Decoding the Carbonatite and Clinopyroxenite Association at Hogenakkal, India: Insights into U–Pb Monazite Geochronology, <i>In situ</i> Trace Element, and Sr-C-O Isotope Geochemistry

Abstract The Paleoproterozoic Hogenakkal complex (India) consists of carbonatites and clinopyroxenite emplaced within granulite country rocks. U–Pb monazite ages (~2.49 Ga) suggests that the carbonatites were emplaced in a post-collisional setting following arc magmatism and collision events between the Western Dharwar Craton and the Biligiri Rangan block (Southern Granulite Terrane). The carbonatites are of two distinct types: silicate-rich, xenolith-bearing early carbonatite-I, and silicate-poor late carbonatite-II. The absence of alkaline silicate rocks combined with the lack of compositional evolution and the presence of xenocrystal clinopyroxene, together with differing Sr isotopic signatures between clinopyroxenite (clinopyroxene: 0.70147 ± 0.00021; 2σ, n = 19) and carbonatite (apatite: 0.70175 ± 0.00015; 2σ, n = 17) suggests that these lithologies were not derived from a common carbonate-bearing alkaline silicate magma or represent an antiskarn. Additionally, the C–O isotopic signature (δ13C = −5.56 to −8.02‰; δ18O = 9.50–9.90‰) combined with the absence of dolomite and primary Fe–Mg silicates suggests that the carbonatites originated from mantle-derived calcitic magma, possibly by partial melting of a carbonated eclogite source. Subsequent to emplacement there was extensive phlogopitization of the pre-existing clinopyroxenite. Magmatic-to-carbohydrothermal evolution is governed by fractionation of magmatic calcite (Cal-1a) and apatite (Ap-1a), as well as interactions between the carbonatite and pre-existing clinopyroxenite and K-feldspar pegmatite. These processes collectively controlled the formation of lithology-specific REE mineralization. In carbonatite-I, the magmatic stage begins with the crystallization of LREE-rich calcite (Cal-1a) [(La/Yb)Cn: 18–36] and apatite (Ap-1a) [(La/Yb)Cn: 45–98]. The transition to the late-magmatic-to-carbohydrothermal stage involved the formation of phlogopite, actinolite, and allanite-(Ce), which co-precipitated with LREE-depleted Cal-1b [(La/Yb)Cn: 0.6–5.8] and Ap-1b [(La/Yb)Cn: 11–31]. Carbonatite-II has high REE and Sr enrichment, with REE-rich Ap-2 and Sr-REE mineralization, including celestine, monazite-(Ce), and hydroxylbastnäsite-(Ce). During the carbohydrothermal stage, LREE-depleted Cal-2 [(La/Yb)Cn: 1.6–6.1] crystallizes together with intergrowths of monazite-(Ce), celestine, and hydroxylbastnäsite-(Ce), and monazite-(Ce) rims on Ap-2. Externally derived hydrothermal fluids overprint the carbohydrothermal assemblage within carbonatite-I, clinopyroxenite, and K-feldspar pegmatites, forming Sr-LREE-poor Cal-3 [(La/Yb)Cn < 1], REE-Sr-bearing epidote, LREE-depleted Ap-3 [(La/Yb)Cn: 0.7–6.2], allanite-(Ce), and actinolite. Elevated δ18O (11.50‰), radiogenic Sr in calcite and some apatite (particularly Cal-3 and Ap-3), combined with prominent negative-Ce anomalies in Cal-3, indicate the involvement of low temperature, externally derived oxidizing hydrothermal fluids.

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