H. Schandelmeier

Pan-African reworked early/middle Proterozoic crust in NE Africa west of the Nile: Sr and Nd isotope evidence

Whole-rock Rb-Sr studies on six suites of highly metamorphosed basement rocks from southern Egypt and northern Sudan, west of the River Nile, yield Pan-African ages (562–918 Ma): these ages are interpreted as reset. Orthogneisses give Nd model ages of 1600 to 2600 Ma for their calc-alkaline plutonic precursors. A gneiss derived from sedimentary precursors gave a mean crustal residence age of 2200 Ma. A series of I-type, late-tectonic, granitoids were intruded during a period of uplift, erosion, and wrench faulting at 560–620 Ma. Geochemical characteristics and Nd model ages (1200–1700 Ma) indicate that these rocks were derived from a mixed source of juvenile Pan-African mantle material and older continental crust of mainly early/middle Proterozoic age. Negative ɛNdt, values for granitoid samples (–18.9 to –5.3) show that the Pan-African episode in NE Africa, west of the Nile, involved considerable reworking of pre-existing crust.

Tectonic evolution of the Neoproterozoic Adola Belt of southern Ethiopia: evidence for a Wilson Cycle process and implications for oblique plate collision

Abstract The Adola Belt of southern Ethiopia comprises three major lithotectonic units: (i) metamorphosed passive continental margin sediments, mafic-ultramafic rocks and associated pelitic metasediments of the Kenticha terrain; (ii) high-grade gneisses and schists, intruded by syn-tectonic calc-alkaline magmatic rocks in the central and western part of the Adola Belt; and (iii) low-grade metavolcano-sedimentary and mafic-ultramafic rocks, and associated granitoids of the Megado terrain. The geochemical signatures and structural features of the rock associations of the Adola Belt may be interpreted to reflect a Wilson Cycle process, i.e., evolution of a passive continental margin and formation of ocean floor in the Kenticha Terrain, W-directed subduction, arc development in the Megado Terrain, closure of an oceanic basin of unknown size and collision of crustal blocks. Oblique plate convergence led to a sequence of continuous deformation events, namely (i) subduction-related folding and thrusting (D 1 ), which culminated in the obduction of mafic-ultramafic assemblages onto the passive continental margin sediments, ii) collision of crustal blocks (D 2 ), leading to re-folding of D 1 structures, development of upright NS-trending folds and generation of reverse faults and shear zones, and finally (iii) evolution of sinistral strike-slip shear zones (D 3 ) with N and NW orientations, the latter being interpreted as antithetic Riedel Shears. All structures of the Adola Belt are compatible with a NW-oriented stress regime and hence can be interpreted to reflect sinistral transpression. In a larger geodynamic framework, structures related to this collision event and the stress regime that produced them are consistent with the position of East and West Gondwana during the Neoproterozoic.

Evolution of the Neoproterozoic Delgo suture zone and crustal growth in Northern Sudan: geochemical and radiogenic isotope constraints

In the Delgo basement area of northern Sudan, low to medium grade metamorphosed volcanic, sedimentary and plutonic rocks are surrounded by high grade gneisses. A NNE-SSW trending suture zone can be defined by the lithological, chemical and structural characteristics of several distinct units. The early Proterozoic gneiss terrain is overlain by metasedimentary units, the metamorphism of which has been dated by the Sm-Nd whole rock-mineral technique (702 ± 27 Ma in the west, 592 ± 16 Ma in the east). In the central part, the Abu Sari volcanic rocks show geochemical signatures of formation at an arc, with a protracted tholeiitic, calc-alkaline and shoshonitic evolution. The overlying El Hamri ophiolite contains chemical features of a back-arc tectonic environments. The ophiolite was dated by the Sm-Nd whole rock method on metagabbros at 752 ± 48 Ma. The further extension of this oceanic basin into the Jebel Rahib in the south-west was dated at 707 ± 54 Ma (Sm-Nd whole rock and minerals).Widespread suite of syn-tectonic granitoid intrusives displays subduction-related characteristics. They where emplaced between 650 to 760 Ma (Pb-zircon evaporation method). Their Nd and Sr isotopic compositions indicate a changing pattern of island arc to active continental margin character along an east-west transect and suggest a west to north-west dipping subduction zone. All units were juxtaposed at the minimum age of ≈ 600 Ma and rearranged during an extensional event, which was dated by the Rb-Sr thin slab technique (546 ± 19 Ma) on a migmatite. The Delgo suture provides evidence of a complex terrane pattern in north-east Africa and crustal growth during the Pan-African event by the addition of oceanic material to pre-existing continental crust.

Petrology, geochemistry and structural development of the Bir Safsaf-Aswan uplift, Southern Egypt

Abstract The Bir Safsaf-Aswan uplift is an east-west striking major basement high in SW Egypt. It consists mainly of granitic gneisses intercalated with amphibolites, marbles and calc-silicates. It is intruded by syntectonic S-type granites and late-tectonic I-type granitoids and by alkaline basaltic plugs and different dyke generations. Data on the metamorphic evolution revealed an early high- T (∼800°C) event, followed by amphibolite-migmatite facies and then by greenschist facies conditions. The following preliminary geological evolution is proposed: the East African craton in SW Egypt was formed and metamorphosed under high T conditions in Pre-Pan-African times. The accretion of the Arabian-Nubian shield led to the ckening of the adjacent continental African crust which caused migmatization and formation of S-type granites. Late tectonic uplift resulted in a low-grade metamorphic overprint and in the formation of I-type granites at around 570 Ma. The end of the Pan-African development is documented by the intrusion of igneous dykes at 521 Ma. The Pan-African generated fracture system was frequently reactivated in the Phanerozoic, as indicated by the ages of fracture bound continental volcanic rocks (193 ± 5 Ma; 155 ± 4 Ma; 87−81 Ma).

Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

Abstract The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.

Atmur-Delgo suture: A Neoproterozoic oceanic basin extending into the interior of northeast Africa

Field and remote-sensing data have enabled identification of a previously unrecognized discontinuous belt of ophiolitic nappe remnants in northeastern Sudan. The belt crops out for 200 km in a west southwest to southwest direction from the western margin of the Arabian-Nubian shield into the gneissic terrane previously accepted as part of the Archean to Paleoproterozoic Nile craton. Although highly dismembered and metamorphosed, the belt contains all the components of a Phanerozoic-type ophiolite. The ophiolite belt is interpreted as a suture zone (Atmur-Delgo suture), manifesting collision (∼700 Ma) between a passive-margin terrane in the south with an island-arc terrane in the north, following the consumption of an oceanic basin along a north-dipping subduction zone. Our results indicate that previously recognized crustal provinces in northeastern Africa must be reassessed critically.

Late Proterozoic magmatism in the Nakasib suture, Red Sea Hills, Sudan

The south-western part of the NE-SW-trending Nakasib suture in the Red Sea Hills of Sudan is characterized by a distinct lithostratigraphic and geochemical zonation from north to south. The southern part of the suture zone is underlain by the Ariab volcanic arc which is separated from the adjacent Oshib ophiolitic melange to the north by a major NW-verging thrust. Basalts of the Oshib ophiolite complex vary from north to south from typical mid-ocean-ridge (MORB) through oceanic island (OIB) to boninitic lavas. The volcanic sequences of the Ariab island are consist of minor tholeiitic and major calc-alkaline intermediate to felsic volcanic and plutonic rocks. Approximately 810 Ma old, subduction-related calc-alkaline granitoid rocks have been intruded into the northern margin of the 890–840 Ma old Haya terrane along the southern boundary of the Nakasib suture zone. This lithostratigraphic arrangement and the geochemical zonation of the ophiolites indicate that the Nakasib ocean basin was consumed in the Late Proterozoic by a southerly dipping subduction zone.

Plutonic and metamorphic rocks from the Keraf Suture (NE Sudan): A glimpse of Neoproterozoic tectonic evolution on the NE margin of W. Gondwana

The Keraf Shear Zone (KSZ) of NE Sudan formed in Neoproterozoic time when juvenile crust of the Arabian–Nubian Shield collided with reworked older crust of the Saharan Metacraton. Orthogneisses and deformed granitoids which intrude carbonate sediments are calc-alkaline, medium-K, metaluminous and I-type diorites and granodiorites, with subduction-related trace element signatures. 207 Pb/ 206 Pb zircon single grain ages are ∼730 Ma for the high-grade gneiss and ∼710 Ma for granitoids. K/Ar ages are 660 Ma for hornblende from the granitoids and 560 Ma for biotite from the high-grade gneiss and the plutons. The geochronologic data indicate that the crust formed ∼730–710 Ma and that orogenic activity ended by ∼565 Ma. Cooling occurred over much of NE Africa at about the same time, possibly caused by orogenic collapse or unroofing caused by extensive glaciation. Pb isotopic ratios indicate an oceanic source for both the magmatic and metamorphic rocks in the KSZ. Sm/Nd model ages are 620–770 Ma for the post-tectonic plutons and 830–900 Ma for the gneiss, indicating negligible involvement of a pre-Neoproterozoic material in the evolution of Keraf igneous rocks. The ( 87 Sr/ 86 Sr)i values range from 0.7026 to 0.7046, which are indistinguishable from initial ratios for juvenile crust of the Arabian Nubian Shield and are lower than initial Sr isotopic compositions for the Saharan Metacraton. The isotopic data indicate that Keraf igneous rocks are juvenile additions to the crust and that the boundary between the Saharan Metacraton and the Arabian–Nubian Shield lies to the west.

Deformational History of the Neoproterozoic Keraf Zone in NE Sudan, Revealed by Shuttle Imaging Radar

The location of the boundary between juvenile Neoproterozoic crust of the Arabian-Nubian Shield in the Red Sea Hills and older crust of the Nile craton to the west is defined by the Keraf zone in northern Sudan, but little is known about its deformation history. Shuttle Imaging Radar (SIR-A), Landsat Thematic Mapper (TM), and Large Format Camera (LFC) images, combined with ground investigation, were used to carry out the first detailed study of the Keraf zone. This N-trending zone is ~45 km wide and is defined by multiply deformed carbonate-rich turbidites and volcanogenic sediments. Six deformational phases (D1 to D6) were identified, associated with two tectonic events: (1) D1 and D2 are related to emplacement of SSE-verging ophiolitic nappes due to collision between the Haifa and Bayuda terranes along the ENE-trending Atmur suture at ~800-700 Ma. The Atmur suture marks the site of a former Neoproterozoic oceanic re-entrant that extended WSW from the Mozambique ocean into the interior of the Nile craton. D1 produced SSE-verging tight folds and thrusts, whereas D2 deformed the D1-structures into ENE-trending upright folds. (2) D3 to D6 reflect shortening across the Keraf zone. D3 produced N-trending, upright, isoclinal to open folds. D4 was superimposed on D3 as coaxial W-verging tight folds. D5 refolded older structures about steeply, east- to ENE-plunging fold axes. D6 manifests development of local NE-trending dextral and NW-trending sinistral shear zones. We suggest that this deformation reflects oblique collision between composite arc terranes of the Arabian-Nubian Shield and the Nile craton along the Keraf zone at ~750-650 Ma.

Outline of the geology of magmatic and metamorphic units between Gebel Uweinat and Bir Safsaf (SW Egypt/NW Sudan)

Abstract The Uweinat basement uplift system extends from Gebel Uweinat in the west to Bir Safsaf in the east underlying an area of roughly 40,000 km 2 in which crystalline rocks arc exposed. The distribution of the basement rocks was mapped by means of field traverses combined with an interpretation of Landsat satellite images. General foliation trend is NE-SW in the Uweinat area changing to NNE-SSW in northern direction and to WNW-ESE in the Gebel Kamil area. In the eastern part (Bir Safsaf) it is NW-SE. Metamorphic rocks are migmatites and granitic to granulitic gneisses with subordinate amounts of amphibolite and marble. Low grade metamorphic rocks were found only in the southern area (NW Sudan). Plutonic rocks include different types of granitoids and dykes, volcanic rocks are alkali olivine basalt, phonolite, trachytes, and rhyolites. All age data for the area are compiled and together with three newly obtained ages (673 ± 65 m.y., migmatites, 431 ± 33, granite, 216 ± 5, rhyolite) it can be shown that magmatic activity occurred in several periods during the Phanerozoic. Some of the magmatic rocks are concentrated along fracture zones.