الفهرس 
Chapter 1. IntroductionOnly 14 pages are availabe for public view
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Abstract
The Neoproterozoic rocks of the Wadi Hamad area, North Eastern Desert, Egypt, are mainly volcanic and plutonic rocks. These rocks represent part of crustal section of Neoproterozoic continental island arc, which is intruded by late to post-collisional granites. The volcanic rocks are represented by the Dokhan Volcanics, which comprise lava flows and their pyroclastics, while the plutonic rocks are dominated by granitoids with minor gabbro.
The medium- to high-K calc-alkaline subduction-related Dokhan volcanic rocks of the Wadi Hamad area comprise earlier andesitic (with subordinate basaltic) and later dacitic pulses, which correspond to two flare-up events during the history of Neoproterozoic continental island arc. The geochemical characteristics of the andesites are inconsistent with generation by fractional crystallization of the same magmas that produced the basalts. Similarly, the overlap in trace elements abundances and comparable (La/Yb)n values of dacitic and andesitic volcanic rocks rule out the origin of dacites through fractional crystallization of the same magmas that produced the andesites. The parental magmas of the Dokhan basalts and andesites were generated by variable degrees of partial melting of subduction-metasomatized mantle whereas that of the dacites was produced by mixing of mantle and arc crustal melts. Oscillatory and reverse zoning in clinopyroxenes and/or plagioclase indicates that magma replenishment/mixing and/or convection played a role in the genesis of the basaltic and andesitic rocks. The depth indicator geochemical parameters suggest that the earlier subduction andesites were evolved at deeper levels in the arc crust compared with the later subduction dacites, but not so deep as to stabilize garnet as a fractionating phase. The parental magmas of the Dokhan andesites and dacites were essentially evolved through fractionation of clinopyroxene and plagioclase, which were accompanied by fractionation of apatite and Fe-Ti oxides/amphibole in the more evolved dacitic rocks.
The plutonic rocks include continental island arc gabbro-granitoids and late to post- collisional alkali feldspar granites. The arc gabbro-granitoids comprise earlier subduction gabbro-diorites and later subduction granodiorites, biotite granite and graphic granite (granophyre). The geochemical signatures of these rocks are consistent with their derivation from different magma sources that evolved through fractional crystallization with crustal contamination. During earlier subduction, the introduction of subducted slab-derived fluids to the overlying mantle wedge triggered partial melting to produce mafic magmas that fractionated to give the gabbro and dioritic rocks. The increase in volcanic arc thickness induced lower degrees of partial melting in the mantle wedge and/or in the overlying lower crust, resulting in
production of intermediate magmatism. The intermediate magmas evolved through fractional crystallization with crustal contribution to produce granodiorite-granite. The relatively rapid ascent of some fractionated magmatic bodies produced the porphyritic granodiorite and graphic granites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate lower/middle crustal sources during the collisional stage. The earlier subduction gabbro-diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites-granites, which display more LILE-enriched spider diagrams with shallower Ta-Nb troughs, reflecting variation of magma composition with arc evolution.
Estimated values of the arc crust thickness indicate that the crust thickened from ~35 km to ~50 km from the time of basalt eruption to the time of andesite eruption, probably implying a high rate of crustal growth. A flare-up event producing 15 km of juvenile crust would this be associated with production of about 500 m of volcanic section assuming a ratio of plutonic emplacement to volcanic eruption greater than 30:1 during flare-up events of continental arcs. The estimated arc crust thickness during the later subduction flare-up is comparable to that of the earlier subduction flare-up, implying arc crust delamination before the production of later subduction dacites. The subduction geochemical characteristics of the Dokhan volcanic rocks suggest that the collision between East and West Gondwana in the tip of NE Africa did not occur before 600 Ma. Re-melting of arc crustal sources during the collisional stage results in differentiation of the continental arc crust and drives the composition of its upper part towards that of continental crust.
Keywords:
Continental island arcs; Arc Magma Genesis; Collisional granites; Continental crust; Gondwana Assembly; Eastern Desert; Arabian-Nubian Shield.