Variscan orogen witnesses in the internal Rif belt (northern Morocco): Metamorphic evolutions, ages and tectonic evolution during Pangaea amalgamation.

Variscan orogen witnesses in the internal Rif belt (northern Morocco): Metamorphic evolutions, ages and tectonic evolution during Pangaea amalgamation.

 

Corsini M. ¹Lardeaux J.M.^1,2, El Bakili^1,3, Bosse V. ⁴, Homonnay E.¹, Bosch D. ⁵ Chalouan A.³, Münch P.⁵, Ouazzani-Touhami, M.⁶

1- Observatoire de la Côte d’Azur, Géoazur, Université Côte d’Azur, IRD, CNRS, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France.

2- Czech Geological Survey, Centre for Lithospheric Research- Klárov 3, 118 21 Prague 1- Czech Republic.

3- Faculté des sciences Rabat - Université Mohammed V - 4 avenue Ibn Batouta,B.P. 1014 Rabat- Morocco.

4- Université Clermont Auvergne, Observatoire de Physique du Globe de Clermont Ferrand, Laboratoire Magmas et Volcans, 6 Avenue Blaise Pascal, 63178 Aubière, France.

5- Université Montpellier 2, Géosciences Montpellier, UMR 5243, CC 060, place Eugène Bataillon, 34095 Montpellier cedex 5, France.

6- Département de Géologie, Université Abdelmalek Esaadi, 93003 Tetouan, Morocco.

Since the late 1970s it is widely recognized that Variscan orogen witnesses are identifiable in the internal zones of the Rif–Betic orogenic system located in the peri-Mediterranean recent (i.e. Alpine cycle) mountain belts. The Rif belt (northern Morocco) is thus a polyorogenic system involving the superimposition of an Alpine cycle upon a previous Variscan cycle. Deciphering the Variscan palaeo-position and tectonic evolution of northern Morocco is therefore challenging and the link between the southern European Variscan belt and the Rif belt is still matters of debates.

We present and discuss the main results obtained, in the internal Rif, during more than 20 years of cooperation between Moroccan and French geologists. Our database integrates (1) high-resolution lithologic and tectonic mapping of the two zones recognized in the internal Rif (from top to bottom Ghomarides and Sebtides), (2) modern petrologic investigations (i.e. geothermobarometry combined with thermodynamic modelling) performed on relict phases and/or shielded mineral inclusions within large poikiloblasts preserved within alpine metamorphic assemblages in the Sebtides and (3) geochronologic investigations including (a) in situ U‒Th‒Pb dating of both frozen monazite inclusions in large sized garnets and monazites oriented in the main regional foliation from high-grade Beni-Bousera metapelites (Lower Sebtides) and (b) ⁴⁰Ar–³⁹Ar dating of white mica porphyroclasts from amphibolite facies metagreywackes recognized west of the Beni Bousera peridotite massif (Upper Sebtides) and from greenschist facies of the Ghomarides metapelites.

We document:

• The late Oligocene to early Miocene age of the MP granulite-facies metamorphism, responsible for crustal anatexis and melts production, coeval with the development of the main regional foliation depicted in the Lower Sebtides,
• the discovery, in the Upper Sebtides, of an amphibolite facies metamorphic event coeval with the Beni Bousera peridotite emplacement during the Upper Triassic in relation to the rifting of Pangea,
• the late Carboniferous–early Permian age of a HP granulite facies metamorphism (1.5–2.0 GPa for 760–820°C) recognized in the Beni Bousera metapelites. These conditions indicate a palaeogradient typically developed during crustal thickening in a collision belt.

These new constraints are confronted first with those recently obtained on the Betic orogen and the southern Europe Variscides and second to unified full plate reconstruction models in order to better understand the involvement of northern Morocco in Pangaea amalgamation.