A late Neoproterozoic magmatic core complex in the Eastern Desert of Egypt: emplacement of granitoids in a wrench-tectonic setting

Abstract

Core complexes within the Eastern Desert of Egypt evolved in the course of orogen-parallel extension during Neoproterozoic plate convergence. In the Sibai Core complex, more than 90% of the exposed surface consists of magmatic rocks. In this study four magmatic suites have been distinguished. They are geochemically characterised and their intrusion ages are determined using the Pb/Pb single zircon evaporation technique. Results show that all magmatic suites formed during Neoproterozoic tectonics. The oldest, calc-alkaline rocks of group (I) gave ages of 670–690 Ma and are related to subduction processes in an island-arc setting. Subsequent orogen-parallel extension was accompanied by the intrusion of group (II) and (III) granitoids yielding ages of 655 and 645 Ma, respectively. The younger, felsic granites of group (IV) intruded along pre-existing structures and exhibit variable composition. We relate emplacement of various granitoids within the Sibai to progressively evolving structures. Group (I) represents the deep level of an island arc and exhibits nappe stacking structures related to oblique convergence in the late Neoproterozoic. Group (II) granitoids have been emplaced at mid-crustal levels and portray transpression and orogen-parallel extrusion dynamics. Related structures are represented by an older set of sinistral shear zones. Granitoids of group (III) intruded upper crustal levels during exhumation of the Sibai Core complex. Their emplacement was controlled by the pull-apart regime of a younger set of sinistral shear zones and associated normal shear zones. Denudation due to ongoing orogen-parallel extension is evident from deposition of sediments within the adjacent intramontane Kareim molasse basin. The sediments were intruded by post-tectonic granitoids of group (IV). Close relations between core complex formation and ongoing magmatic activity define the Sibai as a magmatic core complex.