Characteristics and Petrogenesis of Neoproterozoic Mafic-Ultramafic Intrusions in the Arabian-Nubian Shield and the Question of Layered versus Alaskan-Type Intrusion

Late Neoproterozoic mafic-ultramafic rocks exposed in the Gabal Um Selim–Zeiatit area, northernmost Arabian-Nubian Shield include an old ophiolite sequence and a young mafic-ultramafic intrusion. The ophiolite sequence is represented by serpentinized peridotite and metagabbro, which are highly deformed, metamorphosed, and tectonically thrust over island arc metavolcanics. The mafic-ultramafic intrusion, known as the Um Selim–Zeiatit mafic-ultramafic intrusion (SZMUI), shows no evidence of deformation or metamorphism. The SZMUI consists of cumulate layered rocks and is composed of an upper suite of mafic rocks and a basal suite of ultramafic rocks. The upper suite consists of olivine gabbro, gabbronorite, and normal gabbro, while the basal suite includes lherzolite and olivine websterite. The SZMUI forms a lopolith-like intrusion and displays modal layering at outcrop scale as well as cryptic layering due to the variations in mineral compositions. The lherzolite contains green spinel (pleonaste) characteristic of layered intrusions. The spinel and olivine compositions of the ophiolitic serpentinites are consistent with residual mantle rocks that experienced high degrees of partial melt extraction typical for suprasubduction-zone forearc ophiolites, whereas those in the SZMUI ultramafic layers are typical of stratiform complexes and clearly different from those found in ophiolitic and Alaskan-type complexes. The differences in whole-rock Mg# between the ultramafic samples are consistent with a distinction based on mineral chemistry, with higher values in the ophiolitic peridotite than in the SZMUI peridotite. The mafic and ultramafic units of the SZMUI are clearly related and derived from a common parent magma, but they were not emplaced coevally and represent different pulses of magma. The ultramafic units of the SZMUI are cumulates and are derived from a high-Mg tholeiitic parent magma. Geochemical characteristics of the SZMUI confirm that it is not an Alaskan-type body but is more akin to a layered mafic-ultramafic intrusion. It was emplaced at a late stage of orogeny, during a transition from a subduction to an intraplate setting.