The Rb-Sr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield

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Abstract
Rb-Sr whole-rock isochron ages have been determined for six igneous rock suites from the Ras Gharib. These represent all major crustal components of the northern Nubian Shield and are, from oldest to youngest: diorite-tonalite, extrusive rocks, granodiorite-adamellite and leucogranite, muscovite trondhjemite (orogenic Pan-African suites), dyke swarms and anorogenic peralkaline granites. The ages obtained are: 881 ± 58 Ma, 620 ± 16 Ma, 552 ± 7 Ma, 516 ± 7 Ma, 493 ± 7 Ma and 476 ± 2 Ma. The initial 87 Sr/ 86 Sr ratios of the Pan-African suites and the dykes are in the range 0.7039 to 0.7047. The initial ratio of the anorogenic peralkaline granites is 0.7110 ±0.0012, higher than any previously reported for igneous rocks from the Egyptian belt. The 493 ± 7 Ma age of the dyke swarms represents a younger limit for the Pan-African compression–accretion event and an upper (older) limit for an extensional tectonic regime. Linked with this is the anorogenic peralkaline granite magmatism (476 ±2 Ma), which is the last manifestation of igneous activity in this region. The low initial ratios of the Pan-African suites provide no support for the existence of an older (Archaen) sialic continental crust in the region. The anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier Pan-African suites. The age data are used to construct a tentative correlation scheme for shield rocks of Egypt, Sudan and Saudi Arabia.