The I- and -S type granites from the Shissa pluton, Central African Fold Belt in Cameroon: petrogenesis and geodynamic implication

Situated at the central part of the Central African Fold Belt in Cameroon, the Shissa pluton consists of two-mica granites and biotite granites. All the granites are shoshonitic in nature and present high K 2 O/Na 2 O ratios (1.50–2.42). The biotite granites are subaluminous and present I-type character (A/CNK ranges 1.0–1.1). The two-mica granites are typically two-mica alkali granites with S-type signature (A/CNK ranges 1.1–1.21) and Al 2 O 3 -rich (Al 2 O 3 = 12.86–13.40 wt.%) than the biotite granites (Al 2 O 3 11.24–13.26 wt.%). The Shissa granites multi-element patterns show enrichment in LILE such as Rb, Th, U, and Pb and are depleted in HFSE such as Ti, Ba, and P, suggesting the crustal source with an inherited arc signature. All the granites present the Nb/Th ratio (0.2–1.2) significantly lower than that of the mantle (> 15), indicating the crustal source for the granites. The chemistry of the biotites suggests that the biotite granites are generated from the partial melting of already differentiated metabasite to metatonalitic source under high oxidizing conditions at the temperature between 908 and 1009 °C, while the two-mica granites were formed under moderated oxidizing conditions between 836 and 917 °C from the assimilation of felsic crustal component by the biotite granite magma during its ascent. The Shissa granites formed at a relatively hydrous environment (4 wt.% of the water content). The upwelling of the asthenosphere through slab break-off may have favored the partial melting of the meta-igneous lower crust by supplying the thermal anomaly as it is the case in the Central African Fold Belt in Cameroon. The whole-rock geochemistry and the biotite mineral chemistry suggest the collisional tectonic setting for the Shissa granite as several granitoid plutons in the CAFBC.