Evolution métamorphique des métapélites du Massif hercynien des Rehamna (Maroc): implications tectonothermales

Abstract

conditions. The structural and metamorphic study of the syn-peak mineral assemblages apparently suggests a similar tectonothermal evolution for the Skhour and Devonian-Visean formations. Nevertheless, mineral compositions in the developed metamorphic assemblages are indicative of higher metamorphic conditions in the first formation. Thermobarometric P-T conditions of the regional thermal peak were calculated at 560 Q C and 9 kbar in the Skhour Nappe, and 532 QC and 7 kbar in eastern Rehamna. These P-T conditions are also mainly coincident with those of the pressure peak. This suggests a very limited influence of decompressive thermal relaxation in the final tectonothermal evolution of the Rehamna Massif, interpreted as due to rapid exhumation of the belt as a result of syn-orogenic crustal extension. The syn-extensional part of the P-T path shows slightly retrogressive decompression, which finally reached low pressure conditions in the andalusite stability field .

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

“…The Central Zone is formed by Devono‐Carboniferous intracontinental basins and underlying Proterozoic basement (Figure ). These basins are intensively reworked by late Paleozoic deformation that took place in a relatively high Barrovian geothermal gradient [ Aghzer and Arenas , ; Michard et al ., ]. This Variscan metamorphic zone crops out in three separated crystalline massifs which are from the north to the south: the Central Massif, Rehamna, and Jebilet.…”

“…Two main metamorphic events have been described in the Rehamna. The first one is a syntectonic metamorphism affecting the lower metamorphic unit and reaching the staurolite stability field at around 6–9 kbar and 410–560°C [ Diot , ; Aghzer and Arenas , , ; El Mahi et al ., ]. This is followed by a syntectonic to late tectonic metamorphism penecontemporaneous with the intrusion of the Ras El Abiod and Sebt‐Brikiine granitoids, respectively [ Hœpffner et al ., ; Diot and Bouchez , ; Lagarde et al ., ; Baudin et al ., ].…”

“…Compositional layering S0 is sometimes well preserved in weakly metamorphosed metasediments (Figures b and i). Biotite, garnet, and staurolite progressively grow in the prograde gradient as shown by textural relationships (e.g., garnet enclosed in staurolite porphyroblasts, Figure c) and by the normal zonation of garnet porphyroblasts [ Aghzer and Arenas , , ; El Mahi et al ., ].…”

The Variscan Belt

“…The main recent references are indicated in the legends of the figures (especially Fig. 3.23), with additional references as follows: Kharbouch et al (1985), Lagarde & Michard (1986), , Aghzer & Arenas (1995), Youbi et al (1995), Huvelin & Mamet (1997), Ouanaimi & Petit (1992), El Hassani et al (1994), Bouabdelli & Piqué (1996), Razin et al (2001), Chèvremont et al (2001), Baudin et al (2001Baudin et al ( , 2003 Graham & Sevastopulo (2007). For more specific information on the structure and/or geochemistry of the Variscan granites, see Lagarde et al (1990), Bouchez & Diot (1990), Mrini et al (1992), Oukemeni et al (1995), Gasquet et al (1996), Ajaji et al (1998), Watanabe (2002), Essaifi et al (2003), El Hadi et al (2003, 2006b), Boummane & Olivier (2007), with references therein.…”

“…-(D) Idealized kinematic model (Lagarde & Michard, 1986): the intense stretching deformation (type 1 = c) occurs within major shear zones cutting through a thick tectonic pile in an area of high geothermal gradient Bedding-parallel extensional structures (low angle normal faults) have been observed in the non-metamorphic Ordovician formations of the Zaer anticlinorium, and ascribed to an extensional deformation following the main Variscan contraction and crustal thickening . Likewise, late orogenic extensional tectonics could have favoured the exhumation of the relatively deep (15-20 km) metamorphic zones of Central Rehamna to the same level as the less metamorphic units (Aghzer & Arenas 1995;Baudin et al, 2003). Even if such a process is likely, its modalities and importance are disputable.…”