ISOTOPIC AND GEOCHEMICAL INVESTIGATION OF THE CHILWA-ISLAND CARBONATITE COMPLEX, MALAWI - EVIDENCE FOR A DEPLETED MANTLE SOURCE REGION, LIQUID IMMISCIBILITY, AND OPEN-SYSTEM BEHAVIOR

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 me lt derivation from a Rb/Sr- and Nd/Sm-depleted but Th/Pb- and U/Pb-enr iched mantle source. Initial Nd-143/Nd-144 (0.51265-0.51270) isotope r atios from the Chilwa Island carbonatites are relatively constant, but their initial Sr-87/Sr-86 (0.70319-0.70361) ratios are variable. The delta(18)O(SMOW) (9.53-14.15 parts per thousand) and delta(13)C(PDB) ( - 3.27 to - 1.50 parts per thousand) isotope ratios of the carbonates are enriched relative to the range of mantle values, and there is a ne gative correlation between delta(18)O and Sr isotope ratios. The varia tions in Sr, C, and O isotopic ratios from the carbonatites suggest se condary processes, such as interaction with meteoric groundwater durin g late-stage carbonatite activity. The initial Nd-143/Nd-144 (0.51246- 0.51269) and initial Sr-87/Sr-86 (0.70344-0.70383) isotope ratios from the intrusive silicate rocks are more variable, and the Sr more radio genic than those from the carbonatites. Most of the Pb isotope data fr om Chilwa Island plot to the right of the geochron and close to the oc eanic regression line defined by MORBs and OIBs. Initial Pb isotopic r atios from both carbonatites (Pb-207/Pb-204 15.63-15.71; Pb-206/Pb-204 19.13-19.78) and silicate rocks (Pb-207/Pb-204 15.61-15.72; Pb-206/Pb -204 18.18-20.12) show pronounced variations, and form two groups in P b-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the Ch ilwa Island carbonatites and intrusive silicates suggest that these me lts underwent different evolutionary histories. The chemical data, inc luding isotopic ratios, from the carbonatites and olivine nephelinites are consistent with magmatic differentiation of a carbonated-nephelin ite magma. A model is proposed in which differentiation of the carbona tite magma was accompanied by fenitization (metasomatic alteration) of the country rocks by carbonatite-derived fluids, and subsequent alter ation of the carbonatite by hydrothermal activity. The chemical and is otopic 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 increm ental batch melting of a depleted source and subsequent crustal contam ination.