LP-HT metamorphism in the Jebilet Massif (Moroccan variscan belt): from Paleothetys opening to Pangea formation

In the westernmost part of the Variscan belt, petro-structural observations, mineral equilibria modelling and U-Pb age determination were carried in the Jebilet massif (Morocco), in order to precise the deformation–metamorphism of NW Africa during Late Paleozoic Variscan events. A first episode corresponds to the emplacement of the main Oulad Ouaslam intrusion at 335.2 ± 0.8 Ma (U-Pb LA-ICP MS on zircon) within a Visean intracontinental basin. A second episode correspond S to SE convergence resulting into nappe stacking and upright folding in the supracrustal level together with the progressive development of a sub-horizontal metamorphic foliation around the pre-heated pelitic country rocks of the intrusion up to the sillimanite zone. Crd-And growth during this event in metapelite indicate indicating heating and increase of pressure up to 600–625 °C and 1.6–2.0 kbar. The timing of this sequence in metapelite is constrained by the U-Pb monazite age of 323.4 ± 3.6 Ma, interpreted as minimum age of metamorphic peak conditions, ca. <15 Ma younger than the emplacement of the intrusion. This very moderate thickening is followed by WNW convergence, sub-orthogonal to the first one, affecting all the previous structures and continuing probably up to the Cisuralian, similar to neighboring massifs. The first episode coincides with emplacement of magma within Visean intracontinental basins at the western tip of the ongoing opening Palaeo-Tethys Ocean, whereas the monazite dating document for the first time the precocious onset, during Late Serpukhovian–Early Bashkirian of the convergence in the NW African Variscan segment.