Geochemistry of the early Cambrian succession in the western Anti-Atlas, Morocco: implications on provenance and paleoredox conditions

The Igoudine and Amouslek formations (TerreneuvianâCambrian Epoch 2 boundary) in the western Anti-Atlas of Morocco record the replacement of stromatolite-dominated microbial consortia by thrombolite-metazoan consortia. Carbonate and calcareous shales of both formations have been analyzed for major, trace, and rare earth elements to study their geochemical characteristics and evaluate the provenance of the terrigenous fraction and paleoredox conditions. Discrimination diagrams for the source rocks based on major elements and selected trace elements indicate that the terrigenous fractions of the sediments were likely derived from predominantly felsic rocks, and the source rocks have been identified to be the PaleoproterozoicâNeoproterozoic granites and metasediments of the Kerdous inlier. Paleoredox proxies such as U/Al, V/Al and Mo/Al suggest that the Igoudine and Amouslek formations were deposited in the oxic environment. Our data show that the local water column was prevailingly oxidized before, during and after the transition from the microbial consortium (stromatolite-dominated biota) to the thrombolite-archaeocyathan consortium and shelly metazoans within the studied interval. This implies that the seawater redox status was not driving this change in these biological communities

Response on the Comment of "The Mo-W-Cu, garnet, wollastonite, pyroxene and vesuvianite skarn of Azegour (High Atlas, Morocco)"

International audienceDans l’introduction de leur comment nos commentateurs (Touil A. et Hibti M.) estiment que « Les relations chronologiques et réactionnelles entre les différents stades hydrothermaux ainsi que leurs relations avec les minéralisations ne sont pas bien illustrées » dans notre article et ils proposent ce qu’ils appellent « des éléments de réponse et éventuellement de nouvelles données permettant une meilleure compréhension des altérations hydrothermales et leurs rôles dans la mise en place et la formation des minéralisations liées aux skarns d’Azegour ». Le problème de fond de ce comment est que dans notre article nous n’avons pas abordé l’étude des minéralisations, ni du point de vue descriptif ni du point de vue génétique. Ce n’était pas l’objectif de notre article, la mine étant aujourd’hui inaccessible, ce qui est clairement écrit dans les premières lignes de notre article

Le skarn Mo-W-Cu à grenat, wollastonite, pyroxène et vésuvianite d’Azegour (Haut-Atlas, Maroc)

International audienceThe Mo-W-Cu skarn of Azegour is generated at the expense of a sequence dominated by calcareous sedimentaryrocks Cambrian in age, within the metamorphic aureole of a Permian granite. Metasomatized layers constitutingthe skarn are several meters thick, and mainly composed of massive garnetite and/or wollastonitite and seldomly ofpyroxenite. Garnets belong to three different types easy to recognize due to their different colors: green-black,brown-black and orange, this later one being present only in the northern part (Toulkine zone) of the skarn. Thegreen-black garnet is very common, forming crystals up to 8 cm, and composed of andradite with locally a core ofAl-rich andradite (up to 5% Al2O3). The very common brown-black garnet form thick layers associated with quartz. Itoften reveals a core of pure andradite surrounded by a Al-rich andradite (up to 10% Al2O3), close to the grossular domain.Orange garnet is only grossular. Pourcentage of spessartine is always weak (4% maximum). Wollastonite isslighly manganiferous, while vesuvianite is F- and Cl-rich (up to 3.3% and 1.2% respectively). Mineralogical associationallow to establish the formation of the skarn around 620-650oC, under a pressure of 1.7-2 kbar, and a fCO2 of31 mole %, in a reducing environment (fO2 of 10-18 to 10-17 atm.). The skarn of Azegour marks the end of Hercynianorogeny and whitnesses of the perennity of a reducing environment highly favorable to sulphide crystallization.Mo-skarn of Azegour seems to have formed at higher temperatures than other Hercynian W-skarn such as Salau(France).Le skarn minéralisé en Mo, W et Cu d’Azegour se forme aux dépens d’une série cambrienne essentiellementsédimentaire carbonatée dans l’auréole de métamorphisme d’un granite permien. Les bancs métasomatisés plurimétriquesdu skarn sont pour l’essentiel des grenatites et wollastonitites massives, plus rarement des pyroxénites. L’étude minéralogiquemontre que les grenatites sont composées pour l’essentiel d’andradite (grenat vert-noir et grenat brun-noirzoné) plus rarement de grossulaire (grenat orangé). Le pourcentage de spessartite est toujours faible (maximum 4 %).La wollastonite est légèrement manganésifère, la vésuvianite fluorée (jusqu’à 3,3 %) et chlorée (1,2 %). Les paragenèsespermettent d’établir les conditions de formation du skarn aux alentours de 620-650 oC pour une pression de 1,7 à2 kbar, avec une fCO2 de 31 mole %, en contexte réducteur (fO2 de 10-18 et 10-17 atm.). Le skarn d’Azegour est unmarqueur de la fin de l’orogenèse hercynienne, et dénote la persistance de conditions réductrices, un environnementtrès favorable à la cristallisation de sulfures. Le skarn d’Azegour semble ainsi se former à des températures plus élevéesque d’autres skarns à tungstène hercyniens comme Salau (Ariège, France)

Petrographic and geochemical study of Jurassic-Cretaceous intrusive massifs (Gabbros-syenites) of the Eastern High Atlas, Morocco (Rich-Talsint axis)

Fieldwork, petrographic description and geochemistry are presented for the Upper Jurassic-Lower Cretaceous magmatic intrusions from the Mesozoic Eastern High Atlas in Morocco. The magmatic complexes are ellipsoid-shaped and a diapiric-dominated mechanism is suggested for the emplacement of the intrusions into Paleozoic faulted basement as well as in the overlying Triassic-Jurassic sedimentary rocks. The intrusions are massive and composed of mafic (olivine-, biotite-bearing gabbro, and ordinary gabbro), intermediate (diorite) and felsic (syenite) units. The major and trace element contents indicate transitional to alkaline affinities for the mafic and intermediate intrusions and alkaline for the felsic ones, evolved in an intracontinental setting. The data indicate a heterogeneous mantle source, likely metasomatized sub-continental lithosphere that had been enriched by plume-type melts. The Eastern High Atlas intrusives are part of a continuum of magmatic events that spanned from the Triassic Central Atlantic Magmatic Province to the late-Jurassic to early-Cretaceous magmatism of the High Atlas system

A c. 1710 Ma mafic sill emplaced into a quartzite and calcareous series from Ighrem, Anti-Atlas – Morocco: Evidence that the Taghdout passive margin sedimentary group is nearly 1 Ga older than previously thought

International audienceThe Taghdout Group is a passive margin sequence deposited during rifting and break-up of the northern margin of the West African craton (WAC), culminating with the creation of an oceanic basin between the northern edge of the WAC and an unknown terrane. However, the age of this passive margin has been poorly constrained. It was previously thought to be c. 800–1000 Ma on the basis of age of the contact metamorphosed host rocks of the associated mafic dykes (Rb/Sr, 789 ± 10 Ma). However, with the U-Pb dating of numerous dyke swarms in the Anti-Atlas Inliers, at c. 870, 1416-1380, 1650, 1750, and 2040 Ma, it was suggested by Youbi et al. (2013) that the Taghdout Group could be Mesoproterozoic in age, with a preference for an age of 1750 Ma. In order to test this idea, a mafic sill within the Taghdout Group has been dated by the ID-TIMS U-Pb method on baddeleyite, yielding an approximate age of c. 1710 Ma. This preliminary age confirms that the Taghdout Group is much older than previously thought. Further geochronology work is required to determine whether this c. 1710 Ma age represents a new intraplate event in the WAC or whether more concordant data could yield an age closer to the known WAC LIP event of c. 1750 Ma. With this result we propose a new lithostratigraphic framework for the Proterorozoic in the Anti-Atlas

U-Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas Draa Inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton

In the Bas-Draa Inlier (Anti-Atlas, Morocco), the Paleoproterozoic basement which is cut by the Ediacaran Taourgha granite is also crosscut by numerous dykes of a variety of trends, mostly of uncertain age. Two doleritic dykes are dated by the ID-TIMS U-Pb method on baddeleyite and yield emplacement ages of 1381 +/- 8 Ma (MSWD = 0.84) and 1384 +/- 6 Ma (MSWD = 1.4) determined for a N135 degrees E and a N40 degrees E trending dyke, respectively. These dates represent the first geochronological evidence of a Mesoproterozoic magmatic event in the Anti-Atlas. This magmatic event falls in the previously considered ca 1.7-1.0 Ga (Mesoproterozoic) gap in geological activity in the Anti-Atlas. The poorly dated Taghdout and Taarotihate sequences could represent remnants of the ca. 1380 Ma magmatism and rift-related sedimentation. The Mesoproterozoic sedimentary succession of the Atar Group in the Taoudeni basin (Mauritania) could also represent a good candidate for rift-related sedimentation but it postdates the 1380 Ma magmatic event by 270 Ma. The dated 1380 Ma dykes are transitional to mildly alkaline basalts, not unlike some Hawaiian lavas. However, these dykes have a distinct negative Nb anomaly (a common features in many Large Igneous Provinces, LIPs), and this requires interaction with the lithosphere. This interaction may have occurred at the level of the lithospheric mantle or the crust. These newly dated 1380 Ma dykes may converge to the north, speculatively suggesting a magmatic center (associated with a 1380 Ma mantle plume?) along the northern margin, and possibly linked to rifting and possible breakup on that margin, and also to a regional uplift that largely removed the evidence of a 1380 Ma cover sequence. Contemporaneous 1380-1390 Ma magmatism is reported elsewhere on other crustal blocks, and that in northeastern Laurentia (northern Greenland), northern Siberia (Anabar shield), and Baltica (southern Urals) can be reconstructed with that of the Bas Draa Inlier (Anti Atlas region of the West African Craton, WAC) into a single Large Igneous Province (LIP) extending over an area of >1 million km(2), and associated with the final fragmentation of the Columbia (Nuna) supercontinent. (C) 2012 Elsevier B.V. All rights reserved

Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/ chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.info:eu-repo/semantics/publishedVersio

Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/ chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.info:eu-repo/semantics/publishedVersio

Contrasting Paleoproterozoic granitoids in the Kerdous, Tagragra d’Akka, Agadir-Melloul and Iguerda inliers (western Anti-Atlas, Morocco)

International audienceThe northern margin of the West African Craton is characterized by Paleoproterozoic siliciclastic sediments intruded by ca. 2.1-2.0 Ga Eburnean granitoids. New geochronological data highlight the occurrence of a Rhyacian (2.10 Ga) tonalitic to trondhjemitic magmatic suite, following a coeval Orosirian (2.040-2.035 Ga) magmatic suite composed of calc-alkaline diorites, mesocratic granites and leucogranites.Whole-rock major and trace element compositions indicate that: (i) the Rhyacian tonalites and trondhjemites suggest a subduction environment, an island-arc arc or an active continental margin; (ii) the Orosirian plutonic rocks show a geochemical heterogeneity; (iii) the chemical features of diorites correspond to calc-alkaline M-type suites; (iv) granodiorites and granites are typical calc-alkaline I-type granitoids, and (iv) leucogranites, with peraluminous composition, reflect a crustal melting.While the Rhyacian magmatic event indicates a subduction environment, the Orosirian magmatic event is more complex, and the geochemical signatures imply involvement of mantellic magmas and crustal melting. The contemporaneity of these Orosirian magmatic suites and their siliciclastic sedimentary host-rocks suggest a back-arc geodynamic context, similar to what was described in the Reguibat Rise, Mauritania