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

Recycling of the Proterozoic crystalline basement in the Coastal Block (Moroccan Meseta): New insights for understanding the geodynamic evolution of the northern peri-Gondwanan realm

Detrital zircon age spectra from the siliciclastic rocks of the Lalla Mouchaa Calcschists and El Jadida Dolomitic formations (the Coastal Block of the Moroccan Meseta) are dominated by Paleoproterozoic and Ediacaran ages. The provenance of these two formations is a composite Proterozoic crystalline basement. El Jadida rhyolite (584.2 ± 4.8 Ma) represents the Ediacaran crystalline basement of the El Jadida dome. El Jadida rhyolite is unconformably overlain by the microbreccia, arkosic sandstone and dolostone of the El Jadida Dolomitic Formation with a maximum depositional age of ca. 539 Ma (Lower Cambrian). Detrital zircon-age spectra from El Jadida Dolomitic Formation (ca. 583–582 Ma) suggest direct recycling of El Jadida rhyolite as an exclusive original primary source. However, in the Western Rehamna massif, detrital zircon-age spectra from pre-Middle Cambrian microbreccia and arkosic sandstone of the Lalla Mouchaa Calcschists Formation (ca. 2.05–2.03 Ga) indicate exclusive recycling of the ca. 2.05 Ga-aged crystalline basement rocks (original primary source). Detrital zircon contents of the siliciciclastic rocks from these two formations of the Coastal Block are consistent with derivation from either Eburnian (Paleoproterozoic) or Cadomian/Pan-African (Ediacaran) igneous rocks. The discovery of this composite Proterozoic crystalline basement in the Moroccan Meseta stresses that Cadomian/ Pan-African magmatic arcs were built on an Eburnian basement in a paleoposition close to the West African craton, as part of the northern peri-Gondwanan realm

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

Archaean crustal evolution in West Africa: A new synthesis of the Archaean geology in Sierra Leone, Liberia, Guinea and Ivory Coast

A new synthesis of the geology and geochronology of the little-known Archaean rocks in Sierra Leone, Liberia, Guinea and Ivory Coast is presented in order to better understand the processes of Archaean crustal evolution in this region, and to attempt to interpret these data in the light of our current understanding of Archaean crustal evolution. In addition, this study seeks to identify those aspects of Archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies, given the economic importance of this region in terms of the mineral deposits hosted in the Archaean rocks. These include greenstone-belt hosted iron ore, lode gold, chromite and columbite-tantalite and younger diamondiferous kimberlites intrusive into Archaean felsic gneisses. The new results show that this cratonic nucleus comprises of four main geological units: 1. The oldest crust is made up of 3.5-3.6 Ga TTG (tonalite-trondjemite-granodiorite) gneisses. These only outcrop in the east of the craton in Guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks. 2. The major rock type found throughout the craton is 3.26-2.85 Ga TTG gneiss. In detail these magmas are thought to have formed in two episodes one between 3.05-3.26 Ga and the other between 2.85-2.96 Ga. The presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses. 3.4 Ga eclogite xenoliths in kimberlite derived from the sub-continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the TTG magmas. 3. Supracrustal rocks form linear belts infolded into the TTG gneisses and metamorphosed to amphibolite and granulite grade. They are of different sizes, contain a variety of lithological sequences and may be of several different ages. The larger supracrustal belts in Sierra Leone contain a thick basalt-komatiite sequence derived by the partial melting of two different mantle sources, unconformably overlain by a sedimentary formation. They are seen as an important resource for gold, iron-ore, chromite and columbite-tantalite. 4. A suite of late Archaean granitoids formed by the partial melting of the TTG gneisses in a craton wide deformation-metamorphic-partial melting event at 2800 +/- 20 Ma. This thermal event is thought to be responsible for the stabilisation of the craton. This new synthesis highlights major geological and geochronological similarities between the Archaean rocks of Sierra Leone, Liberia, Guinea and Ivory Coast and those in the Reguibat Shield in the northern part of the West African Craton suggesting that the two regions were once more closely related.UoD URS

Le magmatisme Precambrien du nord de la boutonniere de Zenaga (Anti-Atlas marocain) : etude petrologique et structurale

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc