Formation of juvenile continental crust in northern Nubian Shield: New evidence from granitic zircon U-Pb-Hf-O isotopes

Abstract

The Neoproterozoic Arabian-Nubian Shield (ANS) consists of continental crust formed prior to and during the collision between East and West Gondwana. The Eastern Desert of Egypt (i.e., northern Nubian Shield) constitutes the north-western part of the ANS. Neoproterozoic magmatism in the Eastern Desert region occurred between ∼ 800 Ma and ∼ 550 Ma. Although there is a broad consensus that ∼ 800–650 Ma pre-collisional igneous rocks comprise the main magmatic stage contributing to Eastern Desert crustal evolution, whether this crust is purely of a juvenile composition, or was generated by recycling/reworking of much older continental crust, remains debated. Here, we present new whole-rock geochemical and Sr-Nd isotopic data and an integrated zircon U-Pb-Hf-O-trace element dataset for two granitoids (Um Khariga and Genina Gharbia) from the Eastern Desert. Samples from the Um Khariga granitoids yield zircon U-Pb ages of 746 ± 4 and 753 ± 4 Ma that overlap within error, and the Genina Gharbia granitoids have an age of ∼ 690 Ma, indicating that both granitoids belong to different phases of pre-collisional magmatism. Zircon hafnium isotopic data of both granitoids yield weighted mean εHf(t) values ranging from + 8.01 ± 0.23 to + 10.52 ± 0.13, indicating that both granitoid suites were derived by melting of a juvenile source. SIMS oxygen isotope data for zircon show that the magmatic zircon has mantle-like δ18O values with weighted means ranging from 4.73 ± 0.02 to 5.04 ± 0.08 ‰. Based on previously published zircon U-Pb-Hf-O-trace element data for granitoids and volcanic rocks of the Eastern Desert, plus our new isotopic datasets obtained from the Um Khariga and Genina Gharbia granitoids, we show that the ∼ 800–650 Ma magmatism is characterized by high zircon εHf(t) (+8 to + 15), and mantle-like zircon δ18O values (∼+5‰), implying that the continental crust of the Eastern Desert was either extracted directly from a depleted mantle and/or reworked from accreted juvenile oceanic crust or oceanic arc with no evidence for the input of old continental material. The overall zircon εHf(t) and δ18O evolution trends for the northern Nubian Shield show that juvenile mantle input dominated the early phases (ca. 800–670 Ma) of crustal growth and evolution in this part of the ANS, coinciding with the break-up stage of the supercontinent Rodinia. The involvement of continental crustal recycling only started to play a significant role since the ca. 670 Ma regional collisional event during the early stage of Gondwana assembly.