Abstract

A detailed 4oAr/ 39Ar study, of mineral separates from the Jurassic Atlantic Continental Tholeiites (JACT) of Guyana (French Guyana and Surinam, South America), and Guinea (West Africa) related to the initial opening of the Central Atlantic, has been carried out. In French Guyana, plateau ages of 196.0 _I 5.7 Ma and 196.1 i 7.5 Ma were obtained on single, small amphibole grains from NNW-SSE trending dykes. In Guinea, single biotite grains from intrusive formations from the Kakoulima and Fouta Djalon areas yielded plateau ages of 200.4 k 0.2 Ma and 194.8 + 0.5 Ma, concordant with high temperature apparent ages on other biotites. The bulk plagioclase samples display disturbed age spectra due to alteration and excess argon. However, intermediate temperature, weighted mean plagioclase ages are similar in both regions of Guyana and Guinea, ranging from 200.2 + 2.4 Ma to 188.7 + 1.9 Ma, partly in agreement with the amphibole and biotite data. These data, combined with previous 40Ar/ 39Ar and U/Pb results from the northern part of the Central Atlantic margins, indicate intense magmatic activity distributed over a large area from Iberia to Liberia (ca. 4500 km long) for a short period of time (204-195 Ma, perhaps less for the bulk of the magmatisml during the initial break-up of Pangea continent. These data do not support an initiation of the magmatism from a radial volcano-tectonic system centred in the south of the region, as suggested by May [ 11, and the initial break-up seems to affect the whole Central Atlantic during a period of 9 Ma.