Origin of Neoproterozoic anorthosites in the Wadi El Dib ultramafic-mafic layered intrusions, Egypt

Abstract

The Neoproterozoic ultramafic–mafic intrusions (NUIs) in the Wadi El Dib (WED) region of the Northeastern Desert, Egypt, consist of a gabbro-serpentinite sequence overlain by anorthosite intrusions. The WED anorthosites show two texturally distinct types of framework-dominated and intercumulus-rich and made of cumulus plagioclases (anorthite percentage: An_60-45) and intercumulus mafic minerals along with granophyric materials. They are associated with rock types typically found alongside anorthosites within layered mafic intrusions. Zircon U-Pb age dating yielded Concordia age of 591.5 ± 3.5 Ma for both types of WED anorthosites. The measured crystallization age of the WED anorthosites matches the post-collisional stage of the Pan-African orogeny and coinciding with the emplacement of A-type granites in the Arabian Nubian Shield. The geochemical evidence suggests that gabbros and anorthosites likely formed from a common mafic parental magma. This is indicated by progressive decreases in concentrations of highly compatible elements and Mg^# from gabbros to anorthosites, relatively high total REE contents in anorthosites, chondrite-normalized REE patterns with positive versus negative Eu anomalies, and the crystallization of transitional magmatic to metamorphic orthopyroxene in anorthosites. The low (⁸⁷Sr/⁸⁶Sr)_i ratios and strongly positive εNd_(t) values of the WED gabbros and anorthosites validate their genesis from an isotopically depleted mantle source. Trace-element geochemical modelling suggests that the older gabbros are denser mafic cumulates segregated from partially crystallized high alumina (high-Al) basaltic parent magma originated from low degree partial melting of mantle peridotite, whereas anorthosites settle out of the cooling residual melt through imperfect fractional crystallization process. The Sr-Nd systematics of the anorthosites coupled with distinct An% variations of cumulus plagioclases and the existence of both magmatic and inherited zircon in the granophyric materials verify a small contribution of lower crustal melts through the entrapment of felsic melt pockets within intercumulus spaces. The data and models indicate that the formation of WED anorthosites at the top of NUIs is triggered by post-collisional crustal thickening and asthenosphere upwelling consistent with previously proposed tectono-magmatic model for the NED.