Petrogenetic Evolution of the Neoproterozoic Igneous Rocks of Egypt

Abstract

The Late Neoproterozoic basement exposures in the Sinai Peninsula and Eastern Desert of Egypt represent the northern part of the Nubian Shield, which was contiguous with the Arabian shield before the opening of the Red Sea. These two Shields together form the Arabian-Nubian Shield (ANS). The ANS, formed during the Neoproterozoic Pan-African orogeny due to collision between East and West Gondwana, is the best-preserved juvenile continental crust of Neoproterozoic age on Earth. The Nubian shield evolved through a series of stages that can be characterized as (1) supercontinent rift and drift (1000–900 Ma), (2) subduction and consumption of intervening ocean basins (870–750 Ma), (3) continental collision and orogeny (750–630 Ma), and (4) post-orogenic extension and collapse (620–580 Ma plus minor later activity extending to 540 Ma). The Neoproterozoic (870–580 Ma) development of the juvenile continental crust specifically of the Egyptian Nubian Shield segment of the ANS, however, is most logically discussed in three main stages: pre-collisional, collisional, and post-collisional. The pre-collisional phase (~870–700 Ma) produced complete oceanic ophiolite assemblages coexisting with intra-oceanic island arcs. All the Egyptian Eastern Desert ophiolites are strongly deformed, metamorphosed, and affected by several types of alteration. They occur as dismembered, tectonized bodies and mélanges of pillowed metabasalt, gabbro, and variably altered ultramafic rocks. Along shear zones, the ophiolitic ultramafics are highly altered into talc-carbonates, magnesite, and listvenite. The collisional stage (670–630 Ma) includes a subduction period that produced volcano-sedimentary island arc successions and calc-alkaline gabbro–diorite complexes, followed by development of Cordilleran-style calc-alkaline gabbros and granodiorites and their extrusive equivalents, ending with the merger of West and East Gondwana. All the units of the collisional stage are weakly deformed and experienced only low-grade metamorphism. The post-collisional phase (~620–580 Ma) was characterized by intracrustal melting that first generated calc-alkaline granitoids and then gradually shifted to a terminal stage that produced relatively small volumes of alkaline magma, preserved as both plutonic and volcanic units. Temporally, the later stages of post-collisional calc-alkaline magmatism and the alkaline magmatism overlapped. This chapter discusses the evolution of the Nubian from an igneous petrology perspective, including the plutonic (granitoid and mafic–ultramafic) and volcanic records. Granitoids were emplaced in Egypt during each phase of evolution of the Nubian Shield: pre-collisional granitoids include highly deformed trondhjemite, tonalite, and granodiorite; syn-collisional granitoids are weakly deformed granodiorite and less commonly granite; post-collisional granitoids include undeformed calc-alkaline and alkaline monzogranite, syenogranite, alkali feldspar granite, and alkaline/peralkaline granites. The Neoproterozoic mafic–ultramafic complexes in the Egyptian Nubian Shield include older and younger complexes. The older mafic–ultramafic complexes either form an integral part of obducted ophiolite sequences or constitute members of subduction-related, calc-alkaline gabbro–diorite complexes. The younger mafic–ultramafic complexes are mostly fresh, undeformed, and unmetamorphosed, and were emplaced in post-orogenic settings. Four major volcanic episodes have been recognized in the Neoproterozoic crust of Egypt, including ophiolitic metavolcanic rocks, island arc metavolcanic rocks, the Dokhan volcanic series, and post-collisional alkaline volcanics (Katherina Volcanics).

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