Geochemical and Isotopic Constraints on Tonian Alkaline Magmatism Associated With Crustal Extension in the Southern Granulite Terrane, India

ABSTRACT

The Southern Granulite Terrane (SGT) of India hosts a chain of Tonian–Cryogenian alkaline intrusions emplaced along major crustal‐scale shear zones, representing a widespread episode of Neoproterozoic magmatism. Here, we investigate the Kamaneri alkaline pluton through field relationships, petrography, whole‐rock geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopes to constrain its petrogenesis and geodynamic significance. The Kamaneri pluton is concentrically zoned, with relatively less evolved grey syenites at the core grading outward into porphyritic syenites and pink granites. This compositional zoning is accompanied by geochemical variations consistent with progressive magmatic differentiation within a long‐lived, open magmatic system. Mafic microgranular enclaves (MMEs) occurring within the pluton suggest injection of mafic magma and magma mingling. The Kamaneri rocks are alkaline in character and range from shoshonitic to ultrapotassic compositions, evolving from metaluminous syenites to peraluminous granites. They are enriched in large‐ion lithophile elements (LILE) and display pronounced Ti–Nb–Ta depletion. Zircon U–Pb geochronology yields Tonian crystallisation ages of 823 ± 12 Ma and 800 ± 16 Ma. Zircon Lu–Hf isotopic compositions show strongly negative εHf( t ) values (−18.1 to −21.2) and Neoarchean–Paleoproterozoic model ages, indicating involvement of ancient crustal components and long‐term evolution of an enriched source reservoir. Geochemical and isotopic characteristics, together with comparisons with coeval alkaline complexes in the SGT, suggest derivation from an enriched lithospheric mantle source and involvement of recycled crustal components. The magma evolved through clinopyroxene–amphibole fractionation, periodic mafic recharge, feldspar accumulation, and limited crustal interaction. The Kamaneri pluton forms part of the widespread Tonian alkaline magmatic event recognized across the SGT. Although the ultimate geodynamic trigger remains uncertain, the spatial distribution of Tonian alkaline plutons along long‐lived crustal‐scale sutures suggests that magma generation was linked to Neoproterozoic lithospheric extension and reactivation of ancient enriched lithospheric mantle domains.