New 207Pb/206Pb-Zr minimum evaporation, metamorphic 87Rb/86Sr- WR-Bt ages and tectonic imprints in the Archean So’o Group (Ntem Complex/Congo Craton, SW Cameroon)

Abstract

New ages were obtained from charnockites and tonalites collected in the So’o Group in the Ntem Complex. The rocks were analyzed for their petrography, tectonics and 207Pb/206Pb zircon minimum ages of their zircons as well as metamorphic 87Rb/86Sr isochron ages. The charnockites yielded zircon ages with a mean value of 2739±18 Ma interpreted as their intrusion age. This age is in agreement with previously published zircon Archean ages of charnockites and TTG from the NC. The So’o Group has been subjected to the D1 N-S compression emplacing the S1 sub-vertical foliation and L1 lineation, followed by a D2 E-W compression. The D2 induced subgrain reduction, area reduction feldspar and quartz dynamic recrystallization, N-S large-scale F2 folds and an “Archean nappe” on a largescale. The charnockites and tonalites metamorphic 87Rb/86Sr ages were significantly younger than the Archean 207Pb/206Pb zircon ages, showing that they were overprinted by later processes. Metamorphic 87Rb/86Sr isochron ages of charnockites and tonalites differ. The older age of 1969±170 Ma, which corresponds to the Eburnean orogeny
(2400~1800 Ma) was obtained in charnockites located near the Nyong Complex emplaced during the Congo-São Francisco craton collision. The younger 87Rb/86Sr isochron age of 1129±13 Ma, determined in tonalites located close to the border of the Yaounde Group represents the Kibarian/Greevalian orogeny. Both 87Rb/86Sr isochron ages are
assigned to the Eburnean and Kibarian/Greevalian cooling ages respectively.