Relationship between compositions of basaltoids in geosynclinal and orogenic volcanic associations

During the early stages of North Atlantic rifting, in the Late Triassic and Early Liassic, intracontinental basins with the eruption of tholeiitic lavas developed along the Atlantic margin of Morocco and in the Atlas domain. K-Ar dating of different grain-size fractions of Cambrian schists from the Hercynian basement and of sediments from the Triassic cover yield apparent ages which decrease with grain size, from 330-300 Ma for the coarse fractions to about 220-200 Ma for the clay-size fractions. These apparent ages indicate either K-Ar resetting of Hercynian minerals, or the development of new minerals during one or more Upper Triassic to Lower Liassic thermal or hydrothermal events. Theoretical calculations indicate that a temperature of 185-210 ° C maintained for 15 ± 10 m.y. can account for the isotopic rehomogenization of the fine-grained Hercynian micas of the High Atlas. The probable cause was a rise of the upper mantle in a thin continental crust during the first stages of Atlantic rifting.