Isotopic and Geochemical Investigation of the Chilwa Island Carbonatite Complex, Malawi: Evidence for a Depleted Mantle Source Region, Liquid Immiscibility, and Open-System Behaviour

Initial Nd, Pb, and Sr isotopic data from carbonatites and associated intrusive silica-undersaturated rocks from the early Jurassic, Chilwa Island complex, located in southern Malawi, central Africa, suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted but Th/Pb- and U/Pb-enriched mantle source. Initial 143Nd/144Nd (0.51265–0.51270) isotope ratios from the Chilwa Island carbonatites are relatively constant, but their initial 87Sr/86Sr (0.70319–0.70361) ratios are variable. The δ18Osmow (9.53–14.15%0) and δ13CPDB (−3.27 to −1.50%0) isotope ratios of the carbonates are enriched relative to the range of mantle values, and there is a negative correlation between δ18O and Sr isotope ratios. The variations in Sr, C, and O isotopic ratios from the carbonatites suggest secondary processes, such as interaction with meteoric groundwater during late-stage carbonatite activity. The initial 143Nd/144Nd (0.51246 0.51269) and initial 87Sr/86Sr (0.70344–0.70383) isotope ratios from the intrusive silicate rocks are more variable, and the Sr more radiogenic than those from the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the right of the geochron and close to the oceanic regression line defined by MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites (207Pb/204Pb 15.63–15.71; 206Pb/204Pb 19.13–19.78) and silicate rocks (207Pb/204Pb 15.61–15.72; 206Pb/204Pb 18.18–20.12) show pronounced variations, and form two groups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the Chilwa Island carbonatites and intrusive silicates suggest that these melts underwent different evolutionary histories. The chemical data, including isotopic ratios, from the carbonatites and olivine nephelinites are consistent with magmatic differentiation of a carbonated-nephelinite magma. A model is proposed in which differentiation of the carbonatite magma was accompanied by fenitization (metasomatic alteration) of the country rocks by carbonatite-derived fluids, and subsequent alteration of the carbonatite by hydrothermal activity. The chemical and isotopic data from the non-nephelinitic intrusive silicate rocks reveal a more complex evolutionary history, involving either selective binary mixing of lower-crustal granulites and a nephelinite magma, or incremental batch melting of a depleted source and subsequent crustal contamination.