Lu–Hf and O isotopic compositions on single zircons from the North Eastern Desert of Egypt, Arabian–Nubian Shield: Implications for crustal evolution

Abstract

Neoproterozoic juvenile crust is exposed in the Eastern Desert of Egypt, between the Nile and the Red Sea, forming the basement to Cambrian and younger sedimentary strata in the northernmost part of the Arabian–Nubian Shield (ANS). In order to reveal how the crust of this vast region was formed, four examples of widespread Neoproterozoic (653–595Ma) calc-alkaline and alkaline intrusive rocks in the northwestern most exposures, in the NE Desert of Egypt (NED) were studied. Single zircon Hf–O isotopic compositions of these intrusives were used to characterize the Neoproterozoic syn- and post-collisional granitoids in the NED. The ~653Ma Um Taghir syn-tectonic granodiorite (I-type) displays isotopic characteristics of a depleted mantle source, such as high εHf(t) (+9.1 to +11.2) and mantle δ18O (mean=+5.12‰). In contrast, the ca. ~600Ma post-collision A-type granites (Al-Missikat, Abu Harba, and Gattar) show slightly higher δ18O values (+5.15 to 6.70) and slightly lower εHf(t) values (+6.3 to +10.6, mean=+8.6). We interpret these isotopic data to reflect melting of a juvenile Neoproterozoic mantle source that assimilated slightly older Neoproterozoic crustal material during magma mixing. The involvement of crustal component is also supported by Hf-crustal model ages (0.67–0.96Ga) and by the occurrence of xenocrystic zircons with U–Pb ages older than the crystallization ages, indicating melting of predominantly Late Neoproterozoic crustal protoliths.