30 research outputs found
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 âParadoxides Shale Formationâ 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
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. 583â582 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. 2.05â2.03 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
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 NeoproterozoicâCambrian stratigraphy in the region of GoĂ€ida
(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 NeoproterozoicâCambrian 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 GoÀida granite and a pebble of granite included in the conglomeratic levels of the volcanicsedimentary
complex at Sidi Ali dome (central Rehamna massif). In the GoÀida 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 CryogenianâEdiacaran in the
range ~ 640â600 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
A new early Visean coral assemblage from Azrou-Khenifra Basin, central Morocco and palaeobiogeographic implications
A new early Visean coral assemblage has been recorded from turbidite facies in the southern part of the Azrou-Khenifra Basin, northwest of Khenifra, central Morocco. The newly discovered Ba Moussa West (BMW) coral fauna includes Siphonophyllia khenifrense sp. nov., Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, Cravenia rhytoides, Turnacipora megastoma and Pleurosiphonella crustosa. The early Visean age of the coral assemblage is supported by foraminiferal and conodont data, with the recognition of the basal Visean MFZ9 Zone. This confirms that the first transgression in the Azrou-Khenifra Basin was during the earliest Visean. The allochthonous coral assemblage was recovered from coarse-grained proximal limestone debris flow and turbidite beds within a fault-bounded unit, lying to the west of a thrust syncline containing upper Visean limestones. No evidence exists of the former early Visean shallow-water platform from which the corals were derived. All other in situ platform carbonate rocks around the southern margin of the Azrou-Khenifra Basin are probably of late Visean (AsbianâBrigantian) age. The early Visean Ba Moussa West coral fauna can be compared with that at Tafilalt in eastern Morocco, as well as in other Saharian basins of Algeria. Many of the genera and species in the Ba Moussa West assemblage are identical to those in NW Europe, with which it must have had marine connections. The new rugose species described, Siphonophyllia khenifrense, is probably endemic to North Africa. Its ecological niche in NW Europe was occupied by S. cylindrica or S. aff. garwoodi
Le Massif des Rehamna (Meseta marocaine) : tĂ©moin dâun changement majeur de la dynamique des plaques au CarbonifĂšre supĂ©rieur - Permien infĂ©rieur au sein de lâorogĂšne varisqueâ allĂ©ghĂ©nien
Lâabsence dâĂ©tude moderne dans le varisque marocain associant Ă©tude structurale, mĂ©tamorphique et gĂ©ochronologique freine toute avancĂ©e significative sur son Ă©volution tectonique et sur les corrĂ©lations gĂ©odynamiques avec les autres branches adjacentes de lâorogĂšne varisque-allĂ©ghĂ©nien en Europe et en AmĂ©rique du Nord au PalĂ©ozoĂŻque supĂ©rieur. Dans le massif des Rehamna (Meseta marocaine), trois Ă©pisodes tectoniques ont Ă©tĂ© mis en Ă©vidence. (1) Charriage vers le SSO de formations ordoviciennes sur le socle nĂ©oprotĂ©rozoĂŻque Ă cambrien et ses bassins intracontinentaux dĂ©vono-carbonifĂšres. Cet Ă©vĂ©nement entraĂźne un cisaillement ductile horizontal et un mĂ©tamorphisme prograde Barrovien au sein des roches enfouies. (2) Il rĂ©sulte de ce raccourcissement la formation dâun dĂŽme syn-convergent dâallongement ~EâO permettant lâextrusion des unitĂ©s infĂ©rieures et le dĂ©tachement des unitĂ©s supĂ©rieures mĂ©tamorphiques. Ces Ă©pisodes sont contraints Ă 310â295 Ma par les Ăąges 40Ar/39Ar de refroidissement et de cristallisation mĂ©tamorphique dâamphiboles et de micas. (3) Une convergence de direction ONO, orthogonal au prĂ©cĂ©dent, permet lâaccrĂ©tion finale de toutes les unitĂ©s sur le socle continental plus Ă lâouest. Les Ăąges 40Ar/39Ar de refroidissement dâamphiboles dâun leucogranite syn-tectonique et de son encaissant, ainsi que les Ăąges de cristallisation de muscovites dâune mylonite, dĂ©montrent que cet Ă©pisode prend place entre 295 et 280 Ma. La fin de lâorogĂšne varisque dans cette partie de la Meseta marocaine est contrainte par lâĂąge de refroidissement dâun batholithe granitique post-tectonique Ă 275 Ma. Ces Ă©vĂ©nements, confrontĂ©s dâabord Ă lâĂ©volution de la Meseta marocaine, sont enfin mis en relation avec la gĂ©odynamique globale des continents Laurentia et Gondwana Ă la fin de lâorogĂšne varisqueâallĂ©ghĂ©nien, montrant le changement des contraintes aux limites dans la chaĂźne au CarbonifĂšre supĂ©rieurâPermien infĂ©rieur au Maroc, en Europe et en AmĂ©rique du Nord (Appalaches).Non peer reviewe
Tectonometamorphic evolution of the Rehamna dome (Morocco)
Volume: 42 Host publication title: The 2014 CETeG Conference "LÄ
dek" Host publication sub-title: The Orlica-ĆnieĆŒnik Dome and the Upper Nysa KĆodzka Graben, the Sudetes 23-26 April 2014, LÄ
dek ZdrĂłj, Poland : Proceedings and Excursion GuideNon peer reviewe
Contrasting reef patterns during the evolution of the carboniferous azrou-khenifra basin (Moroccan Meseta)
Five types of reefs are described from the northern and southern parts of the Azrou-Khenifra Basin generated by the interactions of microbes and coral communities. The type 1 microbial reefs grew in both shallow- and deep-water settings, with a strong control by glacioeustasy. Type 2 microbial reefs developed in more tranquil periods, associated with common intermounds, and where only a single major regressive-transgressive sequence is recognised. Type 3 microbial reefs developed in constant deeper water conditions, generated by higher rates of subsidence in the basin, and creating an overall deepening-upward sequence. Type 4 microbial reefs recognised in the northern part of the basin have no clear counterparts in southern outcrops, but they are likely the capping strata observed in the latter area. Rugose corals allow to define a Type 5 reef, unrelated to microbial facies, and are recorded in oolitic-bioclastic backshoals or quiet inner platform settings. The presence of similar reefs in both the northern and southern parts of the basin demonstrates that conditions were not as different as previously proposed, and a lithostratigraphical, environmental uniformity occurs, which permits the analysis of different subsidence rates and glacioeustastic influence. In the Azrou-Khenifra Basin, the reefs, as well as other regional features, suggest that the basin, overall, evolved from an extensional tectonic regime during the early Brigantian into a complex extensional or compressional regime during the early Serpukhovian, passing into a predominantly compressional phase during the late Serpukhovian in a polyphase tectonic inversion during the onset of the Variscan Orogeny in the region
Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny
Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310â295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.Peer reviewe