The late Neoproterozoic–early Palaeozoic extension along the West African craton and the Peri-Gondwanan terranes
Published:January 01, 2008
2008. "The late Neoproterozoic–early Palaeozoic extension along the West African craton and the Peri-Gondwanan terranes", The Boundaries of the West African Craton, Nasser Ennih, Jean-Paul Liégeois
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An interval of episodic carbonate productivity, lithostratigraphically recognized as the ‘Calcaires inférieurs’ (upper member of the Adoudou Formation), took place across the Neoproterozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the ‘Calcaires inférieurs’ was highly variable: in relatively stable substrates, a peritidal-dominated mixed platform is recorded where deposition was primarily controlled by autocyclic processes and accommodation space availability, whereas, in unstable substrates, the tectonic activity associated with the inherited block-faulting basement led to deposition of complex slide sheets composed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part of the ‘Calcaires inférieurs’ displays a negative δ13C shift reaching values of −6.5‰. This shift may represent the δ13C excursion to −6‰ that marks the Neoproterozoic–Cambrian boundary in the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. They consist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO2 gap between 52 and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. The felsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, P and Ti anomalies, and were probably generated as a result of either fractional crystallization coupled with relative crustal contamination, or from a different magmatic source. The lower basic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatism of the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.
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The Boundaries of the West African Craton
The boundaries of rigid cratons can be affected by subsequent orogenic events, leading to ‘metacratonic’ characteristics not often properly recognized and still poorly understood. Major lithospheric thickening is absent and early events such as ophiolites are preserved; however, metacratonic boundaries are affected by major shear zones, abundant magmatism and mineralizations, and local high-pressure metamorphism.
West Africa, marked by the large Eburnian (c. 2 Ga) West African craton, the absence of Mesoproterozoic events, the major Pan-African (0.9–0.55 Ga) mobile belts that generated the Peri-Gondawanan terranes, and the weaker but enlightening Variscan and Alpine orogenies, is an excellent place for tackling this promising concept of metacratonization.
The papers in this book consider most of the West African craton boundaries, from the reworking of the Palaeoproterozoic terranes, through the Pan-African encircling terranes, the late Neoproterozoic-early Palaeozoic extension period and the Peri-Gondwanan terranes, the Variscan imprint to the current situation.