Arabian Shield Research Articles

Development of the Arabian-Nubian Shield along the Marsa Alam-Idfu transect, Central-Eastern Desert, Egypt: geochemical implementation of zircon U-Pb geochronology

The magmatic complex along the Marsa Alam-Idfu transect, Central-Eastern Desert of Egypt, represents the northern segment of the Arabian–Nubian Shield (ANS), which developed within the framework of the East African Orogen. The basement rocks of the Arabian-Nubian Shield have been developed through three distinct phases of magmatic activity: the island-arc, the syn-orogenic, and the post-orogenic phases. Transitioning of the magmatic phases from the syn-orogenic to the post-orogenic, identifies changing the tectonic regime from a compressional to an extensional setting. The scarcity of comprehensive regional geochronological data that rely on precise isochron methods, such as the zircon U-Pb technique, could limit the comprehensive understanding of this region’s geological and tectonic history. That would raise a number of uncertainties ranging from the timing of the different magmatic activities and timing of changes in the tectonic regime to the existence of the pre-Pan-African crust in the CED. Our study provides new insights into the aforementioned uncertainties through zircon U-Pb dating of different rock units along the Marsa Alam-Idfu transect, CED, Egypt. The resulting ages ranged from 729 ± 3 Ma to 570 ± 2 Ma, constraining the temporal evolution of the ANS in the studied region into (1) the island-arc phase, represented by a metamorphic sample with an age of 729 ± 3 Ma. (2) the syn-orogenic phase, represented by calc-alkaline and alkaline granitic samples with ages ranging from 699 ± 4 Ma to 646 ± 2 Ma. These two phases indicate initiation of the compressional subduction regime in the CED since 729 ± 3 Ma and being dominated till 646 ± 2 Ma. (3) the post-orogenic phase, represented by metavolcanics, volcanic rocks, and alkaline plutonic samples with ages ranging from 623 ± 3 Ma to 570 ± 2 Ma. This phase suggests dominance of the compressional-to-extensional tectonic transition setting from 623 ± 3 Ma to 600 ± 1 Ma along with the Dokhan volcanism and activation of post-collision tensional regime activated at 582 ± 3 Ma. Our findings discourage the proposed dominance of the island-arc and syn-orogenic phases in the CED and the classical restriction of older magmatic activity to calc-alkaline granitic rocks and younger magmatic activity to alkaline granitic rocks. Additionally, we identified evidence of local magmatic sources by dating five grains with Mesoproterozoic (pre-Arabian–Nubian Shield) xenocrysts with ages ranging from 1549 ± 4 to 1095 ± 25 Ma.

Neoproterozoic Tectonics of the Arabian-Nubian Shield: Insights from U–Pb Zircon Geochronology, Sr–Nd–Hf Isotopes, and Geochemistry of the Deki Amhare Complex Granitoids, Central Eritrea

The Deki Amhare complex is located in central Eritrea, within the Arabian–Nubian Shield (ANS). It consists of an inner core of monzogranite porphyry and diorite enclaves (MMEs), surrounded outwardly by granodiorite and quartz diorite. The zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Hf isotopic compositions of the Deki Amhare complex granitoids were used to discuss the Neoproterozoic tectonics of the ANS. The Late Tonian granodiorite and quartz diorite are metaluminous and calc-alkaline to slightly high-K calc-alkaline I-type plutons, with ages of 811.2 ± 4.8 Ma and 811.6 ± 5.7 Ma, respectively. They exhibit positive εHf(t) (7.6–9.5) and εNd(t) (3.9–4.7) values and relatively low (87Sr/86Sr)i ratios (0.70374–0.70463), indicating that they derived from the partial melting of a metasomatized mantle wedge during intra-oceanic subduction. The Ediacaran monzogranite porphyry and MMEs are subalkaline to alkaline A2-type granitoids with ages of 620.0 ± 4.3 Ma and 614.8 ± 3.9 Ma. These display positive εHf(t) (5.3–8.7) and εNd(t) (4.2–4.7) values, as well as low (87Sr/86Sr)i ratios (0.70310–0.70480), implying that they formed through crust–mantle magma mixing related to post-collisional slab break-off. Based on these data, three stages of regional tectonic evolution can be described: (1) from ~1200 Ma to ~875 Ma, the mafic oceanic crust was derived from depleted mantle during the opening of the Mozambique Ocean; (2) from ~875 Ma to ~630 Ma, intra-oceanic subduction and arc formation occurred with the development of I-type batholiths; and (3) from ~630 Ma to ~600 Ma, crustal and lithospheric reworking took place post-collision, leading to the formation of A2-type granitoids.

Rodingitization of Neoproterozoic Al-Barramiya ophiolite metagabbro in the Eastern Desert of Egypt, Arabian-Nubian Shield

ABSTRACT The present work studies the rodingite that is recorded for the first time as being associated to Al-Barramiya Neoproterozoic ophiolite at Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different styles of alterations included carbonatization, listvenitization, chloritization and rodingitization. The Neoproterozoic ophiolitic rocks of Al-Barramiya district consists of serpentinized peridotite and metagabbros. Serpentinized peridotites are highly altered to assemblages of talc-carbonate rocks, listvenite and magnesite, while metagabbro is partly converted into rodingite. The field studies indicate that serpentinite and rodingite in Al-Barramiya area display high-strain cataclastic deformation. Rodingite consists of garnet, diopside, vesuvianite and chlorite with minor epidote, prehnite, actinolite and opaque minerals. The garnets and vesuvianite are mostly concentrated near the peripheries of the studied rodingite masses, while clinopyroxene, chlorite, epidote and prehnite gradually increase inwards due to progressive decreasing water/rock ratio as fluids originally buffered by serpentinite move towards equilibrium with gabbroic bulk chemistry. Some rodingite samples preserve a few relics of igneous textures of ophiolitic metagabbros. Garnets are distinguished into andradite, hydroandradite and grossular. Pyroxene is represented mainly by secondary clinopyroxene beside primary fresh relics of diopside; the latter has high Mg# (0.92–0.97) similar to ophiolitic clinopyroxene in the ANS. Rodingite is enriched in CaO, Fe2O3 and MgO but depleted in SiO2, Al2O3 and Na2O compared to associated metagabbro. The association of rodingite with serpentinites and metagabbros suggests that Al-Barramiya rodingite is genetically related to serpentinization of ultramafic rocks. The rodingitization occurred during ocean floor alteration contemporaneous with the serpentinization process. The superposition of cataclastic deformation on the altered rocks indicates that alteration preceded obduction and occurred on the seafloor of the suprasubduction zone where Al-Barramiya ophiolite was originally emplaced. During serpentinization the olivine in the peridotite broke down and released Mg2+ in the metasomatic fluids and clinopyroxene broke down producing Ca-rich fluids. These fluids react with the mafic protoliths and enrich them in MgO and CaO. Also, serpentinization fluids react with the plagioclase of the metagabbro, releasing Ca from the mafic source and concentrated it in the rodingitized samples. The positive Eu anomalies beside high Sr contents of the rodingite can be inherited from a plagioclase-rich protolith such as ophiolitic metagabbro. The enrichment of rodingite in Th and U indicates two distinct solutions affected the gabbroic protolith because the alteration of depleted peridotite to serpentinite cannot be an efficient source to enrich these elements. The present study indicates that the rodingitization process underwent lower greenschist facies conditions associating metamorphism that caused serpentinization and released Ca-rich metasomatic fluids.

Continental-scale sediment mixing and dispersal across northern Gondwana: detrital zircon U-Pb-O-Hf isotopic evidence from the Cambro-Ordovician sandstones overlying the Arabian Shield

ABSTRACT The juvenile Neoproterozoic basement of the Arabian Shield is overlain with angular unconformity by a voluminous Cambro-Ordovician cover sequence known as the Saq Formation and Wajid Group. Provenance studies of this vast siliciclastic cover over the Saudi Arabian part of the Arabian-Nubian Shield (ANS) have to date been based solely on U-Pb zircon data and heavy minerals. We present the first combined in-situ U-Pb, δ18O and Lu-Hf isotopic data for detrital zircon from the Saq and Wajid units exposed along the northeastern margin and southern part of the Arabian Shield. U-Pb age spectra reveal prominent age peaks at ca. 0.8–0.55 Ga and 1.1–0.9 Ga with subordinate peaks at ca. 2.2–1.7 Ga and 2.7–2.5 Ga. The δ18O secular variation mirrors global compilations with Archaean zircon defining a restricted δ18O range of ca. 4.0–8.0 ‰ and younger zircon showing wide variation in δ18O up to ca. 14 ‰. The ca. 0.8–0.55 Ga age peak has the largest variation in εHf(t) with about half of all these Neoproterozoic zircon grains being juvenile (εHf(t) > 5), which are interpreted to be sourced from the juvenile terranes of the ANS. Neoproterozoic zircon with evolved εHf(t) signatures require a more distal source beyond the ANS. As extensive ca. 1.1–0.9 Ga crust is lacking in the vicinity of the ANS, the ca. 1.1–0.9 Ga age peak is interpreted to be derived from either contemporaneous orogenic belts in Central Africa or recycled from older sedimentary rocks containing these age components. Extreme variations in δ18O of post – Archaean zircon, together with the evolved εHf(t), indicate crustal thickening and increased incorporation of supracrustal material associated with collisional orogenesis. The remarkable similarities in age spectra and isotopic compositions of the Saq Formation and Wajid Group sandstones with those from other regions in northern Gondwana indicate a continental–scale homogenization and dispersion process.

Petrology, mineral chemistry and geochemistry of chloritite associated with Neoproterozoic ophiolitic ultramaficsin the Eastern Desert of Egypt, Arabian-Nubian Shield

This study presents for the first time, the field observations, petrography, mineral chemistry and geochemistry of chloritite hosted in the Al-Barramiya Neoproterozoic ophiolite of the Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). The Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different types of alterations including carbonatization, listvenitization, chloritization and rodingitization. The Al-Barramiya chloritite occurs as thin layers associated with highly serpentinized peridotite. It is a fine-grained rock entirely composed of chlorite (85–95 vol.%) with minor talc and accessory minerals (epidote, rutile, titanite, corundum and opaque minerals). The chlorite minerals in the chloritite are represented mainly by diabantite, while those in the serpentinites include ripidolite. Depending on the chemical composition of the chlorites, the chlorite in chloritite formed at temperatures ranging between 200 and 250°C, which are lower than those of the disseminated chlorite in the serpentinite (310–345oC), indicating their formation in different hydrothermal stages. The chloritite samples are rich in total REE contents (17.9–27.3 ppm) compared with the associated serpentinites (0.69–0.87 ppm). They are characterized by slightly depleted LREE relative to HREE [(La/Lu)n = 0.8–0.9], with a moderately negative Eu-anomaly [(Eu/Eu*)n = 0.4–0.5]. The negative Eu-anomalies are derived from chloritization fluids or reflect the presence of talc in the chloritite. Based on field work, petrography, mineralogical and geochemical data, the studied chloritite has been interpreted as being derived from the associated serpentinized ultramafics by hydrothermal alterations. This is supported by an enrichment of chloritite in compatible trace elements (Cr = 2031–2534 ppm, Ni = 1264–1988 ppm, Co = 76–101 ppm) similar to that which is observed in the associate serpentinite.

Magnesite hosted by the Neoarchean ultramafic rocks in Attappadi, southern India: Insights from spectral and stable isotope investigation

Magnesite is an economically important mineral commonly found in ultramafic complexes worldwide, primarily in Archean to Proterozoic ultramafic complexes. This study focuses on the chemical and spectral characterization of magnesite found in the Neoarchean ultramafic rocks in the Attappadi region in the Southern Granulite Terrane of southern India. The research utilizes x‐ray diffraction analysis, hyperspectral, laser Raman, Fourier Transform Infrared, and Isotope Ratio Mass Spectrometry. The studied ultramafic rocks are part of a well‐exposed ophiolitic suite known as the Agali ophiolitic complex. Magnesite primarily occurs as veins, veinlets, and lenses within weathered ultramafic rocks. The hyperspectral analysis of the magnesite samples shows absorption bands in the shortwave infrared region, particularly around 2.3 and 2.5 μm, which correspond to the stretching and bending of the CO bond in the (CO3)2− ion in MgCO3. The laser Raman spectra show intensity peaks at 1095, 738, and 330 cm−1, which may be attributed to the translational and librational vibrations. The Fourier transform infrared data reveal transmittance at 1434, 880, and 747 cm−1, corresponding to MgO bond stretching and asymmetrical CO stretching. The x‐ray powder diffraction spectra exhibit diffraction peaks at 32°, 35°, 42°, 46° and 53°, characteristic of pure magnesite. The spectroscopic parameters derived from various analyses indicate that the magnesite is high quality and free from gangue minerals. Stable isotope analysis of the magnesite samples yielded δ13C values ranging from −5‰ to −9‰ and δ18O values in the range of 21‰–25‰. The estimated water temperature from which the magnesite has been precipitated is ~59 ± 3.9°C. Based on the field relations, mode of occurrence and isotopic signatures, the mineralization is considered to have been formed by the low‐temperature alteration of ultramafic rocks facilitated by CO2‐rich fluids in the near‐surface environment. This study compares the characteristics of magnesite from the study area with a few Neoproterozoic serpentinite‐hosted magnesite veins in the ophiolitic sequence of the Egyptian Eastern Desert, which is part of the Arabian Nubian shield. The research aims to contribute to understanding magnesite formation in Archaean to Proterozoic mafic–ultramafic rocks on the Earth's crust. It also provides insights into the geological processes that govern the genesis of ultramafic‐hosted magnesite globally, particularly in East Gondwana fragments. This information can enhance mineral exploration and resource evaluation in these regions, helping to identify economic prospects and assess the feasibility of magnesite resource extraction and utilization in East Gondwana fragments.

Geochronological Assessment of the Arabian-Nubian Shield plutonic intrusions in the arc assemblages along the Qift-Quseir transect, Central Eastern Desert of Egypt

The ophiolitic mélange and granitic intrusions along the Qift‒Quseir transect in the Central Eastern Desert (CED) of Egypt are parts of the Egyptian Nubian Shield (ENS), which is the northern segment of the East African Orogeny (EAO). The Arabian-Nubian Shield (ANS) is the longest Neoproterozoic belt on Earth, and it was formed during the East and West Gondwanaland collisions within the framework of the EAO. The ANS basement rocks were developed during three distinct phases of magmatic activity: the island arc and syn-collisional phases, identifying a compressional tectonic regime, and a post-collisional phase that identifies changing the tectonic regime into an extensional type. The geochronological assessment of these magmatic activities is essential for understanding the regional geology and tectonic development of the ANS. In our study, we dated different rock units along the Qift‒Quseir transect and revealed ages ranging from the Late Tonian (820 ± 8 Ma) to the Late Ediacaran (563 ± 4 Ma). These ages were associated with three different magmatic pulses: (1) a seafloor spreading and island arc phase (ophiolite and related rocks), represented by sample QQ05, which was dated from 820 ± 8 Ma; (2) a syn-collisional phase, represented by samples QQ08 and QQ10, dated from 733 ± 10 Ma and 729 ± 10 Ma, respectively; and (3) a post-collisional phase, represented by all the other samples, dated from the Ediacaran at 603 ± 9 Ma to 563 ± 5 Ma. These results showed that the post-collisional phase was dominant, especially in terms of the alkali-feldspar granites, relative to ophiolitic rocks, and the syn-collisional granites in the CED. Initiation of the Dokhan Volcanic eruptions at 639 ± 2 Ma gave us the date of the compressional-to-extensional tectonic transition setting, and the post-collisional tensional regime was activated at 603 ± 9 Ma. Additionally, we identified evidence of local magmatic sources by dating 11 grains of Paleo- to Meso-Proterozoic xenocrysts with ages ranging from 1876 ± 18 to 1070 ± 13 Ma (i.e., components of the pre-Arabian-Nubian Shield).

Composite Granitic Plutonism in the Southern Part of the Wadi Hodein Shear Zone, South Eastern Desert, Egypt: Implications for Neoproterozoic Dioritic and Highly Evolved Magma Mingling during Volcanic Arc Assembly

The Abu Farayed Granite (AFG), located in the southeastern desert of Egypt, was intruded during the early to late stages of Pan-African orogeny that prevailed within the Arabian–Nubian Shield. The AFG intrudes an association of gneisses, island arc volcano–sedimentary rocks, and serpentinite masses. Field observations, supported by remote sensing and geochemical data, reveal a composite granitic intrusion that is differentiated into two magmatic phases. The early granitic phase comprises weakly deformed subduction-related calc–alkaline rocks ranging from diorite to tonalite, while the later encloses undeformed granodiorite and granite. Landsat-8 (OLI) remote sensing data have shown to be highly effective in discriminating among the different varieties of granites present in the area. Furthermore, the data have provided important insights into the structural characteristics of the AFG region. Specifically, the data indicate the presence of major tectonic trends with ENE–WSW and NW–SE directions transecting the AFG area. Geochemically, the AFG generally has a calc–alkaline metaluminous affinity with relatively high values of Cs, Rb, K, Sr, Nd, and Hf but low contents of Nb, Ta, P, and Y. The early magmatic phase has lower alkalis and REEs, while the later phases have higher alkalis and REEs with distinctly negative Eu anomalies. The AFG is structurally controlled, forming a N–S arch, which may be due to the influence of the wadi Hodein major shear zone. The diorite and tonalite are believed to have been originally derived from subduction-related magmatism during regional compression. This began with the dehydration of the descending oceanic crust with differential melting of the metasomatized mantle wedge. Magma ascent was long enough to react with the thickened crust and therefore suffered fractional crystallization and assimilation (AFC) to produce the calc–alkaline diorite–tonalite association. The granodiorite and granites were produced due to partial melting, assimilation, and fractionation of lower crustal rocks (mainly diorite–tonalite of the early stage) after subduction and arc volcanism during a late orogenic relaxation–rebound event associated with uplift transitioning to extension.

Crustal rheological properties provide evidence for large-scale heterogeneity in the extended Arabian Shield Crust at the Red Sea Margin

Crustal rheological properties provide evidence for large-scale heterogeneity in the extended Arabian Shield Crust at the Red Sea Margin

Hf Isotopes and Detrital Zircon Geochronology of the Silasia Formation, Midyan Terrane, Northwestern Arabian Shield: An Investigation of the Provenance History

ABSTRACTU–Pb ages and Hf isotopic data from detrital zircon of the Silasia Formation in the Midyan Terrane record evidence for the provenance and tectonic evolution of the northern Arabian Shield. Given that the youngest acknowledged age of these detritus sediments is 735 ± 13 Ma, it is likely that the Silasia Formation was deposited during the closure of the Mozambique Ocean. The U–Pb ages define a major Mesoproterozoic peak, with two minor peaks of Neoproterozoic and Archean age. Combined with zircon Hf isotopic compositions, the sedimentary detritus of the Silasia Formation was mainly derived from source rocks formed during the Grenville Orogeny during the assembly of Rodinia, with a minor contribution from Archean and Paleoproterozoic crustal material, in addition to a limited arc‐basement supply related to the early Mozambique Ocean. The youngest Concordia age of 735 ± 13 Ma with highly variable εHf(t) values (11 to −35) indicates a complex mixture of sources from juvenile to extremely ancient. The Concordia ages at 1113 ± 11 and 1046 ± 10 Ma have positive hafnium isotope signatures (up to +10.45) that are consistent with juvenile source rocks formed during the Grenville Orogeny. Several detrital zircons with ages of 2622 ± 22 Ma and 2690 ± 7 Ma are similar to those reported in Yemen, whereas 1818 ± 19 Ma, 2071 ± 8 Ma and 2001 ± 19 Ma Paleoproterozoic ages are similar to dated outcrops in the Khida terrane in the eastern Arabian Shield.

The Gulf of Suez rifting: implications from low-temperature thermochronology

ABSTRACT The Gulf of Suez is a continental rift basin that has been developed during the Oligocene-Miocene as a northern extension of the Red Sea rift system. The Oligo-Miocene rift-activated flanks are represented by exposures of the Neoproterozoic basement rocks of the Arabian-Nubian Shield. An additional thermal overprint (i.e. mantle plume) and the preceding tectono-thermal events that also affected the rifting dynamics and the uplift of its flanks remain uncertain. We here present thermochronological data for 20 basement samples collected from the Gabal Somr ElQaa area at the northwestern flank of the Gulf of Suez. The resulted zircon fission-track ages could be divided temporospatially into two groups: the Cambrian and the Silurian-Devonian with average ages of 503 ± 14 Ma and 407 ± 11 Ma, respectively. While apatite fission-track data reveal three different age groups of the Devonian-Carboniferous, the Permian-Triassic, and the Late Cretaceous, with average ages of 349 ± 10 Ma, 246 ± 6 Ma, and 81 ± 5 Ma, respectively. The modelled t-T histories indicate four cooling pulses during the Neoproterozoic, the Devonian-Carboniferous, the Cretaceous, and the Oligocene-Miocene. Integrating our findings with the regional tectonism and depositional records indicates that these cooling pulses could be attributed to synchronous activities of the post-accretion erosional event, the Variscan tectonic event, the Mid-Atlantic Ridge spreading, and the Gulf of Suez rifting. The Gulf of Suez is a passive rift, formed by a single and rapid rifting pulse within the framework of the Red Sea rift system. An additional east-southward far-field thermal overprint from the Arabian marginal plume partially affected the Suez rifting process. This thermal print affected only the rift’s southern portion of its eastern flank, causing syn-rift cooling ages, highly elevated rift flanks, and an increased heat flow. The western rift flank and the northern segment of the eastern flank are characterized by pre-rift cooling ages, modest rift flanks, and reduction in heat flow.

Prolonged arc accretion during growth of juvenile crust in the Arabian Nubian Shield: Insights from the granitoids of northern Ethiopia

The Arabian Nubian Shield (ANS) is one of the largest juvenile continental crust formed after the Archean. To determine the timing and understand the mechanism of its early crustal growth, we conducted geochronological and geochemical studies on granitoids from northern Ethiopia. A plagiogranite, dated at 929.7 ± 2.2 Ma, represents one of the early granitoids from the ANS. It shows low K2O, TiO2, and REE contents, flat REE pattern and absence of Eu anomaly. Moreover, it exhibits (87Sr/86Sr)i, ƐNd(t) and ƐHf(t) values of 0.70341, +5.0 and +8.4, respectively. Its zircon δ18O value is 4.8‰, lower than the mantle value. These characteristics suggest that it was derived from partial melting of altered oceanic crust. The oxygen fugacity obtained from zircon trace elements is ΔFMQ+1.3, consistent with a subduction setting.Other plutons have much younger ages ranging from 860 to 840 Ma and are mainly medium K, I-type granitoids. These granitoids show low MgO, Ni and Cr contents, low Sr/Y, (La/Yb)N, Nb/Ta and Zr/Sm ratios. They exhibit whole-rock (87Sr/86Sr)i values of 0.70236–0.70304, ƐNd(t) values of 4.2–5.2, ƐHf(t) values of 5.8–7.6 and zircon ƐHf(t) values of 5.5–10.4, slightly lower than coeval depleted mantle. Furthermore, they exhibit fractionated REE patterns, enrichments in LILE and depletion in HFSE. These geochemical features suggest that these granitoids were generated by remelting of arc crust. Compiled data show that the associated basaltic rocks are of arc affinity, which confirms the coeval development of arc magmatism in the southern ANS. Our results suggest that the growth of juvenile crust in the southern ANS started from the early Neoproterozoic and peaked at ca. 880 to 800 Ma, much earlier than those in the northern ANS. The prolonged history accounts for the growth of the vast juvenile crust in the ANS.

Ediacaran to Jurassic geodynamic evolution of the Alborz Mountains, north Iran: geochronological data from the Gasht Metamorphic Complex

The Alborz Mountains in north Iran underwent several tectono-metamorphic events during opening and closure of the Palaeotethys and Neotethys Oceans. These events are recorded by rare and discontinuously exposed metamorphic rocks, such as the HP-LT Asalem-Shanderman Complex and the Gasht Metamorphic Complex (GMC), that are considered to have been metamorphosed during the closure of the Palaeotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions, along with smaller volumes of granites. Different dating methods were applied to selected samples of the GMC basement to constrain the geological evolution of this part of the Alborz Mountains. A metagranite yielded two LA-ICP-MS U–Pb zircon ages of 638.4 ± 4.1 Ma and 590.3 ± 4.8 Ma that possibly date protolith crystallisation and later deformation and metamorphism, respectively, and a granite yielded a late Ediacaran 551 ± 2.5 Ma U–Pb zircon crystallisation age. A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for this unit. Derivation of the GMC from Turan Block basement is unlikely, as this has a different crustal makeup and is probably composed of Paleoproterozoic and early Neoproterozoic material. The zircon ages are similar to published ages from the Arabian-Nubian Shield, indicating that this part of the Alborz basement may have belonged to the northern margin of Gondwana in the Neoproterozoic before rifting and drifting away along with other Iranian blocks (the Cimmerian terranes) during opening of the Neotethys Ocean. Chemical Th-U-total Pb ages for metamorphic monazites from two metapelite samples yielded a very large range of spot ages, of which c. 80% falls between 200 and 250 Ma, that do not allow to distinguish between Eo-Cimmerian and Main Cimmerian events in the GMC. However, they may indicate that the amphibolite-facies peak metamorphism of the GMC occurred sometime in the Triassic, in any case much later than the Carboniferous metamorphism in the neighbouring Asalem-Shanderman Metamorphic Complex to the north. Peak-metamorphic amphibole from amphibolite, retrograde white mica and foliation-defining biotite from metapelites and magmatic white mica from granite yielded much younger 175.1 ± 0.5 to 177.0 ± 0.4 Ma 40Ar/39Ar plateau ages. The Toarcian 40Ar/39Ar ages for minerals with different nominal closure temperatures reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastward, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It was probably triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent.

Hydrothermal alteration processes in monzogranite: a case study from the Eastern Desert of Egypt: implications from remote sensing, geochemistry and mineralogy

The South Eastern Desert (SED) of Egypt is one of the most promising areas in Egypt; it is widely explored for exploring the rare earth elements (REEs) and uranium-bearing ores. It is a main part of the Arabian-Nubian Shield (ANS). Therefore, the present study concerns with Sikait-Nugrus area as one of the most prolific sites in this region. The study provides a detailed geological, structural, and mineralogical investigation of the monzogranites to describe and characterize the various alteration types and sequence. For this purpose, remote sensing, geochemical and petrographical techniques were applied. The remote sensing technique helped in constructing a detailed geologic map of the study area to follow up strictly the alteration zone of the Sikait-Nugrus area. Petrographically, the granites predominates in the study area, they are described as slightly and highly altered monzogranites. The slightly altered one is composed mainly of quartz (~ 20–35%), alkali feldspar (~ 25–30%), plagioclase (~ 25–30%), and mica (~ 5–15%), while accessory minerals are represented by zircon and monazite. On the other hand, the portion of this granite close to the shearing zone is intensively altered and characterized by sericitization as the main alteration processes. This sheared portion is characterized by accessory minerals as, uranothorite, allanite, fluorite and Nb-minerals (ishikawaite). Minerlogically, the altered monzogranites are predominated by the following mineral groups: (1) radioactive minerals as uranyl silicates (soddyite, uranophane and kasolite), and thorium minerals (thorite and uranothorite), (2) Nb–Ta minerals (betafite, plumbobetafite, columbite, fergusonite, and aeschynite), (3) REE minerals (monazite, cheralite and xenotime), and (4) zircon and fluorite as accessory minerals. Geochemically, the recorded pattern of the REEs tetrad effect (M-type) for the highly altered samples indicate that these granites are highly evolved and affected by late stage of hydrothermal alteration and the effective water-rich alteration processes that connected to intensive physico-chemical changes. The total REE concentrations equal 241.8 and 249.75 ppm for the highly and slightly altered samples. A significant mass change (MC) was analyzed by the isocon technique (22.95 & 11.11) and volume change (VC) (1.8 &-7.99) for the highly and slightly altered samples, respectively. The mass balance calculations and the isocon diagrams revealed that some major oxides were removed from the slightly altered monzogranites and transformed later into highly altered monzogranites with increasing the alteration intensity due to the impacts of hydrothermal alteration processes. The studied area is virgin, where no detailed studies have been applied to this region. It is extendable to other parts of the Arabian-Nubian Shield in around the Red Sea in Egypt, Sudan, Saudi Arabia and Yemen. The applied technical workflow is also extendible to other surface analogues everywhere.

Geochemistry and petrology of metapyroxenite and metagabbro associated with Neoproterozoic serpentinites in the Arabian-Nubian Shield: fragments of a fore-arc ophiolite

Geochemistry and petrology of metapyroxenite and metagabbro associated with Neoproterozoic serpentinites in the Arabian-Nubian Shield: fragments of a fore-arc ophiolite

Carbonatization and silicification of ophiolitic ultramafic rocks and formation of gold-bearing listvenites in the Arabian-Nubian shield: A case study from the Al-Barramiya district

Late Neoproterozoic mantle section in the Gabal Al-Barramiya area, the northwestern corner of the Arabian-Nubian Shield (ANS), contains variably serpentinized peridotites that are highly altered along shear zones and thrust planes to form gold-bearing listvenites. They can be categorized into carbonate listvenite, silica-carbonate listvenite and silica listvenite (birbirite). Carbonate listvenite is characterized by the presence of schistosity and deformation fabrics similar to the host serpentinites, but such fabrics are absent in the silica-carbonate and silica listvenites, suggesting that they postdate carbonate listvenite and serpentinization. The presence of listvenites along shears zones and the presence of relics of serpentine and Cr-spinel reflects their formation through metasomatism of ultramafic rocks by hydrothermal fluids circulating along the thrust faults. Silica-carbonate listvenite is characterized by the presence of fuchsite and is enriched in Zn, Pb, Cu, Ag, and Au. Rare earth element (REE) contents differ between the studied listvenites. Silica -carbonate listvenite has the lowest total REE (∑₌0.98–2.56 ppm), whereas the silica listvenite contains the highest total of REE (∑ ₌ 15.70–21.42 ppm). Based on the above, carbonate listvenite is the earliest to form by the infiltration of CO2–bearing fluids released during serpentinization of the original fore-arc peridotite slab, followed by formation of silica-carbonate listvenite due to the activities of SiO2–saturation and CO2-bearing fluids released during ophiolite obduction. Fuchsite in silica-carbonate listvenite formed as a result of metasomatic reactions of Si- and K-rich fluids with Cr-spinel due to hydrothermal alteration of serpentinized peridotite. Silica listvenite formed at the final stage by the silicification of the early formed silica-carbonate listvenite. Listvenitization of the mantle section of the Al-Barramiya ophiolite led to concentration of Au, Pb, Zn, Cu and Ag. The silica-carbonate listvenite contains higher concentration of gold (899–2199 ppb) than the carbonate listvenite (119–191 ppb) and silica listvenite (156–233 ppb).

Origin of the post-orogenic dyke swarms of Saharan Metacraton, at Qaret El-Maiyit-Bir Safsaf area, southwest Egypt: Constraints on the magmatic–tectonic processes at the end of the Precambrian

Deep in the Western Desert of Egypt, in the southernmost part, between Qaret El-Maiyit and Bir Safsaf, swarms of dykes cut through the Neoproterozoic rocks. This area lies halfway between the juvenile crust of the Arabian-Nubian Shield (ANS) and the Gebel Kamil terrains, close to the border with Libya. Acidic dykes include rhyolites and trachy-dacites. Intermediate dykes include trachy-andesite, basalt-trachy-andesite, and basalt-andesite, while basic dykes consist of basalt. Felsic dykes are more numerous and younger compared to the mafic (intermediate and basic) dykes. Felsic dykes trend mostly run to the northeast and northwest, while mafic dykes mainly run to the northwest, less frequently to the eastwest. Acidic and intermediate dykes show elevated REE concentrations (up to 164 and 203 ppm, respectively) with highly fractionated patterns (av. (La/Lu)N = 20.56 and 18) and moderately fractionated HREEs (av. (Gd/Lu)N = 2.5 and 3.2) and LREEs (av. (La/Sm)N = 5.7 and 3.6 respectively). The basic dyke samples exhibit modest REE concentrations (up to 112 ppm), weakly fractionated patterns (av. (La/Lu)N = 8), and mildly to weakly fractionated HREE (av. (Gd/Lu)N = 2.3) and LREEs (av. (La/Sm)N = 2.5) patterns. There are no recognizable Eu anomalies in the dyke samples. The magma ascended in an extensional setting and the geochemical features indicate a subduction mode, possibly originating from the Atmur-Delgo suture zone in northern Sudan or by mantle delamination during the early Neoproterozoic. Both the felsic and mafic dykes are derived from a mafic calc-alkaline melt and show fractionation on a single downward line, indicating a genetic relationship. The mafic dykes were formed by partial melting of an enriched mantle source (about 10%) that started at 2.7–3.0 GPa and a solidus temperature of about 1420 °C. The enrichment of the mantle melt beneath the Saharan metacraton, which is the origin of the studied melts, could be related to mantle delamination.

New constraints on the Neoproterozoic geological evolution of the SE corner of the Arabian Plate (NE Oman)

The Saih Hatat Dome is a tectonic window in northeastern Oman with a NW–SE extent of c. 95 km and an east–west extent of c. 50 km, rimmed by the allochthonous Samail Ophiolite and the tectonically underlying nappes composed of sedimentary rocks from the Neo-Tethyan Hawasina Basin. Rocks within the window were affected by high pressure/low-temperature metamorphism during the latest Cretaceous. Stratigraphically, the Saih Hatat Dome contains a basal, several kilometre thick sequence, made up of low-grade metamorphic volcanosedimentary rocks (Hatat Formation). U–Pb zircon laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) data from a quartzdiorite dyke, intruding the basal part of the Hatat Formation yielded a crystallization age of 845 +2/−4 Ma, whereas zircons from a quartz keratorphyre have ages of 870 ± 5 Ma. Thus, the basal schists of the Hatat Formation are Tonian in age, similar to Tonian felsic volcanic rocks and a granodioritic intrusion in the Huqf area, 300 km to the south. Detrital zircons from the Hatat Formation show an age peak of 837 Ma with some Neoarchean and Paleoproterozoic grains, which have no potential source in the Arabian–Nubian Shield. Based on the new results, we propose that the Hatat Formation signifies a Tonian back-arc basin, contemporaneous with calc-alkaline igneous intrusions in the Jabal Ja'alan region. Similar calc-alkaline intrusions of comparable ages have been identified in the Mirbat area of Dhofar and the Huqf region in central eastern Oman. We interpret these outcrops as components of a cohesive island arc, characterized by calc-alkaline volcanic rocks and volcanosedimentary back-arc sedimentation. Our model situates these regions off the coast of NW India during the Tonian.

Maximum depositional ages and provenance analysis of the Precambrian Manyovu redbeds, Tanzania: Implications for Neoproterozoic tectonics

Abstract The Manyovu redbeds are an up to 600 m succession of fine-grained, siliciclastic strata in northwestern Tanzania and are part of the Neoproterozoic Bukoban Supergroup. Previous authors estimated the age of the Manyovu redbeds to be Neoproterozoic or older based on the K-Ar dates of underlying volcanic rocks (ca. 800 Ma). However, no other age constraints exist for these Neoproterozoic units. U-Pb detrital zircon results from six stratigraphic intervals of the Manyovu units, including both sandstone and siltstone samples, indicate maximum depositional ages as young as 614 ± 6 Ma, almost 200 m.y. younger than the underlying volcanics, with primary detrital contributions from Pan-African orogens, which indicates that these units are syn-tectonic accumulations associated with the assembly of Greater Gondwana/Pannotia. Detrital zircon spectra and modal compositions reveal that the sediment that formed these strata was sourced from a range of terranes, including continental blocks (i.e., Tanzania Craton), magmatic arcs (i.e., Mozambique Belt and Arabian-Nubian Shield), and recycled orogens (e.g., Ubendian-Usagaran belts). Together, these data indicate that the Manyovu redbeds accumulated following the Marinoan Snowball Earth event (ca. 635 Ma) and record the initiation of collision along the Mozambique Belt during Pan-African orogenesis and the formation of greater Gondwana.

Geochemical and Isotopic Constraints on the Origin of Rare-Metal Enriched Alkaline A-Type Granites from the Nubian Shield: Role of Mantle Metasomatism

Abstract The U-Pb zircon TIMS age of 637.47 ± 0.23 Ma identifies the oldest anorogenic complex in the northern Nubian Shield as that at Mount Hamr, emplaced as elongated plutons along deep-seated faults, intruding the Pan-African shield rocks. The hypersolvus peralkaline granites of Mount Hamr are dominated by perthite, quartz, and arfvedsonite, along with accessory zircon, chevkinite-(Ce), monazite, aenigmatite, apatite, fluorite, and opaque ilmenite. The rocks are depleted in Al, Ca, and Sr and enriched in Rb, high field strength elements (Zr, 528–1115 ppm), and rare earth elements (ΣREE, 416–1648 ppm), showing fractionated, light rare earth element-enriched patterns [(La/Yb)N = 17]. The rocks are classified as ferroan, reduced A-type granites (A1-subtype) and exhibit age-corrected (143Nd/144Nd)(i) ratios ranging from 0.5115 to 0.5117, with [ϵNd(t) = +5.0 to +5.9] similar to HIMU-OIB, and have lower age-corrected (87Sr/86Sr)(i) ratios (avg. Sr(i) = 0.702). The data yield Nd-TDM2 model ages of 860–929 Ma. High Zr/Hf, (Ce/Pb)N and low Y/Nb, (Th/Nb)N in these rocks reflect OIB-geochemical-signatures. The rocks crystallized at high temperature (TZr = 900–1185 °C) from H2O–depleted melt via extensive low-pressure fractionation of OIB-type parent magma, initially developed from a Na-F-rich metasomatized mantle source. The latter may have led to the formation of similar rare-metal-enriched alkaline intrusions within the vast Arabian-Nubian Shield and possibly within some magmatic provinces occurring in other shields.