U-PB AND SM-ND EVIDENCE FOR EBURNIAN AND PAN-AFRICAN HIGH-GRADE METAMORPHISM IN CRATONIC ROCKS OF SOUTHERN CAMEROON

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: approximately 2900 Ma, approximately 2 050 Ma, and approximately 600 Ma. Zircons from the Ebolowa charnockite in southern Cameroon yield an age of 2896 +/- 7 Ma, consistent with p ublished Rb-Sr total rock data for the region. Sm-Nd crustal formation ages for these rocks are approximately 3000 Ma, indicating that the g ranitoids and Ntem Series gneisses in the north end of the Congo Crato n formed approximately 2900 Ma ago and do not contain major contributi ons from older Archean crust. A second metamorphic event of Eburnian a ge is identified from re-equilibration of Sm and Nd in gneisses and me tadolerites of the Ntem Complex at approximately 2050 Ma. Zircons from the younger Nyong Series gneisses are a complex mixture of cores and overgrowths. The over-growths in most cases formed during a high-grade metamorphic event between 2100 and 2000 Ma and clearly postdate the E bolowa charnockite. The ages of the cores are not precisely known, but we believe most represent detrital zircons from 2900 Ma protoliths. S m-Nd crustal formation ages for samples of the Nyong Series gneisses a re generally 2500 to 2900 Ma, indicating that their precursors were ap parently derived mainly from the Archean craton with variable amounts of younger component. Sm-Nd garnet-total rock isochrons for metamorpho sed mafic rocks in the Nyong Series yield ages of approximately 2050 M a, and zircons from one of them yield an upper intercept age of 2037 /- 10 Ma, further documenting the existence of a major event with magm atism and charnockitization at 2000 to 2100 Ma. We interpret the Nyong Series metasedimentary gneisses as post-2900 Ma cratonic cover that w as deformed and metamorphosed during the 2000 to 2100 Ma Eburnian orog eny; we cannot define precise limits on the ages of sedimentation. Pre liminary data on metamorphosed intrusive rocks associated with the Yao unde 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-roc k ages from the Nyong Series are the consequences of this event, which is further represented by a approximately 590 Ma zircon age from a la te syenitic pluton. Results from Cameroon are consistent with recent r esults from the western part of the Congo Craton farther south and fro m the northeastern part of the Sao Francisco Craton in Brazil. One int erpretation of these results is that the Congo and Sao Francisco crato ns were joined during the Eburnian-Trans-Amazonian orogeny approximate ly 2100 Ma and are bordered to the north by a late Mesoproterozoic rif t basin that was closed during the Pan-African-Brasiliano orogeny appr oximately 600 Ma.