Zircon trace element geochemical constraints on the evolution of the Ediacaran (600–614 Ma) post-collisional Dokhan Volcanics and Younger Granites of SE Sinai, NE Arabian–Nubian Shield

Abstract

The post-collisional stage (620–590Ma) of the Egyptian part of the Arabian–Nubian Shield was characterized by the eruption and emplacement of the Dokhan Volcanics and Younger Granites. This study presents LA-ICP-MS analyses of the trace element abundances of zircons separated from Ediacaran (600–614Ma.) Dokhan Volcanic (rhyolite) and Younger Granite (syenogranite) samples from SE Sinai. Whole-rock geochemical data from these two rock units indicate both are peraluminous, calc-alkaline and with A-type characteristics and indicate their magmas were generated by partial melting of continental crust or underplated crust in a post-collisional regime. Zircons separated from the Dokhan sample contain higher abundances of Hf, ΣREE, Pb, Th and U than those from the Younger Granite sample, which suggest a higher degree of magmatic evolution in the former. The chondrite-normalized REE patterns of zircons from the two rock types are characterized by HREE enrichment relative to LREE and MREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. Compared to unaltered magmatic zircons, most of studied zircons display an evident LREE overabundance, whereas the vast majority of the analyzed zircons have Th/U ratios≥0.5 common in igneous zircons. The Ce and Eu anomalies of both zircon populations indicate that crystallization of the zircon grains from the Dokhan Volcanics in more variable and higher oxygen fugacity conditions than those of the Younger Granites. The relation between the U/Yb ratio vs. Y and Hf contents suggest crystallization of the analyzed zircons in continental crust. The application of the Ti-in-zircon thermometer returns high temperatures of 790–986°C and 781–934°C for zircons extracted from the Dokhan Volcanic and Younger Granite samples, respectively, indicative of deep crustal melting within the lower crust. The zircon saturation temperatures for both rock types are slightly lower than the temperature of zircon crystallization (≈40–55°C). The two zircon populations formed by equilibrium crystallization in a late-magmatic closed system that was progressively enriched with late orthomagmatic LREE-enriched fluids.