Major and trace elements, Sr-Nd isotopic ratios, and zircon U-Pb age data of anorthosites and gabbros from Wadi El Dib, Egypt

Two texturally distinct types of anorthosites have been identified in Neoproterozoic ultramafic-mafic intrusions (NUIs) in the Wadi El Dib region (WED), North Eastern Desert, Egypt. Despite some similarities to anorthosites associated with layered intrusions, they do not fit into the global classification scheme of anorthosites. A Concordia zircon U-Pb age of 591.5 ± 3.5 Ma is interpreted as the crystallization age of the WED anorthosites. The geochemical evidence suggests that both gabbros and anorthosites likely formed from a common mafic parental magma. This is indicated by progressive decreases in concentrations of highly compatible elements from gabbros to anorthosites, relatively high total REE contents in anorthosites, chondrite-normalized REE patterns with opposite Eu anomalies, and the crystallization of transitional orthopyroxene in anorthosites. Low (87Sr/86Sr)i and strongly positive εNd(t) values of the WED gabbros and anorthosites validate their genesis from a primitive mantle magma. Trace-element geochemical modeling suggests that the WED gabbros are denser mafic cumulates segregated from the parent magma, whereas anorthosites result from additional fractional crystallization of the residual melt. 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 contribution of lower crustal melts through the entrapment of felsic melt pockets within intercumulus spaces. These data and models characterize the WED anorthosites as a unique variety, associated with post-collisional crustal thickening and asthenosphere upwelling and formed by three distinctive stages from the crystallization of cumulus to intercumulus and late intercumulus phases.