Late Cryogenian and Early Ediacaran rare metal-rich granites in the Eastern Desert of Egypt: constraints from zircon ages and whole-rock Sr- and Nd- and feldspar Pb- isotopic compositions

Abstract

Rare metals (Nb, Ta, Y, Zr, Sn, U, W and REE) are economically important and new supplies need to be found. In order to understand Neoproterozoic rare metal granites of the Arabian-Nubian Shield (ANS), six samples from five rare metal mineralized alkali feldspar granites, syenogranites and granodiorite from the Central and SE Desert of Egypt were studied for zircon U-Pb ages and O isotopic compositions as well as whole-rock Sr- and Nd- and alkali feldspar Pb- isotopic compositions. These are transitional between I-type and A-type granites, mostly high-K calc-alkaline, peraluminous granites with gullwing-shaped REE patterns and strongly negative Eu anomalies .  Four granites gave mantle-like zircon δ 18 O V-SMOW between 4.2 and 5.96 ‰ and yielded ages of 628-633 Ma. This is about when subduction-related magmatism began to be replaced by collision-related magmatism. Igla Ahmr granites are older, formed at 691.7 - 678.9 Ma with δ 18 O V-SMOW ∼5.95 ‰ . All have positive initial ε Nd values (+3.3 to +6.9) typical for mantle and juvenile crust. Pb isotopic compositions are unusually radiogenic compared to unmineralized ANS granitic rocks. The data indicate similar magmatic sources for ANS mineralized and un-mineralized granites. Exploration for other rare-metal mineralized granites in the ANS should focus on bodies with similar characteristics. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6949402