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Subject: search.filters.subject.Arabian–Nubian Shield

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    New ophiolite occurrences in Sudan and constraint on the western boundary of the Nubian Shield: Petrographical and geochemical evidence
    (Elsevier Ltd, 2010-07) E.A. Ali, E.M. Abdel Rahman
    Important mafic–ultramafic masses have been located for the first time in the intersection area between the Keraf Shear Zone and the Nakasib Suture Zone of the Nubian Shield. The masses, comprising most of the members of the ophiolite suite, are Sotrebab and Qurun complexes east of the Nile, and Fadllab complex west of the Nile. The new mafic–ultramafic masses are located on the same trend of the ophiolitic masses decorating the Nakasib Suture. A typical complete ophiolite sequence has not been observed in these complexes, nevertheless, the mafic–ultramafic rocks comprise basal unit of serpentinite and talc chlorite schists overlain by a thick cumulate facies of peridotites, pyroxenites and layered gabbros overlain by basaltic pillow lavas with dolerite dykes and screens of massive gabbros. Associated with pillow lavas are thin layers of carbonates and chert. The best section of cumulate mafic–ultramafic units has been observed in Jebel Qurun and El Fadlab complexes, comprising peridotites, pyroxenites and layered gabbros. Dolerite dykes and screens of massive gabbros have been observed with basaltic pillow lava sections in Wadi Dar Tawaiy. The basal ultramafic units of the complexes have been fully or partly retrograded to chlorite magnetite schist and talc to talc-carbonate rocks (listowenites), especially in the Jebel Qurun and Sotrebab complexes. Petrographically, the gabbros (layered and massive) and the basaltic pillow lavas show mineral assemblages of epidote amphibolite facies. The mafic members from the three complexes show a clear tholeiitic trend and oceanic floor affinity. The pillow lavas plot in the field of oceanic floor basalt, namely in the back arc field. Primitive mantle normalized spider diagram of the pillow lavas reveals a closer correspondence to Enrich-Mid-Oceanic Ridge Basalt (E-MORB) type, which is confirmed by the flat chondrite normalized Rare Earth Elements (REE) pattern. Field, petrographical and geochemical evidence supports ophiolitic origin of the three complexes. The newly discovered ophiolitic complexes mark the western continuation of the Nakasib Suture Zone.
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    Southeast Bayuda volcano-sedimentary sequences (Kurmut terrane, Sudan): juvenile island arc series within the mega-shear zone marking the eastern boundary of the Saharan Metacraton
    (Springer, 2013-01) E. A. Ali & E. M. Abdelrahman, Wu Guang
    This study was conducted to investigate the volcano-sedimentary sequence of Kurmut terrane in southeast Bayuda Desert and its continuation east of the Nile. The sequence is situated at the inferred transition between the Neoproterozoic juvenile Arabian–Nubian Shield and the old reworked Saharan Metacraton. Lithologically, the rocks are composed of intermediate to basic metavolcanics and volcanoclastics intercalated with tuffeous material and metapelite sediments that formed in cyclic graded bedding, indicating a turbid environment in the active orogenic zone. For the first time in the study area, intraformational conglomerates with cigar-shaped volcanic clasts within the low-grade metasediments suggest a high-energy depositional environment. Minerals and textural examination of rock assemblage indicate an anticlockwise P/T condition from amphibolite facies to retrograde greenschist facies conditions in a large high strain zone of distributed deformation. Geochemical results of metavolcanics indicate derivation from different mantle sources (island arc tholeite, calc-alkaline basalt, mid-oceanic ridge, and back-arc basalt). Within this sequence, field relation and microstructural analysis clearly show the best evidence of superimposition of sinistral shear of the NNE–SSW-trending Keraf Shear Zone on the early dextral NE–SWNakasib Suture Zone. Consequently, based on lithological and structural observations with support of available geochemical data, the volcano-sedimentary sequences SE of Bayuda Desert and east of Nile are similar to their equivalent assemblage of juvenile Neoproterozoic arc lavas association of the Arabian–Nubian Shield.