Date of Award

2009

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

Keywords

Earth sciences, Back-arc basins, Central Eastern Desert, Egypt, Neoproterozoic, Ophiolites

Supervisor

Ali Polat

Supervisor

Brian J. Fryer

Rights

info:eu-repo/semantics/openAccess

Abstract

The geochemistry of the ophiolitic complexes and younger late orogenic sedimentary rocks in the Wadi Ghadir and Fawakhir areas has been studied to constrain the tectonic evolution of the Central Eastern Desert. The Wadi Ghadir ophiolite is composed of gabbros and pillow lavas that are intruded by many dike swarms and all of them display subduction zone chemical signatures. The ophiolitic rocks and the surrounding melange were intruded by younger dikes with Andean-type geochemical signature. The Wadi Ghadir ophiolite has chemical similarities to the oceanic crust formed in back-arc basins. The location of the Wadi Ghadir ophiolite to the northeast of the Nugrus volcanic arc and the appearance of forearc amphibolites to the southwest of the Nugrus arc rocks indicates a NE-dipping subduction zone. The accretion of this arcback-arc system onto the passive margin to the west resulted in the reversal in the subduction direction and the establishment of a new Andean-type continental margin.

The Fawakhir ophiolite comprises serpentinized ultramafic rocks, gabbros, sheeted dike complexes, and pillow lavas. The geochemical characteristics of the Fawakhir ophiolite are comparable to the forearc oceanic crust developed in the Izu-Bonin-Mariana arc system. However, pillow lavas preserved in the melange to the west have a geochemical signature similar to that of a modern back-arc oceanic crust which suggests formation of an intra-oceanic arc system above an east-dipping subduction zone. As in Wadi Ghadir, the Fawakhir ophiolite is intruded by later calc-alkaline dikes possessing geochemical signatures similar to rocks that occur at modern active continental margins. The initiation of the Andean-type calc-alkaline magmatism may indicate a reversal in subduction polarity from east-dipping during arc accretion to west-dipping and formation of an active continental margin.

The Umm Hassa Greywacke member is the top unit of the Hammamat Group which is late orogenic molasse-type sedimentary rocks. Continental arc volcanic rocks and oceanic island arc-ophiolitic sources are the main contributors to the sediments of the Urn Hassa greywackes. Considering the geodynamic evolution of the Central Eastern Desert, the Um Hassa greywackes were deposited in retroarc foreland basin behind the continental arcs developed over a west-dipping subduction zone.

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