Sahara Metacraton Research Articles

Frontier of the Paleo-Tethys Ocean in the western Mediterranean: Isotopic (Sm-Nd) constraints on sources of Devonian units from Menorca Island

The c. 1000-m-thick pre-orogenic Devonian mainly metapelitic sequence of North Menorca Island shows a fairly complete stratigraphic succession. The rocks of this sequence indicate gradually increasing deeper marine conditions of sedimentation towards its uppermost levels. Furthermore, the obtained sedimentary characteristics resemble those related to a deep and narrow basin-associated deposit. Thin sills of Ti-augite-bearing alkaline gabbros occur within the Devonian sequence. The intensity of Variscan deformation increases downwards through the structure. According to the characteristics of the Devonian sequence and its location within the Variscan Orogen, a correlation with similar units located in the southern sectors of the Central Iberian Zone (Iberian Massif) is suggested. The Devonian metapelitic rocks have geochemical characteristics suggesting that they represent moderately recycled mature siliciclastic sediments, generated from erosion of distant source areas belonging to an upper continental crust. The relatively narrow range of variation observed in initial 143Nd/144Nd ratios supports a similar source for the Menorcan slates (0.51165–0.51182). However, a marked trend is observed in these isotope ratios, from lower values at the base of the stratigraphic column (minimum value of 0.511941) to higher values at the top (maximum value of 0.512131). The 147Sm/144Nd ratios vary between 0.1074 and 0.1238, within the range defined for siliciclastic rocks with felsic crustal provenance. The Nd model ages define a narrow range between 1496 Ma and 1754 Ma (Late Paleoproterozoic–Early Mesoproterozoic), and they are consistently younger up-section. These data rule out a provenance from the erosion of the West Africa Craton, as they are more compatible with a provenance from regions located in the Trans-Saharan Belt or Sahara Metacraton. The characteristics of the Menorcan Devonian sequence are compatible with its deposition in a narrow and deep peri-Gondwanan transtensional basin, generated to the south of an advancing Variscan orogenic wedge. Systematic variations in the Nd isotopic composition indicate the progressive and continuous denudation of increasingly more easterly North African sectors in a collisional context between Laurussia and Gondwana with a marked dextral component. These data must be interpreted in the sense that there was not a large oceanic domain during Devonian times to the south of Iberia, able to block the arrival of detrital material from North Africa. A large tract of the Paleo-Tethys Ocean would therefore not have existed during the Devonian south of Iberia. This ocean must therefore have had limited extent in this period towards the westernmost sectors. The Devonian peri-Gondwanan shelf was apparently continuous around Iberia. This platform was progressively affected by Variscan deformation advancing from north to south and incorporated into the Variscan orogenic wedge with the same vergence.

Variability in the sources of North Gondwana Cadomian basins tracked by Nd isotopic systematics (Iberian Massif)

The Cadomian Orogeny encompassed the opening and closure of oceanic domains and associated sedimentary basins in the northern margin of Gondwana from Ediacaran to Early Cambrian time. In the Iberian Massif, these Cadomian basins are included in autochthonous and allochthonous (far-travelled) terranes of the Variscan Orogen. A compilation of Nd isotope information of Ediacaran-Lower Cambrian siliciclastic series supports previous proposal for variability in the sources along the North Gondwanan margin that continued during the Early Paleozoic. Older TDM model ages ranging from 1361 to 2357 Ma of the Variscan allochthonous Ediacaran-Lower Cambrian siliciclastic series indicate proximity to Archean and Paleoproterozoic old crustal sources of the West African Craton and represent deposition on a western Cadomian basin. In turn, the autochthonous Ediacaran-Lower Cambrian siliciclastic series show relatively younger TDM values ranging from 1179 to 2114 Ma, suggesting derivation from more juvenile magmatic rocks that are well represented further east. The Ediacaran-Lower Cambrian siliciclastic series of the autochthonous terranes formed in an eastern Cadomian basin located near the Sahara Metacraton, the Trans-Saharan Orogen and the Tuareg and Arabian-Nubian shields.

U-Pb and Ar-Ar geochronological constraints on timing of deformation and peak metamorphism in the Central Africa Orogenic Belt, Yaoundé Domain, Cameroon

ABSTRACT The Central Africa Orogenic Belt (CAOB) and the Transaharan and Sergipano belts form a major West Gondwana suturing orogen affected by D1–D4 compressional deformation. Microstructural observations, mineral chemistry, zircon, and titanite laser ablation ICP-MS U-Pb dating, and biotite, muscovite, and K-feldspar 40Ar-39Ar age data are used to constrain the timing of rocks formation and D1–D4 events in the Yaoundé Domain of the CAOB, as well as characterize the Neoproterozoic-Cambrian transition. The Yaoundé Domain rocks were deposited on a pre-Neoproterozoic structure that was overprinted by a c. 1000–700 Ma mega arc/back-arc system and related Tonian to Ediacaran magmatism. Older U-Pb dates (1000 up to 2700 Ma) are inheritance from a pre-existing crust. The D1–D4 compressional events relate to the inversion of the c. 1000–700 Ma mega-rift system. New dates at c. 700 Ma, c. 630 Ma, c. 580–555 Ma constrain the timing of D1, D2, D3+D4, respectively. The peak of accompanying metamorphism occurred at 614 ± 4 Ma during D2. This was followed by a c. 115 Ma period of deformation-driven crustal exhumation and cooling that extended into the Cambrian. New data document short episodes of rapid denudation and cooling (up to 0.9 km/Ma and 40°C/Ma at c. 578 Ma in gneiss and up to 5 km/Ma and >125°C/Ma at c. 555 Ma in micaschist). It is proposed that the CAOB and correlative belts are parts of the orogenic hinterland formed from accretion and collision between the West African Craton and the Sahara Metacraton precursor. The correlated belts appear to represent the result of a protracted tectono-metamorphic and magmatic process that started in late Mesoproterozoic (c. 1167 Ma) and continued through the Neoproterozoic to the Cambrian (c. 500 Ma).

The role of the proto-Alpine Cenerian Orogen in the Avalonian- Cadomian belt

Abstract The proto-Alpine Cenerian orogen (Ediacaran-Ordovician) and the Cadomian orogen (Ediacaran-Cambrian), remnants of which are exposed in the central European Variscides, should be defined as two distinct and spatially separated coastal orogens within the Avalonian-Cadomian belt. The Cadomian orogen originally lay in front of the Sahara metacraton. It underwent a change from an active to a passive margin setting during the Cambrian. The Cenerian orogen, represented by intra-Alpine rocks, was located farther east near the Arabian Nubian Shield, from where it inherited a characteristic Tonian/Stenian detrital zircon signal. Subduction persisted in the Cenerian Orogen until the Ordovician. The Cadomian orogen was akin to Andean type whereas the Cenerian orogen was more akin to Alaskan type. This paper explores why the two orogens have such different characteristics and tectonic evolutions despite their probable proximity in the Avalonian-Cadomian belt. One explanation could be that they were at nearly right-angles to each other due to a strong concave bending of the northern Gondwana margin ahead of the Arabian-Nubian Shield.

A review of U-Pb detrital zircon systematics from Cambrian- Lower Devonian rocks of the Moroccan Meseta

U-Pb geochronological analysis of detrital zircon grains is a powerful tool to decipher the pre-Mesozoic paleogeography. In this work, we present a sediment provenance study based on a compilation of 41 Cambrian-Early Devonian siliciclastic samples from the Moroccan Meseta. All of them are characterized by late Tonian-Ediacaran (c. 600 Ma, c. 50% of the data), Rhyacian-Orosirian (c. 2100 Ma, c. 20% of the data), and Neoarchean (c. 2500 Ma, c. 4% of the data)detrital zircon age populations indicating a strong West African Craton affinity. Minor populations are: (i) Cambrian-Ordovician (c. 500 Ma), particularly relevant in Lower Ordovician samples, which were probably sourced by local volcanic centers formed during an aborted rift; (ii) Stenian-early Tonian (c. 1000 Ma); and (iii) Orosirian-Statherian (c. 1800 Ma). These two latter populations were probably fed by paleocurrents from NE-African regions (e.g., Sahara Metacraton and Arabian-Nubian Shield) that reached only intermittently the studied area. Finally, the spatial and chronological distribution of the detrital zircon age populations suggests that the tectonic boundaries normally used to separate internal subdomains in northern Morocco did not imply far-travelled terranes during early-middle Paleozoic times.

Cryogenian crustal evolution in western Nigeria shield: whole-rock geochemistry, Sr-Nd and zircon U-Pb-Hf isotopic evidence from Bakoshi-Gadanya granites

ABSTRACT The Bakoshi-Gadanya area, with widespread Neoproterozoic granites, is located within the Pan-African Trans-Sahara Belt, sandwiched between the West African and the Sahara metacraton. The whole-rock geochemistry, Sr-Nd isotope systematics and LA-(MC)-ICP-MS U-Pb-Hf isotopes of zircon were analysed to shed light on the age and genesis of the Bakoshi-Gadanya granites in the Precambrian basement of the northern western Nigeria shield. The granites are silicic (SiO2: 72.8–77.2 wt.%), ferroan, high-K, calc-alkalic to alkali-calcic in composition, with A-type granite affinities. The ferroan calc-alkalic types range in age from 706.6 ± 1 Ma to 638.4 ± 1 Ma, and the alkali-calcic type is dated at 710 ± 1 Ma. Whole-rock Sr-Nd and zircon Hf isotope systematics have constrained the ferroan calc-alkalic granites to an enriched mantle lithospheric magma source, contaminated by Archaean-Palaeoproterozoic crustal components. On the other hand, the magma that crystallized the alkali-calcic Gadanya granite originated from an enriched mantle source, triggered by reactivation of the lithosphere-scale Kalangai fault during the post 750 Ma major collision between West African Craton and the Saharan metacraton. Both alkali-calcic and calc-alkalic ferroan granites were formed contemporaneously with the last major Pan-African syn-metamorphic deformation in northern western Nigerian shield; thus, they are syn-kinematic granites. Neoproterozoic magmatism at Bakoshi-Gadanya area is related to easterly dipping subduction at the margin of microcontinental blocks in northern western Nigeria shield. Zircon U-Pb-Hf data envisaged both early Neoproterozoic and Cryogenian juvenile magmatic additions in this part of western Nigeria shield.

Transcurrent displacement of the Cadomian magmatic arc

The Ossa-Morena Zone in Iberia, and equivalent terranes in northern France and Central Europe, are thought to be paleogeographically linked to the West African Craton at Ediacaran to early Paleozoic times. Evidence is mainly based on metasedimentary rock detrital zircon age spectra, characterized by two dominant populations of Paleoproterozoic and Cryogenian-Ediacaran ages, with a systematic lack of late Stenian – early Tonian zircon grains. We report here U-Pb-Hf results on detrital zircon grains from six Ordovician-Devonian metasedimentary rocks from the Ossa-Morena Zone. In addition to the Cryogenian-Ediacaran and Paleoproterozoic populations already recognized in previous studies on uppermost Ediacaran and lower Cambrian rocks, the samples ubiquitously show a late Stenian – early Tonian population centered at ≈1 Ga and representing ≈20% of the concordant dates. The εHf versus age plot of the studied samples mainly depicts two vertical arrays corresponding to the Cryogenian-Ediacaran and Stenian-Tonian detrital zircon populations, which spread from εHf values of ≈10 down to ≈−20. The detrital zircon age distribution and Hf isotope signature of the studied rocks point to the Sahara metacraton as the most plausible sediment source. The eastward translation of the continental ribbon represented by the Ossa-Morena Zone and equivalent domains of the Variscides from an original position close to the West African Craton to an Ordovician-Devonian location close to the Sahara metacraton probably occurred at latest Ediacaran – earliest Cambrian times in a dextral strike-slip tectonic setting that post-dated Pan-African collision and Cadomian subduction.

Tectonic setting and isotopic sources (Sm–Nd) of the SW Iberian Autochthon (Variscan Orogen)

Metasedimentary rock successions in Sierra Albarrana Group (SW Iberia Autochthonous Domain) were deposited during the Early Paleozoic influenced by the evolution of a peri-Gondwanan active margin. Their geochemical composition indicates dominant contributions from felsic igneous sources and an upper continental crust provenance. The high mineralogical and geochemical maturity, together with negative εNd(530), ranging from − 11.3 to − 4.5, and old Nd model ages (TDM: 1388–1897 Ma) imply reworked materials from old continental source areas with a limited juvenile contribution for the siliciclastic sedimentary rocks in Sierra Albarrana. Both, geochemical and isotope features were caused by the progressive denudation of rocks bearing the isotopic imprint from an old basement exposed along the Gondwanan margin. The paleobasin for the SW Iberia Autochthon probably occupied an outboard position across the peri-Gondwanan margin during Ediacaran–Cambrian times. In a convergent scenario, the interaction between the peri-Gondwanan trench and the external part of the continent generated tectonic uplift, giving way to exposure of crust formed during earlier stages of the Cadomian Orogeny and erosion. Sediment redistribution through Early Paleozoic times supplied recycled detrital materials, which likely filled a retro-arc basin formed after the switch from an extensional to a compressional regime in the upper plate of the Cadomian Orogen that is recorded throughout the peri-Gondwanan domain of Iberia. A subsequent extensional stage led to a progressive widening of its marginal basins, thus bringing the onset of a passive margin from the Cambro–Ordovician onwards. Nd model ages of the Sierra Albarrana Group overlap those of Early Cambrian series from the southernmost Central Iberian Zone, and are considered an indication for the paleogeographic closeness between all these sequences during Cambrian times, occupying eastern positions closer to Tuareg Shield and the Sahara Metacraton sections along the Gondwanan margin.

Provenance of Ediacaran-Ordovician sediments of the Medio Armorican Domain, Brittany, West France: Constraints from U/Pb detrital zircon and Sm–Nd isotope data

The temporal evolution of the sedimentary source areas of the Armorican Massif, involving Ediacaran to Upper Ordovician strata, is investigated to gain insight into the palaeogeographic affinities and changes that occurred as a result of Cadomian orogenesis. Until now, palaeogeographic reconstructions based on geodynamic, stratigraphic and paleontological data have shown geological continuity between the Armorican Massif and the Iberian and Bohemian massifs and have allowed researchers to locate the Armorican Massif near the West African Craton and the Trans-Saharan Belt. This study goes beyond the interpretations based on lithostratigraphic correlation, which may be influenced by allocyclic factors (e.g., sea-level change) or fauna assemblages that have a wide provincial distribution, to provide a correct assessment of sediment flux. To determine the palaeogeographic location more accurately, the provenance of the siliciclastic sediments was examined in this study using U–Pb LA-MC-ICP–MS geochronology on detrital zircons coupled with whole-rock Sm–Nd and zircon Lu–Hf isotope analysis. This work was carried out on the sedimentary succession of the Medio Armorican Domain. The oldest studied sedimentary rocks were shown to belong to the Brioverian succession, which contains mainly 519–781 Ma old zircons, likely derived from sources that are still present in the Armorican basement. Successively, the lower Paleozoic succession was deposited in the rift stages of the Rheic Ocean, with contributions from a new source of 827–1120 Ma old zircons.A comparison of the zircon populations showed an increase in negative εNd(t) and εHf(t) values of the sedimentary supply in the post-Cadomian samples. Moreover, it revealed that the Medio and North Armorican domains had different locations during the Lower Ordovician, and that some areas of the Iberian Massif and the Medio Armorican Domain close to the Sahara Metacraton and Arabian-Nubian Shield were contiguous.

Preliminary detrital zircon signatures from the southern Asir terrane, Saudi Arabia: A link to Yemen or the Nubian Shield?

The Arabian Shield comprises early Neoproterozoic to Cambrian (∼850–530 Ma) tectonostratigraphic terranes formed by the closure and accretion of juvenile volcanic arcs and back-arc basins associated with Gondwana assembly. Unlike the Nubian Shield which preserves crustal isotopic signatures, the Arabian Shield is distinctly juvenile with the exception of the Paleoproterozoic (∼1800–1670 Ma) Khida subterrane in Saudi Arabia and the terranes of Yemen. This study presents the first combined zircon U-Pb, O and Hf isotope data of metavolcanic and metasedimentary rocks from southwestern most Saudi Arabia, near the Yemen border – a region thought to contain some of the oldest (>815 Ma) lithologies in Saudi Arabia, including the Atura Formation and the Tayyah Belt. One volcaniclastic metasediment sample from the Atura Formation yields zircon U-Pb age peaks of 741, 672, 646 Ma (n = 131), δ18O(V-SMOW) ranging from 4.6 to 8.3‰ and ɛHf (t) from +7.7 to +12.5. Two samples from the Tayyah Belt include an older metasandstone and a late intruding granitic dyke which provides a minimum age for the Tayyah Belt. The former yields two significant U-Pb peaks of 812 (n = 8) and 999 (n = 6) Ma, δ18O and ɛHf (t) values ranging from 4.4 to 9.6‰ and −10.1 to +12.4, respectively; the later yields a concordia age of 645.8 [±1.7] Ma (n = 29), δ18O ranging from 5.7 to 6.6‰, and ɛHf (t) of +5.9 to +9.6. The zircon age and juvenile Hf signatures from the Atura Formation are consistent with the synorogenic phase in the Shield. Sedimentation was likely associated with arc volcanism during the previously documented eastward phase of accretion at ∼740–640 Ma and the closure of the Mozambique Ocean. In contrast, the data from the texturally more mature Tayyah Belt metasediment indicate a more distal, and more evolved crustal input at the time of sediment deposition which is unusual for the Saudi Arabian Shield. Consequently, the Tayyah metasediments are likely sourced from areas with greater continental affinity, such as the cratonic basement and/or reworked crust of the Sahara metacraton in NE Africa.

Neoproterozoic-paleozoic evolution of the Drina formation (Drina-Ivanjica entity)

This paper addresses a Drina-Ivanjica basement member, Drina Formation, characterized by ? controversial Neoproterozoic to Carboniferous age. The Drina Formation is also informally referred to as the ?Lower Drina Formation? and the ?Upper Drina Formation? including the Golija Formation as a conditional analog unit of the latter. A review of the biostratigraphic, sedimentary and paleogeographic constraints identified Drina Formation (Inner Dinarides) as a migrated crustal segment derived from a marginal section of northern Gondwana, being, however, of Neoproterozoic-Early Paleozoic age. The presence of arenites, pelites, conglomerates, scarce limestones, basic (sub)volcanics and tuffs of the volcano-sedimentary Drina Formation metamorphosed up to greenschist and locally up to amphibolite facies, coupled with the absence of felsic volcanism implies a passive margin setting. Considering the age, such environment was probably associated with the perplexed Lower Paleozoic Avalonian-Cadomian arc, situated along the former north Gondwanan active margin. More precisely, the Drina Formation originated from a depositional junction between the Gondwana sediment supplier (Sahara metacraton) and Cadomian arc. A comparison with the regional Early Paleozoic succession of the ?Kucaj Unit? (eastern Serbia) yields the absence of typical anchimetamorphic Silurian to Lower Devonian deep-marine fossil-bearing succession. The volcano-sedimentary passive margin system of Drina Formation is overlain by a late Variscan convergencerelated voluminous clastic sequence allocated as the Golija Formation.

The crustal structure of Egypt and the northern Red Sea region

P-wave receiver functions from 26 stations in the Egyptian National Seismic Network (ENSN) have been modeled using the H-k stacking method and in a joint inversion method with Rayleigh wave group velocities to investigate crustal structure across Egypt and the northern Red Sea region. The new estimates of crustal structure, when combined with previous results, show that along the rifted margins of the Red Sea, Gulf of Suez and Gulf of Aqaba crustal thickness ranges from 25 to 30km, the average crustal Vp/Vs ratio is 1.77, and the average crustal shear-wave velocity is 3.6km/s. Beneath northern and central Egypt, including the Sinai Peninsula, crustal thickness ranges from 32 to 38km, the average crustal Vp/Vs ratio is 1.79, and the average crustal shear-wave velocity is 3.5km/s. Beneath southern Egypt, crustal thickness ranges from 35 to 40km, the average crustal Vp/Vs ratio is 1.76, and the average crustal shear-wave velocity is 3.7km/s. In southern Egypt, the crust is also characterized by a 10–20km thick mafic lower crust. These findings indicate that crust along the rifted margins of the northern Red Sea, and Gulfs of Suez and Aqaba have been thinned by about 5 to 10km. The thick mafic lower crust in southern Egypt can be attributed to suturing during the Neoproterozoic collision of east Gondwana against the Sahara metacraton. Overall, the structure of the crust in Egypt away from the northern Red Sea region is similar to the structure of Precambrian crust in many other parts of Africa.

A hidden Tonian basement in the eastern Mediterranean: Age constraints from U–Pb data of magmatic and detrital zircons of the External Hellenides (Crete and Peloponnesus)

A newly discovered Precambrian basement of the External Hellenides (Peloponnesus) is composed of mica schist, quartzite and meta-greywackes intruded by Cambrian granitoids (524+6/−5Ma). As such Cambrian intrusions are frequent in the eastern Mediterranean, but are lacking in northern Africa, they are interpreted to result from a Cadomian magmatic arc that was situated along the northern margin of East Gondwana. This arc was rifted off from Gondwana in Early Paleozoic times. The sedimentation age of the meta-greywackes is bracketed between the age of their youngest detrital zircons (541±7Ma, Crete; 546±7Ma, Peloponnesus) and the intrusion of the Cambrian granitoids. The detrital zircon age spectra are dominated by two significant age peaks at ca. 600Ma and ca. 1Ga, which are characteristic for Cambrian platform sediments of Jordan, Israel and Lybia and for the Minoan-type terranes with an East Gondwana provenance. The high amount of 1Ga old zircons points to a Tonian basement as provenance, which is situated at the northern margin of the Arabian Nubian Shield (N'Sinai) and the Sahara Metacraton, today hidden under Mesozoic cover. There is a good correlation between the detrital age spectra of the Late Ediacaran/Cambrian basement of the External Hellenides and of the eastern Mediterranean realm. Apart from the Hellenides and Turkey (Menderes, Sakarya), the Minoan-type terranes have also been found in Sicily, Sardinia, and NW Iberia. Our review of detrital zircon ages obtained from Europe and northern Africa revealed four different domains during Early Cambrian times, which have affinities to (1) Baltica, (2) South-America (Avalonian-type terranes), (3) W Africa (Armorican-type terranes), and (4) E Gondwana (Minoan-type terranes and the platform sandstones of E Africa).

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

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