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“The geology of the Egyptian-Nubian Shield”. Springer Nature Switzerland AG
Springer Nature Switzerland AG.
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The Neoproterozoic metavolcanic rocks constitute major rock sequences in the Egyptian Nubian Shield. A recent detailed overview of the volcanisms and volcanic sequences particularly the metavolcanic rock unit is treated in this chapter. The geologic overview is stressed on a brief description of the metavolcanics at 41 better studied occurrences, a brief description of some common petrographic characteristics of these rocks, a discussion of their metamorphism, geochemistry, and age-dating. Two essential types: ophiolitic metavolcanics (Oph Mv) and arc metavolcanics (Arc Mv) have been discussed in detail. They were overprinted by greenschist and lower amphibolite facies metamorphism. The ophiolitic (Old) metavolcanic rocks (Oph Mv) are concentrated in the Central Eastern Desert (CED) and Southern Eastern Desert (SED) of Egypt, comprise pillow lavas and sheeted dykes. The pillowed lavas display pillow structure with abundant amygdales and scarcely massive. The sheeted dykes’ are repeated and continuous vertical styles. The rock types of the pillow lavas include spilite, aphyric and variolitic metabasalt, metabasaltic andesite and/or meta-andesite. The sheeted dykes exhibit subophitic, ophitic, and blastodiabasic metabasalt and rare metadolerite and meta-andesite. The arc (Young) metavolcanic rocks (Arc Mv) are generally widespread in the southern Sinai, NED, CED, and northern parts of the SED of Egypt. They belong to the island-arc mafic to felsic metavolcanic rocks. The Arc Mv includes massive and schistose flow and/or pillow lavas, commonly associated with pyroclastic and tuffs. They comprise two associations of mafic and felsic: blastodiabasic and porphyritic metabasalt and meta-andesite association, and porphyritic meta-andesite, metarhyodacite and metadacite association. Geochemically, the Arc Mv has higher ranges and means of SiO2, Al2O3, CaO, Na2O, Sr, Nb, Zr, and V and lower MgO, MnO, TiO2, Ba, Rb, Cr, and Ni compared with that of Oph Mv, suggest their fractionation sources. On discrimination diagrams, transitional tectonic environment among MORB, within-plate, and island-arc basalt types of the Oph Mv, revealing simulating their back-arc marginal basin. However, N-MORB affinity for the lavas and fore-arc character for the associated ophiolitic metaultramafics have been also reported. These variations reflect source region heterogeneities, and differences in degree of partial melting as well as corroborating the variability of oceanic domain of the ED Mv. The complied REEs data can be grouped into: Oph Mv dominating in the CED and SED and Arc Mv of Southern Sinai, NED and CED. The Oph Mv consequently comprises two subgroups: (i) LEEsdepleted pattern (similar to the N-MORB-type) and (ii) LREEs-enriched pattern (analogous to transitional MORB-IA). On the other hand, the Arc Mv has REEs patterns similar to that of IA (e.g., LREEs-enriched).
These variations also reflect source region heterogeneities, differences in degree of partial melting, and variability of oceanic-continental terrains domain of the Egyptian shield Mv. Bimodal arc associations of both mafic and felsic compositions (Shadli-type) are concentrated in CED and northern parts of the SED. The mafic end-members are tholeiitic basalt and basaltic andesite whereas the felsic ones comprise calc-alkaline dacite and rhyolite. The mafics are comparable to N-MORB and arc-related noncumulative mafic rocks. Felsic end-members are subduction-related source of arc-related magmas of thickened low-K mafic lower crust. The Oph Mv is relatively yielded age ranges from 750–700 Ma, whereas, the Arc Mv is relatively range in ages from 750–610 Ma, suggesting an overlap and no discernible difference in age are found between both sequences. These ages may represent the time of eruption of these metavolcanics in the crustal growth of the Egyptian Nubian Shield.
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