Petrogenesis and tectonic significance of the Late Proterozoic unmetamorphosed mafic dyke swarms from the Salvador area (NE Brazil)
Bellieni, G.; Petrini, R.; Piccirillo, E. M.; Brito, C. M.; Figueiredo, A. M. G.; Marques, L. S.; De Min, A.; Melfi, A. J.
published: Jul 22, 1998
ArtNo. ESP154017303004, Price: 29.00 €
Archean to Early Proterozoic granulites from the Sao Francisco Craton, which represents the western extension of the Congo Craton (W Africa), are crosscut by abundant Early (2.4-2.0 Ga) and Late Proterozoic (1.2-1.0 Ga) mafic dyke swarms. The Late Proterozoic unmetamorphosed dyke swarms which outcrop in the Salvador (SA) and Olivenrca-Ilheus-Itabuna-Camaca (OIC) areas (NE Brazil) are composed of twopyroxene tholeiites and are subdivided into two groups characterized by different TiO2 (i.e. TiO2 > 3wt.% = HTi to TiO2 < 2wt.% = LTi) and incompatible element contents. SA basalts are geochemically similar to those from the OIC area (both LTi and HTi types). LTi basalts from SA have higher initial Sr and lower Nd isotopic ratios than those of HTi type. Relative to Bulk Earth, SA LTi basalts plot in the enriched quadrant of the mantle array, whereas both SA and OIC HTi basalts plot in the depleted quadrant. These different isotope features cannot easily be explained as due to granulite country rock contamination, and probably represent melts derived from heterogeneous (lithospheric) mantle sources. SA and OIC basalts have isotope and chemical characteristics which may be explained as due to various melting degrees (spinel/garnet peridotite), gabbro fractionation and metasomatic processes, considering that crustal contamination played a minor role. All SA and OIC dyke swarms have geochemical features representative of intraplate continental magmatism, indicating that the Late Proterozoic mobile belt of the Sao Francisco (and Congo) Craton was of ensialic type, unrelated to suture/collision plate margin processes.