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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 H 2 O, 87 Sr/ 86 Sr and large ion lithophile element contents and relative depletion in Nb. High positive initial ɛ Nd (+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 900 Ma. [ABSTRACT FROM AUTHOR]

The geodynamic settings of the Precambrian mafic–ultramafic complexes in the Eastern Desert of Egypt have important bearing on understanding the geotectonic evolution of the Arabian Nubian Shield. We present a detailed petrological study on a layered mafic–ultramafic intrusion that is located at the contact between the Precambrian continental crust and the Miocene Red Sea oceanic crust. The Motaghairat layered intrusion consists of basal lherzolite, orthopyroxenite, troctolite, olivine gabbro and anorthosite on the top. Variations in modal mineralogy and mineral chemistry along with the chemical composition of these units suggest their derivation from a common high-Mg tholeiitic parent melt through fractional crystallization processes. The parental magma was derived from a metasomatised mantle source. The primitive mantle-normalized patterns of the calculated melts exhibit enrichment in U relative to Th and Ba relative LREE which indicate that the enriched lithospheric mantle source was metasomatised by fluids derived from a subducted oceanic crust rather than by a sediment melt. Geological and petrological evidences suggest that the layered Motaghairat intrusion was emplaced during post-orogenic extension following subduction break-off and lithospheric delamination after the collision between the amalgamated island arc terranes and the Saharan Metacraton. The heat source required to melt the metasomatised lithospheric mantle was derived from the upwelling of hot asthenosphere after the subduction-break-off. [ABSTRACT FROM AUTHOR]