Geochemical characteristics of Cretaceous carbonatites from Angola |
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| Authors: | A Alberti F Castorina P Censi P Comin-Chiaramonti CB Gomes |
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| Institution: | 1 Dipartimento di Science della Terra, Trieste University, Via E. Weiss 8, I-34127 Trieste, Italy;2 Dpartimento di Scienze della Terra, “La Sapienza” University, Piazzale A. Moro 5, I-00185, Rome, Italy;3 Istituto di Mineralogia, Petrografia e Geochimica, Palermo University, Via Archirafi 36, I-90123 Palermo, Italy;4 Dipartimento di Ingegneria Chimica, dell'Ambiente e delle Materie Prime, Trieste University, Piazzale Europa 1, I-34127, Trieste, Italy;5 Istituto de Geociĉias, São Paulo University, USP, CP 11348, 05422-970 São Paulo, Brazil |
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| Abstract: | The Early Cretaceous (138–130 Ma) carbonatites and associated alkaline rocks of Angola belong to the Paraná-Angola-Etendeka Province and occur as ring complexes and other central-type intrusions along northeast trending tectonic lineaments, parallel to the trend of coeval Namibian alkaline complexes. Most of the Angolan carbonatite-alkaline bodies are located along the apical part of the Moçamedes Arch, a structure representing the African counterpart of the Ponta Grossa Arch in southern Brazil, where several alkaline-carbonatite complexes were also emplaced in the Early Cretaceous. Geochemical and isotopic (C, 0, Sr and Nd) characteristics determined for five carbonatitic occurrences indicate that: (1) the overall geochemical composition, including the O---C isotopes, is within the range of the Early and Late Cretaceous Brazilian occurrences from the Paraná Basin; (2) the La versus 
relationships are consistent with the exsolution of COi2-rich melts from trachyphonolitic magmas; and (3) the 
and 
initial ratios are similar to the initial isotopic ratios (129 Ma) of alkaline complexes in northwest Namibia. In contrast, the Lupongola carbonatites have a distinctly different 
initial ratio, suggesting a different source.The Angolan carbonatites have Sr---Nd isotopic compositions ranging from bulk earth to time-integrated depleted sources. Since those from eastern Paraguay (at the western fringe of the Paraná-Angola-Etendeka Province) and Brazil appear to be related to mantle-derived melts with time-integrated enriched or B.E. isotopic characteristics, it is concluded that the carbonatites of the Paraná-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. |
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