Sm-Nd and Rb-Sr isotope geochemistry and petrology of Abu Hamamid intrusion, Eastern Desert, Egypt: An Alaskan-type complex in a backarc setting

Alaskan-type complexes are uplifted fragments of the deep levels of island arcs. These complexes usually reveal chemical diversity that cannot be assigned to a unique setting within arcs. Here we report the petrology, Rb-Sr and Sm-Nd isotopic data of the Abu Hamamid intrusion from the Eastern Desert of Egypt as an example of Neoproterozoic backarc Alaskan-type complex. The intrusion comprises clinopyroxene dunite, olivine clinopyroxenite, amphibole clinopyroxenite and gabbro. All rock units represent cumulates crystallized from a high-MgO hydrous magma with geochemical characteristics intermediate between mid-ocean ridge basalt and island arc magmas. Slab signatures of the parental magma comprise elevated H2O, 87Sr/86Sr and large ion lithophile element contents and relative depletion in Nb. High positive initial εNd (+6.2 to +11.0) is consistent with the Abu Hamamid magma being extracted from a highly depleted mantle source. The parental magma was generated by 10% partial melting of a mantle source modified by 10% MORB (ecolgite) derived fluid. The Abu Hamamid Alaskan-type complex and the surrounding related volcanic rocks were formed in a Neoproterozoic backarc setting. The magmas forming the lower crust of the Arabian-Nubian shield were liberated from a depleted mantle before 900Ma.