Hybrid granite magmatism during orogenic collapse in the Eastern Desert of Egypt: Inferences from whole-rock geochemistry and zircon U–Pb–Hf isotopes

Granites can provide detailed insights into crust formation and reworking during the different stages of orogenic cycles. New field and petrographic observations are integrated with whole-rock geochemical and zircon U-Pb and Hf isotopic data of granitic and subvolcanic felsite intrusions associated with the Atalla Shear Zone (ASZ). Formation of the ASZ is attributed to regional oblique convergence, late in the tectono-magmatic evolution of the Central Eastern Desert of Egypt. The new data reveal three stages of post-collisional felsic magmatism, with a change in magma type from I- to A-type granites. The evolution commenced with formation of quartz-monzodiorite and granodiorite at 639 ± 2 Ma (stage 1), followed by a suite of granodiorite to syenogranite intrusions between 619 ± 2 and 612 ± 2 Ma (stage 2), and ceased with the Atalla felsites at 607 ± 2 Ma (stage 3). During stages 1 and 2, the ASZ was intruded by hybrid granites as a result of mixing of lower crust material with mantle-derived mafic sanukitoid-like melts, in addition to nearly pure crustal melts during stage 2. The Atalla felsites, with distinct A-type affinities, are interpreted to have resulted from fractional crystallization of hybrid magmas in shallow crustal levels prior to emplacement. The identical initial Hf isotope compositions of all ASZ-associated magmatic rocks imply that the crust and mantle reservoirs had similar Hf isotopic composition at ~640–610 Ma (εHf(t) = 8 ± 1). This implies that mantle enrichment was controlled largely by subducted Neoproterozoic oceanic crust, volcanic arc, and syn-collisional rocks. The new U-Pb ages combined with the geochemical characteristics of the ASZ granitic rocks are consistent with a prolonged period of decompression melting of hitherto enriched mantle and lower crust. Melting was likely triggered by rejuvenation of thoroughgoing transcurrent shearing and shear-driven asthenospheric mantle upwelling during the orogenic collapse stage.