Kamal A. Ali

Distribution and Significance of pre-Neoproterozoic zircons in juvenile Neoproterozoic igneous rocks of the Arabian-Nubian shield

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.

U–Pb zircon dating and Sr–Nd–Hf isotopic evidence to support a juvenile origin of the ~ 634 Ma El Shalul granitic gneiss dome, Arabian–Nubian Shield

The calc-alkaline, gneissic El Shalul granite is the westernmost gneiss dome or core complex within the Arabian–Nubian Shield. Previous studies have indicated that it represents either a window into the underlying pre-Neoproterozoic Sahara metacraton or a melt derived from the metacraton. U–Pb LA-ICP-MS dating of magmatic zircons from two samples of the variably foliated El Shalul pluton gives ages of 637 ± 5 Ma and 630 ± 6 Ma, excluding it from representing exhumed cratonic rocks. The ages are, however, indistinguishable from the age of the Um Ba9anib pluton, constituting the core of the Meatiq Gneiss Dome, as well as several other plutons in the Eastern Desert, indicating an important magmatic pulse in the Arabian–Nubian Shield in Late Cryogenian time. Major and trace element data indicate a within-plate setting. Bulk rock Nd-isotope and Hf-isotope data on zircons from the El Shalul pluton indicate derivation of the primary melt from a relatively juvenile source, either the lower crust of a mid-Neoproterozoic volcanic arc or as a result of fractionation of a mantle-derived mafic melt. Sm–Nd bulk rock isotopic data indicate a model age of c . 720 Ma for the protolith from which the melt was derived. Time-corrected Hf-isotope data obtained on the magmatic zircons indicate that the bulk of the source rock was extracted from the mantle around 810 Ma.

The Wadi Zaghra metasediments of Sinai, Egypt: new constraints on the late Cryogenian–Ediacaran tectonic evolution of the northernmost Arabian–Nubian Shield

The depositional age of the Wadi Zaghra metasediments (Zaghra Formation) of the Sa’al-Zaghra metamorphic complex (Sinai) in the Arabian–Nubian Shield (ANS) has been constrained by LA-ICP-MS dating of zircons from (1) metaconglomerate granitoid clasts, (2) the metaconglomerate matrix, (3) biotite-schist, and (4) diorites and granites intruding the metasediments. Three granitoid boulders from a metadiamictite of the Zaghra Formation give crystallization ages of 651 ± 3, 647 ± 3, and 640 ± 4 Ma respectively. Detrital zircon age populations from laminated siltsones and conglomerate matrix vary somewhat with stratigraphic position, a variation best explained by changes in depositional environment and source area with time. Three distinct zircon age populations are identified in the matrix; at ca. 1000, 750, and 630 Ma, with less pronounced peaks at ~780 and ~800 Ma. The youngest detrital zircons indicate deposition after 630–625 Ma. The emplacement age of the post-depositional intrusives is enigmatic due to a large population of xenocrystic zircons, but is interpreted to be as old as ca. 615 Ma. This implies deposition and deformation of the Zaghra Formation in the time interval 625–615 Ma, thus older than the inferred depositional age of the Hammamat deposits in the Eastern Desert of Egypt, which is commonly correlated. The inferred depositional age is similar to the deposits in nearby Rutig Basin, where conglomerates were deposited in two pulses; at ca. 620–610 and 600–590 Ma. The appearance of late Mesoproterozoic (ca. 1.1 to –1.0 Ga) detrital zircons in the fine-grained laminated matrix facies of the Zaghra Formation demonstrates that ~1.0 Ga basement rocks (Sa’al terrane?) were exposed when Zaghra basin sediments were being deposited.

Chapter 22 Evidence for Early and Mid-Cryogenian glaciation in the Northern Arabian–Nubian Shield (Egypt, Sudan, and western Arabia)

Abstract Evidence of Early- to Mid-Cryogenian (c. 780 Ma and c. 740 Ma) glacial activity is summarized for the northern Arabian–Nubian Shield (ANS), including structural framework, stratigraphy, lithological descriptions and relationships with younger and older units, banded iron formation chemostratigraphy, other characteristics, geochronological constraints, and discussion. The ANS is a broad tract of juvenile continental crust, formed from accreted arc-backarc basin terranes developed around the margins of the Mozambique Ocean. As a result, these successions formed in marine environments at some distance from continental margins. Deposits include banded iron formation (BIF) and possibly glacial diamictite scattered over broad regions of the Central Eastern Desert of Egypt, NW Arabia and possible correlative units in NE Sudan. The older (c. 780 Ma) examples (Meritri group, NE Sudan; basal Mahd group, Arabia) occur in the central ANS, on the southern flank of an important lithospheric boundary, an ophiolite-decorated suture zone. Mahd group diamictite is thin (1–5 m thick) and rests above the earliest (Cryogenian) ANS unconformity. The Meritri group interval near Port Sudan is much thicker and part of a deformed passive margin. Both Mahd and Meritri group deposits need further study before they are accepted as glaciogenic; confirmation of this interpretation would indicate that Neoproterozoic glacial activity began at least as early as 780 Ma ago. The younger (c. 740 Ma) glacial deposits include diamictite and BIF: the Atud diamictite and BIFs of the Central Eastern Desert of Egypt and the correlative Nuwaybah diamictite and BIF of NW Arabia. Northern ANS-BIF is a well-layered chemical sediment of interlaminated hematite-magnetite and jasper. A glacial origin for the Atud-Nuwaybah diamictites is inferred because large clasts and matrix zircons have ages (Palaeoproterozoic and Neoarchean) and compositions (especially quartzite, arkose, and microdiamictite) that require transport from outside the ANS Cryogenian basin. Northern ANS-BIF may also reveal glacial influence, having been deposited in response to reoxygenation of a suboxic ocean. The 740 Ma diamictite and/or BIF may correlate with Tambien Group diamictites in Ethiopia (Miller et al. 2011). Northern ANS diamictite and BIF were deposited in an oceanic basin of unknown size, as indicated by association with abundant ophiolites; they are strongly deformed, obscuring many primary features. There is no strong evidence for or against Ediacaran glaciation in the ANS, largely because the region was uplifted at this time. The c. 600 Ma ANS peneplain may have been partly cut by Ediacaran glaciation. Some of the post-accretionary basins of Arabia could preserve glaciogenic deposits of Ediacaran age, but assessing this possibility requires further investigation.

Short note of field workshop on Neoproterozoic ophiolites, ophiolitic mélanges and other rock units in the Eastern Desert of Egypt and comparison with the Central Asian Orogenic Belt of Central Asia (18–24 February 2016)

The Neoproterozoic Arabian–Nubian Shield (ANS) and the latest Mesoproterozoic to late Palaeozoic Central Asian Orogenic Belt (CAOB) are two large and longlived accretionary orogens. Both are mostly well exposed and consist of island arcs, ophiolites and older continental fragments. There are significant similarities between the rock types and evolution of these belts since both developed largely through accretion and ended by collision. However, there are also significant differences, particularly in their tectonic evolution. For example, the CAOB contains many well-defined ophiolite-decorated suture zones with highand ultra-high pressure assemblages that reflect subduction and collision processes during the Neoproterozoic to Palaeozoic. Ophiolitedecorated sutures are common in the ANS, but blueschists, eclogites and other UHP metamorphic rocks have not been found. In the western ANS, known as the Nubian Shield, the tectonic evolution is dominated by large-scale obduction and extensive mélange development. We held a 5-day field workshop to compare and contrast the evolution of the ANS and CAOB, and compared the evolution of these terranes with convergent plate margins of the present circum-Pacific region. The workshop was based out of a seaside resort 60 km north of Marsa Alam, and for each of 5 days went into the Eastern Desert of Egypt to study exceptionally well-preserved ophiolites or their tectonically dismembered components and mélanges between latitudes 26° 00′ and 25° 00′ N, and longitudes 33° 00′ and 35° 00 ′E. We opened the workshop in the evening of 18 February 2016, with three presentations about the ANS. Prof. Alfred Kröner presented an overview on the tectonics of the ANS. Dr Kamal Ali presented his research on the evolution of the ANS by highlighting the controversy resulting from discrepancies between mafic volcanic rock whole-rock Nd isotopic data (which indicated derivation of these melts from depleted mantle) and Hf isotopic signature of zircons (which indicated involvement of pre-Neoproterozoic zircons). Prof. Abu Anbar presented a general overview of the field workshop programme and an overview of the Precambrian in Egypt (Pan-African and Archaean) and the PanAfrican tectonic episode. The field excursions mostly examined Cryogenian rocks of the Central Eastern Desert. It started on 19 February by visiting the Esel dismembered ophiolite sequence in Wadi Esel, the Wadi El-Dabbah banded iron formation and the G. Sebai alkali granite (Figure 1). The participants visited the complete Wadi Ghadir ophiolite as one of the best preserved Precambrian ophiolites on the Earth and ophiolitic mélange on the second day (20 February 2016) (Figure 1). The ophiolite sequence consists of serpenitinized peridotites, layered gabbro, isotropic gabbro, sheeted diabase dikes and pillow basalts. In the evening, Prof. Alexander Kuzmichev presented a talk about ophiolites in southern Siberia of Russia. The Mubarak dismembered ophiolite, including components such as pillow lavas and serpentinites and mélange, was targeted on the third day (Figure 1). The mélange is dominated by serpentinites with subordinate metagabbros, metavolcanic and amphibolites. The research group examined diamictite in the mélange which include clasts suggested to be a glacial in origin, and visited Al-Umrah granodiorite pluton and Kadaburah El Hamara pluton. On the fourth day, the participants (Figure 2) visited the Sefen chromites and Um Khariga serpentines along the Idfu-Marsa Alam road and the Atud diamictite at the type locality of Jabal Atud (Figure 1). The Atud diamictite is composed mainly of conglomerates and minor graywackes and mudstones.