elevated (La/Yb)ₙ ratios and comparatively flat HREE segments. The absence of systematic Nb–Ta depletion, together with low Th/Ce ratios (0.03–0.1), suggests limited crustal contamination and argues against a typical subduction-related affinity. Variable Eu anomalies are interpreted to reflect feldspar-controlled fractionation and crystal accumulation during magma evolution. Trace-element systematics indicate derivation from an enriched, metasomatized mantle source, predominantly within the spinel lherzolite stability field, possibly close to the garnet–spinel transition zone, and generated through low to moderate degrees of partial melting. The geochemical characteristics further indicate melting of a phlogopite ± amphibole-bearing metasomatized subcontinental lithospheric mantle, with probable interaction between enriched lithospheric mantle and asthenospheric melts during intracontinental extension. The nepheline syenites are interpreted to have formed in an intraplate extensional setting along the Bastar Craton–Eastern Ghats Mobile Belt margin and were subsequently affected by deformation and metamorphic reworking during Pan-African tectonism associated with reactivation of the terrane boundary zone. The younger intrusive phase records continued alkaline magmatism during prolonged tectonic reactivation of the TBSZ. Collectively, the Khariar Alkaline Complex forms part of a regionally developed Mesoproterozoic alkaline magmatic province along the EGMB–Bastar Craton margin and provides important insights into lithosphere–mantle interaction, mantle metasomatism, continental rifting, and subsequent Pan-African tectono-metamorphic reworking during the Proterozoic evolution of the Eastern Indian Shield.