Apatite compositional constraints on the magmatic to hydrothermal evolution of lamproites from Raniganj Basin, eastern India

A mineralogical study of early Cretaceous lamproite sill intrusions from the Raniganj Gondwana sedimentary basin in eastern India shows that apatite occurs as both phenocrystic and groundmass phase. Based on texture and compositional zoning patterns of apatite in lamproites from the Rajpura and Ramnagore collieries, three paragenetic stages of apatite are identified. Early-magmatic apatite (Ap-I), which forms the core of zoned grains, is Sr-rich–LREE-poor fluorapatite. This apatite underwent resorption prior to the growth of a second generation of magmatic fluorapatite (Ap-II). In Rajpura, Ap-II overgrowth rim is richer in Sr and LREE compared to Ap-I core. The increase in LREE is explained by the substitutions: (Na,K)⁺ + ∑LREE³⁺ = 2Ca²⁺, and [2∑LREE³⁺ + ₶ = 3Ca²⁺]. Ramnagore Ap-II overgrowth rim is oscillatory-zoned with fluctuations in Sr and LREE, which likely resulted from slow rate of diffusion of these elements relative to fast growth of crystals. Apatite of the third generation (Ap-III) forms the outermost rim of zoned grains and is marked by enrichment in Na, K and Ba. The substitutional schemes which explain the increase in Na and K from Ap-II to Ap-III are: (Na,K)⁺ + CO₃^2– = Sr²⁺ + PO₄^3– and [(Na,K)⁺ + (F,OH)^– = ₶ + ₶]. The role of carbonate in the former substitution is supported by high content of stoichiometrically calculated carbon (0.21–0.30 apfu) in Ap-III. The formation of Ap-III is attributed to metasomatic alteration of Ap-II by CO₂-bearing hydrothermal fluid and is associated with sodic metasomatism. Microporous texture has developed in Rajpura Ap-III which suggests a dissolution–reprecipitation mechanism for its development. This study demonstrates that compositional variations among different generations of apatite provide a meaningful record of melt evolution from early magmatic to magmatic-hydrothermal stages.