Arabian Shield Research Articles

Unpaired ophiolite belts in the Neoproterozoic Allaqi-Heiani Suture, the Arabian-Nubian Shield: Evidences from magnetic data

Under oblique convergence, the intraoceanic subduction zones tend to have a tectonically-escaped accretionary prism, incorporates exotic materials from different sources and oceanic crust fragments. The western part of the Allaqi-Heiani suture (AHS) is the type locality for a significant accretionary prism extends for ⁓ 200 km; this prism is mainly metavolcanic, and metasedimentary rocks have interwoven serpentinite, marble blocks and intruded with different phases of plutonism. Geologic studies of the western part of the AHS have suggested the allochthonous origin for this complex, and terrane accretion happened earlier than ⁓ 700 Ma and subsequently thrusted over Gabgaba terrane to the south. Magnetic data was applied to follow the deep-seated structures and the connectivity between ophiolite belts in the AHS.Magnetic data unravelled the presence of a deep-seated strike-slip faults aligned along N–S and NNE trends underneath the northern belt, and this fault zone was erased at the surface by a major zone of the post-accretionary left-lateral strike-slip faults. In the southern part, E-W and WNW-ESE faults are dominant at the edge of the autochthon Gabgaba terrane. At greater depths, thrusts related to the emplacement of the northern belt disappeared, while those to the south aligned with the suture zone becomes significant. In conclusion, there are two ophiolite belts are located in the western part of the AHS; the southern belt is extended along the edge of the Gabgaba terrane to the south and the Eastern Desert (ED) to the north. This south belt is the western extension of a major arc-arc suture zone that extends to the east into the Arabian shield. The northern ophiolite belt was emplaced onto the southern part of the ED ⁓ 70 km away from the arc-arc to the south.

Geochemical analysis and tectonic evaluation of the Miocene–Pliocene sequence at Al Rehaili area, Northern Jeddah, Saudi Arabia

Geochemical data and approach play a vital role in constraining and evaluating provenance of sedimentary rocks and tectonic settings. It provides valuable information on both the rock compositions of the provenance and the depositional setting. This study is aimed to predict provenance, source rocks, and tectonic settings of the non-marine successions of Bathan Formation (Miocene–Pliocene siliciclastic sequence) exposed along the road cuts at Al Rehaili area, north of Jeddah. The non-marine successions of Bathan Formation were divided into three main sedimentary units (units I, II, and III). From these units, 17 sandstone samples were selected and used to carry out the geochemical analysis using major oxides and trace elements. The examined samples are interpreted as quartz-rich sandstones, which are mainly sourced from intermediate rocks of igneous, sedimentary, and meta-sedimentary rocks of the Arabian Shield. Furthermore, the provenance of the studied sandstones is interpreted as that of mixed granite, granodiorite, rhyolite, and basaltic source rocks. Most of the tectonic discrimination plots indicate that the tectonic settings of the study area are defined as an active continental margin, which approves the interpretation of previous studies.

Shear Velocity Structure Beneath Saudi Arabia From the Joint Inversion of P and S Wave Receiver Functions, and Rayleigh Wave Group Velocity Dispersion Data

AbstractWe develop a new 3‐D shear velocity model for the lithosphere and sublithospheric mantle under Saudi Arabia by jointly inverting P wave receiver functions, S wave receiver functions, and fundamental‐mode Rayleigh wave group velocities. P and S receiver functions are calculated from earthquakes recorded between 2012 and 2015 at 156 Saudi National Seismic Network stations operated by the Saudi Geological Survey. Rayleigh wave dispersion data are extracted from independent tomographic studies. Our model reveals significant lateral variations in crustal and upper‐mantle S velocity below Saudi Arabia. Particularly, a low‐velocity zone, with minimum S velocity of ~4.0 km/s in the depth range of 70‐190 km, is observed under the Arabian Shield coinciding with Cenozoic surface volcanism. The low‐velocity zone is found consistent with the presence of partial melts in the mantle and interpreted as a potential deep magma source for the volcanism in western Arabia. We propose that magmas responsible for the Arabian volcanism may be derived from multiple sources. Both lithospheric thinning and the Afar plume trigger magma production beneath southwestern Arabia, while decompression melting caused by the lithospheric thinning may be the main factor in the central and northern portions. Furthermore, we find evidence for localized crustal low shear velocity anomalies (2–4% reductions) that appear spatially correlated to the volcanism; these may be due to fractures caused by magma ascent and small amounts of partial melt in the crust. This spatial distribution of S velocity reductions may indicate the plumbing system for magma ascent underneath western Arabia.

Emplacement levels and pre-existing structures control mechanisms and host rock interactions of three granitic plutons, western Arabian Shield

Field relations, structural measurements, and microfabric analyses are used to explain the emplacement mechanisms of granitic plutons in the Asir terrane. These circular-to-elliptical granitic intrusions are prominent plutons scattered across the Arabian–Nubian Shield. Various structural and tectonic fabrics affecting three plutons in the western Arabian Shield encouraged studying the effect of pre-existing structures and emplacement levels on the emplacement mechanisms and related solid-state deformation. Two of these plutons, called Al Sayl and Um Ar Raka, are circular and enveloped by ductile or brittle solid-state deformation. Pre- and post-emplacement tectonic fabrics controlled the distribution and superposition of the solid-state deformation. Lateral expansion of magma during the emplacement of these plutons induced significant radial shortening that was associated with either high or low heat transfer into the host rocks and developed brittle or ductile solid-state deformation, respectively. Heat transfer was controlled by the emplacement level and magma pressure of each pluton. The third, oval-shaped Al Hawiyyah pluton was structurally controlled, where magma was emplaced first into twofold hinges on both sides of a thrust fault under low-to-intermediate differential stress. Syn-emplacement shortening was accommodated mainly by upward and lateral magma flows. Continuous magma flow caused uplift of the pluton and its surrounding host rocks and developed stretching structures in these rocks.

Mixed and recycled detrital zircons in the Paleozoic rocks of the Eastern Moroccan Meseta: Paleogeographic inferences

The paleogeographic evolution of the Moroccan Variscides has been a matter of discussion for several decades, with current theories mostly based on classical geological correlations. In this regard, the scarce number of studies devoted to U-Pb geochronological analyses of detrital zircon populations is particularly limiting when trying to ascribe the different domains to a single continental piece either derived from the West African Craton or to different sources, with some located in the Nubian Shield or the Saharan Metacraton. In this work, detrital zircon grains from 10 samples of sandstones from the Paleozoic (Ordovician to Devonian) sequence of the Eastern Meseta and Middle Atlas were dated in order to identify possible sediment sources and elucidate the paleogeography of this easternmost portion of the Moroccan Variscides. The main detrital zircon populations have Ediacaran-Cryogenian ages (610–670 Ma, related to the Cadomian and/or Pan-African orogeny) and middle Paleoproterozoic ages (1980–2080 Ma, related to the Eburnean orogeny), which are in agreement with previous data from the Western Meseta, suggesting similarity between both Mesetas, and strong West African Craton affinity. Such an affinity verifies the most accepted paleogeographic interpretation considering that the Moroccan Mesetas remained attached to northern Gondwana during the entire Paleozoic period. The main differences between our samples and those from the Western Meseta concern the minor detrital zircon populations, such as the Cambro-Ordovician and the Tonian-Stenian ones. In particular, Eastern Meseta and Middle Atlas samples lack a Cambro-Ordovician detrital zircon population, usually interpreted as related to the rifting that opened the Rheic Ocean. This population is locally reported in the Western Meseta and widely described in southwestern Europe, where magmatism of this age is well known. Furthermore, the most northeastern samples are also characterized by a Tonian-Stenian detrital zircon population (up to 30% of the data), which might imply northeastern African sources (Saharan Metacraton and/or Arabian-Nubian Shield).

Multi-tracer study of continental erosion and sediment transport to the Red Sea and the Gulf of Aden during the last 20 ka

Mineralogical compositions and grain-size distributions combined with 87Sr/86Sr and εNd values of the detrital fraction were studied on cores recovered from the Gulf of Aden (MD92-1002) and the Red Sea (MD92-1008) basins in order to document past changes in Indian monsoon and northwesterly winds during the last glacial-interglacial transition (the last 20 ka), encompassing the African Humid Period (AHP). The εNd vs. 87Sr/86Sr plot indicates that sediments result from the mixing of two main sedimentary sources corresponding to the Afar volcanic rocks in Ethiopia and to the Arabian-Nubian Shield. Variations of sediment isotopic and mineralogical composition point to a diminution of the volcanic source contribution during the last deglaciation. Changes of mineral-accumulation rates and grain-size distributions denote a decline in the aridity of the source regions during the Holocene, particularly of the Afar volcanic region. In this area, the reduction of detrital supply, from 15 cal ka BP, can be explained by an increase of precipitations during the AHP, which resulted in an expansion of the vegetation cover and lake extensions in East Africa. In the Arabian Peninsula, precipitations were confined to the south, allowing sediments to be transported even during the Holocene. Our data suggest that the southwest monsoon was not the main carrier of aeolian sediments to the Red Sea and Gulf of Aden basins, but the Northwesterlies. In the Red Sea, the isotopic and mineralogical tracers reveal a contribution from Saharan dust between 16 and 12 cal ka BP, transported from the Nile catchment after aridification during Heinrich event 1.

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.

Geometric and tectonic analysis of Ad-Damm mega-scale fold: implication of Neoproterozoic Transpressive Regime in the west-central Arabian Shield

A mega-scale fold is exposed near Jeddah city in the western-central part of the Arabian Shield. This fold extends NE for about 35-km long and 12-km width and occupies the northwestern block of a distinct linear feature known as Ad-Damm Shear Zone. It is composed essentially of foliated, sheared, and mylonitized metamorphic and plutonic rocks. Para and ortho-gneisses were an amphibolite facies condition whereas greenschist facies units were represented by metasedimentary and metavolcanics. Image processing and field-based structural analysis, in addition to micro structural investigations of the mega-scale fold, indicating that the ductile dextral-sense of shear was overprinted by a brittle regime associated with the evolution of Ad-Damm shear zone. The Ad Dam mega-scale fold is a shear-related structure, north plunging overturned anticline. Mylonitization and dextral shear-related structures were developed during a transpressional regime. The Ad-Damm fold is a structural system that illustrates the evolution history of the Arabian Shield associated with the development of the NE-wrench fault. This fault started as an early plastic followed later by a brittle tectonic regime. The present study indicates that Ad-Damm shear zone and several sub-parallel dextral shears of NE-trends which located in the western side of the Arabian Shield represent a conjugate antithetic shear to the NW- trending sinistral shear of the Najd fault system.

Structural analysis of the central and southwestern parts of the Neoproterozoic Salalah Crystalline Basement, Sultanate of Oman

The Salalah Crystalline Basement (SCB) is the largest Precambrian exposure in Oman located on the southern margin of the Arabian Plate at the Arabian Sea shore. It is thought to represent one of the eastern-most exposures of the Arabian-Nubian Shield. We used detailed structural analysis to establish the deformational history of the SCB by focusing on its central and southwestern parts. This part of the SCB is dominated by the Juffa complex (820 ± 10 Ma) which is made-up of two-mica gneisses, amphibolites and ultramafic lenses, and the Sadh complex (816 ± 12 Ma) which is composed of biotite and amphibolite gneisses. Our work found that the SCB evolved through four deformational events that shaped its final architecture: (1) Folding and thrusting event that resulted in the emplacement of the Sadh complex atop the Juffa complex. This event resulted in the formation of possibly N-verging nappe structure. Associated with this event was the development of mesoscopic asymmetrical tight folds and thrusts with various orientations because of folding during subsequent event. However, the dip of these structures fluctuates between SE and SW suggestive of an initial N-verging nappe structure; (2) Regional folding event around SE- and SW-plunging axes that deformed the regional fabric developed during the N-verging nappe structure. This event produced map-scale SE- and SW-plunging antiforms resulting in shaping the Juffa and Sadh complexes into a semi-dome structure; (3) Strike-slip shearing event that produced a conjugate set of NE-trending sinistral and NW-trending dextral strike-slip shear zones. The NW-trending dextral strike-slip shear zones dissected the northeastern part of the Juffa-Sadh semi-dome whereas the NE-trending sinistral strike-slip shear zones dissected the southwestern part of the semi-dome. Strong mylonitic fabric, stretching lineation and associated kinematic indicators were developed during this event; and (4) Localized SE-directed extension manifested by top-to-the-southeast kinematic indicators and the development of stretching lineation in the form of stretched amphibole crystals overprinted on older planar fabric. This is particular apparent in the hinge of the map-scale SE-plunging antiform.

The Effects of Structural Elements on Groundwater of Wadi Yalamlam, Saudi Arabia Using Integration of Remote Sensing and Airborne Magnetic Survey

The present study aimed to develop an integrated approach that utilized field geology, remote sensing and airborne magnetic data to identify the Quaternary aquifers impounded by dykes and explain the dykes/lineaments relationship and their effects on the storage and flow of the groundwater of Sadiyah sub-basin, lower part of Wadi Yalamlam basin, Western Arabian Shield, Saudi Arabia. Field visits supported by Landsat ETM + ratio images (5/7, 5/4 and 5/1) revealed the presence of three main dyke types that can be chronologically arranged from older to younger as: Sheared mafic and acidic dykes (D1); Damm dyke complex (D2); and Ghumayqah dykes (D3). Two dykes referred to the first type (D1) were traced with depth using airborne magnetic survey crosscutting the main course of Wadi Yalamlam around Sadiyah village at high angle. According to water table elevation measurements, the aquifer of lower Wadi Yalamlam area is divided into three parts namely I, II and III. The water table elevations decreased from 158 m in the first part (I) to reach 66 m in the third part (III) of the aquifer. This drop is attributed to the effect of dykes which act as natural barriers. The study area was affected by three major fault systems as confirmed by field study and total magnetic map. Two of these systems have NE–SW to ENE–WSW and N–S to NNW–SSE directions, whereas the third one represents the Najd fault system trends in NW–SE direction. The present study identifies two locations valid for agricultural expansion lie just to the north of Sheikh Said and Abu Helal Farms.

The mafic–ultramafic complex of Salem, southern India: An analogue for Neoproterozoic Alaskan‐type complex

The Salem Mafic–Ultramafic Complex occurs within the northern part of Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex occurs as semicircular to elongated linear body with a thick mantle section and comparatively well‐developed crustal section comprising different lithological units, also referred to as the Chalk Hills of Salem. The major rock types in the complex comprise ultramafic cumulates of dunite, peridotite, wherlite, pyroxenite, and hornblendite, mafic and felsic intrusions of gabbros and amphibolites, and quartzo–feldspathic alkaline rocks together with several ultrapotassic dykes. Geochemical studies of amphibolites from this complex indicate tholeiitic parent magma with enrichment of LIL elements (Rb, Ba, Th, and Sr) and are relatively more enriched than N‐MORB as compared with arc‐related rocks. The enrichment of LIL elements over the HFS elements and with distinct Nb, Ta, and Ti depletion relative to other HFS elements suggest the involvement of subduction‐related components in the depleted mantle source and that the magmatism occurred in an oceanic convergent realm. The mineral chemistry of dunite and peridotite shows high content of forsterite (86.9–92.2) in olivine and high Cr# value (80.85–98.73) for spinel. The tectonic discrimination plots of spinel and olivine mineral chemistry together with clinopyroxene chemistry of amphibolites reveal arc signature typical of Alaskan type of complex. The U–Pb zircon analysis of quartz monzonite intrusion within the hornblendite from the complex yielded a weighted mean age of 819 ± 2.4 Ma, suggesting that the complex formed during the Neoproterozoic and can be correlated to the Alaskan‐type complexes described from the Arabian–Nubian Shield.

From the Libyan border to the Nile – Neoproterozoic magmatism and basement evolution of southern Egypt

ABSTRACTIn southern Egypt, Neoproterozoic granitoids intrude the crust from Libya in the west to the Nile River in the east. Best exposed at Gebel Uweinat, Gebel Kamil, Bir Safsaf, Gebel El Asr and Gebel Umm Shaghir, these rocks represent the transition between the rejuvenated Archean to Paleoproterozoic crust of the east Saharan craton and the juvenile Neoproterozoic crust of the Arabian–Nubian Shield to the east. We present U-Pb zircon geochronology and whole rock geochemistry from 16 Neoproterozoic (c. 628–580 Ma) granitoids across southern Egypt which document the synchronous production of diverse compositions indicative of an intraplate or intracontinental tectonic setting. Our data better constrain the age relationships, petrogenesis, and Pan-African crustal growth associated with the amalgamation of Gondwana and contribute to understanding the crust-modifying processes that led to the development of the Saharan metacraton in the Ediacaran.

Occurrence and genesis of the exposed Oligo-Miocene Ghar Formation in Kuwait, Arabian Gulf

A sequence dominated by fawn cross bedded sandstones that constitutes the basal part of the post Eocene Kuwait Group siliciclastics and has been assigned as Ghar Formation is well exposed within Bahrah coastal plain in the northeast of Kuwait. The geology of these exposures had been briefly described in the middle of the late century. The purpose of this paper is to assess in detail its sedimentology and specifically the diagenesis of its rocks. The exposures of this sequence have been investigated at several locations. A simplified geologic map was prepared and representative samples of the studied sequence were subjected to detailed petrographic examination and geochemical analysis including stable isotopes. Four main facies were recognized within this sequence; sandstones, palustrine dolomicrite, mudstone and carbonates. Structural, textural and geochemical characteristics of the sandstone facies indicated that the detrital constituents are mostly derived from the Arabian Shield to the west of the study area and transported for a long distance within relatively low to moderate relief terrains during tropical sub-humid to humid climate. The palustrine dolomicrite facies was precipitated within semi-closed lagoons that may be influenced by seawater intrusion. The occurrence of thin carbonate beds within this sequence suggests short lived marine incursions intermittent with the prevailed fluvial conditions. Sedimentation was episodic and significantly controlled by the regional tectonics particularly Red Sea rifting. It is, therefore, concluded that the studied sequence has been deposited by fluvial braided system close to a paleo-shore line. Due to its peculiar sedimentological and petrographic characteristics it is suggested to name it as Bahrah Formation. further detailed sedimentological and stratigraphic assessment of this Formation is recommended.

Geochemical Constraints of the Ediacaran Volcano‐Sedimentary Succession of the Sa'al Metamorphic Complex at Wadi Zaghra, South Sinai, Egypt

AbstractThe volcano‐sedimentary succession around Wadi Zaghra in Sinai, at the northernmost segment of the Arabian Nubian Shield, comprises volcanic rocks interbedded with rather immature sediments. The succession is dominated by intermediate to silicic volcanics of medium‐ to high‐K calc‐alkaline affinity. It is divided into two units, the lower unit includes intermediate rocks and dacites interbedded with graywackes, semi‐pelites and pelites and topped by polymict conglomerates. This unit is subjected to folding and regional metamorphism (up to garnet zone) and is intruded by quartz diorite‐granodiorite inducing, locally, low‐pressure contact thermal metamorphism. The unmetamorphosed upper unit encompasses acid volcanics intercalated with litharenite, sublitharenite and minor arenite. The rhyolites of this unit pertain to the highly fractionated granites and are characterized by an agpaitic index (NK/A) ranging from 0.87 to 0.96. They may reflect either extensive interaction of subduction‐related magmas with the continental crust or a change in the tectonic regime. The present lithological and geochemical characteristics of the studied sediments together with available zircon ages indicate rather distal provenance of their detritus. This detritus comprises fluvial‐alluvial sediments accumulated in the intermontane basins, which are half‐grabens or tilted fault blocks. The tectonic setting of the depositional basins is active continental margin and continental island arcs. Geochemical patterns of the Zaghra volcano‐sedimentary succession indicate their correlation with the Dokhan Volcanics‐Hammamat Clastics sequence of the Eastern Desert of Egypt. Also, the Zaghra volcanics display geochemical similarities with those exposed in Sinai, at the Rutig, Ferani and Iqna Shar'a areas. The Zaghra succession is dated as Ediacaran but is not related either to the ensimatic island arc assemblage or to the rift‐related assemblage formed during the early stages of the break‐up of Rodinia as previously thought.

Detrital zircon U–Pb–Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin

Combining U–Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U–Pb/Lu–Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA–(MC)–ICP–MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 ± 5.5 and 556.8 ± 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55–0.65 Ga zircons with considerably scattered εHf(t) from −19 to +14; (ii) varied amounts of 0.65–1.0 Ga zircons showing dominantly positive εHf(t) with the exception of the c. 1.0 Ga zircons; and (iii) ≥1.8 Ga zircons (mostly between 1.9–2.1 and 2.5–2.8, up to 3.2 Ga) with εHf(t) ranging between +5 and −7. Multidimensional scaling of our U–Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60–75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume.

Northern Arabian Shield shear zones: strain analysis comparison

Ajjaj-Qazaz-Hanabiq shear zones (AQH-SZ) are ductile to semi-ductile shear zones that are located within the Midyan terrane in the northwestern part of the Arabian Shield. They expose Neoproterozoic rock assemblages that comprise an elongated ridge of ophiolitic rocks, arc-metavolcanics, and contemporary arc-related intrusions. They are unconformably overlain by the post-amalgamation molasses-type sediments and finally intruded by late- to post-tectonic intrusions. The strain analysis results of the AQH-SZ are consistent with no significant differences. The average of RsXZ values in the three shears ranges from 2 to 2.22, while the RsYZ average ranges from 1.7 to 1.9. The average of stretching in the three shears ranges from 34 to 42%, while the shortening ranges from 33 to 37%. The stretching direction is oriented parallel to the shear trends; WNW directions in the Ajjaj and Qazaz shear zones, and NNE direction in the Hanabiq shear zone. The change in direction of stretching is suggested to be due to the exhumation of gneissic cores which also might have caused the presence of a local dextral sense in Hanabiq shear zone. Based on the similarity in the strain analysis results and the absence of overprinting relationships at the conjunction areas of the three shears, it is concluded that the three shears are one major shear.

Provenance and depositional environment of the Middle-Late Jurassic shales, northern Iraq

The Middle–Late Jurassic formations are regarded to be an important source rock in the Jurassic petroleum system of North Iraq. The major and trace element analysis on the Middle and Late Jurassic shales in the Imbricated Zone, Iraq was analyzed. This study investigated the shales provenance and depositional environment, in addition to evaluate the intensity of weathering in the source area. The chemical index of alteration (CIA), index of compositional variability (ICV) and the Al2O3-CaO + Na2O-K2O diagram of the shales suggest increasing in the intensity of chemical weathering from the Middle to the Late Jurassic due to the climatic change and/or tectonic activity. The elemental ratios: Al2O3/TiO2, Th/Sc, La/Th, Co/Th, (Gd/Yb)n, Eu/Eu*, and REE pattern; and tectonic discrimination diagrams indicated that they were derived from felsic source rocks in the crystalline basement of the northern fringe of the Arabian Shield and Rutba Uplift. The Sargelu, Naokelekan and Barsarin formations are also supplied from the intermediate source rocks from the Mid Oceanic Ridge and deposited in an active setting. The geochemical parameters suggest the lower part of the Middle Jurassic and the shales from upper part of the Late Jurassic were deposited in an oxic shallow marine environment; and other ages were deposited in an anoxic deep marine environment.

U-Pb detrital zircon constraints on the depositional age and provenance of the dinosaur-bearing Upper Cretaceous Wadi Milk formation of Sudan

Cretaceous continental deposits in Sudan have long been recognized as important archives of continental vertebrate fossils in central Africa. A number of different sedimentary units including the Wadi Milk, Shendi and Kababish formations from northern and central Sudan are known to yield dinosaur and other vertebrate fossils. The ages of these deposits are poorly constrained, and traditionally assumed to be of Albian to Santonian age based on biostratigraphic evidence. However, recent palynological analyses suggest a Campanian–Maastrichtian age for the Shendi Formation. Not only are the ages of these units poorly resolved; but stratigraphic correlations within and between them and similar aged units in central Africa, remains tenuous. To address these issues a detailed sedimentary fingerprinting and provenance analysis was conducted on the putatively correlative Shendi and Wadi Milk formations using sandstone petrography and a multifaceted detrital zircon investigation combining U-Pb geochronology, Lu-Hf isotope analysis and trace element geochemistry. The objective was to provide constraints on the age of deposition, sediment sources and paleofluvial drainage patterns. Based on laser ablation ICP-MS U-Pb detrital zircon geochronology of 18 sandstone samples (>1400 detrital zircons), we identified a population of six Late Cretaceous zircons from the Wadi Milk Formation yielding a maximum depositional age of 79.2 ± 2.4 Ma (MSWD = 0.65, probability 0.62). Specifically, these data restrict the depositional age to Campanian or younger for the Wadi Milk Formation, which significantly refines our understanding of the age of this unit and its fauna. The Lu-Hf-isotope and trace element analyses of the remainder of the dated zircons show a provenance dominated by Neoproterozoic crustal sources and minor Paleoproterozoic and Archean sources for both units, which we interpret as being sourced from the Arabian-Nubian Shield to the south and southeast of the study area. These findings, coupled with the recently updated biostratigraphic age of the Shendi Formation, confirm that both units are correlative to one another and were probably deposited synchronously by north-north westerly flowing fluvial system draining into the Tethys Sea during the Late Cretaceous.

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.

Multiple Stages of Carbonation and Element Redistribution during Formation of Ultramafic-Hosted Magnesite in Neoproterozoic Ophiolites of the Arabian-Nubian Shield, Egypt

AbstractWe present a study of the serpentinized peridotites of the Ghadir-Mohagar-Ambaut area, Egypt. They represent the mantle section of a dismembered ophiolite, tectonically emplaced over a volc...