Geochemical characteristics of Cretaceous carbonatites from Angola

The Early Cretaceous (138-130 Ma) carbonatites and associated alkaline rock s of Angola belong to the Parana-Angola-Etendeka Province and occur as ring complexes and other central-type intrusions along northeast trending tecto nic lineaments, parallel to the trend of coeval Namibian alkaline complexes . Most of the Angolan carbonatite-alkaline bodies are located along the api cal part of the Mocamedes Arch, a structure representing the African counte rpart of the Ponta Grossa Arch in southern Brazil, where several alkaline-c arbonatite complexes were also emplaced in the Early Cretaceous. Geochemica l and isotopic (C, O, Sr and Nd) characteristics determined for five carbon atitic occurrences indicate that: (1) the overall geochemical composition, including the O-C isotopes, is within the range of the Early and Late Creta ceous Brazilian occurrences from the Parana Basin; (2) the La versus La/Yb relationships are consistent with the exsolution of CO2-rich melts from tra chyphonolitic magmas; and (3) the Nd-143/Nd-144 and Sr-87/Sr-86 initial rat ios are similar to the initial isotopic ratios (129 Ma) of alkaline complex es in northwest Namibia. In contrast, the Lupongola carbonatites have a dis tinctly different Nd-143/Nd-144 initial ratio, suggesting a different sourc e. The Angolan carbonatites have Sr-Nd isotopic compositions ranging from bulk earth to time-integrated depleted sources. Since those from eastern Paragu ay (at the western fringe of the Parana-Angola-Etendeka Province) and Brazi l appear to be related to mantle-derived melts with time-integrated enriche d or B.E. isotopic characteristics, it is concluded that the carbonatites o f the Parana-Angola-Etendeka Province have compositionally distinct mantle sources. Such mantle heterogeneity is attributed to 'metasomatic processes' , which would have occurred at ca 0.6-0.7 Ga (Angola, northwest Namibia and Brazil) and ca 1.8 Ga (eastern Paraguay), as suggested by Nd-model ages. ( C) 2000 Elsevier Science Limited. All rights reserved.