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Introduction to the JEBEL volume of Precambrian Research
Precambrian Research 239 (2013) 1–5
Contents lists available at ScienceDirect
Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Editorial
Introduction to the JEBEL volume of Precambrian Research
The Arabian-Nubian Shield (ANS) is one of the largest tracts of juvenile Neoproterozoic crust on Earth, extending >3500 km north–south and >1500 km east–west over parts of Jordan, Saudi Arabia, Yemen, Israel, Egypt, Sudan, Eritrea, Ethiopia, Somali, and Kenya (Fig. 1). It is the product of a ∼300 million year cycle of crustal growth, bracketed by break up of the supercontinent Rodinia and assembly of the supercontinent Gondwana, and composed of ophiolitic complexes, oceanic arcs, granitoid plutons and batholiths, and terrestrial to shallow-marine volcanosedimentary basins. Formation of the shield began with the creation of early Cryogenian oceanic crust some 10 km thick in the Mozambique Ocean between rifted parts of Rodinia and ended with a block of Ediacaran lithosphere comprising continental crust some 45 km thick and lithospheric upper mantle some 80–120 km thick. At times, divergent national scientific objectives and differing priorities of geoscience organizations have made systematic geologic study and synthesis of the entire ANS problematic, but the breadth and detail of current geologic research and developing opportunities for transnational cooperation are fostering clearer insight into the geologic evolution of the shield. The present volume is an outgrowth of a five-year collaboration by geologists from Jordan, Egypt, Saudi Arabia, Sweden, Norway, Germany, Australia, and the United States of America within the JEBEL Project. JEBEL was funded by the Swedish International Development Agency specifically to promote scientific exchange with the Middle East and North Africa (SIDA-MENA). Our collaborative work has been facilitated by national geoscience organizations and institutions. Among these, we particularly wish to thank the Saudi Geological Survey, the National Materials Authority of Egypt, and the University of Jordan for hospitality and logistical help. Geologic investigations in the ANS are making a fundamental contribution to knowledge of Neoproterozoic Earth history. The shield rocks are well exposed and easily accessible, they are mostly less deformed and metamorphosed than contemporary rocks elsewhere in the world, and retain unambiguous evidence of their depositional, volcanic, intrusive, and structural character and relationships. The region is a prime natural laboratory for researching: (i) the origins and character of Neoproterozoic oceanic crust; (ii) depositional and magmatic processes in Neoproterozoic subduction systems; (iii) changes in Neoproterozoic weathering conditions; of isotopic excursions in Neoproterozoic seawater and atmosphere; and (iv) the effect on structure and magmatism of a Neoproterozoic tectonic transition from unstable coalescing volcanic arcs to stable continental crust. Tectonically, the ANS is an accretionary orogen, part of the larger East African Orogen encompassing the ANS in the north
0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.10.003
and the Mozambique Belt in the south. The shield has a reasonably well-defined margin with the Archean-Neoproterozoic Saharan Metacraton in the west and with high-grade metamorphic rocks of the Mozambique Belt in the south. To the east in Kenya and southern Ethiopia, the shield is in contact with the Azania ribbon continent, a belt of Archean-Paleoproterozoic schist and gneiss that extends from Madagascar into Somalia. In Arabia, Archean and Paleoproterozoic gneiss is also present and may represent a northward continuation of Azania. However, the structural character of the Archean-Paleoproterozoic gneiss changes from south to north. In Yemen, Archean-Paleoproterozoic gneiss is structurally intercalated with Neoproterozoic arc assemblages, and in Saudi Arabia is pervasively reworked, as evidenced by small exposures of intact Paleoproterozoic granite, gneiss and schist and the imprint of Paleoproterozoic lead, strontium, and neodymium isotopes on Neoproterozoic granite. The Yemen exposures are referred to as the Abas and Al Mahfid terranes; reworked preNeoproterozoic crust in Saudi Arabia is referred to as the Khida terrane. Another change is that Neoproterozoic juvenile arc rocks re-occur east of the Archean-Paleoproterozoic gneiss in northern Somali, Yemen, and Saudi Arabia, where they reflect subduction in a late Cryogenian-Ediacaran ocean east of Azania. In the north, isotopic and geochronologic data strongly indicate a contact with pre-Neoproterozoic crust at the far northern edge of shield exposures in Sinai or in basement beneath Phanerozoic rocks farther north. The time-space history of the ANS is based on a comprehensive data set of U–Pb and Rb–Sr ages and a smaller number of Sm–Nd and 40 Ar/39 Ar ages. Current U–Pb dating programs in the shield mostly utilize Secondary Ion Mass Spectrometry (SIMS), Thermal Ionization Mass Spectrometry (TIMS), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses of single zircons. Some programs use Pb/Pb evaporation techniques on zircon or U–Pb spot methods on titanite. Early-middle Cryogenian arc assemblages dominate the southern part of the ANS, locally enclosing small amounts of Tonian arc rock in the Makkah batholith (Mk: Fig. 1); Erkowit batholith (E); the Kurmut Series (K); and in the Bulbul region of southern Ethiopia (Bu). Middle to late Cryogenian arc rocks dominate the north and east, and late Cryogenian-Ediacaran assemblages occur in the farthest most east in the Arabian Shield (Ar Rayn terrane) and the farthest most west in the Nubian Shield (Aswan area). Oceanic crust in the shield, represented by mostly forearc ophiolite complexes, ranges from 845 to 675 Ma; arc assemblages range from ∼870 to 615 Ma. Arc amalgamation and suturing occurred between ∼780 and 600 Ma, and accretion between the ANS and
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Editorial / Precambrian Research 239 (2013) 1–5
Fig. 1. The Arabian-Nubian Shield showing its exposure in Arabia, Northeast Africa, and the East African coast and illustrating its relation to older crust on its margins (after Fritz et al., 2013). Ophiolites schematically shown after Berhe (1990). A, Allaqi; Ad, Adola; Ak, Akobo; B, Baragoi; BU, Bi’r Umq; Bu, Bulbul; E, Jabal Ess; G, Gebel Gerf; H, Halaban; K, Kinyiki; M, Moyale; MS, Moroto-Sekerr; N, Nuba; Na, Nakasib; S, Sol Hamed; T, Jabal Thurwah; Ta, Jabal Tays; Tu, Bi’r Tuluhah; TY, Tuludimtu-Yubdo; U, Jabal Uwayjah; W, Jabal Wask. Arrows show sense of sinistral transpressive shear during final collision of ANS and the Saharan Metacraton.
the Saharan Metacraton reflecting terminal collision of the ANS and western Gondwana blocks, occurred ∼650–580 Ma. Recent dating campaigns help clarify the magmatic history of the ANS. Six pulses of magmatism and, in some cases, associated migmatization and deformation, are recognized in the Nubian shield in Egypt: (1) 705–680 Ma, (2) ∼660 Ma, (3) 635–630, (4) 610–604, (5) 600–590, and (6) 550–540 Ma (Lundmark et al., 2012). High-resolution ionprobe dating shows that post-tectonic granitoids in Sinai result
from early Ediacaran calc-alkaline magmatism at ∼635–590 Ma and middle Ediacaran alkaline magmatism at ∼608–580 Ma (Be’eriShlevin et al., 2009). Significantly, the ages define a 15 million-year overlap of the two magmatic types at ∼605–590 Ma such that the onset of alkaline magmatism apparently coincides with transitions in the calc-alkaline suite from mafic to felsic magmatism and a voluminous pulse of granodiorite to granite plutonism at 610–600 Ma. This refined dating demonstrates that a commonly accepted model
Editorial / Precambrian Research 239 (2013) 1–5
of a transition from calc-alkaline to within-plate alkaline to peralkaline magmatism in the northern ANS at about 610 Ma (Beyth et al., 1994) is an oversimplification. Recent dating likewise provides stronger constraints on magmatic modeling than hitherto available in the southern ANS, identifying 4 principal magmatic events: 890–840 Ma (late Tonian-early Cryogenian); 790–700 Ma (middle Cryogenian); 660 Ma (late Cryogenian); and 630–500 Ma (Ediacaran) (Stern et al., 2012). A defining characteristic of the ANS, with the exception of the obvious areas of pre-Neoproterozoic rocks and reworked crust, it that shield rocks have magmatic formation ages close to their neodymium model ages; they formed from material separated from the mantle shortly before their emplacement. This juvenile character of shield rocks is displayed by remnants of oceanic crust represented by ophiolite complexes, by suprasubduction and rift-related volcanic assemblages, by arc-related diorite, tonalite, trondhjemite, and granodiorite, and by younger late- to posttectonic granite emplaced in the evolving crust of the shield. An emerging feature is the growing body of evidence that many of the arc-related volcanosedimentary and plutonic associations have an adakitic affinity (Fig. 2). Also notable, is evidence that volcanic and plutonic host rocks of gold, silver, base-metal, molybdenum, and tungsten occurrences in the ANS are adakites. This places the ANS at the center of a debate about the origin and petrogenetic implication of adakites. Due to the abundance of alkali-feldspar granite in the ANS, the shield is also central to the debate about the origin and petrogenetic implication of alkali-feldspar granite and alkaline magmatism. Granite dominates late Cryogenian-Ediacaran magmatism in the ANS. It constitutes the most abundant rock type of this period exposed at the surface and, on the basis of seismic-refraction data, is inferred to be the main component of the upper 20 km of lithospheric crust in the Arabian Shield (Gettings et al., 1986). Granite is a likely similar component of the Nubian Shield crust. In total, therefore, a massive emplacement of granitic magma affected the entire upper crust of the Arabian-Nubian Shield during the late Cryogenian and Ediacaran. Among the granite intrusions, alkali-feldspar A-type granite plutons and batholiths are particularly conspicuous. Despite this, there is no consensus about the origin of alkalifeldspar granite in the shield, similar to the lack of consensus in the global geoscience community. Although originally believed to be the product of magmatism in extensional settings, it is now known that A-type granites occur in a variety of settings ranging from anorogenic, within-plate to plate boundaries, and are variously considered to originate from anhydrous high-grade metamorphic rock or from mafic to intermediate calc-alkaline precursors. Proposals for A-type granite origins in the ANS include: (1) the melting of earlier arc rocks followed by anhydrous fractionation that in some cases gives rise to magmas that have a residual calc-alkaline signature; (2) partial melting of phlogopite-bearing spinel lherzolite, perhaps from OIB-type lithospheric–asthenospheric mantle present in subduction-modified mantle wedges concurrent with or following cessation of subduction and slab-break-off or delamination; and (3) partial melting of Rb-depleted crustal sources in shear zones. The ANS is also important for calibrating Neoproterozoic climate change, oceanic isotopic excursions, glaciation, and biologic radiation. In this regard, a triad of lithologies is significant – diamictite, carbonates units, and banded iron formation (Fig. 3). At least four diamictites are known in the Arabian Shield: in the Nuwaybah formation (∼740 Ma), at the base of the Mahd group (∼750 Ma), in the Hadiyah group (∼695 Ma), and in the Jibalah group (∼590–560 Ma), and three occurrences are known in the Nubian Shield: in the Meritri group (∼780 Ma), the Atud Conglomerate (∼740 Ma), and the Tambien Group (∼750–720 Ma). Of these, the Atud and Nuwaybah diamictites have strong evidence for a
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glacial origin and the diamictite at the base of the Mahd group is possibly glaciogenic. All three are broadly contemporary with the Kaigas event. The Meritri diamictite was deposited shortly after the ∼800 Ma negative ␦13 Ccarb (‰) excursion recorded in the global carbon isotope curve; the Hadiyah group and Jibalah group diamictites are approximately contemporaneous with the Sturtian and Gaskiers events. The Atud Conglomerate diamictite is particularly significant because it is stratigraphically beneath Cryogenian Algoma-type banded-iron formation (BIF) in the shield. BIF is not abundant in the ANS but occurs in several locations in the northwestern part of the shield in Egypt and Saudi Arabia as beds of alternating iron- and silica-rich laminae. A stratigraphic relationship between glacial diamictite and BIF figures in the debate about global glaciations, e.g. – the Snowball Earth hypothesis. No direct evidence of glaciation is found in the ANS BIF examples and the BIF laminae may primarily reflect seasonal changes in deposition of iron and silica (Stern et al., this volume), but depending on the timing adopted for global glacial events, the ANS BIF was deposited soon after the Kaigas event or shortly before the Sturtian event. The third lithology in the ANS triad significant for calibrating Neoproterozoic climate change is carbonate, important for recording variations or excursions in carbon and strontium isotope values such as the negative ␦13 Ccarb (‰) excursions known to be associated with profound carbon flux changes that accompany widespread glacial transitions. Such excursions have been studied in the Tambien Group in Ethiopia and the Jibalah group in Saudi Arabia. Tambien Group limestone underlies putative glacial diamictite and records a pronounced negative carbon excursion through the transition to diamictite from +7 to −2‰ consistent with a glacial environment (Miller et al., 2003). Carbon isotope values in Jibalah group carbonate in contrast are predominantly positive, averaging +2.4 ± 2.3‰, more consistent with deposition in the interval either before or after the Gaskiers Shuram anomaly (∼551–542 Ma) rather than during the Gaskiers event itself (Miller et al., 2008). Many other carbonate intervals occur in the ANS that are appropriate targets for additional C, Sr, and O isotopic investigations. This Special Issue of Precambrian Research provides examples of ongoing research into the geology of the ANS confirming the value of the ANS to global geologic understanding of Neoproterozoic events. The volume begins with the Ediacaran, progresses back through Cryogenian time, and concludes with contributions on the pre-Cyrogenian geological framework of the region. Contributions reflecting Ediacaran sedimentation and igneous rocks include two papers by Yaseen et al. and Boskabadi et al. The first paper presents a SIMS U–Pb detrital zircon investigation of a posttectonic basin of the ANS. Such basins are fascinating records of the basement exposed via orogenesis and post-orogenic extension. Yaseen et al.’s study contains surprising results indicating for the first time igneous activity of a previously unrecognized age, with important ramifications for the onset and duration of magmatism during a period previously regarded as amagmatic. The preservation of syn- to post-tectonic basins (or fragments thereof) provides important information on the timing of ANS orogenic uplift, erosion, and burial. Boskabadi et al. investigate alteration associated with the c. 610 Ma Tarr carbonatite–albitite complex (Sinai, Egypt). Geochemical analyses indicate that the complex is a trace-element-poor carbonatite, probably reflecting its mantle source. Breccia associated with the complex indicates the addition of carbonate and other secondary minerals as fluid-mobilized alteration products. Their fluid inclusion studies indicate that alteration occurred at c. 500 ◦ C and c. 1 kbar, probably during post-intrusion cooling. The second group of papers in this volume relates to Cryogenian crustal growth, metamorphism, and climate change (Ali et al., Jarrar et al., Stern et al.). New Hf-isotopic analyses of zircons from Egypt’s
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Fig. 2. Y versus Sr/Y plots of data from volcanic and plutonic rocks in the Arabian-Nubian Shield showing the adakitic affinity that is now well established for arc-related assemblages in the shield. (A) Plot of three examples of data from northern Eritrea (Teklay et al., 2001), the eastern Arabian Shield (Doebrich et al., 2007), and western Arabian Shield (Hargrove, 2006). (B) A plot of data from volcanic and plutonic rocks in the Arabian Shield illustrating the association between metallic-mineral occurrences and host rocks of adakitic affinity (after Thiéblemont, 2000).
Fig. 3. Stratigraphic relationships of a triad of lithologies in the Arabian-Nubian Shield significant for calibrating Neoproterozoic climate change and the evolution of atmospheric and oceanic chemistry. Composite seawater carbon-isotope curve on right modified from Halverson and Shields-Zhou (2011); timing of global glacial events (gray bands) after MacGabhann (2005).
Editorial / Precambrian Research 239 (2013) 1–5
Eastern Desert, combined with Nd isotopic data, indicate that the post-collisional (c. 600–630 Ma) granites were derived from a juvenile source, whereas results from the metavolcano-sedimentary rocks indicate involvement of pre-Neoproterozoic material in the formation of some of these rocks (Ali et al.). The search for older components within the ANS is ongoing, yet to date very little has been documented so this important contribution adds to the limited body of evidence characterizing these rare occurrences. The oldest part of the ANS in Jordan is represented by orthoand paragneiss intruded by later granitoids of the Abu-Barqa Metamorphic Suite (ABMS). Dating of these rocks indicates the ABMS evolved between 600 and 800 Ma (SIMS U–Pb zircon). Maximum P–T conditions (5–6 kbar/c. 700 ◦ C) were achieved at c. 625 Ma, with peak conditions recording c. 3.2 kbar/540 ◦ C; this was followed by decompression and a thermal pulse associated with intrusion of post-tectonic granitoids at c. 610 Ma; and finally the whole complex was cooled and uplifted to the surface at ∼600 Ma (Jarrar et al.). The final paper in this group addresses the formation of the c. 750 Ma ANS BIF. Stern et al. present petrographic, geochemical, and Nd and Pb isotopic compositions of Neoproterozoic oxide facies from the NE margin (Sawawin, NW Saudi Arabia) of the now dismembered ANS BIF basin and its correlative Egyptian counterparts in Wadi Dabbagh and Wadi Kareim. They document the oxidation of this initially anoxic basin and, in conjunction with consideration of BIF of similar age on other paleocontinents, suggest that a major change in marine redox occurred before Sturtian glaciation. The implications of such a change for the Neoproterozoic Oxidation Event during ‘Kaigas-Sturtian’ time may be significant and need to be investigated further. The final two papers in our volume address the pre-Cryogenian framework of the ANS. Flowerdew et al. evaluate the significance of the Nabitah fault zone in the Saudi Arabian Shield, previously regarded as a major suture separating distinct terranes. Using geochronology, feldspar Pb and whole rock geochemistry, and Sm–Nd isotopic results, they suggest that the Nabitah suture separates two juvenile oceanic arc terranes of different ages and geochemical characters. They conclude that the fault zone is not the major suture separating the juvenile terranes of west Gondwana from more evolved terranes of east Gondwana, as previously supposed. The oldest part of the ANS in Sinai (Egypt) is represented by the Feiran–Solaf gneiss complex. In the contribution from Abu El-Enin & Whitehouse this metamorphic complex is investigated using SIMS dating. Their results indicate that the complex is older than previously thought, from <600–1000 Ma. Metasediments also contain numerous older zircons. The authors conclude that the Fieran–Solaf metamorphic complex reveals the existence of pre-Pan African crust, as well as four Pan-African episodes of magmatism (799–785 Ma, 713–692 Ma, 629–611 Ma, and 606–583 Ma) and two metamorphic peaks synchronous with the youngest pulses. Again, due to the paucity of known older crust in the ANS, this important contribution adds to the limited body of evidence characterizing these rare occurrences. We hope the contributions presented in this volume will promote further collaboration on important topics and questions associated with the ANS and its Neoproterozoic evolution. We would like to take this opportunity to extend our thanks to those who donated their time and energy to reviewing the manuscripts submitted to this volume. Their willingness to
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participate in the peer review process contributes to maintaining a high scientific standard through constructive criticism and feedback. Reviewers include Arild Andresen, Kamal Ali, Yaron Be’eri-Shlevin, Joachim Jacobs, Ian Miller, Aley El-Shazly, William Peltier, Jun-Iche Kimura, Kathryn Moore, as well as anonymous reviewers. References Be’eri-Shlevin, Y., Katzir, Y., Whitehouse, M., 2009. Post-collisional tectonomagmatic evolution in the northern Arabian-Nubian Shield; time constraints from ionprobe U–Pb dating of zircon. J. Geol. Soc. Lond. 166, 71–85. Berhe, S.M., 1990. Ophiolites in the northeast and east Africa: implications for Proterozoic crustal growth. J. Geol. Soc. London 147, 41–57. Beyth, M., Stern, R.J., Altherr, R., Kröner, A., 1994. Late Precambrian Timna igneous complex, southern Israel: evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma. Lithos 31, 103–124. Doebrich, J.L., Al-Jehani, A.M., Siddiqui, A.A., Hayes, T.S., Wooden, J.L., Johnson, P.R., 2007. Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African Orogen. Precambrian Res. 158, 17–50. Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A.S., Fowler, A.R., Ghebreab, W., Hauzenberger, C.A., Johnson, P.R., Kusky, T.M., Macey, P., Muhongo, S., Stern, R.J., Viola, G., 2013. Orogen styles in the East African Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution. J. Afr. Earth Sci. 86, 65–106. Gettings, M.E., Blank, H.R., Mooney, W.D., 1986. Crustal structure of southwestern Saudi Arabia. J. Geophys. Res. 91, 6491–6512. Halverson, G., Shields-Zhou, G., 2011. Chemostratigraphy and the Neoproterozoic glaciations. In: Arnaud, E., Halverson, G., Shields-Zhou, G. (Eds.), The Geological Record of Neoproterozoic Glaciations, vol. 36. Geological Society, London, Memoirs, pp. 51–66. Hargrove, U.S., 2006. Crustal evolution of the Neoproterozoic Bi’r Umq suture zones, Kingdom of Saudi Arabia: Geochronological, isotopic, and geochemical constraints, University of Texas at Dallas doctoral dissertation, Richardson, TX, 343 pp. Lundmark, A.M., Andresen, A., Hassan, M.A., Augland, L.E., El-Rus, M.A.A., Boghdady, G.Y., 2012. Repeated magmatic pulses in the East African Orogen in the Eastern Desert, Egypt: an old idea supported by new data. Gondwana Res. 22, 227–237. MacGabhann, B.A., 2005. Age Constraints on Precambrian Glaciations and the Subdivision of Neoproterozoic Time. IUGS Ediacaran Subcommission Discussion Document, August 2005. Miller, N., Johnson, P.R., Stern, R.J., 2008. Marine versus non-marine environments of the Jibalah group, NW Arabian shield: a sedimentologic and geochemical survey and report of possible metazoan in the Dhaiqa formation. Arabian J. Sci. Eng. 33, 55–77. Miller, N.R., Alene, M., Sacci, R., Stern, R.J., Kröner, A., Conti, A., Zuppi, G., 2003. Significance of the Tambien Group (Tigrai, N. Ethiopia) for Snowball Earth events in the Arabian-Nubian Shield. Precambrian Res. 121, 263–283. Stern, R.J., Ali, K.A., Abdelsalam, M.G., Wilde, S.A., Zhou, Q., 2012. U–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield. Precambrian Res. 206–207, 159–167. Teklay, M., Kröner, A., Mexger, K., 2001. Geochemistry, geochronology and isotope geology of Nakfa intrusive rocks, northern Eritrea: products of a tectonically thickened Neoproterozoic arc crust. J. Afr. Earth Sci. 33, 283–301. Thiéblemont, D., 2000. A geochemical database for the Proterozoic magmatism of the Arabian-Nubian Shield: Final report Arabian-Nubian Shield project, in GIS Arabia, Bureau de Recherches Géologiques et Minières, http://www.gisarabia.brgm.fr
Victoria Pease ∗ Tectonics & Magmatism Reseach Group, Department of Geological Sciences, Stockholm University, 10691 Stockholm, Sweden Peter R. Johnson 6016 SW Haines Street, Portland, OR 97219, USA ∗ Corresponding author. Tel.: +46 8 674 7321. E-mail address: [email protected] (V. Pease)
26 September 2013 Available online 30 October 2013
Contents lists available at ScienceDirect
Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Editorial
Introduction to the JEBEL volume of Precambrian Research
The Arabian-Nubian Shield (ANS) is one of the largest tracts of juvenile Neoproterozoic crust on Earth, extending >3500 km north–south and >1500 km east–west over parts of Jordan, Saudi Arabia, Yemen, Israel, Egypt, Sudan, Eritrea, Ethiopia, Somali, and Kenya (Fig. 1). It is the product of a ∼300 million year cycle of crustal growth, bracketed by break up of the supercontinent Rodinia and assembly of the supercontinent Gondwana, and composed of ophiolitic complexes, oceanic arcs, granitoid plutons and batholiths, and terrestrial to shallow-marine volcanosedimentary basins. Formation of the shield began with the creation of early Cryogenian oceanic crust some 10 km thick in the Mozambique Ocean between rifted parts of Rodinia and ended with a block of Ediacaran lithosphere comprising continental crust some 45 km thick and lithospheric upper mantle some 80–120 km thick. At times, divergent national scientific objectives and differing priorities of geoscience organizations have made systematic geologic study and synthesis of the entire ANS problematic, but the breadth and detail of current geologic research and developing opportunities for transnational cooperation are fostering clearer insight into the geologic evolution of the shield. The present volume is an outgrowth of a five-year collaboration by geologists from Jordan, Egypt, Saudi Arabia, Sweden, Norway, Germany, Australia, and the United States of America within the JEBEL Project. JEBEL was funded by the Swedish International Development Agency specifically to promote scientific exchange with the Middle East and North Africa (SIDA-MENA). Our collaborative work has been facilitated by national geoscience organizations and institutions. Among these, we particularly wish to thank the Saudi Geological Survey, the National Materials Authority of Egypt, and the University of Jordan for hospitality and logistical help. Geologic investigations in the ANS are making a fundamental contribution to knowledge of Neoproterozoic Earth history. The shield rocks are well exposed and easily accessible, they are mostly less deformed and metamorphosed than contemporary rocks elsewhere in the world, and retain unambiguous evidence of their depositional, volcanic, intrusive, and structural character and relationships. The region is a prime natural laboratory for researching: (i) the origins and character of Neoproterozoic oceanic crust; (ii) depositional and magmatic processes in Neoproterozoic subduction systems; (iii) changes in Neoproterozoic weathering conditions; of isotopic excursions in Neoproterozoic seawater and atmosphere; and (iv) the effect on structure and magmatism of a Neoproterozoic tectonic transition from unstable coalescing volcanic arcs to stable continental crust. Tectonically, the ANS is an accretionary orogen, part of the larger East African Orogen encompassing the ANS in the north
0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.10.003
and the Mozambique Belt in the south. The shield has a reasonably well-defined margin with the Archean-Neoproterozoic Saharan Metacraton in the west and with high-grade metamorphic rocks of the Mozambique Belt in the south. To the east in Kenya and southern Ethiopia, the shield is in contact with the Azania ribbon continent, a belt of Archean-Paleoproterozoic schist and gneiss that extends from Madagascar into Somalia. In Arabia, Archean and Paleoproterozoic gneiss is also present and may represent a northward continuation of Azania. However, the structural character of the Archean-Paleoproterozoic gneiss changes from south to north. In Yemen, Archean-Paleoproterozoic gneiss is structurally intercalated with Neoproterozoic arc assemblages, and in Saudi Arabia is pervasively reworked, as evidenced by small exposures of intact Paleoproterozoic granite, gneiss and schist and the imprint of Paleoproterozoic lead, strontium, and neodymium isotopes on Neoproterozoic granite. The Yemen exposures are referred to as the Abas and Al Mahfid terranes; reworked preNeoproterozoic crust in Saudi Arabia is referred to as the Khida terrane. Another change is that Neoproterozoic juvenile arc rocks re-occur east of the Archean-Paleoproterozoic gneiss in northern Somali, Yemen, and Saudi Arabia, where they reflect subduction in a late Cryogenian-Ediacaran ocean east of Azania. In the north, isotopic and geochronologic data strongly indicate a contact with pre-Neoproterozoic crust at the far northern edge of shield exposures in Sinai or in basement beneath Phanerozoic rocks farther north. The time-space history of the ANS is based on a comprehensive data set of U–Pb and Rb–Sr ages and a smaller number of Sm–Nd and 40 Ar/39 Ar ages. Current U–Pb dating programs in the shield mostly utilize Secondary Ion Mass Spectrometry (SIMS), Thermal Ionization Mass Spectrometry (TIMS), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses of single zircons. Some programs use Pb/Pb evaporation techniques on zircon or U–Pb spot methods on titanite. Early-middle Cryogenian arc assemblages dominate the southern part of the ANS, locally enclosing small amounts of Tonian arc rock in the Makkah batholith (Mk: Fig. 1); Erkowit batholith (E); the Kurmut Series (K); and in the Bulbul region of southern Ethiopia (Bu). Middle to late Cryogenian arc rocks dominate the north and east, and late Cryogenian-Ediacaran assemblages occur in the farthest most east in the Arabian Shield (Ar Rayn terrane) and the farthest most west in the Nubian Shield (Aswan area). Oceanic crust in the shield, represented by mostly forearc ophiolite complexes, ranges from 845 to 675 Ma; arc assemblages range from ∼870 to 615 Ma. Arc amalgamation and suturing occurred between ∼780 and 600 Ma, and accretion between the ANS and
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Editorial / Precambrian Research 239 (2013) 1–5
Fig. 1. The Arabian-Nubian Shield showing its exposure in Arabia, Northeast Africa, and the East African coast and illustrating its relation to older crust on its margins (after Fritz et al., 2013). Ophiolites schematically shown after Berhe (1990). A, Allaqi; Ad, Adola; Ak, Akobo; B, Baragoi; BU, Bi’r Umq; Bu, Bulbul; E, Jabal Ess; G, Gebel Gerf; H, Halaban; K, Kinyiki; M, Moyale; MS, Moroto-Sekerr; N, Nuba; Na, Nakasib; S, Sol Hamed; T, Jabal Thurwah; Ta, Jabal Tays; Tu, Bi’r Tuluhah; TY, Tuludimtu-Yubdo; U, Jabal Uwayjah; W, Jabal Wask. Arrows show sense of sinistral transpressive shear during final collision of ANS and the Saharan Metacraton.
the Saharan Metacraton reflecting terminal collision of the ANS and western Gondwana blocks, occurred ∼650–580 Ma. Recent dating campaigns help clarify the magmatic history of the ANS. Six pulses of magmatism and, in some cases, associated migmatization and deformation, are recognized in the Nubian shield in Egypt: (1) 705–680 Ma, (2) ∼660 Ma, (3) 635–630, (4) 610–604, (5) 600–590, and (6) 550–540 Ma (Lundmark et al., 2012). High-resolution ionprobe dating shows that post-tectonic granitoids in Sinai result
from early Ediacaran calc-alkaline magmatism at ∼635–590 Ma and middle Ediacaran alkaline magmatism at ∼608–580 Ma (Be’eriShlevin et al., 2009). Significantly, the ages define a 15 million-year overlap of the two magmatic types at ∼605–590 Ma such that the onset of alkaline magmatism apparently coincides with transitions in the calc-alkaline suite from mafic to felsic magmatism and a voluminous pulse of granodiorite to granite plutonism at 610–600 Ma. This refined dating demonstrates that a commonly accepted model
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of a transition from calc-alkaline to within-plate alkaline to peralkaline magmatism in the northern ANS at about 610 Ma (Beyth et al., 1994) is an oversimplification. Recent dating likewise provides stronger constraints on magmatic modeling than hitherto available in the southern ANS, identifying 4 principal magmatic events: 890–840 Ma (late Tonian-early Cryogenian); 790–700 Ma (middle Cryogenian); 660 Ma (late Cryogenian); and 630–500 Ma (Ediacaran) (Stern et al., 2012). A defining characteristic of the ANS, with the exception of the obvious areas of pre-Neoproterozoic rocks and reworked crust, it that shield rocks have magmatic formation ages close to their neodymium model ages; they formed from material separated from the mantle shortly before their emplacement. This juvenile character of shield rocks is displayed by remnants of oceanic crust represented by ophiolite complexes, by suprasubduction and rift-related volcanic assemblages, by arc-related diorite, tonalite, trondhjemite, and granodiorite, and by younger late- to posttectonic granite emplaced in the evolving crust of the shield. An emerging feature is the growing body of evidence that many of the arc-related volcanosedimentary and plutonic associations have an adakitic affinity (Fig. 2). Also notable, is evidence that volcanic and plutonic host rocks of gold, silver, base-metal, molybdenum, and tungsten occurrences in the ANS are adakites. This places the ANS at the center of a debate about the origin and petrogenetic implication of adakites. Due to the abundance of alkali-feldspar granite in the ANS, the shield is also central to the debate about the origin and petrogenetic implication of alkali-feldspar granite and alkaline magmatism. Granite dominates late Cryogenian-Ediacaran magmatism in the ANS. It constitutes the most abundant rock type of this period exposed at the surface and, on the basis of seismic-refraction data, is inferred to be the main component of the upper 20 km of lithospheric crust in the Arabian Shield (Gettings et al., 1986). Granite is a likely similar component of the Nubian Shield crust. In total, therefore, a massive emplacement of granitic magma affected the entire upper crust of the Arabian-Nubian Shield during the late Cryogenian and Ediacaran. Among the granite intrusions, alkali-feldspar A-type granite plutons and batholiths are particularly conspicuous. Despite this, there is no consensus about the origin of alkalifeldspar granite in the shield, similar to the lack of consensus in the global geoscience community. Although originally believed to be the product of magmatism in extensional settings, it is now known that A-type granites occur in a variety of settings ranging from anorogenic, within-plate to plate boundaries, and are variously considered to originate from anhydrous high-grade metamorphic rock or from mafic to intermediate calc-alkaline precursors. Proposals for A-type granite origins in the ANS include: (1) the melting of earlier arc rocks followed by anhydrous fractionation that in some cases gives rise to magmas that have a residual calc-alkaline signature; (2) partial melting of phlogopite-bearing spinel lherzolite, perhaps from OIB-type lithospheric–asthenospheric mantle present in subduction-modified mantle wedges concurrent with or following cessation of subduction and slab-break-off or delamination; and (3) partial melting of Rb-depleted crustal sources in shear zones. The ANS is also important for calibrating Neoproterozoic climate change, oceanic isotopic excursions, glaciation, and biologic radiation. In this regard, a triad of lithologies is significant – diamictite, carbonates units, and banded iron formation (Fig. 3). At least four diamictites are known in the Arabian Shield: in the Nuwaybah formation (∼740 Ma), at the base of the Mahd group (∼750 Ma), in the Hadiyah group (∼695 Ma), and in the Jibalah group (∼590–560 Ma), and three occurrences are known in the Nubian Shield: in the Meritri group (∼780 Ma), the Atud Conglomerate (∼740 Ma), and the Tambien Group (∼750–720 Ma). Of these, the Atud and Nuwaybah diamictites have strong evidence for a
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glacial origin and the diamictite at the base of the Mahd group is possibly glaciogenic. All three are broadly contemporary with the Kaigas event. The Meritri diamictite was deposited shortly after the ∼800 Ma negative ␦13 Ccarb (‰) excursion recorded in the global carbon isotope curve; the Hadiyah group and Jibalah group diamictites are approximately contemporaneous with the Sturtian and Gaskiers events. The Atud Conglomerate diamictite is particularly significant because it is stratigraphically beneath Cryogenian Algoma-type banded-iron formation (BIF) in the shield. BIF is not abundant in the ANS but occurs in several locations in the northwestern part of the shield in Egypt and Saudi Arabia as beds of alternating iron- and silica-rich laminae. A stratigraphic relationship between glacial diamictite and BIF figures in the debate about global glaciations, e.g. – the Snowball Earth hypothesis. No direct evidence of glaciation is found in the ANS BIF examples and the BIF laminae may primarily reflect seasonal changes in deposition of iron and silica (Stern et al., this volume), but depending on the timing adopted for global glacial events, the ANS BIF was deposited soon after the Kaigas event or shortly before the Sturtian event. The third lithology in the ANS triad significant for calibrating Neoproterozoic climate change is carbonate, important for recording variations or excursions in carbon and strontium isotope values such as the negative ␦13 Ccarb (‰) excursions known to be associated with profound carbon flux changes that accompany widespread glacial transitions. Such excursions have been studied in the Tambien Group in Ethiopia and the Jibalah group in Saudi Arabia. Tambien Group limestone underlies putative glacial diamictite and records a pronounced negative carbon excursion through the transition to diamictite from +7 to −2‰ consistent with a glacial environment (Miller et al., 2003). Carbon isotope values in Jibalah group carbonate in contrast are predominantly positive, averaging +2.4 ± 2.3‰, more consistent with deposition in the interval either before or after the Gaskiers Shuram anomaly (∼551–542 Ma) rather than during the Gaskiers event itself (Miller et al., 2008). Many other carbonate intervals occur in the ANS that are appropriate targets for additional C, Sr, and O isotopic investigations. This Special Issue of Precambrian Research provides examples of ongoing research into the geology of the ANS confirming the value of the ANS to global geologic understanding of Neoproterozoic events. The volume begins with the Ediacaran, progresses back through Cryogenian time, and concludes with contributions on the pre-Cyrogenian geological framework of the region. Contributions reflecting Ediacaran sedimentation and igneous rocks include two papers by Yaseen et al. and Boskabadi et al. The first paper presents a SIMS U–Pb detrital zircon investigation of a posttectonic basin of the ANS. Such basins are fascinating records of the basement exposed via orogenesis and post-orogenic extension. Yaseen et al.’s study contains surprising results indicating for the first time igneous activity of a previously unrecognized age, with important ramifications for the onset and duration of magmatism during a period previously regarded as amagmatic. The preservation of syn- to post-tectonic basins (or fragments thereof) provides important information on the timing of ANS orogenic uplift, erosion, and burial. Boskabadi et al. investigate alteration associated with the c. 610 Ma Tarr carbonatite–albitite complex (Sinai, Egypt). Geochemical analyses indicate that the complex is a trace-element-poor carbonatite, probably reflecting its mantle source. Breccia associated with the complex indicates the addition of carbonate and other secondary minerals as fluid-mobilized alteration products. Their fluid inclusion studies indicate that alteration occurred at c. 500 ◦ C and c. 1 kbar, probably during post-intrusion cooling. The second group of papers in this volume relates to Cryogenian crustal growth, metamorphism, and climate change (Ali et al., Jarrar et al., Stern et al.). New Hf-isotopic analyses of zircons from Egypt’s
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Fig. 2. Y versus Sr/Y plots of data from volcanic and plutonic rocks in the Arabian-Nubian Shield showing the adakitic affinity that is now well established for arc-related assemblages in the shield. (A) Plot of three examples of data from northern Eritrea (Teklay et al., 2001), the eastern Arabian Shield (Doebrich et al., 2007), and western Arabian Shield (Hargrove, 2006). (B) A plot of data from volcanic and plutonic rocks in the Arabian Shield illustrating the association between metallic-mineral occurrences and host rocks of adakitic affinity (after Thiéblemont, 2000).
Fig. 3. Stratigraphic relationships of a triad of lithologies in the Arabian-Nubian Shield significant for calibrating Neoproterozoic climate change and the evolution of atmospheric and oceanic chemistry. Composite seawater carbon-isotope curve on right modified from Halverson and Shields-Zhou (2011); timing of global glacial events (gray bands) after MacGabhann (2005).
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Eastern Desert, combined with Nd isotopic data, indicate that the post-collisional (c. 600–630 Ma) granites were derived from a juvenile source, whereas results from the metavolcano-sedimentary rocks indicate involvement of pre-Neoproterozoic material in the formation of some of these rocks (Ali et al.). The search for older components within the ANS is ongoing, yet to date very little has been documented so this important contribution adds to the limited body of evidence characterizing these rare occurrences. The oldest part of the ANS in Jordan is represented by orthoand paragneiss intruded by later granitoids of the Abu-Barqa Metamorphic Suite (ABMS). Dating of these rocks indicates the ABMS evolved between 600 and 800 Ma (SIMS U–Pb zircon). Maximum P–T conditions (5–6 kbar/c. 700 ◦ C) were achieved at c. 625 Ma, with peak conditions recording c. 3.2 kbar/540 ◦ C; this was followed by decompression and a thermal pulse associated with intrusion of post-tectonic granitoids at c. 610 Ma; and finally the whole complex was cooled and uplifted to the surface at ∼600 Ma (Jarrar et al.). The final paper in this group addresses the formation of the c. 750 Ma ANS BIF. Stern et al. present petrographic, geochemical, and Nd and Pb isotopic compositions of Neoproterozoic oxide facies from the NE margin (Sawawin, NW Saudi Arabia) of the now dismembered ANS BIF basin and its correlative Egyptian counterparts in Wadi Dabbagh and Wadi Kareim. They document the oxidation of this initially anoxic basin and, in conjunction with consideration of BIF of similar age on other paleocontinents, suggest that a major change in marine redox occurred before Sturtian glaciation. The implications of such a change for the Neoproterozoic Oxidation Event during ‘Kaigas-Sturtian’ time may be significant and need to be investigated further. The final two papers in our volume address the pre-Cryogenian framework of the ANS. Flowerdew et al. evaluate the significance of the Nabitah fault zone in the Saudi Arabian Shield, previously regarded as a major suture separating distinct terranes. Using geochronology, feldspar Pb and whole rock geochemistry, and Sm–Nd isotopic results, they suggest that the Nabitah suture separates two juvenile oceanic arc terranes of different ages and geochemical characters. They conclude that the fault zone is not the major suture separating the juvenile terranes of west Gondwana from more evolved terranes of east Gondwana, as previously supposed. The oldest part of the ANS in Sinai (Egypt) is represented by the Feiran–Solaf gneiss complex. In the contribution from Abu El-Enin & Whitehouse this metamorphic complex is investigated using SIMS dating. Their results indicate that the complex is older than previously thought, from <600–1000 Ma. Metasediments also contain numerous older zircons. The authors conclude that the Fieran–Solaf metamorphic complex reveals the existence of pre-Pan African crust, as well as four Pan-African episodes of magmatism (799–785 Ma, 713–692 Ma, 629–611 Ma, and 606–583 Ma) and two metamorphic peaks synchronous with the youngest pulses. Again, due to the paucity of known older crust in the ANS, this important contribution adds to the limited body of evidence characterizing these rare occurrences. We hope the contributions presented in this volume will promote further collaboration on important topics and questions associated with the ANS and its Neoproterozoic evolution. We would like to take this opportunity to extend our thanks to those who donated their time and energy to reviewing the manuscripts submitted to this volume. Their willingness to
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participate in the peer review process contributes to maintaining a high scientific standard through constructive criticism and feedback. Reviewers include Arild Andresen, Kamal Ali, Yaron Be’eri-Shlevin, Joachim Jacobs, Ian Miller, Aley El-Shazly, William Peltier, Jun-Iche Kimura, Kathryn Moore, as well as anonymous reviewers. References Be’eri-Shlevin, Y., Katzir, Y., Whitehouse, M., 2009. Post-collisional tectonomagmatic evolution in the northern Arabian-Nubian Shield; time constraints from ionprobe U–Pb dating of zircon. J. Geol. Soc. Lond. 166, 71–85. Berhe, S.M., 1990. Ophiolites in the northeast and east Africa: implications for Proterozoic crustal growth. J. Geol. Soc. London 147, 41–57. Beyth, M., Stern, R.J., Altherr, R., Kröner, A., 1994. Late Precambrian Timna igneous complex, southern Israel: evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma. Lithos 31, 103–124. Doebrich, J.L., Al-Jehani, A.M., Siddiqui, A.A., Hayes, T.S., Wooden, J.L., Johnson, P.R., 2007. Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African Orogen. Precambrian Res. 158, 17–50. Fritz, H., Abdelsalam, M., Ali, K.A., Bingen, B., Collins, A.S., Fowler, A.R., Ghebreab, W., Hauzenberger, C.A., Johnson, P.R., Kusky, T.M., Macey, P., Muhongo, S., Stern, R.J., Viola, G., 2013. Orogen styles in the East African Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution. J. Afr. Earth Sci. 86, 65–106. Gettings, M.E., Blank, H.R., Mooney, W.D., 1986. Crustal structure of southwestern Saudi Arabia. J. Geophys. Res. 91, 6491–6512. Halverson, G., Shields-Zhou, G., 2011. Chemostratigraphy and the Neoproterozoic glaciations. In: Arnaud, E., Halverson, G., Shields-Zhou, G. (Eds.), The Geological Record of Neoproterozoic Glaciations, vol. 36. Geological Society, London, Memoirs, pp. 51–66. Hargrove, U.S., 2006. Crustal evolution of the Neoproterozoic Bi’r Umq suture zones, Kingdom of Saudi Arabia: Geochronological, isotopic, and geochemical constraints, University of Texas at Dallas doctoral dissertation, Richardson, TX, 343 pp. Lundmark, A.M., Andresen, A., Hassan, M.A., Augland, L.E., El-Rus, M.A.A., Boghdady, G.Y., 2012. Repeated magmatic pulses in the East African Orogen in the Eastern Desert, Egypt: an old idea supported by new data. Gondwana Res. 22, 227–237. MacGabhann, B.A., 2005. Age Constraints on Precambrian Glaciations and the Subdivision of Neoproterozoic Time. IUGS Ediacaran Subcommission Discussion Document, August 2005. Miller, N., Johnson, P.R., Stern, R.J., 2008. Marine versus non-marine environments of the Jibalah group, NW Arabian shield: a sedimentologic and geochemical survey and report of possible metazoan in the Dhaiqa formation. Arabian J. Sci. Eng. 33, 55–77. Miller, N.R., Alene, M., Sacci, R., Stern, R.J., Kröner, A., Conti, A., Zuppi, G., 2003. Significance of the Tambien Group (Tigrai, N. Ethiopia) for Snowball Earth events in the Arabian-Nubian Shield. Precambrian Res. 121, 263–283. Stern, R.J., Ali, K.A., Abdelsalam, M.G., Wilde, S.A., Zhou, Q., 2012. U–Pb zircon geochronology of the eastern part of the Southern Ethiopian Shield. Precambrian Res. 206–207, 159–167. Teklay, M., Kröner, A., Mexger, K., 2001. Geochemistry, geochronology and isotope geology of Nakfa intrusive rocks, northern Eritrea: products of a tectonically thickened Neoproterozoic arc crust. J. Afr. Earth Sci. 33, 283–301. Thiéblemont, D., 2000. A geochemical database for the Proterozoic magmatism of the Arabian-Nubian Shield: Final report Arabian-Nubian Shield project, in GIS Arabia, Bureau de Recherches Géologiques et Minières, http://www.gisarabia.brgm.fr
Victoria Pease ∗ Tectonics & Magmatism Reseach Group, Department of Geological Sciences, Stockholm University, 10691 Stockholm, Sweden Peter R. Johnson 6016 SW Haines Street, Portland, OR 97219, USA ∗ Corresponding author. Tel.: +46 8 674 7321. E-mail address: [email protected] (V. Pease)
26 September 2013 Available online 30 October 2013