Petrology and CHIME geochronology of Pan-African high K and Sr/Y granitoids in the Nkambe area, Cameroon

The Central African Belt in the Nkambe area, northwestern Cameroon represents a collisional zone between the Saharan metacraton and the Congo craton during the Pan-African orogeny, and exposes a variety of granitoids including foliated and massive biotite monzogranites in syn- and post-kinematic settings. Foliated and massive biotite monzogranites have almost identical high-K calc-alkaline compositions, with 73-67 wt.% SiO2, 17-13 wt.% Al2O3, 2.1-0.9 wt.% CaO, 4.4-2.7 wt.% Na2O and 6.3-4.4 wt.% K2O. High concentrations of Rb (264-96 ppm), Sr (976-117 ppm), Ba (3680-490 ppm) and Zr (494-99 ppm), with low concentrations of Y (mostly< 20 ppm with a range 54-6) and Nb (up to 24 ppm) suggest that the monzogranites intruded in collisional and post-collisional settings. The Sr/Y ratio ranges from 25 to 89. K, Rb and Ba resided in a single major phase such as K-feldspar in the source. Garnet was present in the source and remained as restite at the site of magma generation. This high K2O and Sr/Y granitic magma was generated by partial melting of a granitic protolith under high-pressure and H2O undersaturated conditions where garnet coexists with K-feldspar, albitic plagioclase. CHIME (chemical Th-U-total Pb isochron method) dating of zircon yields ages of 569 ± 12-558 ± 24 Ma for the foliated biotite monzogranite and 533 ± 12-524 ± 28 Ma for the massive biotite monzogranite indicating that the collision forming the Central African Belt continued in to Ediacaran (ca 560 Ma).