Crustal Derivation of the ca. 475~Ma Eppawala Carbonatites in Sri Lanka

Although a mantle origin of carbonatites has long been advocated, a few carbonatite bodies with crustal fingerprints have been identified. The Eppawala carbonatites in Sri Lanka are more similar to orogenic carbonatites than those formed in stable cratons and within plate rifts. They occur within the Pan-African orogenic belt and have a formation age of ca. 475 Ma newly obtained in this study with no contemporary mantle-related magmatism. These carbonatites have higher (87Sr/86Sr)i ratios (0·70479–0·70524) and more enriched Nd and Hf isotopic compositions than carbonatites reported in other parts of the world. Model ages (1·3–2·0 Ga) of both Nd and Hf isotopes [apatite ɛNd(t) = −9·2 to −4·7; rutile εHf(t) = −22·0 to −8·02] are in the age range of metamorphic basement in Sri Lanka, and the carbon and oxygen isotopic compositions (δ13CPDB = −2·36 to −1·71; δ18OSMOW = 13·91–15·13) lie between those of mantle-derived carbonatites and marble. These crustal signatures are compatible with the chemistry of accessory minerals in the carbonatites, such as Ni-free olivine and Al- and Cr-poor rutile. Modeling results demonstrate that the Eppawala carbonatite magmas originated from a mixture of basement gneisses and marbles, probably during regional metamorphism. This interpretation is supported by the occurrence of the carbonatites along, or near, the axes of synforms and antiforms where granitic gneiss and marble are exposed. Therefore, we propose that the Eppawala carbonatites constitute another rare example of a carbonatitic magma that was derived from melting of a sedimentary carbonate protolith. Our findings suggest that other orogenic carbonatites with similar features should be re-examined to re-evaluate their origin.