2883 Ma commencement of BIF deposition at the northern edge of Congo craton, southern Cameroon: New zircon SHRIMP data constraint from metavolcanics

New SHRIMP data for zircons separated from a metadacite interbedded with banded iron formation (BIF) in the Mbalam region of southern Cameroon provide a 2883+/-20Ma age for the commencement of BIF deposition in the Congo craton. Separated zircons from the associated metadacite are brown and euhedral to subheral in form with distinct oscillatory zoning typical of magmatic signatures. These zircons lack internal resorption features and serrated zone boundaries as should be expected of zircons inherited from the surrounding basement rocks or multiple magmatic plumbing events. However, zircons from a surface BIF sample collected similar to 60km west of the sequence from which the metavolcanics were sampled show a different age spectrum (Palaeoproterozoic, 1.0-2.0 Ga) suggesting a latter stage of BIF formation in the region which is not the focus of this work. Geochemical features of the metavolcanic samples analyzed identify the rocks to be rhyolitic, dacitic to andesitic in composition and possibly formed from a single magma by simple fractional crystallization. Trace element plots show negative Nb, Ta, Ti and Ho peaks that are characteristic of arc magmas. The study shows that initial BIF deposition in the Congo Craton was associated with early arc magmatism in the Archean. These ages correlate reasonably with early BIF deposition across the Atlantic in Brazil which was once linked to the Congo craton prior to the breakup of Gondwana.