Mineralogy, in situ trace element geochemistry, and C–O isotopic composition of contaminated allanite-bearing Khaderpet carbonatite: insights regarding magmatic and supergene processes

Abstract The Khaderpet carbonatite (15°58′N, 77°33′E) occurs as a small plug-like intrusion (45 m × 60 m) within altered ultramafic volcaniclastic breccia of unknown parentage. Both the carbonatite and its host rock contain crustal xenoliths of granite and quartz monzonite. Although the absence of primary silicate and oxide phases obscures its direct genetic link with the host rocks, the carbonatite preserves clear evidence of magmatic crystallization subsequently overprinted by hydrothermal alteration, crustal assimilation and supergene oxidation. The rock is dominated by calcite, which occurs in three distinct generations. Early Sr-Ba-rich calcite-1 (0.8–2.1 wt.% SrO and 0.4–2.2 wt.% BaO) phenocrysts co-crystallized with rounded fluorite at temperatures above ∼600°C, and are hosted within a Ba-Sr-poor calcite-2 matrix. Mantle-like bulk-rock δ 13 C values (–4.21 to –4.62 ‰, VPDB), together with (La/Yb) Cn (>1–100) and Y/Ho (24–34) ratios in calcite-1 and calcite-2, support a primary magmatic origin. Evidence for crustal assimilation includes REE-Si enrichment in apatite (up to 1.3 wt.% SiO 2 ) by a britholite-type substitution, increased allanite abundance near xenolith contacts, Si-rich pyrochlore and interstitial quartz. Coarse calcite-3 veins crosscut the calcite-2 matrix and comprise Mn-Fe-Mg-rich bright calcite-3a cores and nearly pure, dark calcite-3b peripheries. Elevated Mn-Fe-Mg contents and high Y/Ho ratios (up to 64) in calcite-3a reflect rapid crystallization during waning hydrothermal stages. High δ 18 O values (+9.17 to +11.54 ‰ VSMOW) indicate low-temperature H 2 O-rich, CO 2 -poor meteoric fluid alteration. Negative Ce anomalies in apatite (Ce/Ce*: 0.8–0.3) and calcite (Ce/Ce*: 0.8–0.4), most pronounced in calcite-3b (Ce/Ce*: 0.2–0.6), together with apatite trace element compositions, indicate supergene alteration. Textural evidence of supergene alteration includes replacement of pyrochlore-1 by pyrochlore-2, pyrochlore-1 and pyrite by goethite, allanite-(La) by ferriallanite-(Ce), and late precipitation of baryte, REE-fluorocarbonates and vanadinite, indicating involvement of F – , SO 4 2– , Pb and V in oxidizing hydrothermal fluids.