bluuebunny/crossref_metadata_2025_split huggingface dataset:
The boundaries of the West African craton, with special reference to the basement of the Moroccan metacratonic Anti-Atlas belt
Authors: Ennih Nasser , Liégeois Jean-Paul 
Published: May 2008

Abstract

The West African craton (WAC) was constructed during the Archaean and the
c.
2 Ga Palaeoproterozoic Eburnian orogeny. Mesoproterozoic quiescence at
c.
1.7–1.0 Ga allowed cratonization. In the absence of Mesoproterozoic activity, there are no known WAC palaeogeographical positions for that time. At the beginning of the Neoproterozoic, the WAC was affected by several extensional events suggesting that it was subjected to continental breakup. The most important event is the formation of the Gourma aulacogen in Mali, and the Taoudeni cratonic subcircular basin and deposition of platform sediments in the Anti-Atlas. At the end of the Neoproterozoic, the WAC was subjected to convergence on all its boundaries, from the north in the Anti-Atlas, to the east along the Trans-Saharan belt, to the south along the Rockelides and the Bassarides and to the east along the Mauritanides. This led to a partial remobilization of its cratonic boundaries giving rise to a metacratonic evolution. The WAC boundaries experienced Pan-African Neoproterozoic to Early Cambrian transpression and transtension, intrusion of granitoids and extrusion of huge volcanic sequences in such as in the Anti-Atlas (Ouarzazate Supergroup). Pan-African tectonism generated large sediment influxes around the WAC within the Peri-Gondwanan terranes whose sedimentary sequences are marked by distinctive zircon ages of 1.8–2.2 Ga and 0.55–0.75 Ga.

WAC rocks experienced Pan-African low grade metamorphism and large movements of mineralizing fluids. In the Anti-Atlas, this Pan-African metacratonic evolution led to remobilization of REE in the Eburnian granitoids due to the activity of F-rich fluids linked to extrusion of the Ouarzazate Supergroup. During the Phanerozoic, the western WAC boundary was subjected to the Variscan orogeny, for which it constituted the foreland and was, therefore moderately affected, showing typical thick-skin tectonics in the basement and thin-skin tectonics in the cover. During the Mesozoic, the eastern and southern boundaries of the WAC were subjected to the Atlantic opening including Jurassic dolerite intrusion and capture of its extreme southern tip by South America. The Jurassic is also marked by the development of rifts on its eastern and northern sides (future Atlas belt). Finally, the Cenozoic period was marked by the convergence of the African and European continents, generating the High Atlas range and Cenozoic volcanism encircling the northern part of the WAC. The northern metacratonic boundary of the WAC is currently uplifted, forming the Anti-Atlas Mountains.
The boundaries of the WAC, metacratonized during the Pan-African orogeny have been periodically rejuvenated. This is a defining characteristic of the metacratonic areas: rigid, stable cratonic regions that can be periodically cut by faults and affected by magmatism and hydrothermal alteration – making these areas important for mineralization.
Gold mineralization in the Proterozoic Bleida ophiolite, Anti-Atlas, Morocco
Authors: Belkabir Abdelhay , Jébrak Michel , Maacha Lhou , Azizi Samir M. Rachid , Madi Atmane 
Published: May 2008

Abstract

The newly discovered (1998) West Bleida gold mineralization (3 tonnes metal Au) lies west of the main Moroccan Bleida copper deposit (1981–1991) in the central Anti-Atlas (southern Morocco). It is hosted by metamorphosed and deformed mafic to intermediate volcanic rocks that are part of the Neoproterozoic tholeiitic volcanosedimentary series forming the stratigraphically upper part of the Bou Azzer ophiolite sequence. Strong sericitization and local silicification are associated with mineralization. These altered rocks represent a proximal hydrothermal alteration halo around the West Bleida ore zones. Normative chlorite characterizes the metamorphic assemblage away from the ore zones. Gold mineralization primarily occurs as deformed gold-bearing quartz veins and disseminations in Cu-rich chert zones (chalcopyrite–malachite), Fe-rich lithofacies and breccia zones. Gold is accompanied by small amounts of copper sulphides (<1% modal chalcopyrite). Scanning electron microscope–energy dispersive spectrometry analyses of gold grains from veins and disseminations reveal the presence of palladium as inclusions of Pd–As–Sb, Pd–Bi–Se and Pd–Te mineral phases. An electron microprobe study confirms the presence of two types of gold. The first is an alloy of Au–Ag–Pd, typically bordered by small grains of Pd and Bi (Te,Sb) phases and associated with a metamorphic assemblage. Isomertieite, Pd
11
(Sb
2
,As
2
), was identified as one of the phases. The second type of gold is electrum (10% Ag, 90% Au), which is always associated with fractures and occurs with hematite and white mica. Based on its form and habits, West Bleida gold reflects two distinct generations of fluid activity. The primary event precipitated Au–Ag–Pd alloys from Au–Pd-bearing hydrothermal fluids and produced auriferous quartz veins and disseminations within mafic rocks of the Bleida ophiolitic accretionary complex. It was structurally and lithologically controlled. This early event is preserved in the deeper (and thus fresher) zones more than 80 m below the surface. Intense tectonic overprinting obscures the genetic relationship between vein and disseminated styles of mineralization, both of which contain Pd-rich gold, but some of the auriferous quartz veins are observed to crosscut disseminated mineralization. Two possible hypotheses are considered: the pre-tectonic root of a volcanogenic massive sulphide system, or a late tectonic orogenic (mesothermal) deposit. The presence of Pd minerals and anomalous cobalt concentrations suggest a source in ultramafic rocks. The second event, characterized by inclusion-free electrum, occurred much later and represents the alteration and weathering of the primary Pd-rich gold assemblage by oxidizing surface fluids. It affected all mineralized units and structures to a depth of 80 m. This post-tectonic surficial alteration also caused leaching of Cu-sulphides, which may explain their low abundances in the upper parts of the ore zones.
Petrology and geochemistry of the Neoproterozoic Siroua granitoids (central Anti-Atlas, Morocco): evolution from subduction-related to within-plate magmatism
Authors: Touil Ahmed , Hafid Ahmid , Moutte Jacques , Boukhari Abdelmajid El 
Published: May 2008

Abstract
The Siroua massif includes many plutons of Neoproterozoic age. The mineralogical and geochemical character of the plutons allows us to describe an evolution of the magmatism, in space and time, from a subduction-related type in the northern part, to a within-plate subalkaline type in the southern part. The first magmatic activity coeval with the closing of the Khzama oceanic basin in the north is little evolved and of oceanic type (dominantly gabbros and basalts). It is followed by a low potassic calc-alkaline magmatism (gabbro–diorites, tonalites and trondhjemites of Nebdas pluton) and by a voluminous highly potassic calcalkaline magmatism (Askaoun and Ifouachguel plutons) that marks the collisional period. The end of crustal uplift and the beginning of the extension is marked in the south by a sub-alkaline magmatism corresponding to the Ida ou Illoun, Imdghar and Affela N'ouassif granites. Magmatic activity, in the Siroua massif, is marked at the end of the Neoproterozoic (PIII) by a continental tholeiite with an alkaline affinity, which occurs as dykes crosscutting the Neoproterozoic granites, and later by dominantly alkaline granites.
The Cambrian volcano-sedimentary formations of the westernmost High Atlas (Morocco): their place in the geodynamic evolution of the West African Palaeo-Gondwana northern margin
Authors: Pouclet A. , Ouazzani H. , Fekkak A. 
Published: May 2008

Abstract
In the westernmost part of the High Atlas, two Palaeozoic formations, rich in mafic volcanic rocks, are distinguished. They belong to different structural blocks created during the Variscan orogeny. New U–Pb dating yields an Early Cambrian age. The basaltic lavas have the composition of continental tholeiites and the magmatic signature of an initial rifting tectonic setting. They are related to the western Moroccan Cambrian rift. Their geodynamical context could be a passive margin initiated from an active rift that aborted in the Middle Cambrian.
The magmatic evolution at the Moroccan outboard of the West African craton between the Late Neoproterozoic and the Early Palaeozoic
Authors: Ezzouhairi H. , Ribeiro M. L. , Ait Ayad N. , Moreira M. E. , Charif A. , Ramos J. M. F. , de Oliveira D. P. S. , Coke C. 
Published: May 2008

Abstract
The Late Neoproterozoic Ouarzazate Group crops out on the north margin of the West African craton (WAC). In this group an important post-collisional magmatism is characterized by a great diversity in plutonic and volcanic rock types of the high-K calc-alkaline series. This series evolved mainly by crystal fractionation and by an important crustal contamination from an anomalous mantle source. The Early Cambrian magmatism began at the same time on both sides of the Anti-Atlas Major Fault, the southwestern side (Kerdous region) and northeastern side (Ouarzazate-Agdz region), interbedded in the Early Cambrian Basal Series and spread later to the Western High Atlas of the Morocco northern WAC outboard areas. This magmatism changes from a continental tholeiitic series (HPT and LPT) at the beginning to an alkaline series at the top (Adoudou and ‘Lie de vin’ formations). Fractional crystallization and pelagic or crustal contamination were the most important processes in the magma differentiation. The geochemical inversion from calc-alkaline to tholeiitic magmatism between the Late Neoproterozoic and the Early Cambrian is documented, as is the major extension of the tholeiitic activity on both sides of the South Atlas Fault. This geochemical variation indicates a transition of the tectonic regime from compressive to extensional. The late local Jbel Boho alkaline magmatism indicates the sink of the source and the mitigation or closure of the extensional cycle at this time.
Zircon U–Pb geochronology of paragneisses and biotite granites from the SW Iberian Massif (Portugal): evidence for a palaeogeographical link between the Ossa–Morena Ediacaran basins and the West African craton
Authors: Pereira M. F. , Chichorro M. , Williams I. S. , Silva J. B. 
Published: May 2008

Abstract

Sensitive high-resolution ion microprobe U–Th–Pb age determinations on detrital and inherited zircon from the Évora Massif (SW Iberian Massif, Portugal) provide direct evidence for the provenance of the Ossa–Morena Ediacaran basins (Série Negra) and a palaeogeographical link with the West African craton. Three samples of the Série Negra paragneisses contain large components of Cryogenian and Ediacaran (
c
. 700–540 Ma) detrital zircon, but have a marked lack of zircon of Mesoproterozoic (
c
. 1.8–0.9 Ga) age. Older inherited zircons are of Palaeoproterozoic (
c
. 2.4–1.8 Ga) and Archaean (
c
. 3.5–2.5 Ga) age. The same age pattern is also found in the Arraiolos biotite granite, which was formed by partial melting of the Série Negra and overlying Cambrian rocks. These results are consistent with substantial denudation of a continental region that supplied sediments to the Ediacaran Ossa–Morena basins during the final stages of the Cadomian–Avalonian orogeny (peri-Gondwanan margin with principal zircon-forming events at
c
. 575 Ma and
c
. 615 Ma). Combined with the detrital zircon ages reported for rocks of the same age from Portugal, Spain, Germany and Algeria, our data suggest that the sediment supply to the Ediacaran–Early Palaeozoic siliciclastic sequences preserved in all these peri-Gondwanan regions was similar. The lack of Grenvillian-aged (
c
. 1.1–0.9 Ga) zircon in the Ossa–Morena and Saxo-Thuringia Ediacaran sediments suggests that the sediment in these peri-Gondwanan basins was derived from the West African craton.
Orogen-parallel tectonic transport: transpression and strain partitioning in the Mauritanides of NE Senegal
Authors: Dabo M. , Gueye M. , Ngom P. M. , Diagne M. 
Published: May 2008

Abstract
New structural, metamorphic, finite-strain analysis and kinematic data from the southern part of the Mauritanides in Senegal (Bakel area) have revealed a history reflecting different tectonic regimes. The structural framework is expressed in the form of shear-zone systems comprising rectilinear NE–SW trending thrusts and NE–SW-trending ductile sinistral strike-slip faults associated with north–south- to NE–SW-trending transpression belts. The analysis of ductile fabrics reveals the presence of lineation trajectories that show a change from a tangential deformation towards the marginal zones in the Northern and Southern Domains of the belt to sinistral oblique transpression in the Central Domain. The deformation within the belt is strongly partitioned into shear zones distributed across the belt defining structural domains with accompanying large-scale folds and thrusts. This exemplifies wrench-style tectonics dominated by crustal-scale strike-slip shear zones in the orogen core and temporal and spatial progression outwards to the marginal zones. Thus, the Variscan orogeny in the southern part of the Mauritanides belt in Senegal developed through sinistrally oblique transpressive shearing along the borders of the orogen core leading to the exhumation of ultramafic rocks between large shear zones.
Deep structure of the southeastern margin of the West African craton from seismic reflection data, offshore Ghana
Authors: Attoh Kodjopa , Brown Larry 
Published: May 2008

Abstract

Seismic reflection data, acquired as part of an oil exploration survey offshore Ghana, have been reprocessed to reveal the deep structure of the Proterozoic continental crust on the continental shelf, off West Africa. The seismic profiles presented are located across the Romanche transform margin, which transects the West African craton (WAC) margin where it is bounded by the Pan-African Dahomeyide orogen. The seismic data reveal highly reflective middle–lower crust of the WAC but a nearly transparent upper crust of
c
. 10 km thickness. Bundles of discontinuous reflections are located at the base of the unreflective upper crust and locally define the trough of a synformal structure interpreted as the keel of a Palaeoproterozoic (Birimian) greenstone belt exposed onshore. The deepest crust imaged is characterized by more continuous, subhorizontal reflections that are very similar to those observed in the lower crust of cratonic areas such as the Superior Province. Near the WAC margin the subhorizontal reflections are truncated by east-dipping reflections in the Pan-African domain, which are correlated with cratonward directed fold-and-thrust structures formed during the
c
. 600 Ma Pan-African orogeny. These seismic observations represent, to date, the first deep seismic reflection images of the West African craton margin and the continental lithosphere in the region.
Geochronology and metamorphic
P
-
T
-
X
evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution
Authors: Bendaoud Abderrahmane , Ouzegane Khadidja , Godard Gaston , Liégeois Jean-Paul , Kienast Jean-Robert , Bruguier Olivier , Drareni Amar 
Published: May 2008

Abstract

Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with
in situ
U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic
P–T
conditions of 860±50 °C and 7–8 kbar (M
1
) were followed by a decrease of pressure to 4.3±1 kbar and of temperature to around 700 °C, associated with the development of migmatites (M
2
). After cooling, a third thermal phase at
c
. 650 °C and 3–4 kbar (M
3
) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151±8 Ma, the granulite-facies and migmatitic metamorphisms (M
1
–M
2
) at 2062±39 Ma and the medium-grade metamorphic assemblage (M
3
) at 614±11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan-African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution.
Review of the orogenic belts on the western side of the West African craton: the Bassarides, Rokelides and Mauritanides
Authors: Villeneuve Michel 
Published: May 2008

Abstract
The West African craton is fringed along its western side by a 3000 km long orogenic belt subdivided into three separate orogens: the Bassaride (Pan-African I orogeny), Rokelide (Pan-African II orogeny) and Mauritanide (Hercynian orogeny) thrust belts. The Bassarides are cut to the north by the Mauritanides and to the south by the Rokelides but parts of this Bassaride belt are incorporated in the other two younger belts. This review presents the main geological, geophysical and geochronological results from the western side of the West African craton, collected over the past 90 years, concentrating on those from the last 15 years. Former geological investigations underlined the thin-skinned structure model within these thrust belts, whereas the geophysical results gave prominence to the major importance of block faulting resulting from the Pan-African I orogeny and its strong influence on the subsequent orogenic belt features. The geochronological data allow us to distinguish major tectonothermal events related to the Pan-African I (660–650 Ma), Pan-African II (550–530 Ma) and Hercynian (330–300 Ma and 280–270 Ma) orogenies. However, they also reveal five other tectonothermal events (at 1200–1000, 750–700, 600–580, 510–480 and 450–380 Ma), which are still very poorly understood. The 1200–1000 Ma tectonothermal event recently revealed in the northern Mauritanides may correspond to a remanent orogenic belt segment that witnessed the Grenvillian orogeny.
Neoproterozoic garnet-glaucophanites and eclogites: new insights for subduction metamorphism of the Gourma fold and thrust belt (eastern Mali)
Authors: Caby R. , Buscail F. , Dembélé D. , Diakité S. , Sacko S. , Bal M. 
Published: May 2008

Abstract

The Neoproterozoic Gourma fold and thrust belt exposed in eastern Mali includes in its inner part high-pressure, low-temperature metasediments and scarce metabasites. This high-pressure metamorphic unit is characterized in the Ansongo region by garnet–glaucophane–paragonite assemblages and eclogites of basaltic derivation, whereas phengite–garnet–rutile mineral assemblages characterize the metapelites. Thermobarometric estimates on the metabasites suggest peak pressure around 13–15 kbar and temperature of 500±50 °C for the Seyna Bela garnet glaucophanite and glaucophane-bearing eclogite, and 16 kbar at 600±50 °C for the Tin Hama phengite eclogite, values indicative of palaeogeothermal gradients of about 10 °C km
−1
typical of subduction settings. The high-pressure unit may represents a giant allochthon emplaced on top of very low-grade metasediments. It represents the southern extension of the ultrahigh-pressure rocks. Garnet pyriclasites from the Amalaoulaou massif, which represents the roots of a
c
. 800–730 Ma Neoproterozoic island arc, underwent a medium-temperature metamorphic overprint characterized by barroisite–paragonite assemblages; that is, of same grade as the decompression that affected the eclogites and garnet-blueschists. The Gourma high-pressure metamorphic belt formed as a consequence of the east-dipping subduction of the Neoproterozoic passive palaeo-margin of the West African craton. The presented
P
–
T
estimates suggest that subduction-related palaeogeothermal gradients during the late Neoproterozoic period along the main Pan-African suture were similar to those reconstructed for Tertiary Alpine-type belts.
Time Course of Early Audiovisual Interactions during Speech and Nonspeech Central Auditory Processing: A Magnetoencephalography Study
Authors: Hertrich Ingo , Mathiak Klaus , Lutzenberger Werner , Ackermann Hermann 
Published: May 2008

AbstractCross-modal fusion phenomena suggest specific interactions of auditory and visual sensory information both within the speech and nonspeech domains. Using whole-head magnetoencephalography, this study recorded M50 and M100 fields evoked by ambiguous acoustic stimuli that were visually disambiguated to perceived /ta/ or /pa/ syllables. As in natural speech, visual motion onset preceded the acoustic signal by 150 msec. Control conditions included visual and acoustic nonspeech signals as well as visual-only and acoustic-only stimuli. (a) Both speech and nonspeech motion yielded a consistent attenuation of the auditory M50 field, suggesting a visually induced “preparatory baseline shift” at the level of the auditory cortex. (b) Within the temporal domain of the auditory M100 field, visual speech and nonspeech motion gave rise to different response patterns (nonspeech: M100 attenuation; visual /pa/: left-hemisphere M100 enhancement; /ta/: no effect). (c) These interactions could be further decomposed using a six-dipole model. One of these three pairs of dipoles (V270) was fitted to motion-induced activity at a latency of 270 msec after motion onset, that is, the time domain of the auditory M100 field, and could be attributed to the posterior insula. This dipole source responded to nonspeech motion and visual /pa/, but was found suppressed in the case of visual /ta/. Such a nonlinear interaction might reflect the operation of a binary distinction between the marked phonological feature “labial” versus its underspecified competitor “coronal.” Thus, visual processing seems to be shaped by linguistic data structures even prior to its fusion with auditory information channel.
The Attentional Blink Modulates Activity in the Early Visual Cortex
Authors: Hein Grit , Alink Arjen , Kleinschmidt Andreas , Müller Notger G. 
Published: May 2008

Abstract
The attentional blink (AB) documents a particularly strong case of visual attentional competition, in which subjects' ability to identify a second target (T2) is significantly impaired when it is presented with a short SOA after a first target (T1). We used functional magnetic resonance imaging to investigate the impact of the AB on visual activity in individually defined retinotopic representations of the target stimuli. Our results show reduction of neural response in V3 and marginally in V2 and V1, paralleling the behavioral AB effect. Reduction of visual activity was accompanied by reduced neural response in the inferior parietal cortex. This indicates that attentional competition modulates activity in higher-order parietal regions and the early visual cortex, providing a plausible neural basis of the behavioral AB effect.