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Stern R.J.,University of Texas at Dallas | Ali K.A.,University of Texas at Dallas | Liegeois J.P.,Royal Museum for Central Africa | Johnson P.R.,6016 SW Haines Street | And 3 more authors.
American Journal of Science | Year: 2010

Igneous rocks of the Arabian-Nubian Shield (ANS) have lithologic associations (ophiolites, calc-alkaline igneous rocks, immature sediments) and radiogenic isotopic compositions consistent with formation as juvenile continental crust as a result of accreting intraoceanic arc systems during 880 to 630 Ma, with crustal differentiation continuing until ∼ 570 Ma. ANS igneous rocks locally contain zircons with ages that are much older than this, leading some researchers to infer the presence of pre-Neoproterozoic crust at depth in spite of Nd isotopic evidence that ANS crust is overwhelmingly juvenile. The ANS is flanked by pre-Neoproterozoic crust but geochronology and isotopic compositions readily identify such tracts. We have compiled U-Pb zircon ages for 302 samples of ANS igneous rocks that have been analyzed for the age of individual zircons (2372 ages) and find that a significant proportion (∼ 5%) of these have ages older than 880 Ma (zircon xenocrysts). Zircon xenocrysts are more common in volcanic than plutonic rocks and mafic relative to felsic igneous rocks. Four explanations are considered: 1) contamination during sample processing; 2) involvement of pre-Neoproterozoic crust; 3) incorporation of detrital zircons from sediments; and 4) inheritance from a mantle source. Possibilities 1 and 2 are discounted, and we conclude that the presence of pre-880 Ma zircon xenocrysts in ANS igneous rocks with mantle-like isotopic compositions indicates either incorporation of sediments or inheritance from the mantle source region, or both.

Johnson P.R.,6016 SW Haines Street | Andresen A.,University of Oslo | Collins A.S.,University of Adelaide | Fowler A.R.,United Arab Emirates University | And 4 more authors.
Journal of African Earth Sciences | Year: 2011

During the late Cryogenian-Ediacaran (650-542. Ma), the Arabian-Nubian Shield (ANS) underwent final assembly and accretion to the Saharan Metacraton concurrent with the assembly of eastern and western Gondwana. At the end of the Precambrian it lay at one end of the East African Orogen, with its northern margin (present coordinates) forming a low-relief stable shelf facing an open ocean; to the south the ANS transitioned into the Mozambique Belt. The geologic history of the ANS during this period provides insight into the closing developmental stages of one of the world's largest accretionary orogens. Following a 680-640. Ma orogenic event reflecting amalgamation of a core grouping of island-arc terranes (the proto-Arabian-Nubian Shield; pANS), the region underwent extensive exhumation, erosion, and subsidence. Depositional basins formed in the northern and eastern pANS, with those in the east below sea level and connected to an ocean. Periodic basin closure and formation of new basins in other parts of the ANS followed. Many basins were filled by terrestrial, molasse-type sediments interfingering with subordinate to predominant amounts of volcanic rocks. Magmatism was extensive throughout the period, initially characterized by tonalite-trondhjemite-granodiorite (TTG) and granite (monzogranite, syenogranite), but also characterized, from ~610. Ma on, by increasing amounts of alkali-feldspar granite and alkali granite. The plutons are largely undeformed, except where cut by brittle-ductile shear zones. The magma sources of the late Cryogenian-Ediacaran granitoids were dominated by juvenile crust and(or) depleted mantle and magmas mostly originated in anorogenic, post-collisional, commonly extensional, settings. They were derived by melting and fractionation of anhydrous high-grade metamorphosed lower crust, mafic- to intermediate calc-alkaline crust, and(or) subduction-modified mantle wedges associated with slab break-off or delamination.By ~630. Ma, the region was affected by oblique (transpressional) convergence of continental blocks that formed eastern and western Gondwana-the pANS was approaching the Saharan Metacraton; north-trending shear and shortening zones developed in the southern ANS; and northwest-trending strike-slip shear zones of the Najd fault system dominated farther north. In the northwestern ANS, convergence and Najd transpression buckled the crust causing structural highs with domes of gneissic infracrust overlain by supracrust composed of ophiolitic and volcanosedimentary assemblages dating from the Tonian-middle Cryogenian period of island-arc activity. The supracrust was extensively translated to the northwest above a high-strain zone. Extension and tectonic escape augmented exhumation of the gneissic infracrust particularly between ~620-580. Ma. In the northeastern ANS, linear belts of gneiss formed from reworked older intrusive bodies or syntectonic intrusions that were emplaced along Najd faults. By ~620. Ma a marine basin on the eastern margin of the pANS (present coordinates) was beginning to close. A thick sedimentary assemblage (Abt formation) in this basin underwent metamorphism and folding, and subduction-related magmatism and volcanism farther into this basin (Al Amar arc; >690-615. Ma) was coming to an end. Amalgamation of the Abt formation, Al Amar arc, and the pANS occurred between ~620 and ~605. Ma, and terminal collision between the pANS and the Saharan Metacraton was complete by ~580. Ma. At this time, the ANS was fully assembled. Granite magmatism continued until ~565-560. Ma and orogeny ceased by ~550. Ma. During these terminal events, the region underwent strong chemical weathering and became a vast low-relief surface on which Lower Paleozoic sandstone was eventually deposited. © 2011 Elsevier Ltd.

Johnson P.R.,6016 SW Haines Street | Halverson G.P.,McGill University | Kusky T.M.,China Three Gorges University | Stern R.J.,University of Texas at Dallas | Pease V.,University of Stockholm
Geosciences (Switzerland) | Year: 2013

The Arabian-Nubian Shield (ANS) includes Middle Cryogenian-Ediacaran (790-560 Ma) sedimentary and volcanic terrestrial and shallow-marine successions unconformable on juvenile Cryogenian crust. The oldest were deposited after 780-760 Ma shearing and suturing in the central ANS. Middle Cryogenian basins are associated with ~700 Ma suturing in the northern ANS. Late Cryogenian basins overlapped with and followed 680-640 Ma Nabitah orogenesis in the eastern ANS. Ediacaran successions are found in pull-apart and other types of basins formed in a transpressive setting associated with E-W shortening, NW-trending shearing, and northerly extension during final amalgamation of the ANS. Erosion surfaces truncating metamorphosed arc rocks at the base of these successions are evidence of periodic exhumation and erosion of the evolving ANS crust. The basins are evidence of subsequent subsidence to the base level of alluvial systems or below sea level. Mountains were dissected by valley systems, yet relief was locally low enough to allow for seaways connected to the surrounding Mozambique Ocean. The volcanosedimentary basins of the ANS are excellently exposed and preserved, and form a world-class natural laboratory for testing concepts about crustal growth during the Neoproterozoic and for the acquisition of data to calibrate chemical and isotopic changes, at a time in geologic history that included some of the most important, rapid, and enigmatic changes to Earth's environment and biota. © 2013 by the authors; licensee MDPI, Basel, Switzerland.

Stern R.J.,University of Texas at Dallas | Mukherjee S.K.,University of Texas at Dallas | Miller N.R.,University of Texas at Austin | Ali K.,King Abdulaziz University | Johnson P.R.,6016 SW Haines Street
Precambrian Research | Year: 2013

Neoproterozoic Banded Iron Formation (BIF) from Sawawin, NW Saudi Arabia and the Central Eastern Desert of Egypt define the 200×100km Arabian-Nubian Shield (ANS) BIF basin. ANS BIF formed ~750Ma, prior to the Sturtian glacial episode (which began ~716Ma). BIF deposition occurred in a marine basin associated with arc/backarc basin volcanism and immature clastic sedimentation. Beds are composed of alternating iron- and silica-rich laminae, which may reflect seasonal changes in deposition of Fe vs. Si. Fe-rich layers are dominantly composed of primary fine-grained hematite "dust" and minor apatite, with abundant secondary magnetite. Rapid deposition is revealed by: (1) major and trace element data indicating that ANS-BIF are very pure (<20% detrital input) chemical sediments in spite of being deposited in a basin with high sedimentation rates, and (2) pervasive evidence for soft-sediment deformation, suggesting that rapid sedimentation of dense, weak materials resulted in slumping. Nd and Pb isotopic compositions are predominantly mantle-like, indicating the dominance of hydrothermal sources or weathering of juvenile ANS crust for these elements. REE data show HREE-enriched patterns typical of modern seawater, with small positive Eu and small negative Ce anomalies. Low abundances of transition elements that are commonly abundant in proximal hydrothermal deposits of modern oceans may indicate that ANS-BIF formed at some distance from hydrothermal vents, or may reflect prior sulfide scavenging by euxinic and sulfidic deep ocean waters. REE data and Zn/Co share characteristics of both modern seawater and hydrothermal vent fluids suggesting derivation from a mixture of shallow suboxic seawater with a dilute, low-T hydrothermal vent fluid. Considered in conjunction with BIF of similar age on other paleocontinents, these observations support that rapid and widespread re-oxygenation of Fe+2 in previously anoxic or suboxic seawater led to rapid precipitation of hematite "dust" and BIF deposition ~750Ma. Sulfate limitation or diminished bacterial sulfate reduction, required to form BIF instead of pyrite, may reflect large-scale glaciation but evidence for deep ferruginous conditions prior to Cryogenian glaciations suggests than any scenario that substantially reduced continental weathering (i.e., hard snowball or slushball) could have primed the oceans for BIF deposition. The likely short duration of ANS BIF deposition (a few to 10s of kyr) and apparent timing well before the "Sturtian" glaciation suggest that conditions favoring Neoproterozoic BIF formation could have existed over an extended period (10s of myrs) and that Neoproterozoic Oxidation Event began during the "Kaigas-Sturtian" time frame. © 2013 Elsevier B.V.

Fritz H.,University of Graz | Abdelsalam M.,Oklahoma State University | Ali K.A.,King Abdulaziz University | Bingen B.,Geological Survey of Norway | And 12 more authors.
Journal of African Earth Sciences | Year: 2013

The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ~850 and 550. Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ~850 and 620. Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ~650-620. Ma granulite-facies metamorphism. Completion of the nappe assembly around 620. Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620. Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ~600-500. Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ~600-550. Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ~550-480. Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of distinctly different orogen styles. The Arabian-Nubian Shield is an accretion-type orogen comprising a stack of thin-skinned nappes resulting from the oblique convergence of bounding plates. The Eastern Granulite-Cabo Delgado Nappe Complex is interpreted as a hot- to ultra-hot orogen that evolved from a formerly extended crust. Low viscosity lower crust resisted one-sided subduction, instead a sagduction-type orogen developed. The regions of Tanzania and Madagascar affected by the Kuungan Orogeny are considered a Himalayan-type orogen composed of partly doubly thickened crust. © 2013 The Authors.

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