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Distribution and tectonic setting of post-kinematic igneous complexes in the Red Sea Hills of Sudan and the Arabian—Nubian shield
A41
DISTRIBUTION A N D TECTONIC SETTING OF POST-KINEMATIC IGNEOUS COMPLEXES IN THE RED SEA HILLS OF S U D A N A N D THE A R A B I A N - - N U B I A N SHIELD
J.R. VAIL
Portsmouth Polytechnic, Portsmouth (Great Britain)
Numerous igneous complexes, many in the form of ring structures, are located in the Red Sea Hills of northern Sudan and form part of an extensive post-tectonic igneous province in Sudan, Egypt and Saudi Arabia. Many are bimodal calc-alkaline granite-gabbro associations, others are alkali granites, syenites and trachytes. The complexes pierce a late Proterozoic basement of greenschist facies andesitic volcanic rocks with interbedded sediments, which are crossed by three NE trending ophiolitefilled suture zones. The basement units are disrupted by syn- to late-tectonic granodiorite emplacements of batholithic dimensions. The calc-alkaline post-orogenic igneous complexes are concentrated along bands parallel to the ophiolite zones, the location of individual intrusions tending to favour the volcano-sedimentary assemblage rather than the surrounding granitoid batholiths. The NE trending grain of the basement is intersected by N--S and NW fractures parallel to the Red Sea Coast. A second, younger group of syenite and trachyte intrusive complexes is associated with the meridional fractures and is probably related to the Red Sea rift zone. The geological structure of the Red Sea Hills of Sudan is also reflected in Bouguer gravity and aeromagnetic anomalies. The regional pattern of ring complexes and anorogenic igneous masses across the combined Arabian--Nubian shield is illustrated, and is shown to be an important element of the crustal evolution. Younger Granite-type ring intrusions also occur in early Proterozoic gneisses to the west of the volcano-sedimentary greenschist assemblages, suggesting that there is a third type of post-tectonic igneous complex present in Sudan, and probably also in Egypt and Saudi Arabia.
THE N O R T H E A S T BRANCH OF THE MOZAMBIQUE BELT
A.J. WARDEN
Montanuniversitat, Leoben (Austria)
The Mozambique belt bifurcates in southern Ethiopia, and a branch extends northwards into the Horn of Africa. Separating these predominantly ensialic paragneisses which sustained rather uniform medium-pT metamorphism in the amphibolite facies is variablymetamorphosed crust of different origin derived from the closure of island arc basins. This consists mainly of tholeiitic and calc-alkaline volcanics and immature clastics with ophiolitic slices indicating occlusion of some oceanic crust and mantle. The northeast branch of the Mozambique belt comprises an extension of the succession in Kenya. It can be traced northwards via isolated inliers into the coastal strip of northern Somalia (with the exception of the rocks of the Bur region inlier which may predate the gneisses of the Mozambique belt). The belt continues northwards into SW Arabia where it is truncated by a NW-trending shear zone. A northwestward transition is evident from the
DISTRIBUTION A N D TECTONIC SETTING OF POST-KINEMATIC IGNEOUS COMPLEXES IN THE RED SEA HILLS OF S U D A N A N D THE A R A B I A N - - N U B I A N SHIELD
J.R. VAIL
Portsmouth Polytechnic, Portsmouth (Great Britain)
Numerous igneous complexes, many in the form of ring structures, are located in the Red Sea Hills of northern Sudan and form part of an extensive post-tectonic igneous province in Sudan, Egypt and Saudi Arabia. Many are bimodal calc-alkaline granite-gabbro associations, others are alkali granites, syenites and trachytes. The complexes pierce a late Proterozoic basement of greenschist facies andesitic volcanic rocks with interbedded sediments, which are crossed by three NE trending ophiolitefilled suture zones. The basement units are disrupted by syn- to late-tectonic granodiorite emplacements of batholithic dimensions. The calc-alkaline post-orogenic igneous complexes are concentrated along bands parallel to the ophiolite zones, the location of individual intrusions tending to favour the volcano-sedimentary assemblage rather than the surrounding granitoid batholiths. The NE trending grain of the basement is intersected by N--S and NW fractures parallel to the Red Sea Coast. A second, younger group of syenite and trachyte intrusive complexes is associated with the meridional fractures and is probably related to the Red Sea rift zone. The geological structure of the Red Sea Hills of Sudan is also reflected in Bouguer gravity and aeromagnetic anomalies. The regional pattern of ring complexes and anorogenic igneous masses across the combined Arabian--Nubian shield is illustrated, and is shown to be an important element of the crustal evolution. Younger Granite-type ring intrusions also occur in early Proterozoic gneisses to the west of the volcano-sedimentary greenschist assemblages, suggesting that there is a third type of post-tectonic igneous complex present in Sudan, and probably also in Egypt and Saudi Arabia.
THE N O R T H E A S T BRANCH OF THE MOZAMBIQUE BELT
A.J. WARDEN
Montanuniversitat, Leoben (Austria)
The Mozambique belt bifurcates in southern Ethiopia, and a branch extends northwards into the Horn of Africa. Separating these predominantly ensialic paragneisses which sustained rather uniform medium-pT metamorphism in the amphibolite facies is variablymetamorphosed crust of different origin derived from the closure of island arc basins. This consists mainly of tholeiitic and calc-alkaline volcanics and immature clastics with ophiolitic slices indicating occlusion of some oceanic crust and mantle. The northeast branch of the Mozambique belt comprises an extension of the succession in Kenya. It can be traced northwards via isolated inliers into the coastal strip of northern Somalia (with the exception of the rocks of the Bur region inlier which may predate the gneisses of the Mozambique belt). The belt continues northwards into SW Arabia where it is truncated by a NW-trending shear zone. A northwestward transition is evident from the