Northern Arabian Nubian Shield Research Articles

A newly recorded Cryogenian-Ediacaran Dokhan Volcanic caldera with resurgent uplift in the Arabian-Nubian Shield, southwest Safaga, Egypt

The late Neoproterozoic Dokhan Volcanics (DV) of the Egyptian Eastern Desert (northern Arabian-Nubian Shield), were investigated at Gebel Nuqara, near the boundary between the Northern and Central Eastern Deserts. In particular, the nature of the peculiar circular and annular regional structures at Nuqara, were targeted for study. This included exploring the relationships of DV and intrusive units to each other and to regional dikes and faults. The central part of the Nuqara DV exposures consists of two volcanic units: 1) “Older DV” (intermediate varieties, dominantly andesites; 659 ± 16 Ma), with associated arc granitoids (663.2 ± 8.4 Ma), followed by andesite dikes (636.9 ± 7.2 Ma); and 2) “Younger DV” (acidic varieties; 602.3 ± 4.4 Ma for rhyolites, and 589.4 ± 6.1 Ma for rhyolite porphyry). This central area of DV units is enclosed within an annular intrusion of granite. The geochemical data for the Nuqara DV reveal intermediate low-silica adakitic (basaltic-andesite and andesite) and acidic high-silica adakitic (rhyolite and rhyolite porphyry). Both adakite groups share a common geochemical trend, and show evidence for magma diversification via fractional crystallization. The Nuqara DV is identified as a caldera sequence, with resurgent uplift, based on a positive comparison of the volcanic morphology, lithology, structure and mechanism of DV eruptions in the study area with those of other resurgent calderas. The geodynamic setting, volcanic architecture, and geochemistry of the DV, as well as its in-situ zircon LA–ICP–MS estimated ages, allow recognition of a series of eruptive stages, leading to collapse to form the Nuqara caldera, followed by resurgent uplift of the caldera floor during intrusion of rhyolite porphyry domes. The resurgent caldera model assigns a greater significance for the rhyolite porphyry as the exposed top of a sub-volcanic magma chamber. Resurgence is viewed as resulting from increased pressure or volume changes in the magma reservoir. A consequence of resurgence was uplift and erosion of the previous collapse-related topography.

Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

The Janub Metamorphic Complex (JMC) in southern Jordan provides new correlative data constraining the transition from compressional to extensional tectonics in the northern Arabian-Nubian Shield. This constraint comes from the identification of extensional mylonitic shear zones affecting both the JMC and some intruding granitoids. The JMC comprises metamorphosed andesitic-dacitic-rhyolitic flows and pyroclastics, meta-volcanogenic sediments, and hornfels. Volcanism ceased by 618 ± 5 Ma. The volcanosedimentary sequence formed in an intra-arc basin in a mature island arc and was then buried and regionally metamorphosed at lower greenschist facies conditions in the waning accretionary phase between 618 and 615 Ma. This is based on the age of crosscutting plutons of the Rumman Suite, which triggered local high-T/low-P metamorphism at ∼ 615 Ma. Rumman granitoids and JMC rocks were deformed by narrow gently-dipping mylonitic shear zones, interpreted as extensional, that ceased operating by 605 Ma, the age of crosscutting undeformed dikes and plutons of the Yutum Suite. At 596 Ma, a late granite intruded the JMC and thermally metamorphosed some of the meta-sediments. This granite was deformed in a Najd-related brittle shear zone at ∼ 590 Ma as recorded by 40Ar/39Ar geochronology. The JMC can be correlated with the higher-grade Abu-Barqa Metamorphic Complex (ABMC), despite tectonics relations being obscured by younger intrusive rocks. The main metamorphic phase in the ABMC occurred at a depth of 18 km at ∼ 620–615 Ma, followed by gradual uplift and high-T metamorphism at 615–610 Ma and exhumation to the surface at ∼605 Ma. Extensional shearing in the JMC and exhumation of the ABMC probably developed in a core-complex like setting triggered by orogenic collapse. Ductile shearing related to collapse ceased before 605 Ma. The 590 Ma brittle shear zone is the first dated Najd-related structure from the basement of Jordan. The central setting of our data allows correlation of metamorphic complexes in Sinai and Saudi Arabia and is consistent with the final assembly of Gondwana, initiation of orogenic collapse, and transition to extensional tectonics occurring at ∼ 620–610 Ma in the northern Arabian-Nubian Shield.

Tonian–Ediacaran Tectonometamorphic History of the Sa'al Complex, Sinai (Egypt): Implications for the Tectonostratigraphic Framework of the Northern Arabian–Nubian Shield

AbstractThe Sa'al Metamorphic Complex (SMC; southern Sinai) encompasses the oldest arc rocks in the Arabian–Nubian Shield, comprising two non‐consanguineous metavolcanic successions (the Agramiya Group and the Post‐Ra'ayan Formation) separated by the metasediments of the Ra'ayan Formation. It experienced three distinct deformational events (D1–D3) and two low‐medium grade regional metamorphic events (M1–M2). The Agramiya Group and the Ra'ayan Formation experienced all tectonometamorphic events (D1–D3 and M1–M2), whereas the Post‐Ra'ayan volcanic rocks were only affected by the D3 and M2 events. D1 is an extensional event and is connected to the late Rodinia break‐up (∼Tonian; 900–870 Ma). The M1 metamorphism variably affected the older Agramiya Group, the rhyolitic tuffs experiencing lower to upper greenschist facies conditions and the basic and intermediate volcanic rocks undergoing amphibolite facies metamorphism. The Ra'ayan Formation metasediments experienced upper greenschist to amphibolite facies metamorphism. The upper greenschist facies M2 affected the youngest Post‐Ra'ayan volcanic rocks and other stratigraphic successions. The compressive D2 and D3 events were coeval with the accretion of dismembered terranes in the assembly of Gondwana. D2 can be linked to the Tonian–Cryogenian arc‐arc assembly (∼880–760 Ma; in Elat and Sinai), whereas D3 and the accompanying M2 is constrained to 622–600 Ma (Ediacaran).

The anorogenic late Ediacaran granite-rhyolite porphyries of Gabal Abu Durba, Sinai: termination of magmatism in the Neoproterozoic Arabian-Nubian Shield crust

ABSTRACT Anorogenic magmatism in the northern Arabian-Nubian Shield occurred during a long-lasting crustal extension event (<580 Ma), which succeeded the formation of the Pan-African orogenic belt in NE Africa and Arabia. Late Neoproterozoic anorogenic felsic volcanic-subvolcanic rocks, along with post-collisional granitoid suites, are exposed in the Gabal Abu Durba mountain range along the eastern flank of the Gulf of Suez in Sinai. These rocks include granite and rhyolite porphyries, metaluminous to weakly peralkaline with distinct potassic alkaline and ferroan affinities. Major and trace element characteristics such as K2O/MgO >16, total alkalis >8.5 wt.%, (K2O + Na2O)/CaO >10, agpaitic index (NK/A) >0.85 and Zr (>250 ppm), Nb (>20 ppm), Y (>80 ppm), Zn (>100 ppm) and Ce (>100 ppm), and 10,000 × Ga/Al >2.6 and [Zr + Nb + Y + Ce] >350 ppm are suggestive of an A-type granite genesis. Zircon U-Pb dating of two representative samples returned crystallization ages of 569 ± 2.6 Ma and 561.7 ± 3.2 Ma for granite and rhyolite porphyries, respectively. The melting temperatures estimated at 997–1020°C are consistent with high-temperature liquidus conditions. Fractional crystallization, coupled with less significant crustal assimilation, was likely the main mechanism of formation of these rocks from a common primitive asthenospheric mantle-derived trachytic magma in an anorogenic intra-plate setting. The A-type alkali granite and rhyolite porphyries of Gabal Abu Durba are manifestations of mantle upwelling and lower crust underplating in the late Ediacaran anorogenic stage. These felsic subvolcanic intrusions (≈ 569–562 Ma) along with their volcanic counterparts constitute a new unmatched rock unit that most probably represent the last chapter of the anorogenic magmatism in the Neoproterozoic Arabian-Nubian Shield crust before the beginning of Phanerozoic era.

Depositional environments and diagenesis of north Gondwana Ediacaran clays (Zenifim and Elat Conglomerate Formations, Israel)

A thick sequence of arkosic sandstones and conglomerates, locally interbedded with volcanic rocks, was deposited in the piedmont region and within intramountain basins of the northern Arabian-Nubian Shield (ANS) during the late Neoproterozoic Ediacaran period. We have studied the geochemistry and clay mineralogy of these sediments in outcrops and drill cores along a ∼200 km long transect in southern Israel, in order to constrain their depositional environments and the diagenetic processes that have affected them. Whole rock trace element data for the Israeli Ediacaran sediments indicates magmatic island arc provenance, consistent with a source in the ANS. Major elements data indicate the importance of pedogenic processes in the source area and modification by grain size sorting during transport and deposition. In the southernmost part of the study area, clay assemblages of the Ediacaran formations include significant proportions of the mineral illite–smectite (I/S), including of randomly interlayered (R0) structure. In the central and northern parts of the study area, the entire Ediacaran section is dominated by illite, while the minor I/S found in this area is always ordered, and changes from R1- to R3-type structure with increasing depth. This transition from R1- to R3-type I/S seems to shallow towards the north. These trends of clay minerals composition along the Ediacaran strata reflect increased paleo-thermal gradient and/or burial depths toward the north, which is correlated with the formation of late Devonian – Carboniferous thermal structure over the Levant area. The pristine, I/S dominated clay assemblage found in the Ediacaran sediments of southernmost Israel is remarkable given these sediments' relatively old age and offers a unique opportunity to probe the Ediacaran climate in this region, which resided at low latitudes at that time. This I/S dominated, kaolinite lacking, clay assemblage, together with relatively low chemical index of alteration values found for the Ediacaran sediments of southern Israel, reflect limited chemical weathering of ANS basement rocks under relatively dry, potentially cool, climatic conditions. The persistence of such climatic conditions within the tropical belt is consistent with a series of late Ediacaran glaciation events, although negating a complete, prolonged 'Snowball Earth'-type global glaciation.

Cryogenian accretion of the Northern Arabian-Nubian shield: Integrated evidence from central Eastern Desert Egypt

Accretionary geology of the Arabian-Nubian shield (ANS) is critical in deciphering the Neoproterozoic global tectonics. Multidiscipline investigation of ophiolites, sediments, and igneous units of central Eastern Desert Egypt results in a revisited tectono-magmatic scenario. The first baddeleyite U-Pb age (ca. 687 Ma) of the ANS ophiolites is obtained from the layered gabbro of the Wadi Ghadir ophiolite, which limits the back-arc spreading age. Significant Archean-Paleoproterozoic detrital zircon age spectrum, similar to those of the Saharan-Congo craton, is identified from a conglomerate of the Atud turbidites, indicating the existence of craton basement in the shield. Significant ca. 840 Ma and ca. 750 Ma detrital zircon ages with a mixing of juvenile and ancient Hf isotope features approve the provenances to be arc(s) with craton basement. Two episodes of calc-alkaline units represented by the Umm Rus complex (URC) and the El Sibai complex (ESC) are dated at ca. 680 Ma and ca. 660 Ma, respectively. They were both originated from juvenile sources (in situ zircon ɛHft = +6.1 ∼ +12.2) with depleted high-strengthen field elements (e.g., Nb, Ta, Zr, Hf) but enriched large ion lithophile (e.g., Ba, Sr) and radiogenic elements (e.g., Th, U). The Ti-in-zircon temperature of the ESC granitoids (696 ± 16 °C) is lower than that of the older URC (749 ± 21 °C), as well as the inferred ca. 840 Ma and ca. 750 Ma arcs; and this reconciles an increased hydrous melting during the final Gondwana assembly. The older plutons represented by the URC show an E-W elongated distribution consistent with the alignment of the regional imbricated thrusting; whereas the elongation of the younger generation represented by the ESC is N-NE to S-SW and is coherent with the axis of the core complexes and the major strike-slip faults of the region. This possibly coincides a direction-change of paleo-subduction. In summary, the Northern ANS likely represents a superimposed arc accreted at 680–660 Ma during the closure of the Mozambique ocean.

The birth of the Alps: Ediacaran to Paleozoic accretionary processes and crustal growth along the northern Gondwana margin

New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism.

Origin and magmatic evolution of late Neoproterozoic post-accretion high-K calc-alkaline adakitic volcanics in the northern Arabian–Nubian Shield

In the northernmost segment of the Arabian–Nubian Shield, a post-collisional high-K calc-alkaline volcanic sequence is exposed along Wadi Abu Ma’amel, Eastern Desert of the Nubian Shield. It comprises a series of intermediate to silicic volcanics and associated pyroclastics that include the Imperial Porphyry and calc-alkaline volcanics typical of the Dokhan Volcanics. The Imperial Porphyry occurs as subvolcanic sill-like intrusions forming the young member of the Dokhan Volcanics. The volcanic sequence extruded through synorogenic granite and was intruded by post-collisional granite, which also caused thermal contact metamorphism. The red and purple colors of the Imperial Porphyry reflect hydrothermal alterations, which resulted in the formation of dispersed flakes of hematite, epidote, and piemontite. The entire high-K calc-alkaline volcanic sequence, ranging from andesite through dacite and rhyodacite, exhibits post-collisional geochemical characteristics. Most samples of the Imperial Porphyry and some of the typical Dokhan Volcanics have characteristics of adakitic rocks, including high Sr (694–889 ppm), low Y (10.6–18.8 ppm), high Sr/Y (41.1–83.8), (La/Yb)_n (8.6–15.6), and low (Yb)_n (5.4–9.0). The mostly calc-alkaline character and other traits of the studied volcanics that were previously interpreted to indicate arc magmatism reflect, instead, remelting of earlier (pre-collisional) arc-related material. The formation of Wadi Abu Ma9amel volcanics resulted from upwelling of hot asthenospheric material during thinning of the previously thickened lithosphere as a consequence of lithospheric delamination. The parental magma was generated by partial melting of mafic lower crust that mixed with upper-crust-derived magma. It evolved mostly through fractionation of clinopyroxene and plagioclase, accompanied by apatite and Fe–Ti oxides in the more-evolved dacitic and rhyodacitic rocks.

Evolution of a Neoproterozoic island arc in the northern Arabian-Nubian Shield: Volcanic rocks and their plutonic equivalents in the Hamash area, south Eastern Desert, Egypt

A suite of Neoproterozoic island arc volcanic and plutonic rocks are exposed in the Hamash area of the south Eastern Desert of Egypt. The volcanic assemblage includes metabasalt, a group of andesites with subordinate basalts (the Hamash Volcanics), and felsic volcanic porphyry. The plutonic rocks include a diorite-tonalite series, fine-grained granite, and granite porphyry. The volcanic rocks are generally sub-alkaline and feature a strong depletion in Nb and Ta. The metabasalt belongs to the low-K tholeiitic series and has nearly flat rare-earth element patterns. Its compositional characteristics indicate generation by partial melting of a depleted mantle source in an immature oceanic island arc setting. On the other hand, the basalt, andesite and felsic volcanic porphyry have low- to medium-K calc-alkaline affinity and are enriched in light rare-earth elements and large ion lithophile elements. Their parental magmas likely formed by fluid-assisted partial melting of a less depleted mantle source with major subduction input, in a mature island arc setting. The andesite cannot be derived from the basalt by fractional crystallization; their parental magmas likely represent different degrees of melting underneath crust of different thickness. Estimation of the arc crustal thickness during formation of these rocks yields ~ 5 km for the metabasalt, ~20 km for the Hamash basalt, and ~ 30 km for the Hamash andesite, consistent with a progression in maturity of the arc over time. The geochemical characteristics of the diorite-tonalite series suggest that it represents an intrusive equivalent of the medium-K calc-alkaline Hamash andesite, and the fine-grained granite represents residual liquid after continued fractionation of the parental magma of the andesite and diorite-tonalite. We present thermodynamic fractionation models that constrain the conditions of fractionation for the andesite, diorite-tonalite, and fine-grained granite suite. The compositional similarity of the granite and rhyolite porphyries, especially their SiO2 and Al2O3 contents and Y/Nb ratios, indicate generation by partial melting of similar crustal sources, although greater assimilation of upper crust is evident in elevated Rb/Sr ratios and K2O and light rare-earth element concentrations in the granite porphyry. We conclude that the magmatic rocks of the Hamash area represent an arc crustal section that records the evolution of a Neoproterozoic oceanic island arc. The arc was later caught up in the assembly of the northern Arabian-Nubian Shield.

Subduction to post-collisional volcanism in the Northern Arabian-Nubian Shield: Genesis of Cryogenian/Ediacaran intermediate-felsic magmas and the lifespan of a Neoproterozoic mature island arc

The Neoproterozoic intermediate-felsic volcanic rocks in the Wadi Ambagy area, Eastern Desert of Egypt, comprise three units: Dokhan intermediate-felsic volcanics, post-Dokhan felsic volcanics and felsites. The Dokhan lavas are generally porphyritic and composed of andesite and dacite with subordinate basalt, whereas the post-Dokhan felsic volcanics are represented by phyric and aphyric rhyolites. The medium-K calc-alkaline nature, LILE enrichment, Ta-Nb anomalies and LREE-enriched patterns of the Dokhan volcanic rocks are consistent with development in a subduction setting. Arc crustal thickness estimation using average (La/Yb)n is ~ 32 km, indicating that these rocks were generated in a Neoproterozoic island arc with thick crust. The subduction magmas were generated by hydrous mantle melting and they evolved by fractional crystallization. In contrast, the post-Dokhan felsic volcanics and felsites have medium- to high-K and shoshonitic affinities, respectively. They are highly silicic with weak to absent Ta-Nb anomalies and significant Nb, Zr, Hf and REE enrichment, which increases from the post-Dokhan felsic volcanics to the felsites. These geochemical features suggest generation in a post-collisional to within-plate tectonic setting by partial melting of mafic or intermediate arc crustal sources. SHRIMP U-Pb zircon dating indicates a crystallization age of ~ 660 Ma for the subduction-related Dokhan andesite and dacite, reflecting a magmatic arc lifespan longer than previously considered. The change of setting from subduction to post-collisional was accompanied by an increased role of crustal sources in the generation of the felsic magmas, resulting in differentiation of the accreted arc crust.

Neoproterozoic hybrid forearc – MOR ophiolite belts in the northern Arabian-Nubian Shield: no evidence for back-arc tectonic setting

ABSTRACT The Neoproterozoic Arabian-Nubian Shield (ANS) post-dates the Boring Billion (1.85−0.85 Ga), a transition dominated by extensive arc-magmatism, terrane accretion along active subduction zones. Understanding the nature of the oceanic crust fragments preserved in the ophiolite belts is essential in extrapolating the predominant style of plate tectonics during the Neoproterozoic Era. There are two ophiolite belts with controversial genetic models in the Central Eastern Desert (CED) of Egypt, namely Wadi Semna–Fawakhir–Um Gheig belt to the north, and Ghadir–Mubarak–Barramiya belt to the south. Geochemical database and magnetic data are herein integrated to envisage the tectonic setting of the dispersed ophiolite blocks within these two belts. The northern belt is dominated by NW obduction-related thrust faults and is intensely deformed by structures attributed to the Najd fault system (NFS). The southern belt is less affected by the Najd fault with NE obduction-related thrust faults. Based on various discrimination diagrams and REEs pattern, oceanic mafic-ultramafic rocks in these belts can be classified into three types, MOR, forearc and volcanic arc basalt, where the latter is MORB overprinted by arc affinity. Our proposed evolutionary model includes N-dipping subduction zones parallel to the ~ E-W Allaqi-Heiani suture with hybrid forearc-MOR affinities. The data synthesis presented here weighs with advancing accretionary orogeny along the intraoceanic subduction zones as the dominant tectonic style during the Neoproterozoic evolution of the CED rather than the retreating style of the modern western Pacific region.

Petrogenesis of a rare Ediacaran tonalite–trondhjemite–granodiorite suite, Egypt, and implications for Neoproterozoic Gondwana assembly

AbstractMost tonalite–trondhjemite–granodiorite (TTG) suites are Archean–Palaeoproterozoic in age, but those of Neoproterozoic–Phanerozoic age are scarce. A rare Ediacaran high-Al TTG suite has been identified at the Fannani Igneous Complex (FIC) in the northern Arabian–Nubian Shield, which is essentially composed of amalgamated Neoproterozoic island-arc Pan-African composite terranes that contain several ophiolitic sutures. The FIC exhibits a wide range of SiO2, Al2O3, Sr and Zr, shows moderate rare earth element (REE) enrichment, and K, Ti, Nb, Y and heavy REE depletion. It is a subsolvus suite with clear orogenic affinities and strong arc-geochemical signatures. The precise U–Pb zircon thermal ionization mass spectrometry age obtained (607.4 ± 1.95 Ma) indicates oceanic subduction extended to late stages of the East African Orogeny. The FIC exhibits87Sr/86Sr compositions of 0.70346–0.71091 (Sr(i)ratio, 0.70284), and143Nd/144Nd of 0.51254–0.51270 (ϵNd(t) = +5.12 to +7.16), typical of modern oceanic-arc rocks (as Japan-arc basalts), and suggestive of mantle sources and island-arc settings. The FIC possesses low values of Yb (1.55 ppm), Nb (14 ppm) and Y (24 ppm), and high ratios of Sr/Y (27), Zr/Sm (46) and Nb/Ta (11.8), typical of magmas produced by anatexis of a basaltic slab. Partial melting models show that the FIC magma was generated by melting (F= 0.25–0.50) of a subducted oceanic crust transformed into eclogite, leaving 10–25% garnet in the residue. The FIC and similar complexes produced via slab melting during the closure of the Mozambique Ocean formed large juvenile belts along the East African Orogen that sutured East and West Gondwana together into a united supercontinent.

Cadomian (ca. 550 Ma) magmatic and thermal imprint on the North Arabian-Nubian Shield (south and central Israel): New age and isotopic constraints

The Arabian-Nubian Shield (ANS) is a province of mostly juvenile Neoproterozoic continental crust in NE Africa and Arabia, forming the northern extension of the East-African orogen. Subsequent to widespread, mostly late- to post-orogenic early-Ediacaran calc-alkaline granitoid emplacement, the latest magmatic phase in the ANS saw the intrusion of alkaline shallow plutons alongside volcanic massifs and dikes of variable compositions. While some of these intrusions were dated to 550–530 Ma by Rb-Sr and Ar-Ar, U-Pb zircon data has mostly limited ANS magmatism to no younger than ca. 580 Ma. Here we report new age and isotopic data from latest Neoproterozoic intrusions along a N-S transect within the northern ANS and its periphery, substantiating the record of a ca. 550 Ma igneous-thermal event in the area. In the northern Negev of Israel, mafic igneous rocks intruded into the subsurface late Neoproterozoic clastic wedge of the Zenifim Formation produced apatite U-Pb isochron ages between 557 and 545 Ma. In the Amram alkaline massif, 150 km to the south, an alkali-granite was dated to 605 ± 2.5 Ma by zircon U-Pb, but apatite U-Pb ages from all rocks of the massif concentrate at ca. 550 Ma indicating thermal resetting. Apatites from the zircon dated 611 ± 5 Ma Roded quartz diorite in the Elat area, south of Amram, produced a 600 ± 4 Ma U-Pb age, demonstrating that southern segments were not thermally reset at ca. 550 Ma. Nd isotopes in apatite and whole rock show the ca. 550 Ma igneous rocks to be variable in their isotopic composition, ranging between εNd(t)=(-4) – (+2), far less juvenile than typical ANS magmas. The ca. 550 Ma intrusions resemble in their age and isotopic properties Cadomian granites and volcanic rocks that are known from southern Turkey, Iran and Europe. We interpret the ca. 550 Ma igneous overprint and thermal resetting documented in the northernmost ANS as related to the superposition of Cadomian orogeny on the ANS periphery. This can be related to the late Ediacaran subduction of proto-Tethys accompanying the accretion of the Cadomian basement of the Tauride block to NE Africa and Arabia, before the area was overstepped by Cambrian platform sedimentation.

40Ar/39Ar and U-Pb SIMS zircon ages of Ediacaran dikes from the Arabian-Nubian Shield of south Jordan

A spectacular feature of the Arabian-Nubian Shield (ANS) is the abundance of well-exposed and extensive Neoproterozoic dike swarms of variable compositions. Most of these dikes are late to post-orogenic with respect to the East African Orogen (EAO) and are unmetamorphosed. We dated a composite dike with latite margins and a rhyolite core (607 ± 6 Ma, U-Pb), a biotite rhyolite dike (600 ± 4 Ma, 40Ar/39Ar age of biotite), an andesite dike (594 ± 3, 40Ar/39Ar age of amphibole) and a dolerite dike (~579 Ma, 40Ar/39Ar whole rock total gas age). We propose that the first three dikes represent one generation that was emplaced at different episodes extending between 607 and 590 Ma. Time and composition equivalent dikes are common in the northern ANS. The dikes crosscut late collisional granitoids and geochemically display a subduction-related character as evidenced by a negative Nb-Ta anomaly. These dikes are absent in the alkali feldspar A-type Humrat Syenogranite dated at 586 ± 5 Ma in Jordan and equivalent rocks in the northern ANS, which are crosscut only by the (~579 Ma) dolerite dikes. The within-plate character of the dolerite dikes is supported by the absence of the Nb-Ta anomaly and high field strength element geochemistry. We propose that the dolerite dikes are a generation, distinct from the ~607–590 Ma dikes, that reflects a change in tectonic regime and represents the last magmatic activity of the Neoproterozoic in the northern ANS. The ages of the dikes dated in this study agree with the published age range of the transitional stage from late orogenic calc-alkaline to extensional alkaline tectono-magmatic setting for the ANS. We propose that the magmatic activity was terminated ~50 m.y. before the age of the Cambrian unconformity at ~530 Ma. Correlation with ages of dikes and magmatic rocks in the northern ANS favors this supposition. The dike geochemistry and geochronology are compatible with a tectonic model that involves mantle lithosphere delamination from below the northern ANS after a significant crust-mantle thickening caused by the EAO, followed by thermal relaxation, subsidence and gradual denudation until the age of the unconformity at ~530 Ma.

Remote sensing-guided stratigraphic dissection of an Ediacaran terrestrial molasse basin (Kareim basin, Egypt), with implications for sedimentary evolution

The Kareim basin in the Central Eastern Desert is one of the largest Ediacaran post-amalgamation (Hammamat) basins in Egypt. It formed at a time of extensional tectonism (600 ± 20 Ma) in the northern Arabian-Nubian Shield. The Kareim basin preserves a ~6500 m fill of rapidly deposited alluvial fan and lake sediments. Due to monotonous lithology and complex facies variations the stratigraphic architecture of this and other large Hammamat basins is poorly known. In this study, LANDSAT 8 (OLI) data for the Kareim was processed to provide best band ratio and PCA images that allow tracing of sandstone-dominant and conglomerate-dominant packages, revealing a configuration of eight new stratigraphic units (Units 1–8). Units 1–3 are basinwide, while Units 4–8 are localized and partly bounded by intraformational angular unconformities that formed during syn-sedimentary tectonism. Fan palaeoslopes and palaeocurrents were W-directed in Units 1–3, then switched to SW- and S-directed in later units, implying that the provenance areas lay east and north of the basin, not south as in previous basin evolutionary models. This study provides the first RS-guided stratigraphic dissection of a Hammamat basin, and provides a sensitive tool for detecting intraformational unconformities in basins filled with monotonous molasse sequences. It paves the way for the sequence stratigraphic analysis of the Egyptian Hammamat basins.

Petrogenesis of Maktali fractionated calc-alkaline younger granitoids, Central Eastern Desert, Egypt

The Egyptian calc-alkaline younger granitoids represent part of broadly distributed late collisional high-K calc-alkaline granitoids in the northern Arabian-Nubian Shield. In the Eastern Desert of Egypt, these granitoids have a significant economic value that they are commonly associated with rare metal-bearing granites. Maktali granitoids are located in Central Eastern Desert and they constitute a two-phase pluton, which consists of leucogranites and monzogranites. They intrude the country metavolcanics with sharp contacts. These granitoids are slightly peraluminous and moderately to highly fractionated high-K calc-alkaline granitoids. Leucogranites and monzogranites were emplaced at temperatures 699.8-8.28.8°C and 757.2–909.6°C and under pressure 1.22–4.8 and 2.36–2.98 kbars respectively. Leucogranites and monzogranites were emplaced at depths 3.28–22.96 km and 6.36–8.5 km and they were crystallized under log Oxygen fugacity between − 13.2 to − 16.1 and − 11.6 to − 14.7 respectively. They are magnetite-series granitoids and their crystallization was controlled by NNO fugacity buffer. Leucogranites and monzogranites share the same magmatic source and they were emplaced during late-collisional stage by melting of crustal source rocks may occur as a consequence of decompression following delamination of the lithospheric root and slab breakoff. Maktali leucogranites could be generated by partial melting and subsequent fractional crystallization of mafic lower crust with addition of melts from the mantle. Maktali monzogranites show distinctive characteristics of rare metal-bearing granites that they contain Fe-columbite and transitional micas. Field and textural evidences together with chemistry of columbite, micas, and trace element behavior suggest the magmatic origin of monzogranites. Moreover, REEs in monzogranites show a pronounced Eu anomaly and a well-visible tetrad effect, supporting their generation by fractional crystallization and fluid-rock interaction during the late stage of granite crystallization.

The timing of migmatization in the northern Arabian–Nubian Shield: Evidence for a juvenile sedimentary component in collision‐related batholiths

AbstractCollision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.

Metamorphic evolution of Pan-African Wadi El Miyah Metasediments, Central Eastern Desert, Egypt: a distinctive LP/HT metapelitic sequence from the northern Arabian–Nubian Shield

The meta-volcanosedimentary sequence from the SW sector of Wadi El Miyah area constitutes an example of the Late Proterozoic Pan African supracrustal rocks exposed in the Central Eastern Desert of Egypt. This folded meta-volcanosedimentary sequence is juxtaposed along about 8-km-long WNW–ESE-trending thrust contacts with a highly foliated ophiolitic melange comprising serpentinites, metagabbros and quarztofeldspathic blocks and intruded by a gneissose hornblende diorite. Chaistolite-bearing mica schists crop out in three compositionally different metamorphic zones, marked by garnet, andalusite, and staurolite porphyroblasts, respectively. Microstructural data combined with careful mapping indicate that the studied metapelites were subjected to three deformational phases, D1–D3, each having its own impact on the development of planar and linear structures. These rocks witness the coexistence of a less ubiquitous dextral sense of shear and a later sinistral one that correspond to the D2-thrusting and D3-related phases, respectively. Geothermobarometry of the garnet-bearing assemblages combined with Th–U–Pb dating of metamorphic monazite by an electron microprobe indicates a counter-clockwise P–T–t path with a thermal peak assemblage of garnet + staurolite + chiastolite ± fibrolitic sillimanite at 622 °C/4.1 kbars during the D2 phase, with an age around 640–658 Ma. This is concordant with the ages assigned for the arc accretion phase in the Nubian Shield. The determined counter-clockwise P–T–t path of the studied rocks suggests that such a Buchan-type LP/HT sequence may be considered as a typical example of a regional–contact metamorphism which formed in areas that suffer from crustal thickening accompanied by emplacement of granitoids that affected the geothermal gradient of the crust.

Provenance of Austroalpine basement metasediments: tightening up Early Palaeozoic connections between peri-Gondwanan domains of central Europe and Northern Africa

New U–Pb and Lu–Hf detrital zircon data together with whole-rock geochemical and Sm–Nd data were obtained for paragneisses of the Austroalpine basement south of the Tauern Window. Geochemically immature metasediments of the Northern–Defereggen–Petzeck (Otztal–Bundschuh nappe system) and Defereggen (Drauzug–Gurktal nappe system) groups contain zircon age populations which indicate derivation mainly from Pan-African orogens. Younger, generally mature metasediments of the Gailtal Metamorphic Basement (Drauzug–Gurktal nappe system), Thurntaler Phyllite Group (Drauzug–Gurktal nappe system) and Val Visdende Formation (South Alpine Basement) were possibly derived from more distant sources. Their significantly larger abundances of pre-Pan-African zircons record a more advanced stage of downwearing of the Pan-African belts and erosion of older basement when the Austroalpine terrane was part of the Early Palaeozoic Northern Gondwana passive margin. Most zircon age spectra are dominated by Ediacaran sources, with lesser Cryogenian, Tonian and Stenian contributions and subordinate Paleoproterozoic and Neoarchean ages. These age patterns are similar to those recorded by Cambro–Ordovician sedimentary sequences in northeastern Africa between Libya and Jordan, and in some pre-Variscan basement inliers of Europe (e.g. Dinarides–Hellenides, Alboran microplate). Therefore, the most likely sources seem to be in the northeastern Saharan Metacraton and the Northern Arabian–Nubian Shield (Sinai), further supported by whole-rock Sm–Nd and zircon Lu–Hf data.

Mineralogical, geochemical and Sr-Nd isotopes characteristics of fluorite-bearing granites in the Northern Arabian-Nubian Shield, Egypt: Constraints on petrogenesis and evolution of their associated rare metal mineralization

The Central Eastern Desert (CED) of Egypt, a part of Neoproterozoic Arabian Nubian Shield (ANS), embraces a multiplicity of rare metal bearing granitoids. Gabal El-Ineigi represents one of these granitic plutons and is a good example of the fluorite-bearing rare metal granites in the ANS. It is a composite pluton consisting of a porphyritic syenogranite (SG; normal granite) and coarse- to medium-grained highly evolved alkali-feldspar granite (AFG; fluorite and rare metal bearing granite) intruded into older granodiorite and metagabbro-diorite rocks. The rock-forming minerals are quartz, K-feldspar (Or94-99), plagioclase (An0-6) and biotite (protolithonite-siderophyllite) in both granitic types, with subordinate muscovite (Li-phengite) and fluorite in the AFG. Columbite-(Fe), fergusonite-(Y), rutile, zircon and thorite are the main accessory phases in the AFG while allanite-(Ce) and epidote are exclusively encountered in the SG. Texture and chemistry of minerals, especially fluorite, columbite and fergusonite, support their magmatic origin. Both granitic types are metaluminous to weakly peraluminous (A/CNK=0.95–1.01) and belong to the post-collisional A2-type granites, indicating melting of underplated mafic lower crust. The late phase AFG has distinctive geochemical features typical of rare metal bearing granites; it is highly fractionated calc-alkaline characterized by high Rb, Nb, Y, U and many other HFSE and HREE contents, and by extremely low Sr and Ba. Moreover, the REE patterns show pronounced negative Eu anomalies (Eu/Eu∗=0.03 and 0.06) and tetrad effect (TE1,3=1.13 and 1.27), implying extensive open system fractionation via fluid–rock interactions that characterize the late magmatic stage differentiation. The SG is remarkably enriched in Sr, Ba and invariably shows a relative enrichment in light rare-earth elements (LREEs). The SG rocks (569±15Ma) are characterized by relatively low initial 87Sr/86Sr ratios (0.7034–0.7035) that suggest their derivation from the mantle, with little contamination from the older continental crust. By contrast, the AFG has very high 87Rb/86Sr and 87Sr/86Sr ratios that reflect the disturbance of the Rb-Sr isotopic system and may give an indication for the high temperature magma-fluid interaction. The positive εNd(t) values of AFG (+7.40) and SG (+5.17), corresponding to young Nd-TDM2 ages ranging from 707 to 893 Ma, clearly reflect the juvenile crustal nature of Gabal El-Ineigi granitoids and preclude the occurrence of pre-Neoproterozoic continental crust in the ANS. The field relationships, chemical, petrological and isotopic characteristics of El-Ineigi SG and AFG prove that they are genetically not associated to each other and indicate a complex origin involving two compositionally distinct parental magmas that were both modified during magmatic fractionation processes. We argue that the SG was formed by partial melting of a mid-crustal source with subsequent fractional crystallization. In contrast, the AFG was generated by partial melting and fractionation of Nb- and Ta-rich amphibole (or biotite) of the lower crust. The appreciable amounts of fluorine in the magma appears to be responsible for the formation of rare metal element complexes (e.g., Nb, Ta, Sn and REEs), and could account for the rare metal mineralization in the El-Ineigi AFG.