East African Orogen 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.

Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution

The Neoproterozoic Bure adakitic rock in the western Ethiopia shield is a newly discovered magmatic rock type. However, the physicochemical conditions during its formation, and its source characteristics are still not clear, restricting a full understanding of its petrogenesis and geodynamic evolution. In this study, in order to shed light on the physicochemical conditions during rock formation and provide further constraints on the petrogenesis of the Bure adakitic rock, we conduct electron microprobe analysis on K-feldspar, plagioclase, and biotite. Additionally, we investigate the trace elements and Hf isotopes of zircon, and the Sr-Nd isotopes of the whole rock. The results show that the K-feldspar is orthoclase (Or = 89.08~96.37), the plagioclase is oligoclase (Ab = 74.63~85.99), and the biotite is magnesio-biotite. Based on the biotite analysis results, we calculate that the pressure during rock formation was 1.75~2.81 kbar (average value of 2.09 kbar), representing a depth of approximately 6.39~10.2 km (average value of 7.60 km). The zircon thermometer yields a crystallization temperature of 659~814 °C. Most of the (Ce/Ce*)D values in the zircons plotted above the Ni-NiO oxygen buffer pair, and the calculated magmatic oxygen fugacity (logfO2) values vary from −18.5 to −4.9, revealing a relatively high magma oxygen fugacity. The uniform contents of FeO, MgO, and K2O in the biotite suggest a crustal magma source for the Bure adakitic rock. The relatively low (87Sr/86Sr)i values of 0.70088 to 0.70275, positive εNd(t) values of 3.26 to 7.28, together with the positive εHf(t) values of 7.64~12.99, suggest that the magma was sourced from a Neoproterozoic juvenile crust, with no discernable involvement of a pre-Neoproterozoic continental crust, which is coeval with early magmatic stages in the Arabian Nubian Shield elsewhere. Additionally, the mean Nd model ages demonstrate an increasing trend from the northern parts (Egypt, Sudan, Afif terrane of Arabia, and Eritrea and northern Ethiopia; 0.87 Ga) to the central parts (Western Ethiopia shield; 1.03 Ga) and southern parts (Southern Ethiopia Shield, 1.13 Ga; Kenya, 1.2 Ga) of the East African Orogen, which indicate an increasing contribution of pre-Pan-African crust towards the southern part of the East African Orogen. Based on the negative correlation between MgO and Al2O3 in the biotite, together with the Lu/Hf-Y and Yb-Y results of the zircon, we infer that the Bure adakitic rock was formed in an arc–arc collision orogenic environment. Combining this inference with the whole rock geochemistry and U-Pb age of the Bure adakitic rock, we further propose that the rock is the product of thickened juvenile crust melting triggered by the Neoproterozoic Pan-African Orogeny.

North Lhasa terrane in the Precambrian originated from Western Australia

Constraining the origin and Paleogeographic positions of ancient continental fragments plays a crucial role in reconstructing and validating supercontinent cycles. The Lhasa terrane in southern Tibet is the key to revealing the early formation and evolution of the Himalayan–Tibetan orogen. Although previous origin models of the Lhasa terrane are supported by Phanerozoic evidence, Precambrian geological records have not been comprehensively considered. The U–Pb age and Hf isotope data on detrital zircons from newly discovered Precambrian basement in the North Lhasa terrane define two distinctive age populations of ca. 1170 Ma with εHf(t) values identical to the coeval detrital zircons from Western Australia, and ca. 950 Ma with a similar εHf(t) range to the contemporaneous detrital zircons from South Qiangtang and Tethyan Himalaya terranes. The ca. 1170 Ma detrital zircons were most likely derived from Western Australia, whereas the ca. 950 Ma detrital zircons might have been sourced from the norther Indian margin. Compiled detrital zircon data shows that in the early Neoproterozoic, the affinity between northern Lhasa and Western Australia decreased, while the predilection between northern Lhasa and northern India increased. Combined with ca. 930–870 Ma rift, ca. 870–700 Ma arc igneous rocks and ∼650 Ma high-pressure granulites, we propose that the North Lhasa terrane was a ribbon continent that rifted from Western Australia in the early Neoproterozoic (ca. 930–870 Ma), drifted westward along the northern edge of Rodinia in the middle Neoproterozoic (ca. 870–700 Ma), and was emplaced in the Northern East African Orogen during the late Neoproterozoic (∼650 Ma).

Structural mapping and kinematic history of the Qena-Safaga Shear Zone (Egyptian Nubian Shield, East African Orogen)

Structural mapping and kinematic history of the Qena-Safaga Shear Zone (Egyptian Nubian Shield, East African Orogen)

The Cambro-Ordovician Gondwana alluvial megafan in Central Africa: Insights from the Paleozoic sandstones of the Inkisi group, Congo Republic and Democratic Republic of the Congo.

The Paleozoic arkoses of the Inkisi Group in both the Republic and the Democratic Republic of the Congo has been investigated in order to examine whether they can represent an alluvial fan originating from the Gondwana supermountains in Northern and East Africa.This study focuses on outcrops’ description, lithofacies, bounding surfaces, architectural and sequential organization of sedimentary bodies and paleocurrents. The aim is to determine the nature of the Inkisi sandstones, the provenance of its sediments and the depositional setting, in order to test whether the East African orogen was also the sediments source for the formation of sedimentary fan in the western part of Central Africa.The study shows that the Inkisi Group consists of various sandstone lithofacies, amalgamated by conglomerates. The sediments are immature composed of quartz and feldspars associate with lithic fragments and micas. The sediments originated from the erosion of the West-Congolian belt and the Congo Craton, and were transported by braided fluvial system under mainly upper flow regime and secondarily lower flow regime. Sediments were deposited in the Inkisi basin, which was extensional and affected by burial.The Inkisi sediments setting up in a proximal to medial distributary zone of an alluvial megafan.The flows were directed southward, as opposed to paleofans originating from the Gondwana supermountain in Northern and East Africa. Thus, the Inkisi sandstone constitutes a new alluvial megafan distinguished from the Gondwana superfan derived from the Gondwana supermountain. It originated from the Pan-African belt of Mayombe and the Congo Craton. We name this megafan “the Cambro-Ordovician alluvial megafan of Central Africa”.

The Khida terrane – Isotopic evidence for Paleoproterozoic to Neoarchean basement in the eastern Arabian Shield

Pre-Cryogenian crystalline rocks occur only in the eastern part of the Arabian Shield, around Jabal Muhayil in the Khida sub-terrane. These occurrences have been mapped and investigated using U-Pb zircon geochronology, bulk rock Sm-Nd and feldspar Pb isotopes. The previously unrecognized ca. 1.87 Ga Libab gneiss complex contains granitic rocks with inherited zircon up to ca. 2.5 Ga and has both Nd and Pb isotopic evidence for a Neoarchean precursor. The ca. 1.67 Su’ayra alkali feldspar granite similarly requires a Neoarchean precursor but lacks older zircon inheritance. The areally and topographically dominant Muhayil anorthosite cannot precisely be dated but has a minimum age of ca. 750 Ma based on a cross cutting gabbro and granodiorite, the latter also having ca. 1.7 Ga zircon cores. Highly retarded Pb isotopes in the anorthosite require long-term residence in a near-zero U/Pb reservoir, considered most consistent with a ca. 1.7 Ga emplacement age, though a previously suggested genetic link with the Su’ayra granite remains entirely speculative. The consistently ancient isotopic signature that occurs in the younger (850 – 750 Ma) studied samples suggest that these were influenced by Neoarchean continental crust during emplacement of the Siham magmatic arc, which developed along the southern margin of the Afif composite terrane prior to its amalgamation with the Asuir terrane to its west. Correlation with terranes in Yemen showing similar ages and isotopic character suggest a contiguous crustal block separate from the Azania microcontinent that was juxtaposed along the northward extension of the ophiolite-decorated Bayhan suture during Cryogenian closure of the Mozambique Ocean in the northern part of the East African Orogen. Whether the Neoarchean continental crust rifted off of Azania and then re-amalgamated or forms an entirely separate crustal domain remains to be tested.

Fault Striae Analysis and Paleostress Reconstruction of the Northern Tectonic Province (Egyptian Nubian Shield): Insights into the Brittle Deformation History of the Northern East African Orogen

The Egyptian Nubian Shield (ENS), the northwestern continuation of the East African Orogen (EAO), comprises a variety of three lithologically and structurally different tectonic provinces—southern compressional-, central transpressional/wrench-, and northern extensional- provinces. The extensional tectonic province (ETP) extends from Qena‒Safaga shear belt up to the northern tip of the ENS. Several lines of evidence indicate extensional regime in the ETP, such as mantle delamination-induced post-orogenic magmatism and bimodal Dokhan Volcanics, together with the pervasive E‒W dyke swarms and the E‒W (to ENE‒WSW) oriented extensional fractures. The present work is devoted to carry out the fault striae analysis and paleostress reconstruction of the northern tectonic province (ENS) in attempt to add more contribution to the brittle deformation history of the northern EAO. Field measurements collected from six areas (I‒VI) have been processed using Win-Tensor Software. Analysis of fault-slip data revealed four paleostress tensor stages (groups or regimes) that prevailed in the ENS. The stress states and related tectonic regimes are characterized by the stress regime index R' and the horizontal stress axes as follows: 1st stage: transpression (R' = 1.91), E‒W compression (SHmax = N090°‒270° E); 2nd stage: transtension (R' = 1.16), N‒S compression (SHmax = 174°‒354° E); 3rd stage: transpression (R' = 1.81), NE‒SW compression (SHmax = N053°‒233° E); 4th stage: extension (R' = 0.31), E‒W extension (Shmin = N074°‒254° E). The 1st stage can be related to the oblique convergence between E and W Gondwanalands. The 2nd stage was probably concurrent with the N- to NNW-ward migration of the Central Eastern Desert syn-collisional delamination (triggering orogen-parallel extensional collapse) to the North Eastern Desert post-collisional delamination. The 3rd stage was controlled by the generally N‒S shortening which affected the northern and central ENS, post-dating the formation of volcanosedimentary Hammamat Basins. The 4th stage is akin to retreat of the Cadomian arc and the Red Sea rifting.

Expanding Azania at the heart of Gondwana: Terrane correlation from Southern India to Sri Lanka

The reconstruction of megacontinent Gondwana was arguably the first crowning achievement of paleogeography, nonetheless the kinematics of its assembly remains controversial. The now-outmoded concept that Gondwana assembly was simply the amalgamation of East and West Gondwana has been replaced by models involving multiple terrane accretions and continental collisions along different orogenic belts. Although the affinities of large cratons are well established, figuring out the relationships between the multiple “Pan-African” orogenic belts has been hindered by poorly defined smaller intervening terranes. At the center of Gondwana, the polymetamorphic terranes in Sri Lanka have been correlated to various crustal blocks without broad consensus, mainly because of a paucity of high-quality geochronological results. Terrane correlations and figuring out the spatial distributions of “Pan-African” orogenic belts in the central part of Gondwana are, therefore, hampered by the controversies about Sri Lanka. We present new data on the high-grade metamorphic rocks occurring in the Highland Complex of Sri Lanka, which reveal the widespread Paleoproterozoic magmatism with U-Pb ages of ≥ 2465 – 1803 Ma and uniform Archean Hf model ages. Our data confirm the pervasive metamorphic ages of ∼ 610 – 510 Ma, and for the first time, we present multiple episodes of metamorphism at Paleoproterozoic (1854 – 1803 Ma) and Neoproterozoic (∼637 – 635 Ma) which are comparable with those reported from the East African Orogen. Based on a comparison with neighboring crustal blocks, we suggest that these early Precambrian basement components as well as the multiple metamorphism of the Highland Complex in Sri Lanka can be correlated with micro-continent Azania. These new data not only confirm the widespread early Precambrian basement components in Sri Lanka, but also open up new ways for terrane correlations from East Africa through Sri Lanka to East Antarctic.

Proterozoic fold and thrust belt imaged on reflection seismic in Arabian plate basement

Major structures are imaged on 2D reflection seismic data within the Precambrian basement of the western Rub’ Al-Khali of Saudi Arabia for the first time. In this area the base-Cambrian Angudan Unconformity that defines top basement is at a depth of 1–2 km with a gentle easterly dip. Large antiformal structures are clear on 2D reflection seismic within the basement. The antiforms are separated by zones that are interpreted as low-angle faults. No basal detachment is seen and the faults pass below the base of reflection seismic at a depth of approximately 12 km. Individual folds have dimensions in the order of 20 km wavelength and 2 km amplitude and occur between 3 and 10 km depth. The structural style is interpreted as compressional by comparison with thrust systems and accretionary prisms elsewhere. Regional studies indicate that basement is comprised of a series of north-south trending oceanic Proterozoic terranes that were brought together in the Ediacaran East African Orogeny. Based on current regional tectonic maps the newly-imaged fold belt is on the east margin of the Khida-Abas terrane. Depth conversion shows the thrusts dip at approximately 10° north in the line of section. A novel combination of cross-section to structural trend obliquity estimates combined with matched filtering of potential field data indicate the fold and thrust belt trends 015°-195° with ESE vergence. This compressional belt may be some internal structure of the Khida-Abas terrane, or may be a non-outcropping accretionary prism on the margin of that terrane, or may even represent a set of amalgamated microterranes that do not outcrop along trend. Either way, these observations represent a major, previously undescribed element of this sector of the East African Orogen.

Constraints on the lithospheric structure and tectonics of Archean Dharwar Craton, southern India from geophysical and geological data: Evidence for modified lithosphere

• Mantle plume activities eroded the lithospheric keel of the Archean Dharwar Craton. • Small-scale convection destabilized and promoted erosion of lithospheric mantle. • SCLM is metasomatized as indicated by low S-wave velocities (Vs = 4.1-4.4 km/s). • Effective distance of mantle plume activity is ∼1500 km from the source region. Cratons are the stable portions of the continents for at least the past two billion years. Their longevity is generally attributed to their thick, cold, rheologically strong and chemically depleted subcrustal mantle lithosphere. The lithosphere achieved both buoyancy and strength by relatively low thermal regime and compositional differences due to dehydration, presence of melt-depleted peridotites and low iron content. Low temperature of the mantle lithosphere leads to high viscosity, which prevents deformation and thus preserves the cratonic root. However, significant modifications/erosion of the lithospheric root is observed for some cratons from different parts of the globe. The present paper discusses some aspects of lithospheric modification of the Archean Dharwar Craton, southern India. Southern India is a mosaic of several Archean cratonic blocks, the Western and Eastern Dharwar cratons, and Southern Granulite Terrain. It has experienced several tectonic and magmatic events, including mafic dykes of various periods since Paleoproterozoic. The Dharwar Craton was accreted with the Karwar and Coorg blocks on the west and Eastern Ghats and Nallamalai fold belts on the east during the Proterozoic. The Cuddapah, Bhima and Kaladgi basins were formed during the Proterozoic period. A 1.1 Ga kimberlite magmatism and 550 Ma East-African orogen are other important geodynamic events of the region. The western and eastern continental margins, together with the Deccan Large Igneous Province, one of the world's largest igneous provinces, were formed due to mantle plume activities during 130-65 Ma. 2500 km long Himalayan Mountain chain was formed during ∼55 Ma. Active and passive seismic, magnetotelluric, gravity, xenolith and elevation data complimented with geological data were used to understand the lithospheric structure. Subduction, collision and mantle plume activities, which are main processes for crustal evolution are also found responsible for lithospheric destruction in the region. The detached subducted slabs from various geodynamic activities of the region might have led to rheological weakening of the subcontinental lithospheric mantle and thereby craton modification/erosion in the southern India. Mantle plume activities are responsible for epeirogenic uplift and lithospheric thinning in the southern India.

The timing and tectonic context of Pan-African gem bearing pegmatites in Malawi: Evidence from Rb–Sr and U–Pb geochronology

The Pan-African belts of Malawi contain a largely unexplored endowment of gem bearing pegmatites. We present U–Pb in zircon (LA-ICPMS) and Rb–Sr mineral isochron geochronological and isotope data from pegmatites across Malawi. The pegmatites contain tourmaline, beryl, aquamarine, zircon, amethyst and sunstone as gemstone species. Two zircon bearing pegmatites in southern Malawi intruded early in the Pan-African orogenic cycle at 719 ± 5 Ma and 729 ± 4 Ma and are associated with the emplacement of alkaline rocks that formed during an intra-continental rifting episode in the eastern part of former Rodinia. One further zircon pegmatite containing inherited zircon of a similar age (746 ± 44 Ma) was emplaced at 598 ± 15 Ma, after the assembly of Western and Eastern Gondwana and the formation of the East African Orogen (EAO). The majority of the analysed pegmatites, however, are significantly younger. The ∼550 Ma pegmatites were emplaced during the Kuunga Orogeny, correlating with the collision of northern and southern Gondwana cratonic entities. During a prolonged post-collisional period, possibly related to crustal collapse and extension, further gem-mineralised pegmatites formed at ∼520 ± 6 Ma and ∼500–485 Ma. The youngest pegmatite intruded in the southern Malawian Ntcheu area in the Middle Ordovician at ∼460 Ma. A large spread in 87Sr/86Sr initial isotopic ratios between 0.70556 and 0.79018 suggests a variety of magma sources for the Kuunga-related pegmatites with a variably strong crustal affinity.

Age and correlation of the Proterozoic Mafingi Group in Malawi

The low-grade Mafingi Group form a restricted sequence of Proterozoic arenaceous and pelitic metasedimentary rocks which locally lie unconformably on the Palaeoproterozoic Ubendian Domain basement gneisses in northern Malawi. Four samples were taken from different parts of the sequence, in order to obtain detrital zircons for U–Pb geochronology to constrain their maximum age of deposition, provenance and thereby to suggest correlations with other sequences in the wider SE African region. Three samples, including one from the Type Area (Mafingi Hills), and two from different tectonic slivers within the Ubendian gneisses, gave maximum deposition ages of around 1800 Ma. A third sample, also from a tectonic sliver gave a much younger maximum depositional age of ∼1500 Ma. Apart from this, all samples show similar zircon age spectra, with a major composite age-peak at ∼1900 ± 100 Ma from zircon detritus most probably derived from the local Ubendian Domain basement. Three samples show a very small Archaean age component probably derived from the Tanzania Craton which lies to the NE. The most NE sample of strongly recrystallised “Virauli Quartzite”, apparently interlayered with Ubendian gneisses of the Ufipa Subdomain, contained metamorphic zircon grains dated at ∼560 Ma, reflective of a Pan-African overprint within the local western margin of the wider East African Orogen. Significantly, no evidence of ∼1 Ga (Irumide) metamorphism was detected in any of the samples.The three older samples gave maximum depositional ages confirming previous proposed correlations of the Mafingi Group with the Manshya River Group of the Muva Supergroup in Zambia. Despite having a similar detrital age spectrum to the other samples, and an identical field appearance, one contains a population of five much younger detrital grains, suggesting a maximum deposition age of ∼1500 Ma. This is well outside the age-range of the Manshya River Group and might suggest correlation with the youngest sequence of the Muva Supergroup; the Mesoproterozoic Kasama Formation. This inference should, however be treated with extreme caution as dating of that formation itself rests on the age of a single detrital zircon.

Zircon geochronology and Hf isotopic study from the Leo Pargil Dome, India: implications for the palaeogeographic reconstruction and tectonic evolution of a Himalayan gneiss dome

AbstractU–Pb geochronology, Hf isotopes and trace-element chemistry of zircon grains from migmatite of the upper Sutlej valley (Leo Pargil), Northwest Himalaya, reveal a protracted geological evolution and constrain anatexis and tectonothermal processes in response to Himalayan orogenesis. U–Pb geochronology and ϵHf record separate clusters of ages on the concordia plots in the migmatite (1050–950 Ma, 850–790 Ma and 650–500 Ma). The 1050–950 Ma zircon population supports a provenance from magmatic units related to the assembly of Rodinia. A minor amount of Palaeoproterozoic grains were likely derived from the Indian craton. The potential source rock of the 930–800 Ma detrital zircons may be granitoid present in Greater Himalayan rocks themselves and the Aravalli Range, which has 870–800 Ma granitic rocks. The arc-type basement within the Himalayan–Tibet orogen recorded (900–600 Ma) igneous activity, which may depict a northeasterly extension of juvenile terranes in the Arabian–Nubian Shield. The granitoid of 800 Ma may be a potential source for 790 Ma detrital zircons owing to scatter in 206/238 dates. The 650–500 Ma zircon population suggests their derivation from the East African Orogen and Ross–Delamerian Orogen of Gondwana. The Cambrian–Ordovician magmatism during the Bhimphedian Orogeny and observed late Neoproterozoic to Ordovician detrital zircons have been derived to some extent from Greater Himalayan magmatic sources. We found no detrital zircon grains that cannot be explained as coming from local sources. One sample yielded a discordia lower intercept age of 15.6 ± 2.2 Ma, the age of melt crystallization.

The Tectonic Map and Structural Provinces of the Late Neoproterozoic Egyptian Nubian Shield: Implications for Crustal Growth of the Arabian–Nubian Shield (East African Orogen)

The Late Neoproterozoic Egyptian Nubian Shield (ENS) has attracted increasing attention since the establishment of the Egyptian Geological Survey and Mining Authority (EGSMA (1896)), which conducted the first mapping. In the last three decades, rapid improvements in analytical techniques, along with field-oriented studies, have made it possible for the interpretation of the ENS as an integral part of the juvenile Arabian–Nubian Shield (ANS) as a portion of the East African Orogen (EAO). However, a consistent tectonic map of the ENS does not exist. Presentation of such a map is the main objective of the present work where Landsat-based lithological discrimination is combined with the systematic structural investigation and careful reassessment of previously published geological maps. Our interpretation of this map indicates that in accordance with previous work, the basement units of the Eastern Desert (ED) can be divided into three structural domains: Northern, Central, and South-Eastern Desert provinces. The proposed provinces are ascribed to three different plate tectonic far-field boundary conditions. The earliest magmatic, metamorphic, and tectonic history was set off by the approximate north–south convergence of the Gabgaba-Gebeit, Jiddah-Asir terranes, and Eastern Desert-Midyan terranes along the Yanbo-Onib-Sol-Hamid-Gerf-Allaqi-Heiani (YOSHGAH) suture between c. 800 and 620 Ma. The second event between c. 640 and 580 Ma was correlated with the Nabitah Orogeny when the Ad Dawadimi-Ar Rayn terranes in the eastern Arabian Shield accreted to the earlier consolidated arc terranes (Afif and Tathlith terranes). During this period, east–west convergence between northwestern Saudi Arabia and the Central Eastern Desert (CED) in Egypt caused a pronounced strike-slip deformation associated with the displacement along the wider Najd Fault system. The third orogenic phase, which was younger than c. 580 Ma, was controlled by the evolution and retreat of the Cadomian Arc. The northern portions of the ENS and the northern Arabian Shield experienced extension whereas extensive post-orogenic magmatism was related to mantle delamination and associated crustal thinning.

Evolution of the Arabian-Nubian Shield in Gabal Samra area, Sinai; implications from zircon U–Pb geochronology

The basement complex of Sinai represents the northern end of the Arabian-Nubian Shield that formed as part of the East African Orogen. Our study focuses on the genesis and development of the crystalline rocks in the Gabal Samra area, Sinai, Egypt, using LA-ICP-MS zircon U–Pb dating. We delineate three distinct magmatic pulses including: 1) island arc (ca. 871-804 Ma) magmatic activity represented by the Baba metamorphic belt; 2) syn-tectonic (ca. 753-733 Ma) magmatic activity represented by calc-alkaline and alkaline granitic rocks, and; 3) post-tectonic (ca. 622-535 Ma) magmatic activity represented by dykes, calc-alkaline and alkaline granitic rocks. We found limited evidence for pre-Neoproterozoic basement involvement in the study area except for several xenocrysts with ages of 2123 ± 17, 1808 ± 17, 1740 ± 20, 1256 ± 18, and 972 ± 14 Ma. Younger zircon grains yield ages ranges between 509 ± 11 and 469 ± 8 Ma, which indicate younger magmatic events in the region.

Locating Lhasa terrane in the Rodinia and Gondwana supercontinents: A key piece of the reconstruction puzzle

Abstract The debate over global continental reorganization from Rodinia to Gondwana likely stems from some key Neoproterozoic units being overlooked in previous reconstructions. Here we provide a self-consistent set of evidence, based on magmatism and metamorphism, that the Lhasa terrane is the “lost” part of NW India and retains crucial archives, including 760–730 Ma ophiolitic-arc magmatic rocks and evidence of a distinctive metamorphic event at ca. 660 Ma. Hence, the Lhasa terrane together with Madagascar and Seychelles formed in a late Neoproterozoic juvenile arc system laying either along the periphery of rifting Rodinia or outboard of the supercontinent. Subsequently, these terranes made up the northern East African Orogen (EAO) during the initial assembly of Gondwana. As a result of lateral-escape tectonics after collision and dextral strike-slip along the northern margin of Gondwana, the Lhasa terrane escaped from the EAO and had migrated to the northern margin of Australia by the Middle Cambrian.

Neoproterozoic Tectonic Events of Egypt

AbstractThe Egyptian Nubian Shield (ENS) represents the northwestern part of the Arabian‐Nubian Shield and the northern extension of the East African Orogen. The ENS is regarded as being formed due to northward‐directed escape tectonics. It is characterized by mild accretion and suture zones dominated by major strike‐slip zones with a commonly sinistral sense of movement; some shear zones display a dextral sense of shear. It is dominated by gneisses and migmatites in the south, arc volcaniclastic metasediments and highly dismembered ophiolites in the central parts, whereas its northern part is dominated by late‐ to post‐tectonic granitoids. In southern Sinai, the Neoproterozoic rocks are grouped into four complexes, namely Feiran–Solaf, Sa'al–Zaghra, Kid and Taba. The ENS ophiolites were formed between 730–750 Ma, mainly in a supra‐subduction zone setting. The ENS has undergone a Neoproterozoic deformation history involving three successive phases: (1) Early N–S shortening phase (D1), (2) Syn‐accretionary phase (D2) and (3) Post‐accretionary phase (D3). The initial island‐arc stage (780–730 Ma) is a N–S shortening phase initiated by collision between the Eastern Desert tectonic terrane to the north with both the Gebeit and Gabgaba terranes to the south (830–720 Ma). During the arc‐splitting and back‐arc spreading stage (730–620), voluminous syn‐tectonic granitoids intruded into the ENS (750–610 Ma). The E–W‐directed compressional/transpressional phase (620–450 Ma) led to the overall uplift of the central part of the ENS and consequently the development and exhumation of the core complexes in oblique convergent zones. The E–W intense shortening deformation resulted also in the formation of NW‐ and NE‐striking sinistral and dextral strike‐slip shear zones, respectively. The latest periods of the E–W‐directed compressional/transpressional regime were characterized by deposition of the molasse‐type Hammamat Sediments unconformably over the Dokhan Volcanics, or interbedded with them. The combined thrusting, folding and sinistral‐reverse shearing structures have been interpreted to resulted from the E–W‐directed compressional/transpressional phase in response to the oblique shortening of the Arabian‐Nubian Shield between East and West Gondwana.

Age and hafnium isotope evolution of Sudanese Butana and Chad illuminates the Stenian to Ediacaran evolution of the south and east Sahara

The Saharan Metacraton is a poorly known tract of pre-Neoproterozoic continental crust that occupies the area between the juvenile Arabian Nubian Shield, in the east, and the Tuareg Shield to the west. Neoproterozoic orogenesis (i.e. Oubanguides and East African orogenies) affect the west, the south and the east of the metacraton, respectively, which led to deformation, emplacement of igneous bodies, and localised episodes of rift-related magamatism. Details about interior regions of the Saharan Metacraton are poorly known, with much of it covered by Phanerozoic rocks. The basement outcrops in Sudan, Chad, Algeria and Libya have been the subject of few modern geochronological studies. Here we present results from the first zircon geochronology and in-situ zircon hafnium isotope investigations from both the Sudanese Butana and central Chad. The terranes in Butana, formed to the east of pre-Neoproterozoic continental crust of the Saharan Metacraton, with the oldest juvenile magmatism (ƐHf(t) of +4.89 to +7.89) at ca. 839 Ma, followed by subsequent magmatism at ca. 787 Ma. The ca. 787 Ma event (seen elsewhere in the East African Orogen) is interpreted to represent volcanic-arc collision and accretion with the kernel of the Saharan Metacraton. The ca. 839 Ma magmatism is contemporaneous with the accretion of the Tonian (ca. 850 Ma) arc terranes of the Arabian Nubian Shield and marks subduction to the east of the Saharan Metacraton. The magmatic history of the Ouaddaï region in Chad begins in the late Mesoproterozoic, with localised rifting, resulting in the emplacement of juvenile granites (ca. 1030 Ma). The Cryogenian and Ediacaran in the Saharan Metacraton reveal a complicated history of magmatism and deformation. Age data from Chad show the emplacement of granites (from melting of Mesoproterozoic crust: ƐHf(t) = +2.04 and −4.07) at ca. 665–654 Ma, coeval with the main East African Orogeny and accretion of the other ANS terranes to the Saharan Metacraton at ca. 650–580 Ma. The youngest tectonothermal event within the Saharan Metacraton is recorded by emplacement of granites between 580 and 550 Ma (ƐHf(t) values of −17 to −31) in southern Chad.

Tectonic evolution of an Early Cryogenian late- magmatic basin in central Madagascar

Central and southern Madagascar comprise a number of distinctive Archaean crustal blocks (the Antongil-Masora and Antananarivo domains) overlain by Proterozoic supracrustal sequences, preserved in the East African Orogen. Here, we present U–Pb and Lu–Hf isotopic data for two supracrustal units from detrital and metamorphic zircon grains. The lower sequence is comprised of quartzite and calc-silicate units with a major Palaeoproterozoic detrital zircon source and a minor Archaean contribution with a maximum depositional age of ca. 1780 Ma. This sequence reflects a stable shelf sedimentation within the Antananarivo Domain and is correlated with the Itremo Group. U–Pb and Hf data are equivocal in determining the direct sources for the Archaean and early Palaeoproterozoic detrital zircon grains. However, the abundant ca. 2.3–1.8 Ga detrital grains are correlative with the Congo-Tanzania-Bangweulu Block and as these are close to the inferred age of the Itremo Basin, these are interpreted to be single cycle detritus. This implies that the Congo-Tanzania-Bangweulu craton was close to central Madagascar at ca. 1.8–1.6 Ga and the lower sequence would correspond to an originally contiguous late Palaeoproterozoic to early Mesoproterozoic sedimentary basin across central Madagascar. The upper metasedimentary unit has contrasting detrital sources and is represented mostly by biotite-plagioclase paragneiss, with an inferred psammitic protolith interleaved with volcanic/subvolcanic andesitic/rhyolitic dikes. The predominant Tonian-aged population (ca. 860-710 Ma) are igneous zircon grains with εHf(t) values varying from −15.1 to −29.2 and TDM Hf model ages between ca. 3.4 and 2.6 Ga. These grains were derived from the ca. 850-750 Ma Imorona-Itsindro magmatic suite. Their Neoarchaean-Palaeoproterozoic cores are interpreted as xenocrysts, reinforcing that the Imorona-Itsindro magmatism has a prominent continental reworking component. The probable tectonic setting for this Early Cryogenian sedimentary basin would represent a transition from an intra-arc to an intracontinental setting related to an outboard subduction, partially jammed at ca. 710 Ma due to the subduction of a ridge-transform system. The analogue would be the western US, where the Basin and Range region corresponds to a wide rift associated with a major mantle thermal anomaly. The absence of geological units and structures between ca. 720 and 635 Ma in central Madagascar corroborate with this model for a transition to a transform continental setting. The pre-Gondwana amalgamation convergence in the Ediacaran-Cambrian, that deformed and metamorphosed all units in central Madagascar units, is accounted for by ca. 550 Ma metamorphic rims on zircon grains from the quartzites in the Itremo Group.

Cambrian magmatic flare-up, central Tibet: Magma mixing in proto-Tethyan arc along north Gondwanan margin

AbstractAccompanying Gondwana assembly, widespread but diachronous Ediacaran–early Paleozoic magmatism of uncertain origin occurred along the supercontinent's proto-Tethyan margin. We report new geochemical, isotopic, and geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the Gondwana-derived North Lhasa terrane, located in the present-day central Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros, quartz monzonites, granitoids (with mafic microgranular enclaves), and rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that these rocks were probably generated by mixing of mantle-derived mafic and crust-derived felsic melts. The mantle end-member volumes of mafic, intermediate, and felsic rocks are ∼75%–100%, 50%–60%, and 0–30%, respectively. Integration of our new data with previous studies suggests that the North Lhasa terrane experienced long-term magmatism through the Ediacaran to Ordovician (ca. 572–483 Ma), with a magmatic flare-up at ca. 500 Ma. This magmatism, in combination with other Ediacaran–early Paleozoic magmatism along the proto-Tethyan margin, was related to an Andean-type arc, with the magmatic flare-up event related to detachment of the oceanic slab following collisional accretion of Asian microcontinental fragments to northern Gondwana. Diachroneity of the proto-Tethyan arc system along the northern Gondwanan margin (ca. 581–531 Ma along the Arabian margin and ca. 512–429 Ma along the Indian-Australian margin) may have been linked to orogenesis within Gondwana. The North Lhasa terrane was probably involved in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens, based on its paleogeographic location at the northern end of the East African orogen.