PROVENANCE, TECTONIC SETTING AND GEOCHEMISTRY OF GREYWACKES AND SILTSTONES OF THE LATE PRECAMBRIAN HAMMAMAT-GROUP, EGYPT

Greywackes and siltstones from the Neoproterozoic Hammamat Group in th e Wadi Hammamat and Wadi Bali areas, Eastern Desert of Egypt, were ana lyzed for major and trace elements. The sedimentary rocks are characte rized by Na2O/K2O > 1, a wide range of both Al2O3/Na2O (2.5-7) and Al2 O3/TiO2 (13-33) plus a chemical index of alteration between 60 and 44. These data suggest that the Hammamat sedimentary rocks are chemically immature and that there was minimal chemical weathering involved in t heir erosion and deposition. The sedimentary units show two broad comp ositional groupings in the two studied areas. The Wadi Bah sedimentary rocks are characterized by low MgO, Cr, Ni, V, Ba, Nb, Y and Zr and h igh Na2O, CaO, and Sr relative to the Wadi Hammamat sedimentary rocks. These geochemical differences suggest contrasting provenance for the sedimentary rocks in the two areas. The Wadi Hammamat sedimentary rock s were derived from 30% mafic racks, 25% granodiorite, 25% intermediat e volcanics and 20% felsic volcanics, whereas those of the Wadi Ball a rea were derived from a relatively homogeneous source of 90% intermedi ate volcanics and 10% felsic volcanics. The Wadi Hammamat sedimentary rocks were deposited in an intra-arc basin that developed as a result of the creation of considerable topographic and structural relief afte r development of a continental are. The source rocks were continental are volcanics (Dokhan Volcanics) and the uplifted older oceanic tholei ites and island are assemblages. The Wadi Ball sedimentary racks were formed by the same scenario as the Wadi Hammamat sedimentary rocks, bu t the absence of rock fragments older than the Dokhan Volcanics indica tes a closed intra-arc basin where the Dokhan Volcanics were the main source of clasts. (C) 1998 Elsevier Science B.V. All rights reserved.