30 research outputs found
Precambrian isotopic sources of the Anti-Atlas, Morocco
The isotopic data stored in detrital and magmatic zircons are crucial for assessing magma
sources, terrane correlation, paleogeography and plate reconstructions. In many cases the
comparison of the zircon age and isotope signature of a terrane of unknown provenance with
the signature of possible sources, generally old cratonic areas, can resolve questions of origin
and paleoposition. Obviously, a precise knowledge of the zircon characteristics of these old
areas is essential for reliable comparisons.
One of the major sources of sediments of the peri-Gondwanan terranes and of the European
Variscan Belt is the West African craton. The northern boundary of this craton is the Pan-
African Anti-Atlas belt, which is therefore an ideal place to better constrain the zircon isotopic
features of sediments sourced from it. With that aim, we obtained LA-ICM-MS U-Pb and Hf
isotopic data of more than 600 zircons separated from six samples of siliciclastic sedimentary
rocks from the main Neoproterozoic stratigraphic units of the Anti-Atlas belt, from the Sirwa
and Zenaga inliers.
The data suggest that the north part of the West African craton formed during three cycles of
juvenile crust formation, with variable amount of reworking of older crust. The youngest group
of zircons, with a main population clustering around 610 Ma, has a predominantly juvenile
character and evidence of moderate mixing with Paleoproterozoic crust, suggesting that the
igneous and metamorphic rocks in which the zircons originally crystallized were formed in
an ensialic magmatic arc environment. A group of zircons with ages in the range 1.79–2.3
Ga corresponds to the major crust forming event in the West African craton: the Eburnian-
Birimian orogeny. The isotopic data indicate that the provenance area should represent a crustal
domain that separated from a mantle reservoir at ∼2050–2300 Ma, and further evolved with
a time-integrated 176Lu/177Hf of ∼0.013, characteristic of continental crust. The evolution
of the Eburnian orogeny is apparently dominated by new crust formation in a magmatic arc
environment. The Lower Paleoproterozoic and Neoarchean evolution (2.3–2.75 Ga) involves a
group of detrital zircon ages that has not been identified up to now in the igneous or metamorphic
rocks of the north West African craton basement. Their Hf isotopic signature points to reworking
of juvenile crust mixed with moderate amounts of Archean crust. The significance of these ages
is uncertain: they could represent a tectonothermal event not discovered yet in the Reguibat
Shield or the zircons can be far-travelled from an unknown source.Peer Reviewe
Sedimentary provenance of siliciclastic rocks from the Lalla Mouchaa Calcschists Formation (Coastal Block, Western Rehamna): Evidence of denudation of ca. 2 Ga basement in the Moroccan Meseta.
Paleoproterozoic basement rocks (ca. 2Ga) are scarce along the European and North African
Paleozoic mountain chains (Fig.1a). In Morocco, ca. 2.2-2Ga granitic rocks (Gasquet et al.,
2008; Kouyaté et al., 2013) have been exclusively reported in the Western Anti-Atlas at
southwest of the Anti-Atlas Major Fault, (Choubert, 1963). In Eastern and Central Anti-Atlas
and in the Moroccan Meseta, the existence of a Paleoproterozoic basement has only been
recognized through indirect evidence (Gasquet et al., 2008; Michard et al., 2010). In the Anti-
Atlas belt, ca. 2Ga detrital zircon grains are found in the Ediacaran siliciclastic rocks of the
Bou Salda, Saghro and Taghdout groups (Abati et al., 2010). In the Moroccan Meseta,
Paleoproterozoic (ca. 2Ga) zircon grains were extracted from gneiss and granitic xenoliths
found in Triassic lamprophyre dykes, and from Carboniferous granophyric microgranite
intrusions of central Jebilet (Dostal et al., 2005, Essaifi et al., 2003).
Recently, a porphyritic rhyolite from the Rehamna Massif was dated at ca. 2.05Ga (Pereira et
al., 2015), demonstrating for the first time, the exposure of the Eburnian basement in the
Western Meseta. These Eburnian arc-related magmatic rocks, which are exposed to the south
of the Permian Sebt Brikiyine granite in the core of anticlines from the Lalla Mouchaa
Anticlinorium, are allegedly unconformably overlained by transgressive siliciclastic and
carbonate beds (Corsini, 1988; Pereira et al., 2015). At north of the Sebt Brikiyine granite the
probable Lower Cambrian sequence (Lalla Mouchaa Calcschists Formation; Guezou &
Michard, 1976; Corsini, 1988) comprises a basal unit of microbreccias, arkosic sandstones
and siltstones (lower member) that pass towards the top to centimeter-thick beds of calcschists
interbedded with limestones and dolomites (upper member) that are conformably overlain by
the “Paradoxides Shale Formation” composed of siltstone, greywacke and sandstone with
Middle Cambrian fauna.
In order to study the potential sources of the Lalla Mouchaa Calcschists Formation (probable
Lower Cambrian), we have sampled a microbreccia at the Koudiat El Hamra region, for U-Pb
geochronology on detrital zircon. This foliated microbreccia is composed of elongated Kfeldspar
and quartz phenocrysts surrounded by a fine-grained matrix. Detrital zircon grains
gave 206Pb/238Ub ages that in the Probability density plot curve are distributed by two main
age peaks at ca. 2.05Ga and ca. 2.03Ga (Fig.1b), yielding a 206Pb/238U age-weighted mean of
ca. 2.04Ga (El Houicha et al., 2018). The obtained U-Pb results point to a provenance from a
Paleoproterozoic source, suggesting a possible contribution from rocks of the same age of the
2Ga porphyritic rhyolite exposed at south of the Sebt Brikiyine granite. Thus, there is a
possibility that the Paleoproterozoic basement extends bellow the Paleozoic and
Neoproterozoic sequences of the Rehamna massif
Zircon U-Pb geochronology and geochemistry of Cambrian magmatism in the Coastal Block (Oued Rhebar volcanic complex, Moroccan Meseta): Implications for the geodynamic evolutionary model of North-Gondwana
U-Pb dating (SHRIMP) of magmatic zircons from an intermediate-mafic agglomerate of the Oued Rhebar
Volcanic Complex (Coastal Block, Western Meseta) yielded a weighted mean age of 507 ± 5 Ma. The obtained
middle Cambrian age (Series 3, Stage 5) seems to be the best estimate for the crystallization of the ORVC
volcanic rock, providing a maximum depositional age for the overlying Bouznika volcanosedimentary
Formation. The ORVC rock is representative of middle Cambrian crust generation in North-Gondwana, but
contains Ediacaran (ca. 546-542 Ma) and early Cambrian (ca. 536-526 Ma) inherited zircon which might be
derived from recycling of older continental crust. This geochemistry study corroborates the existence of calcalkaline
rhyolites, basaltic andesites and andesites in the ORVC rocks, but also of the existence, albeit to a lesser
volume, of tholeiitic basalt. The relative higher volume of calc-alkaline compared to tholeiitic signature might
reflect, in some extent, contamination of depleted mantle-derived magmas by the upper continental crust, as has
been proposed for the origin of the same age rift-related igneous rocks from North-Gondwana, as the Iberian
correlatives
Flow of Canary mantle plume material through a subcontinental lithospheric corridor beneath Africa to the Mediterranean : comment
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Insights on the crustal evolution of the West African Craton from Hf isotopes in detrital zircons from the Anti-Atlas belt
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Geoheritage, Geoconservation and aspiring Geoparks in Morocco: The Zenaga inlier
Morocco presents a varied and globally important geology reflecting its successive continental-scale geological settings through geological time. Central, Southern and Western Morocco exhibits a rich geological history of crustal, magmatic, tectonic, metamorphic, sedimentary, and palaeontological features from Archaean to Quaternary, from large- to small-scale and varying in significance from international to local. The geostrategic position of Morocco renders it a land where many civilizations met through time leaving archaeological evidence of outstanding values - as such, its geodiversity is even more attractive as is related to historical, archaeological, architectural, cultural and traditional frames which vary with its geology. In this context, we focus on the central Anti-Atlas that consists of numerous Precambrian inliers amongst a matrix? of Palaeozoic strata, and each inlier has the merit to be transformed into thematic geoparks. Zenaga Inlier is a good example of an ensemble of geological features that have a regionally important geological story and geodiversity that can be transformed into a geopark. In this paper, we present geological information on potential geosites in the Zenaga region of the Central Anti-Atlas which could form the bases for future geopark
Géochimie des roches méta-volcaniques de l'ophiolite de Tasriwine, Anti-Atlas, Maroc: nature des sources et environnement tectonique de mise en place
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Implications of petrostructural analysis on the tectonic evolution of the Neoproterozoic Tasriwine ophiolite (Anti-Atlas, Morocco)
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[Pan-African overprint on Eburnian granitoids at the northern boundary of the West African Craton, Zenaga Inlier, central Anti-Atlas, Morocco]
The Zenaga Inlier shows a comprehensive record of the Eburnian and PanAfrican Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the PanAfrican shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas. ((C)) 2002 Elsevier Science Limited. All rights reserved