Timing and tectonic implications of the Pan-African Bangangte syenomonzonite, West Cameroon: Constraints from in-situ zircon U-Pb age and Hf-O isotopes

The Bangangte pluton is a SW-NE elongated (5 x 20 km) massif located in the southeastern part of the Pan-African North Equatorial Fold Belt in Cameroon, consisting of two units with dominant monzonites in the south and syenites in the north. SIMS U-Pb zircon dating yields consistent emplacement ages of 585 +/- 4 Ma and 583 +/- 4 Ma for the southern unit and the northern unit, respectively. The Bangangte rocks display typical shoshonitic compositions characterized by Na2O + K2O > 5 wt%, K2O/Na2O similar to 2, enrichment in LILE and LREE, but depletion in HFSE. Rocks from both units have similar O-Hf isotopes, with the monzonite zircons from the southern unit showing slightly higher delta O-18 (7.0 +/- 0.4 parts per thousand) but lower epsilon Hf(t) (-15.3 +/- 1.4) value than the syenite zircons from southern unit (delta O-18 = 6.0 +/- 0.4 parts per thousand; epsilon Hf(t) = -14.0 +/- 2.0). They were generated by partial melting of an enriched mantle source metasomatized by previous subduction processes, accompanied by crystal fractionation of pyroxene, Ti-Fe oxides and apatite, as well as crustal contamination to varying degrees. These rocks display a transitional geochemical feature of the subduction-related and within-plate shoshonites, suggesting that they were most likely emplaced in a post-collisional setting at the waning stage of the Pan-African orogeny. (C) 2016 Elsevier Ltd. All rights reserved.