Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

Abstract

The Mewongo area of the Nyong Complex is located in the northwestern part of the Congo craton in southwestern Cameroon. The metavolcanosedimentary succession in this area comprises banded iron formations (BIFs), interlayered with amphibolites and gneisses and intruded by metamorphosed dolerite dykes. Within the framework of detailed field investigation, we report whole-rock geochemical data for the BIFs, metadolerites, and gneisses to constrain their provenance and tectonic setting, and present LA-ICP-MS U-Pb-Hf isotopic data for zircons from amphibolite and gneiss to assess the depositional age of BIFs from the Nyong Complex and understand the crustal evolution at the northwestern margin of the Congo craton. The geochemical composition of the Mewongo BIFs indicates a contribution of hydrothermal fluids, derived via submarine alteration of volcanics, and seawater. Geochemical and LA-ICP-MS U-Pb detrital zircon data for the gneisses reveal felsic protolith with ages between 2.97 and 2.65 Ga. Our zircon U-Pb-Hf isotope data suggest that a significant growth of juvenile crust occurred at ca. 2.7–2.2 Ga, followed by a reworking event at ca. 2.1 Ga with only minor juvenile crust addition. The metadolerites, cutting the metavolcanosedimentary succession, range in composition between tholeiitic and calc-alkaline basalts. They can be subdivided into two groups based on their trace element signature. The group I metadolerites have OIB-like geochemical composition (e.g., enriched LILE, HFSE, and LREE), while the group II metadolerites exhibit MORB-like geochemical signature (e.g., flat HFSE and HREE patterns). Metadolerites belong to two different so far undated magmatic events on the Congo craton that were previously recognized on the adjacent Ntem Complex.LA-ICP-MS U-Pb zircon dating of amphibolites, combined with chemical composition of metadolerites and gneisses, indicate deposition of the metavolcanosedimentary sequence at ca. 2.1 Ga in a pro-foreland basin sandwiched between the West and East Gabonian blocks, at the time when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny. The Nyong Complex BIFs were therefore deposited in a deep, open-marine setting during the ca. 2.1–2.0 Ga time interval.