The French Guyana doleritic dykes: geochemical evidence of three populations and new data for the Jurassic Central Atlantic Magmatic Province

A petrographic and geochemical study of 15 Early Jurassic and 7 Proterozoic dolerites of French Guyana, and of one Jurassic dolerite from Ivory-Coast were carried out. The Early Jurassic SSW-NNE trending dykes have doleritic aphyric or gabbroic phyric texture. Their chemical compositions, slightly under-saturated to over-saturated, show moderate to low Mg-ratios (63-36), high TiO2 contents (1.85-3.56 wt.%), weak rare earth element fractionation [1.8 < (La/Yb)(n) < 4.6], negative Sr-anomalies (0.41 < Sr* < 0.91) and positive TiO2-anomalies (1.2 < TiO2* < 2.15). They correspond to sub-alkaline high-Ti basalts (HTiB). The La/Nb ratio (1.18 +/- 0.23) is close to that of the OIB-type Deccan basalts and incompatible element ratios: Rb/Nb (1.07 +/- 0.3), Zr/Y (4.6 +/- 0.8) are intermediate between those of HTiB in Deccan and those of HTiB in Parana. These geochemical signatures and recent isotopic data of Bertrand et al. [Bertrand, H., Liegeois, J.P., Deckart, K., Feraud, G., 1999. High-Ti tholeiites in Guinea and Their Connection with the Central Atlantic CFB Province: Elemental and Nd-Sr-Pb Isotopic Evidence for Preferential Zone of Mantle Upwelling in Cause of Rifting. AGU spring meeting (Abst. p 317)] suggest that their magmatic source is different from that of the other basalts of the Central Atlantic Magmatic Province (CAMP). Such signatures are restricted to a central zone coinciding with the Panafrican Rokelides suture. We propose a model of sub-lithospheric preferential channelling of an asthenospheric ascent in this zone. Two other groups of dykes were identified in French Guyana. Compared to the Jurassic ones the Proterozoic dykes have NNW-SSE and E-W trending direction, more important LILE enrichment, low TiO2 contents (<2 wt%) and NbTa negative anomalies. Their calc-alkaline signature could be the result of a previous subduction and may be related to the 1800 Ma Venturi-Tapajos event, which contaminated the mantle source. (C) 2002 Elsevier Science Ltd. All rights reserved.