Abstract
The Neoproterozoic Sevattur alkaline complex (Southern Granulite Terrain, India) comprises calcite- and dolomite‑carbonatites spatially associated with an alkaline silicate suite including monzodiorite, monzonite, syenite, granite, albitite, and pyroxene hornblendite enclaves, exclusively hosted within monzonite and monzodiorite. The carbonatites are characterized by enrichment in high field strength elements (HFSE), Ba, Sr, and rare earth elements (REE)-bearing phases. Their radiogenic isotopic signatures (εSr(i) = +21.2 to +26.4; εNd(i) = −8.0 to −5.2) are consistent with derivation from an enriched, metasomatized peridotite mantle source. Their mineralogical, geochemical, and isotopic signatures lack evidence of crustal contamination or any genetic link to the associated silicate suite. In contrast, the unaltered silicate rocks display relatively less radiogenic Sr (εSr(i) = +4.6 to +11.6) and Nd (εNd(i) = −9.7 to −6.8) isotopic signatures. Combined with their whole-rock geochemical characteristics and coherent mineral evolutionary trends, these features indicate derivation from an alkali-rich basaltic parent magma that evolved by fractional crystallization. The cumulate texture, similar pyroxene and amphibole compositions, and coherent geochemical and isotopic trends indicate that the pyroxene hornblendite represents cognate cumulates derived from the silicate magma. Evidence of limited crustal assimilation is confined to the most evolved members of the suite, namely the granite and the albitite. The silicate suite records a transition from magmatic to hydrothermal stages: (i) amphiboles crystallized at magmatic conditions (2–6 kbar; 839–986 °C), whereas mica, feldspar, and actinolite formed at shallower levels under hydrothermal conditions (1–2 kbar; 400–715 °C) and (ii) later low-temperature overprint (∼188 °C) in granite, marked by chlorite and clinozoisite which also has resulted in extremely low calculated εSr(i) values (−37.7 to −7.7) in some silicate rocks. Fenitization is largely absent in the silicate suite and is restricted to the localized phlogopite-rich clinopyroxenite, formed through interaction with carbonatite-derived fluids. Comparative mineral chemistry reveals systematic differences between carbonatites and silicate rocks: diopside–hedenbergite, phlogopite–annite, and Ca-rich amphiboles predominate in the silicate suite, whereas carbonatites host aegirine, phlogopite, and alkali-rich amphiboles. These contrasting mineralogical trends, combined with trace-element systematics (Ba/Mn, Nb/Th, Ba/Y) and isotopic signatures, exclude both silicate–carbonate liquid immiscibility and involvement of a common parental magma, and instead indicate derivation from independent magma sources.