UPb and SmN edvidence for Eburnian and Pan-African high-grade metamorphism in cratonic rocks of southern Cameroon

Abstract

UPb analyses of zircons and SmNd mineral and whole-rock analyses of samples from southern Cameroon document three distinct rock-forming or high-grade metamorphic events: ∼2900 Ma, ∼2050 Ma, and ∼600 Ma. Zircons from the Ebolowa charnockite in southern Cameroon yield an age of 2896±7 Ma, consistent with published RbSr total rock data for the region. SmNd crustal formation ages for these rocks are ∼3000 Ma, indicating that the granitoids and Ntem Series gneisses in the north end of the Congo Craton formed ∼2900 Ma ago and do not contain major contributions from older Archean crust. A second metamorphic event of Eburnian age is identified from re-equilibration of Sm and Nd in gneisses and metadolerites of the Ntem Complex at ∼2050 Ma. Zircons from the younger Nyong Series gneisses are a complex mixture of cores and overgrowths. The overgrowths in most cases formed during a high-grade metamorphic event between 2100 and 2000 Ma and clearly postdate the Ebolowa charnockite. The ages of the cores are not precisely known, but we believe most represent detrital zircons from 2900 Ma protoliths. SmNd crustal formation ages for samples of the Nyong Series gneisses are generally 2500 to 2900 Ma, indicating that their precursors were apparently derived mainly from the Archean craton with variable amounts of younger component. SmNd garnet-total rock isochrons for metamorphosed mafic rocks in the Nyong Series yield ages of ∼2050 Ma, and zircons from one of them yield an upper intercept age of 2037±10 Ma, further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 Ma. We interpret the Nyong Series metasedimentary gneisses as post-2900 Ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 Ma Eburnian orogeny; we cannot define precise limits on the ages of sedimentation. Preliminary data on metamorphosed intrusive rocks associated with the Yaoundé Series yield UPb concordant ages around 620 Ma, consistent with SmNd garnet—whole-rock ages from the same series. This dates the Pan African tectono-metamorphic activity in the southern end mobile belt. The Pan African lower intercepts of zircons and SmNd garnet-whole-rock ages from thNyong Series are the consequences of this event, which is further represented by a ∼590 Ma zircon age from a late syenitic pluton. Results from Cameroon are consistent with recent results from the western part of the Congo Craton farther south and from the northeastern part of the São Francisco Craton in Brazil. One interpretation of these results is that the Congo and São Francisco cratons were joined during the Ebumian-Trans-Amazonian orogeny ≈2100 Ma and are bordered to the north by a late Mesoproterozoic rift basin that was closed during the Pan-African-Brasiliano orogeny ∼600 Ma.