U-Pb geochronology of the El Jadida rhyolite and relation to possible Lower Cambrian recycling (Coastal block, Moroccan Meseta).

The El Jadida (Mazagan) dome, whose existence was reported as early as 1934 by Yovanovitch and Freys, constitutes one of the first outcrops of the Moroccan Meseta where the Precambrian (PIII?)-Paleozoic (Lower Cambrian?) boundary was established (Gigout, 1951; Cornée et al., 1984). Since then, it is listed as one of the few locations where the basement of the Moroccan Variscan belt can be observed (Hoepffner et al.. 2005; Michard et al., 2010).Despite, the absence of geochronological and biostratigraphic precise data to constrain the time interval recorded here, there are stratigraphic similarities that allow a correlation with the Ediacaran-Cambrian geological record of Anti-Atlas belt (Cornée et al., 1984). In this study, we developed a petrographic, geochemical and U-Pb geochronological study using zircon extracted from: (i) the El Jadida rhyolite with the aim of characterizing the magma source and estimate the age of crystallization; (ii) a microbreccia sampled at the base of the El Jadida Dolomitic Formation for determining provenance

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

New U-Pb zircon dating of Late Neoproterozoic magmatism in Western Meseta (Morocco)

We present new U-Pb zircon ages from magmatic rocks of the Western Meseta, part of the Moroccan Variscan belt. The Neoproterozoic–Cambrian stratigraphy in the region of Goäida (Aguelmous massif, SE of Moroccan Central Massif) consists of limestones with conglomeratic and felsic volcaniclastic levels, pelites and mafic volcanic rocks assigned to the Cambrian which unconformably overlie rhyolites, andesites and rhyodacites and felsic tuffs associated with the Aguelmous granite of probable Neoproterozoic age. The Neoproterozoic–Cambrian stratigraphy of the region of Sidi Ali is roughly similar and also includes a volcanic-sedimentary complex with limestones, arkoses rhyolites and conglomerates. These conglomerates contain pebbles of granite and rhyolite, whose source may be the Neoproterozoic basement. In order to constrain the age of the Precambrian felsic magmatism we sampled and dated zircons (LA-ICPMS) from the Goäida granite and a pebble of granite included in the conglomeratic levels of the volcanicsedimentary complex at Sidi Ali dome (central Rehamna massif). In the Goäida granite, zircon ages are Ediacaran ranging between ~ 610 Ma and ~ 540 Ma, with a discordia upper intercept age of 598±32 Ma (MSWD=0.04) that could be interpreted as the age of intrusion. However, if we consider only the two youngest ages we obtain a Concordia age of 590±3 Ma (MSWD= 0.34). In the Sidi Ali pebble sample the majority of zircon ages are Cryogenian–Ediacaran in the range ~ 640–600 Ma, with the youngest yielding a Concordia age of 609±2 Ma (MSWD=0.04), indicating the intrusion age of the granite from which the pebble derived. These results support the existence of Cadomian/Pan-African magmatism in the Western Meseta of Morocco, characterized by the intrusion of granites at ~ 609 Ma and ~ 590 Ma. This result is important for studies of sedimentary provenance and to improve paleogeographic reconstructions of the northern margin of Gondwana during the Neoproterozoic

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

Evidence of a Paleoproterozoic basement in the Moroccan Variscan Belt (Rehamna Massif, Western Meseta)

For the first time, an Eburnian magmatic event has been identified in the Rehamna Massif (Moroccan Variscan Belt, Western Meseta) located north of the South Meseta fault. The best estimate of the crystallization age of rhyolitic porphyry is given by a weighted mean 207Pb/206Pb age of 2050.6 ± 3 Ma (Rhyacian-Orosirian). The new U–Pb age obtained for rhyolitic porphyry from the Rehamna Massif is of great relevance for improving geological knowledge about the boundaries ofthe WAC because:(i)it overlaps the older Eburnian magmatic event described in the Anti-Atlas belt and the Icartian magmatic event of the European Variscan Belt; (ii) this suggests that exists Paleoproterozoic basement in the Western Meseta, a hundred kilometers further to the north of the South Meseta fault, as old continental crust slivers preserved in the Cadomian and Variscan belts; and (iii) this means that the Cambrian transgression in the Western Meseta probably took place based on a more complex structural architecture affecting the Precambrian basement composed not only of Ediacaran rocks, as has been suggested in the literature, but also with Paleoproterozoic rocks (ca. 2.05 Ga) as discovered in this study

Tectonometamorphic evolution of an intracontinental orogeny inferred from P-T-t-d paths of the metapelites from the Rehamna massif (Morocco)

International audienceNew petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome-like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re-heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma-rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe

Maximum extent of ice sheets in Morocco during the Late Ordovician glaciation

New field data demonstrate that during the Late Ordovician (Hirnantian) glaciation, an ice sheet expanding northwestwards over the Anti-Atlas range reached into the southern Meseta of northern Morocco. Its growth to a glacial maximum position resulted in extensive subglacial erosion and deformation including the development of soft-sediment striated surfaces and streamlined subglacial bedforms preserved between the High Atlas of Marrakech and Rehamna. These features imply that this ice mass extended >200 km further than previously thought, and increase its size by at least ca. 190, 000 km2 (comparable in area to the UK). Correlation between a measured section in the High Atlas of Marrakech and that of the southern Meseta identifies sedimentary evolution within an ice-contact system common to both. These findings imply that the West African Craton and northern Morocco were in full glaciological communication during the latest Ordovician. Palaeogeographic reconstruction shows that beyond the ice sheet, south and southeastward palaeoslopes persisted on the shelf. A palaeohigh beyond the main ice sheet was a major source for sand, feeding delta systems that grew along the shelf as far as the shelf break. This palaeohigh probably formed as a result of rift shoulder uplift and supported a satellite ice mass. In the eastern Meseta, a thick (350 m) underflow-dominated deep-marine fan was fed both from this shelf delta system and from glaciogenic debris derived fromthe main ice sheet. The occurrence of this unexpected deep-marine area in northern Morocco implies that continued northward advance of the ice sheet was hampered by a dramatic break in bathymetry. Two depositional units are recognised across the Meseta, containing four distinct sedimentary cycles, each recognised as a glacioeustatic response to the waxing and waning of ice masses elsewhere in West Gondwana

Cadomian arc recycling along the northern Gondwana margin: Source-inherited composition of Miaolingian rift-related rhyolitic rocks (Ossa-Morena Zone, SW Iberia)

Rhyolites and rhyolitic tuffs of the Freixo-Segóvia Volcanic-Sedimentary Complex of the Cambrian of the Ossa-Morena Zone (Variscan belt, SW Iberia) were analyzed for petrography, major and trace element geochemistry, Sm–Nd isotopes and U–Pb zircon geochronology in order to deduce magma sources. New U–Pb zircon age data indicate that Freixo-Segóvia rhyolitic rocks, previously assigned to the Terreneuvian, formed during the Miaolingian (ca. 509-505 Ma). These rhyolitic rocks exhibit calc-alkaline signature, LREE enrichment, nearly flat HREE patterns, negative Eu, Nb and Ti anomalies, and are chemically similar to the bulk continental crust. Freixo-Segóvia rhyolitic rocks have negative to slightly positive εNd(T) values (−2.8; 0.5) resulting in TDM model ages (1.0–1.3 Ga) that overlap the range defined by Terreneuvian Malcocinado andesites, formed in the transition of the Cadomian (West-African) arc to continental rifting in northern Gondwana margin. Based on the Sm–Nd isotopic data, the Freixo-Segóvia rhyolitic rocks may have resulted from of partial melting of andesitic crust. The presence of Ediacaran-age zircon in the Freixo-Segóvia rhyolitic rocks indicates inheritance from the Cadomian arc. Inherited zircon grains with West African affinity were probably transferred into the rhyolitic magma from an older igneous source formed in the Cadomian arc. Based on their major and trace element composition, combined with isotopic and geochronological data, the Freixo-Segóvia rhyolitic rocks record recycling of arc crust during a Late Cambrian rifting event along the northern Gondwana margin. The transition from Cadomian accretion to peri-Gondwana break-up leading to the opening of the Rheic Ocean is also known in other parts of the Variscan belt

The impact of the end-Ordovician glaciation on sediment routing systems: A case study from the Meseta (northern Morocco)

Assessment of sediment redistribution by end-Ordovician ice sheets is crucial for the reconstruction of Lower Paleozoic source-to-sink patterns. Focusing on the ice-distal, deepwater Tazekka depocenter (Moroccan Meseta), we performed a provenance study that combined whole-rock geochemistry, petrography and insights from high-resolution detrital zircon ages. The results show that the glacigenic sediments are compositionally — mineralogically and geochemically — more mature than preglacial strata. This observation points to a preferential cannibalization of the “great Lower Paleozoic quartz-rich sandstone sheet”, with a limited input of first-cycle, far-travelled clastic sediments. Differentiation of glacial units is not straightforward, yet the glaciation acme istypified by a highly mature sedimentary source and an age spectrum lacking Mesoproterozoic zircon grains, both features strongly indicating derivation from the Cambrian–Lower Ordovician cover of the Tuareg Shield. More regional sources are expressed during the earlier glaciation stages, during which lowstand remobilizations unrelated to subglacial erosion are also suspected. Subordinate but notable late Tonian (∼0.8 Ga) and latest Stenian to early Tonian (∼1 Ga) zircon populations are also evidenced inMorocco, which may have implications for future paleogeographic reconstructions