Reguibat Shield Research Articles

Sedimentology and stratigraphy of lower Cretaceous fluvial to shallow marine deposits on the central Atlantic passive margin: The Aaiun-Tarfaya Basin, Morocco

This paper presents the first integrated regional outcrop-based sedimentological study of the northern Aaiun-Tarfaya Basin located in Morocco (NW Africa). The Lower Cretaceous Tan-Tan Formation has been subdivided into six new members and placed within a sequence stratigraphic framework that includes two incomplete depositional sequences. Strong thickness variations of individual lithostratigraphic units from north to south suggest differential subsidence during sedimentation and/or the existence of major topography on the basal unconformity that the succession onlaps. The results provide valuable insights into the timing of local tectonics in the Western Anti-Atlas and the control on the evolution of the sedimentary system. Deposition of each of these six units is interpreted to be the result of a complex interplay between an overall eustatic sea-level rise during the early Cretaceous, sediment delivery controlled by tectonic movements in the Western Anti-Atlas and Reguibat Shield and periods of differential subsidence in the basin. The results document the style of evolution of a back-stepping wave-dominated system feeding into the Central Atlantic during the passive margin phase. The improved facies and depositional models together with improved understanding of the evolution of the delta have significant implication for exploring the deep-water equivalents offshore.

Colliding cratons: linking the Variscan Orogeny in West Africa and North America

Abstract The Variscan Orogen was formed during the closure of the Rheic Ocean and the final collision between the North American and West African cratons in the Late Paleozoic. This collision led to the multistage building of the Mauritanide Belt to the east of the Variscan suture and to the building of the well-known Appalachian Belt to the west. Both led to opposite vergences in this part of the Variscan belt. The earliest records of the main collision episode begin at ∼360 Ma and end about 250 myr ago, while a late extensional phase lasted until ∼190 Ma. Three distinct stages are recognized in West Africa. The first stage ( c. 350–300 Ma) records the indentation of the Reguibat Shield into the central Appalachian margin of Laurentia. This indentation led to thrusting of the Souttoufide and Akjoujt ‘nappes’ onto the Reguibat Shield, to southward motion of the Senegalese block (SB), and to strike-slip motion in the Appalachians. The motion of the SB to the south is coeval with: (1) folding of the northern part of the Bové Basin, (2) north–south sinistral strike-slip motions in the central Mauritanides, and (3) the end of sedimentation in the Bové and Taoudeni Basins by the Late Devonian. The second stage ( c. 300–250 Ma) involves the eastward motion of the Western Thrust Block (WTB) against the SB and, likely, some of the westward thrusts in the Appalachians. This second ‘Variscan’ event includes: (1) closure of parts of the lower Diourbel Carboniferous basin, which is now concealed beneath the Senegalo-Mauritanian Basin, (2) thrusting to the east of the Simenti Group over the Koulountou Group in the Bassaride Belt, (3) thrusting to the east of the Wa-Wa Group, (4) thrusting of the Mauritanide Belt onto the Taoudeni Basin in the central Mauritanide Belt, and finally (5) thrusting of the Agualilet Group over the Akjoujt nappes and eastward motion of the western units over the Dhloat Ensour (Late Ordovician to early Devonian) autochthonous unit in the Souttoufides. West of the supposed ‘Variscan’ suture, Appalachian thrusting affected parts of Appalachian Belt. The third stage ( c. 250 to 190 Ma) began with the opening of Triassic rift basins in the Senegalo-Mauritanian basin and also in the north of Florida. As numerous previous correlations across the Variscan system do not include the West African part, our sythesis is intended to enhance these correlations.

A new triradial species from the Neoproterozoic formations of the Chenachene region, north-east of the Taoudeni basin (south-western Algeria)

Precambrian life remains enigmatic, the traces of life of this period discovered throughout the world, have been interpreted differently; the observations made in the Chenachene region will deepen the knowledge and enrich the debate on these fascinating organisms.The Chenachene area represents the north-eastern margin of the Taoudeni Basin, where the Reguibat Shield outcrops with Pleoproterozoic metamorphic, metasedimentary and granitic series, constituting the Eburnian basement of the Eglab, surmounted in major discontinuity by a Meso-Neoproterozoic and Paleozoic sedimentary cover little or no deformed. A trace fossil was collected in a coredrill from the Chenachene region, in the Cheikhia-Bir Amrane Group, with an absolute age obtained by Rb/Sr ranging from 775 to 650 Ma corresponding to the Cryogenian.The diagnosis of the collected specimen allows us to recognize similarities with the genus Rugoconites described by Glaessner and Wade in 1966 from Flinders Ranges, South Australia, with notable differences from the two species already described (R. enigmaticus and R. tenuirugosus), which allows us to present it as a new species: Rugoconites reguibatensis.The exceptional state of preservation of Rugoconites reguibatensis has permitted to characterize it and reconstitute it, presenting great morphological similarities with present-day jellyfish, this revive, once again, the debate on the Precambrian life and the disappearance of this fauna at the end of the Proterozoic.

Tracing the sedimentary provenance of the Mesoproterozoic rocks from Taoudeni Basin (∼ 1.1 Ga) Mauritania: Evidence from Sm/Nd and elemental geochemistry

The Mesoproterozoic era is a crucial period in Earth history characterized by muted oxygen levels in the atmosphere–ocean system. This period is well represented in the non-metamorphosed sedimentary rocks from the Taoudeni Basin, Mauritania. Few studies have been carried out to trace the sources of these sedimentary archives. Here, we examined the sedimentary rocks from the Atar (S4 drill core) and El Mreiti groups (S2 and S3 drill cores). This study focuses on whole-rock elemental geochemistry and Sm/Nd isotope compositions to provide insights into the provenance of the fine-grained siliciclastic sediments from the basal part of the Mesoproterozoic Atar and El Mreiti groups. Paleoweathering indices reveal a moderate degree of chemical weathering in the source areas, consistent with the low compositional maturity of the studied samples. The concentrations of the relatively immobile trace elements suggest mixing sources of sediment provenance, which is dominated by felsic source rocks for the Atar sediments. However, distinctively positive ƐNd(t) values coupled with high 143Nd/144Nd and Sm/Nd ratios and high ƒSm/Nd values of the sediments from the El Mreiti Group indicate derivation from juvenile and less evolved sources rocks. Further, the depleted mantle model ages (TDM) of the provenance protolith (1.95–2.31 Ga) support the argument that the El Mreiti sediments are dominantly sourced from the Paleoproterozoic terrane exposed in the eastern Reguibat Shield while the Atar Group were predominantly from the Archean basement rocks in the western Reguibat Shield. Moreover, Sm/Nd isotope and mixing model results support the major contributions of the felsic Archean rocks of the Reguibat Shield for the Atar Group samples. Although these strata, separated by ∼1000 km, have been described as stratigraphic equivalent, our study demonstrates distinct sediment sources between the Atar Group strata and those of El Mreiti, therefore, reinforcing the importance of sediment provenance in understanding the evolution and reconstruction of modern and ancient sedimentary systems.

Meso-Archean to Mid-Paleozoic Granitoids in Oulad Dlim Massif (the Pericratonic Terrane of the Reguibat Shield, West African Craton, Morocco): Petrology, Geochemistry, Geochronology and Geological Implications

Meso-Archean to Mid-Paleozoic Granitoids in Oulad Dlim Massif (the Pericratonic Terrane of the Reguibat Shield, West African Craton, Morocco): Petrology, Geochemistry, Geochronology and Geological Implications

ОСОБЛИВОСТІ ЛОКАЛІЗАЦІЇ УРАНОВОГО ЗРУДЕНІННЯ В МЕЖАХ ТЕРЕЙНУ ТАЗІАСТ-ТІДЖІРІТ, РЕГІБАТСЬКИЙ ЩИТ

The Reguibat Shield is an area of intensive uranium exploration and about 80 ore occurrences have been discovered in recent decades within its borders. The region of the study is located within the Tasiast-Tijirit terrane, which represents a northwestern part of Archean domain of the Reguibat Shield. The rocks of the terrane are of Mesoarchean age. The main types of rocks are migmatites, gneisses and amphibolites, greenstone associations, granitic intrusions and pegmatites. Samples of pegmatites, granites, ferruginous quartzites and sedimentary rocks that showed an increased radiation level after conducting a ground-based radiometric survey were selected for laboratory studies from the host rocks of the Tasiast-Tijirit terrane. The following types of laboratory analyses were used for the study: 1) spectral analysis by laser-luminescence method to determine the uranium content in samples; 2) atomic emission spectroscopy for the general estimation of concentrations of chemical elements in samples; 3) X-ray fluorescence analysis to determine the chemical composition of samples and its trace elements; 4) electron microprobe analysis to determine the morphology of mineralization and the chemical composition of uranium minerals. Among the Tasiast-Tijirit terrane rocks the increased uranium content was found in the following rock associations: pegmatites and granites; ferruginous quartzites within the banded iron formation; calcretes in sedimentary rocks. According to the results of field and laboratory studies, the main factors of the localization of uranium mineralization in calcretes of the OumDherua area were established. The main geological, physical and chemical conditions essential for the formation of uranium mineralization of the calcrete type within the study region were established. It was found that granites and pegmatites could provide source of uranium for ore mineralization in sedimentary formations, such as surficial and calcrete types. The latter are studied and mined quite intensively in the world including the Reguibat Shield. The study results show the high potential for prospecting and exploration of new uranium occurrences within the Tasiast-Tijirit terrane.

Low‐temperature thermochronology as a control on vertical movements for semi‐quantitative source‐to‐sink analysis: A case study for the Permian to Neogene of Morocco and surroundings

AbstractContinental passive margins and their hinterlands in the Atlantic realm have been the locus of many Low Temperature Thermochronology (LTT) and time‐Temperature (t‐T) modelling studies that evidence pre‐, syn‐ and post‐rift episodic km‐scale exhumation and burial episodes. In this study, we integrate data from over 30 published LTT and t‐T modelling studies from Morocco and its surroundings using a three‐step workflow to obtain: (a) exhumation/burial rates, (b) erosion rates and (c) palaeoreconstructions of source‐to‐sink domains, between the Permian and the Present. Our synthesis of available t‐T modelling results predicts high exhumation rates in the Anti‐Atlas (0.1 km/Myr) during the Early to Middle Jurassic, and in the High Atlas (0.1 km/Myr) and Rif (up to 0.5 km/Myr) during the Neogene. These rates are comparable to values typical of rift flank, domal or structural uplifts settings. During the other investigated periods, exhumation rates in the Meseta, High‐Atlas, Anti‐Atlas and Reguibat shield are around 0.04 ± 0.02 km/Myr. Interpolation of the exhumation rates at the regional scale allow calculation of the volume of rocks eroded. Estimates of erosion rates are between 0.2 x 103 and 7.5 x 103 km3 (in the Meseta and the Reguibat Shield respectively). Ten erosional (quantitative, from interpolation results) and depositional (qualitative, from data synthesis) “source‐and‐sink” maps have been constructed, with emphasis on the Jurassic and Cretaceous periods. The maps integrate the extent of exhumed domains, using information from geological maps, lithofacies and biostratigraphic data from new geological fieldwork and well data from onshore and offshore basins. The results illustrate changes in the source‐to‐sink systems and allow for a better understanding of the Central Atlantic margin hinterlands evolution.

Aeromagnetic modelling of Precambrian subsurface structures of the Tasiast area, NW Mauritania

Abstract The NW sector of Mauritania represents the Archaean province of the Reguibat Shield. It is mainly composed of the typical Archaean cratonic association of granitoid gneisses, greenstone belts and granites. The greenstone belts contain high mineral potential, mainly gold. This is particularly the case of the Chami Greenstone Belt, which hosts the large Tasiast gold mine. The gold deposits are hosted along a NS oriented shear zone with a surface of 70 × 15 km 2 . In this study, a subsurface structural model is proposed based on the application of suitable computations on a high-resolution reduced-to-the-pole aeromagnetic anomaly map of the northern Chami area. Upward continuations, apparent magnetic susceptibility, directional derivatives, analytical signal, 3D Euler deconvolution and spectral analyses were performed to build this model. They were used to decipher the main evidenced structural features with the main NNE–SSW, NW–SE and ∼120° N directions, identified on most all computed maps combined with in situ measurements. They probably correspond to mafic dykes, faults and shears rooted up to 5 km in depth, particularly to the north, SW and NE, and southwards and westwards by spectral analyses and 3D Euler deconvolution, respectively. The basement is deeper towards the west and south and crops out towards the NE in agreement with the geological observations. In addition, the high values of magnetic susceptibility ( χ m ) measured in situ on banded iron formations (25.7–35 × 10 −3 SI) SW of Akjoujt could be the origin of metallotect sources associated with magnetic minerals for exploration of gold-bearing and base metals.

Spatial and temporal distribution patterns of Precambrian mafic dyke swarms in northern Mauritania (West African craton): analysis and results from remote-sensing interpretation, geographical information systems (GIS), Google Earth ™ images, and regional geology

We used remote sensing, geographical information systems, Google Earth™ images, and regional geology in order to (i) improve the mapping of linear structures and understand the chronology of different mafic dyke swarms in the Ahmeyim area that belongs to the Archean Tasiast-Tijirit Terrane of the Reguibat Shield, West African craton, NW Mauritania. The spatial and temporal distributions with the trends of the dyke swarms provide important information about geodynamics. The analysis of the mafic dyke swarms map and statistical data allow us to distinguish four mafic dyke swarm sets: a major swarm trending NE-SW to NNE-SSW (80%) and three minor swarms trending EW to ENE-WSW (9.33%), NW-SE to WNW-ESE (9.06%), and NS (1.3%). The major swarms extend over 35 km while the minor swarms do not exceed 13 km. The Google Earth™ images reveal relative ages through crossover relationships. The major NE-SW to NNE-SSW and the minor NS swarms are the oldest generations emplaced in the Ahemyim area. The NW-SE-oriented swarm dykes which are cutting the two former swarms are emplaced later. The minor E-W to WSW-ENE swarms are probably the youngest. A precise U-Pb baddeleyite age of 2733 ± 2 Ma has been obtained for the NNE-SSW Ahmeyim Great Dyke. This dyke is approximately 1500 m wide in some zone and extends for more than 150 km. The distinct mafic dyke swarms being identified in this study can potentially be linked with coeval magmatic events on other cratons around the globe to identify reconstructed LIPs and constrain continental reconstructions. (Less)

Mapping mafic dyke swarms, structural features, and hydrothermal alteration zones in Atar, Ahmeyim and Chami areas (Reguibat Shield, Northern Mauritania) using high-resolution aeromagnetic and gamma-ray spectrometry data

Analysis of an airborne geophysical data covering the Tasiast-Tijirit Terrane in the western part of the Reguibat Shield (including the 1:200,000 geological sheets of Chami, Ahmeyim and Atar), provided an improved mapping of mafic dyke swarms, structural features, and hydrothermal alteration zones. It also extended the mapping into extensive areas covered by sand. A low-altitude (100 m) airborne survey collected high-resolution magnetic and gamma-ray spectrometry data. The magnetic data were enhanced using techniques such as reduction to the pole, analytic signal and first vertical derivative, and revealed dyke swarms with trends of NNE-SSW to NE-SW, NW-SE to WNW-ESE and E-W. The use of the Euler deconvolution method provided constraints on the continuity and the depth of magnetic sources.Gamma-ray spectrometry which maps the three main radioactive elements, i.e. potassium, uranium and thorium helped discriminate lithological units of the Archean basement, notably felsic intrusions. The radiometric data also helped delineate potassic alteration zones, which could testify to hydrothermal activities of relevance to sulfide mineralisation.

The Cretaceous marine onlap on Palaeozoic deposits (Smara–Lâayoune Basin, South Morocco). Comparison with neighbouring regions

The Cretaceous marine transgression proceeded through successive steps from the Albian to the Turonian (dated with ammonites). The onlapping wedge begins with coastal transgressive–regressive short-term sequences on massive, probably fluvial sandstones to be correlated with the very thick continental Lower Cretaceous succession found in the Puerto Cansado well in the Tarfaya sub-basin to the north. A second step, of probable Cenomanian age, reached the Palaeozoic basement. A third, more pronounced step occurred during the earliest Turonian with platy laminated limestone overlain by marlstone bearing pyritized ammonites. At early Turonian peak transgression, a marine connection was possibly established between the Atlantic and the Tethyan margins, between the Anti-Atlas and the Reguibat Shield. From large-scale correlation integrating what occurred along the southwestern shoulder of the Atlas rift, the South Moroccan Atlantic margin may have undergone a short-lived tectonic uplift around the Cenomanian–Turonian boundary.

Reconstructing Extensional Basin Architecture and Provenance in the Marrakech High Atlas of Morocco: Implications for Rift Basins and Inversion Tectonics

AbstractThe Moroccan Atlas developed through a multiphase tectonic history including Permian‐Jurassic continental rifting and Cretaceous‐Cenozoic intracontinental shortening. New detrital zircon (DZ) U–Pb geochronologic results from the Ourika‐Zat region in the Marrakech High Atlas constrain the sediment provenance of prerift sediment sources and synrift and postrift basin fill. Prerift Precambrian‐Cambrian units contain Neoproterozoic DZ from the Pan‐African orogeny. Ordovician rocks contain diagnostic 0.95‐ to 1.28‐Ga DZ that are absent in the West African Craton, suggesting distal sediment sources. Permian rift basin fill contains Permian zircons that constrain maximum deposition age and diagnostic of northern Meseta domain sediment sources. New mapping and cross‐section construction demonstrate that Permian basin fill accumulated in a half‐graben above a southward dipping normal fault that likely guided subsequent inversion. Triassic rift basin fill thickens southward, above a likely northward dipping normal fault. The change in rift basin geometry was accompanied by catchment reorganization, the introduction of southern rift flank sediment sources, and subbasin integration. The southern catchment expanded to encompass Meso‐Archean rocks of the Reguibat Shield situated >800 km southward. Existing and new provenance results from postrift strata document onset of recycling of Atlas rift basin fill during the mid‐Cretaceous and before the onset of compressional inversion. These results highlight the interaction between deformation and sediment routing in an extensional basin and sediment dispersal patterns across continental margins during multiphase deformation. Stratigraphic, structural, and provenance characteristics reveal that the Atlas rift shares similarities with high‐angle and supradetachment basin models, emphasizing the diverse nature of extensional styles.

High-P amphibolite-facies metamorphism in the Adrar–Souttouf Metamafic Complex, Oulad Dlim Massif (West African Craton margin, Morocco)

The Oulad Dlim Massif represents the northern segment of the Mauritanide belt that thrusts over the western margin of the Reguibat Shield, north of the West African Craton (WAC). This belt includes various metamorphic units of Archean, Neoproterozoic and Palaeozoic ages that were stacked and thrust eastward during the Variscan orogeny. The core of the Oulad Dlim Massif comprises the Adrar–Souttouf Metamafic Complex that represents a large tectonic unit made of high-grade mafic rocks and vast exposures of amphibolites. A characterisation of the metamorphism in these amphibolites is essential to understand the relationships of the Oulad Dlim Massif with the southern segments of the Mauritanide belt and to provide constraints on the geodynamic evolution of the western margin of the WAC. Here we determine the P–T conditions of metamorphism of two samples of garnet amphibolites collected at the northernmost end of the Adrar–Souttouf Metamafic Complex. The samples show a main mineral assemblage of garnet+low-Ti pargasite+oligoclase+phengite+epidote+quartz+rutile±paragonite±K-feldspar. We calculated their P–T conditions using the amphibole–plagioclase NaSi–CaAl exchange thermometer, and the garnet–amphibole–plagioclase–quartz and the amphibole–plagioclase Si–Al partitioning barometers. The thermobarometric results indicate that this mineral assemblage was formed at high-P amphibolite-facies conditions at 650–700°C and 10–13 kbar. The observed stability of paragonite and phengite reveals fluid-absent conditions or the presence of a fluid phase with reduced H2O activity during the peak of metamorphism. We found no relicts of eclogite-facies mineral assemblage in the garnet amphibolites. This contrasts with the eclogite-facies metamorphism found due south in the Tarf Magneïna unit. This suggests that the northernmost end of the Adrar–Souttouf Metamafic Complex may have been buried to shallower depths than the units further south, probably during the Variscan orogeny. However, precise absolute radiometric dating of the high-P amphibolite-facies metamorphism is required to confirm these findings.

Detrital zircon U–Pb provenance and palaeogeography of Triassic rift basins in the Marrakech High Atlas

AbstractThe Marrakech High Atlas contains some of the best exposures of the Triassic early‐rift strata related to Atlantic opening in NW Africa. We present the first detrital zircon U–Pb data of five Triassic redbed samples from the Tizi n'Test basin to quantify sediment provenance, transport and dispersal patterns during early rifting. These U–Pb ages document dominant sediment sourcing from the south, the Anti‐Atlas domain, with very limited to absent input from the Variscan Meseta domain to the north. This combined with stratigraphic and thermochronologic information points to a highly asymmetric palaeogeography during Triassic rifting. Furthermore, the occurrence of Archaean detrital zircon grains in Triassic sandstone, likely recycled from the Reguibat shield, suggests the presence of a fully developed regional drainage system with rivers and catchments reaching hundreds of kilometres into the hinterland of the rift flank.

Mesozoic and Cenozoic thermal history of the Western Reguibat Shield (West African Craton)

AbstractUsing low‐temperature thermochronology on apatite and zircon crystals, we show that the western Reguibat Shield, located in the northern part of the West African Craton, experienced significant cooling and heating events between Jurassic and present times. The obtained apatite fission track ages range between 49 and 102 Ma with mean track lengths varying between 11.6 and 13.3 μm and Dpar values between 1.69 and 3.08 μm. Zircon fission track analysis yielded two ages of 159 and 118 Ma. Apatite (U–Th)/He uncorrected single‐grain ages range between 76 and 95 Ma. Thermal inverse modelling indicates that the Reguibat Shield was exhumed during the Early Cretaceous, Late Cretaceous, Palaeocene–Eocene and Quaternary. These exhumation events were coeval with regional tectonic and geodynamic events, and were probably driven by a combined effect of plate tectonics and mantle dynamics.

Mesozoic evolution of NW Africa: implications for the Central Atlantic Ocean dynamics

The Central Atlantic Ocean opened during the Early Jurassic and represents the oldest portion of the Atlantic Ocean. Although the American margin has been well studied, the onshore evolution of its African counterpart is poorly understood. We investigated the evolution of a c . 1300 km transect across the Reguibat Shield (Morocco, Mauritania, Algeria) in the northern West African Craton using low-temperature thermochronology. Fourteen samples were dated using apatite fission-track analysis. Nine of these samples were also dated using (U–Th–Sm)/He analysis. Fission-track ages range from 118 ± 10 to 497 ± 61 Ma, with mean track lengths between 11.2 ± 0.4 and 12.5 ± 0.2 µm. (U–Th–Sm)/He single-grain ages range from 32 ± 3 to 396 ± 32 Ma. Through forward and inverse thermal modelling, we demonstrate that the craton underwent kilometre-scale exhumation between the Early–Middle Jurassic and the Late Cretaceous. Based on our new results, published data on NW Africa and data from the conjugate eastern North American passive margin, we show that this post-rift Early–Middle Jurassic to Early Cretaceous exhumation affected both margins to a similar areal extent and simultaneously. Transient mantle dynamic support is suggested to account for the major erosional phase recorded on both margins. Supplementary material: The method for thermal modelling, the thermal modelling for all samples and the relationship between apatite chemistry and (U–Th)/He ages are available at https://doi.org/10.6084/m9.figshare.c.3770918

A historical overview of Moroccan magmatic events along northwest edge of the West African Craton

Located along the northwestern edge of the West African Craton, Morocco exhibits a wide variety of magmatic events from Archean to Quaternary. The oldest magmatic rocks belong to the Archean Reguibat Shield outcrops in the Moroccan Sahara. Paleoproterozoic magmatism, known as the Anti-Atlas granitoids, is related to the Eburnean orogeny and initial cratonization of the WAC. Mesoproterozoic magmatism is represented by a small number of mafic dykes known henceforth as the Taghdout mafic volcanism. Massive Neoproterozoic magmatic activity, related to the Pan-African cycle, consists of rift-related Tonian magmatism associated with the Rodinia breakup, an Early Cryogenian convergent margin event (760–700 Ma), syn-collisional Bou-Azzer magmatism (680–640 Ma), followed by widespread Ediacaran magmatism (620–555 Ma). Each magmatic episode corresponded to a different geodynamic environment and produced different types of magma.Phanerozoic magmatism began with Early Cambrian basaltic (rift?) volcanism, which persisted during the Middle Cambrian, and into the Early Ordovician. This was succeeded by massive Late Devonian and Carboniferous, pre-Variscan tholeiitic and calc-alkaline (Central Morocco) volcanic flows in basins of the Moroccan Meseta. North of the Atlas Paleozoic Transform Zone, the Late Carboniferous Variscan event was accompanied by the emplacement of 330–300 Ma calc-alkaline granitoids in upper crustal shear zones. Post-Variscan alkaline magmatism was associated with the opening of the Permian basins.Mesozoic magmatism began with the huge volumes of magma emplaced around 200 Ma in the Central Atlantic Magmatic Province (CAMP) which was associated with the fragmentation of Pangea and the subsequent rifting of Central Atlantic. CAMP volcanism occurs in all structural domains of Morocco, from the Anti-Atlas to the External Rif domain with a peak activity around 199 Ma. A second Mesozoic magmatic event is represented by mafic lava flows and gabbroic intrusions in the Internal Maghrebian flysch nappes as well as in the external Mesorif. This event consists of Middle-Upper Jurassic MORB tholeiites emplaced during opening of the Alpine Tethys ocean. The Central High Atlas also records Early Cretaceous alpine Tethys magmatism associated with the aborted Atlas rift, or perhaps linked to plume activity on the edge of the WAC.Cenozoic magmatism is associated with Tertiary and Quaternary circum-Mediterranean alkaline provinces, and is characterized by an intermittent activity over 50 Ma from the Anti-Atlas to the Rif Mountain along a SW–NE volcanic lineament which underlines a thinned continental lithosphere.

The Archean kalsilite-nepheline syenites of the Awsard intrusive massif (Reguibat Shield, West African Craton, Morocco) and its relationship to the alkaline magmatism of Africa

More than 40% of the known alkaline complexes are reported from Africa. Most are ring complexes composed of syenites and associated or not, lithotypes as carbonatites, granites and mafic rocks. Radiometric dating indicates the presence of alkaline complexes with ages spanning from Precambrian to the present. In terms of outcrops, alkaline complexes are reported from cratonic zones and from belts embedded between cratonic areas. Because of the high economic potential for associated REE deposits, these alkaline complexes have received much attention from Earth scientists. These studies aim mainly to constrain the role of the mantle and the crust (and the interaction between them) in the genesis of this peculiar magmatism, and also to explain the variability observed in lithotypes and geotectonic settings. Among those alkaline complexes, Precambrian occurrences are rare. Up-to-date only a few Proterozoic examples were cited in Africa. The recently studied Awsard complex in Southern Morocco is a peculiar one with a crystallization age of 2.46 Ga and an unusual rock assemblages. This paper is a first approximation to a comparison of geochemical and isotopic fingerprints of the Awsard magmatism (as the oldest one) with other known different ages African complexes from different geotectonic settings, aiming to detect if there is any evolution in this alkaline magmatism through time. A first conclusion is that magma sources for this alkaline magmatism has been probably evaluating over geological time, from parental magmas compositions close to that of primitive mantle in these early geological time to compositions holding more and more depleted mantle and continental crust components. However, to go further in this debate more modern isotopic, geochemical and geochronological data from all these complexes are needed. Nevertheless, this comparison highlighted the peculiar character of the Awsard magmatism with an isotopic composition very close to that of Primitive mantle (values of εNd(2.46Ga) range from −3.5 to 1.2, Nd model ages range from ca.2.5 Ga to 3.0 Ga (the hosting TTG gneiss crystallization age)); an ultrapotassic composition (K2O/Na2O>>10); the oldest crystallization age 2.46 Ga reported until the date in Africa and the unusual occurrence of kalsilite-rich syenites “synnyrites”.

Contrasting SHRIMP U–Pb zircon ages of two carbonatite complexes from the peri-cratonic terranes of the Reguibat Shield: Implications for the lateral extension of the West African Craton

The Oulad Dlim Massif of the Western Reguibat Shield contains several carbonatite complexes of previously unknown age. The largest and best studied are Gleibat Lafhouda, composed of magnesiocarbonatites, and Twihinate, composed of calciocarbonatites. Gleibat Lafhouda is hosted by Archean gneisses and schists. It has a SHRIMP U–Th–Pb zircon crystallization age of 1.85±0.03Ga, a Nd model age of TCR=1.89±0.03Ga, and a Sm–Nd age of 1.85±0.39Ga. It forms part of the West Reguibat Alkaline province. Twihinate, on the other hand, is much younger. It is hosted by Late Silurian to Early Devonian deformed granites and has a zircon crystallization age of 104±4Ma, which is within error of the age of the carbonatites of the famous Richat Structure in the southwest Reguibat Shield. Like these, the Twihinate carbonatites are part of the Mid-Cretaceous Peri-Atlantic Alkaline Pulse. The Twihinate carbonatites contain abundant inherited zircons with ages that peak at ca. 420Ma, 620Ma, 2050Ma, 2466Ma, and 2830Ma. This indicates that their substratum has West African rather than, as previously suggested, Avalonian affinities. It has, however, a Paleoproterozoic component that is not found in the neighboring western Reguibat Shield. The 421Ma to 410Ma gneissic granites hosting Twihinate are epidote+biotite+Ca-rich garnet deformed I-type to A-type granites derived from magmas of deep origin compatible, therefore, with being generated in a subduction environment. These granites form a body of unknown dimensions and petrogenesis, the study of which will be of key importance for understanding the geology and crustal architecture of this region.

History of the West African Neoproterozoic Ocean: Key to the geotectonic history of circum-Atlantic Peri-Gondwana (Adrar Souttouf Massif, Moroccan Sahara)

Bordered by the Archaean basement of the Reguibat Shield to the E and S, the Adrar Souttouf Massif is located in the southern regions of the Moroccan Sahara and represents the northern part of the Mauritanide belt. The central areas of this massif comprise the Dayet Lawda and Sebkha Matallah units that are mainly composed of mafic rocks. Ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology, U-Th-Pb, and Lu-Hf isotope composition. Additionally, 155 apatite grains from six samples were also dated by the U-Th-Pb method. Mostly well rounded zircon grains commonly showed bright cathodoluminescent (CL) overgrowth domains and leaching zones, indicating metamorphic overprint around unaffected oscillatory zoned core areas. All samples but one yielded two significant zircon age populations: ~605 Ma and ~634 Ma with some inherited grains in the range of ~650–740 Ma and very scarcely up to ~1190 Ma. For six samples the Hf isotope composition suggests a major contribution of Neoproterozoic juvenile magmas from depleted mantle source. The zircon Hf data for the remaining samples points to a predominant recycling of older Archaean to Palaeoproterozoic crust. Based on the geochemical composition of the rocks, an island arc setting is assumed at the periphery of the West African Craton close to the Cryogenian-Ediacaran boundary. Subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 Ma. A minor Variscan overprint could be demonstrated for only one sample. This case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the West African Craton for palaeogeographic and geodynamic reconstructions including the peri-Gondwanan terranes during the Late Neoproterozoic.