DUPAL ANOMALY OF BRAZILIAN CARBONATITES - GEOCHEMICAL CORRELATIONS WITH HOTSPOTS IN THE SOUTH-ATLANTIC AND IMPLICATIONS FOR THE MANTLE SOURCE

Geochemical and Sr, Pb, O and C isotopic data are reported for carbona tite samples from five locations in southeast Brazil. Elemental abunda nces and delta(13)C(PDB) data (between -5.8 and -7.2 parts per thousan d) prove that all the samples are derived from the mantle. Dupal isoto pic characteristics in the all carbonatite samples from five locations in southeast Brazil are found in this study, characteristics that hav e not previously been recognized in carbonatites. Brazilian carbonatit es possess average Delta 8/4 values of between 101 and 145, average De lta 7/4 values of between 5.2 and 10.3, and initial Sr isotopic compos itions of between 0.7046 and 0.7062. The Brazilian carbonatites compri se two groups: The northern group is coincident with the passage of th e Trindade non-Dupal hotspot at ca. 80 Ma, while the southern group ma inly corresponds to the passage of the Tristan de Cunha Dupal hotspot at ca. 130 Ma. Although we expected a geochemical correlation between the Brazilian carbonatites and the South Atlantic hotspots, the enrich ed isotopic signature (EM1) of all the carbonatite samples is similar to that of alkali basalts on Tristan de Cunha. The combined O-Sr isoto pic diagram indicates that the southern group carbonatites have neglig ible or only slight crustal contamination. The northern group samples show significantly higher delta(18)O(SMOW) values of 9-14 parts per th ousand, more radiogenic Pb isotopic ratios, and a Sr-87/Sr-86 value of 0.705. Even if these signatures are derived from the contamination of a lower crustal component with mantle sources, it is clear that the p arental magma also has inherent EM1 isotopic characteristics. The inte rpretation of the origin of EM1 in the Brazilian carbonatites (subcont inental lithospheric mantle vs. asthenosphere) is dependent on the mod el of Parana volcanism at ca. 130 Ma, which remains controversial. One possibility is that both the northern and southern carbonatites came from enriched SCLM under a part of Gondwanaland. In this case, a hotsp ot would provide the thermal energy to melt the lithospheric source re gion for both the Parana flood basalts and the alkali carbonatitic vol canism. Another possibility is that the source of the northern carbona tites is also the Tristan plume carbonate-rich material which had once been trapped under the crust and reactivated by the Trindade hotspot, on the assumption that Parana volcanism at ca. 130 Ma was mainly trig gered by the huge Tristan plume activity. If anything, we favour the l atter and believe an asthenospheric mantle plume origin for bath the u ltimate carbonate source and the Dupal anomaly in the Brazilian carbon atites.