New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement

Backscattered electron BSE-CL images, U-Th abundance, and U-Pb ages were obtained for zircons from the Nkogho granitic basement rocks cropping out in the Mamfe Basin (SW Cameroon). These data are used to characterize and classify each zircon, elucidate their geochemical traits. They were also used to formulate a model of their host composition to ascertain the source, and document any preserved postemplacement events. These zircons mainly form long to short prisms that are pyramidal to dipyramidal in shape. They typically exhibit complex oscillatory growth zoning, as well as exhibit sector zoning. These features are mainly compatible with igneous zircons, although a few examples have metamorphic signatures. The U (30 ppm–6 380 ppm), Th (4 ppm–1 280 ppm), and Pb (12 ppm–648 ppm) contents show core to rim variations with most values fall within the range of crustal granitic zircons. The Th/U ratios (0.08–1.23), with core to rim differences mainly encompass values typical of magmatic zircons with a few values characterizing metamorphic zircons that grew in equilibrium with an anatectic melt. The U-Pb ages (108.4±1.7 to 988.4±19.0 Ma) with some core and rim age differences date Early Neoproterozoic, Cryogenian-Ediacarian, Early Cambrian-Ordovician, Devonian-Carboniferous, and Aptian-Albian events. The arc-like Nkogho I-type granitoid crystallized from granitic magma during Cryogenian to Ediacarian times and was later affected by post-Ediacarian Cambrian to Albian magmatic events. Aptian-Albian ages probably reflect opening of the Mamfe Basin.