TRIASSIC-LIASSIC THERMAL EVENTS IN MOROCCO RELATED TO THE OPENING OF THE ATLANTIC-OCEAN

During the late Triassic and the early Lias, intracontinental basins with emission of tholeiitic lavas related to early stages of the North Atlantic rifting developed along the Moroccan Atlantic margin and within the Atlasic domain. K-Ar dating of grain size fractions of Cambrian slates from the Hercynian basement and of sediments from the Triassic cover yields apparent ages decreasing with grain size from 330-300 Ma for the coarse fractions to about 220-200 Ma for the clay size fractions. These latter values indicate either a K-Ar reset of Hercynian minerals or the occurrence of neoformed phases under very-low temperature conditions during the late Triassic - early Lias. Temperatures of 185-210-degrees-C maintained during 15 +/- 10 Ma can be deduced from theoretical calculation. These values can account for the isotopic rehomogenization of fine-grained Hercynian micas of the Haut Atlas. Ascension of the upper mantle, within a thin continental crust during early stages of the Atlantic rifting is considered.