Metamorphic evolution of the metapelites of the Hercynian Massif in Rehamna (Morocco): Tectonothermal implications

In central Rehamna (Moroccan Hercynian Belt), the maximum crustal thickness was attained after the development of the Ouled Zedness Thrust, which emplaced the Jorf Fl Ahmar Formation (Eastern Rehamna) over the Skhour Nappe (central Rehamna) during a metamorphic event of intermediate P/T reaching amphibolite-facies conditions. Later on, the Fl Broumi detachment caused the emplacement of a Devonian-Visean unit (El Broumi-Ouled Hassine and Lalla Tittaf Formations), previously metamorphosed in the low T part of the amphibolite-facies, in the top of the structural ensemble during retrogressive metamorphic 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 degrees C and 9 kbar in the Skhour Nappe, and 532 degrees C 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. (C) 1998 Elsevier Science Limited.