The provenance and tectonic setting of the Neoproterozoic Um Hassa Greywacke Member, Wadi Hammamat area, Egypt: Evidence from petrography and geochemistry

We present new petrographic, and major and trace element data for the Um Hassa Greywacke Member, the uppermost unit of the Hammamat Group, at Wadi Hammamat area, the Eastern Desert of Egypt. The Neoproterozoic Hammamat sedimentary rocks are immature molasse-type rocks that were deposited at the end of the Pan-African Orogeny. The Um Hassa Greywacke Member is bedded, weakly deformed and lies conformably on the polymictic conglomerate of the Um Had Member. The Um Hassa greywackes are poorly sorted and composed mainly of quartz, lithic fragments, and feldspar grains, with an average modal composition of QFL = 53:17:30. Quartz is mostly monocrystalline, feldspars are mainly plagioclase, and lithic fragments are predominantly intermediate to felsic volcanic rocks. The Um Hassa greywackes are characterized by low chemical index of alteration (46–65) and SiO2/Al2O3 (4.6–6.1) that indicate limited chemical weathering of the source rocks and deposition close to the source area.Both point-counting modal analyses and geochemical data suggest that the Um Hassa greywackes were derived from mixed sources. In terms of provenance, continental arc volcanic rocks and oceanic island arc-ophiolitic sources are the main contributors to the sediments of the Um Hassa greywackes. These rock types are exposed in the Eastern Desert as Dokhan-type continental arc volcanic rocks and accreted oceanic arc-ophiolitic mélange, respectively. Considering the geodynamic evolution of the Central Eastern Desert, the Um Hassa greywackes in the Wadi Hammamat area appear to have been deposited in a retroarc foreland basin behind the Dokhan continental arc developed over a west-dipping subduction zone.