Permo-Carboniferous and early Miocene geological evolution of the internal zones of the Maghrebides - New insights on the western Mediterranean evolution

摘要

Geochemical and geochronological analyses were performed on metamorphic rocks from the footwall of an oceanic unit thrusted onto the North African margin. The footwall is mainly constituted by metamorphosed sediments and interlayered mafic rocks with scarce ultramafic lenses. Major and trace elements geochemistry of the sediments is consistent with a passive margin setting (16.9 < K2O/Na2O < 20.9; 18.2 < Zr/Th < 24.6). U-Pb analyses of detrital zircons indicate that deposition of the sedimentary sequence is younger than 307 +/- 7 Ma (2 sigma) and concur with a provenance of the detritus from the Eastern Meseta and High Atlas of Morocco. The age of a crosscutting orthogneiss (281 +/- 4 Ma) further constrains deposition to the upper Carboniferous/lower Permian times (307-281 Ma), in good agreement with the 295 +/- 2 Ma age of an interbedded amphibolite. Geochemistry of the mafic and ultramafic rocks displays subduction related features (LILE and LREE enrichments, HFSE depletion). Pb, Sr, Nd and Hf isotopes further indicate a depleted mantle reservoir modified by a continental crust-derived component brought into the mantle during an ancient subduction event. The range of isotopic compositions and geochemical signatures displayed by the Carboniferous mafic rocks suggest they inherited their geochemical characteristics during melting of a heterogeneous lithospheric mantle containing variable metasomatic additions. Geochronological results indicate that the HT metamorphic evolution of this upper Paleozoic sequence resulted from two distinct events, each followed by fast cooling. The first one, dated at 20.85 +/- 0.34 Ma (2 sigma), followed HP metamorphism and thrusting of the Kef Lakhal oceanic complex onto the Northern margin of Africa. The second event occurred at 17.65 +/- 0.46 Ma (2 sigma) and is coeval with exhumation and anatexis of the lower crustal units of the neighbouring Edough dome. Fast cooling of the footwall rocks, through the closure temperatures of muscovite and biotite, occurred synchronously at 16.94 +/- 0.10 Ma (2 sigma) and 17.02 +/- 0.19 Ma (2 sigma) respectively. This study documents a further example of the complex history recorded by rocks of the Peri-Mediterranean area that contains an imbrication of geological units, witnesses of a protracted history related to Permo-Carboniferous and Miocene events. (C) 2015 Published by Elsevier Ltd.