Les Basaltes de l’Atlas de Béni-Mellal (Haut Atlas Central, Maroc): un Volcanisme Transitionnel Intraplaque Associé aux Stades de L’évolution Géodynamique du Domaine Atlasique

Abstract

RésuméLes roches basaltiques de l'Atlas de Béni-Mellal (Haut-Atlas Central, Maroc), mises en place, dans les différents bassins synclinaux de la région, se manifestent en deux horizons volcaniques B1 et B2. Les deux événements éruptifs sont d'âge Jurassique moyen pour les coulées de l'horizon B1 et d'âge Crétacé inférieur pour celles de l'horizon B2. Les roches sont caractérisées par un faciès des basaltes à olivines. La composition de leurs clinopyroxènes est marquée par deux tendances : augite au niveau des basaltes à olivine du B1 et diopside au niveau de ceux du B2. La composition géochimique sur roche totale dévoile le caractère transitionnel de ces roches. La relative dissimilitude géochimique signalée au niveau des basaltes des deux horizons peut être expliquée par un début du changement de la nature du magmatisme de la région en relation avec l'évolution géodynamique du domaine atlasique. Mots-clés : Atlas de Béni-Mallal; clinopyroxène; basaltes transitionnels; géodynamique du domaine atlasique AbstractThe basaltic rocks emitted at the synclinal basins of the Atlas of Béni-Mellal, show entablatures shapes constituted by interbedded lava flow successions with red silted-sandstone layers (Red Beds). In the synclinal basins of the area, basaltic lavas form two volcanic sequences B1 and B2 which correspond to distinct volcanic events. Pyroclastic deposits are rare. Basaltic outcrops show no pillow lavas. The lava flows were emplaced while the sediments were still soft and unconsolidated. The original flows fissure bet up to the open air, or under very shallow water. Ages of basaltic flows grouped around two distinct values that correspond to the Middle Jurassic (B1 basalts) and to the last of the Lower Cretaceous (B2 basalts). Basaltic rocks consist of olivine basalt. Textures are variable and dependent of the type of structure. Lavas flows typically show porphyritic texture at the bottom of entablatures, fluidal structure in its inner parts and vesicules at its upper zone. Clinopyroxene compositions range from augite in B1 to diopside in B2 basaltic horizons. Whole rock compositions mark the transitional character of these volcanic rocks. Normative composition show low nepheline contents (≤ 5%). LREE/HREE ratios are low and clearly less enriched than those obtained for alkaline magmatic series. Spidergram spectra are similar to those of Oceanic Island basalts (OIB). The relative geochemical dissimilitude reported at the both basaltic sequences B1 and B2, indicate, eventually, the changing of the magmatic regime, which becomes from tholeiitic to purely alkaline in Cenozoic, in relation to the geodynamic evolution of atlasic domain. This geodynamic and geochemical evolution is ruled by two major events: the opening of the Central Atlantic and the Mediterranean convergence.

The oldest durophagous teleosauroid (Crocodylomorpha, Thalattosuchia) from the lower Bathonian of central High Atlas, Morocco

“…Within the Taguelft syncline, the Tillouguit Formation is directly underneath the Guettioua Formation (Guezal et al . ). This formation is characterized by variegated silty marls and sandstones.…”

Microfacies and stable isotope features of the Lower–Middle Jurassic carbonate rocks of Western Saharan Atlas (Aïn Ouarka area, Algeria)

“…Two hydrothermal activity events seem to have occurred in this area: The first one during the Early Jurassic rifting episode was related to the late pulse of the CAMP volcanism (Nomade et al, 2007) linked to the opening of the Hispanic Corridor during the breakup of Pangea (Mercuzot et al, 2019). The second was during the Middle Jurassic (Bajocian–Bathonian) related to a magmatism event as basaltic flows, sills, dykes and intrusion bodies which are either interbedded with or cross‐cut the surrounding carbonated‐detrital formations in the High Atlas of Morocco (Guezal, Baghdadi, & Barakat, 2013). Synsedimentary faults and salt tectonics networks may thus have induced the migration of hydrothermal fluids to the seafloor.…”

Geological Structures Mapping Using Aeromagnetic Prospecting and Remote Sensing Data in the Karstic Massif of Beni Mellal Atlas, Morocco.

“…The Beni Mellal Atlas is an open and flattened anticline that abruptly dominates Beni Moussa plain through a fault system, the most important one is the North Atlasic Fault (Tadla overlap). It is the key area of Central High Atlas (Guezal et al, 2013), its geological history is therefore linked to this intracontinental chain (Michard,1976;Mattauer et al, 1977;Ziegler et al, 1995), whose various structural elements are a result of geodynamic evolution that began with distal tectonics at the end of Paleozoic and during Mesozoic by the opening of Tethys sea (Du Dresnay, 1972;Laville and Harmand, 1982;Beauchamp,1988 andPiqué et al, 1998). Its northern border is characterized by a succession of overlapping structural units that rest on the Liassic and Tertiary thrusts of Dir (Figure 2c).…”

“…This Figure 2-a) Geological map of the study area (Benzaquen, 1963;Monbaron, 1985 andVerset, 1985) important detrital deposit has been accompanied by Juarrasic-Cretaceous magmatic occurrences. The latter, outcrop in the synclinal basins of Beni Mella (Guezal et al, 2013 ) represented as two successive horizons interbedded in the sandstone-detrital series of "red beds" (Jenny et al, 1981;Haddoumi, 1988;Souhel, 1996) (Figure 2b):…”