Whole Rock Geochemistry and Geodynamic Evolution of Paleoproterozoic Gneisses, Ako’ozam-Njabilobe Area, Southwestern Cameroon

Abstract

The Ako’ozam–Njabilobe area, which is part of the Nyong group, is characterized mainly by highly deformed gneisses that host amphibolites, and metadolerites as enclaves. The gneisses within the Ako’ozam-Njabilobe area were investigated to constrain their geotectonic environment as well as understand their geodynamic evolution within the Nyong group. Three litho-types of gneisses outcrop within the Ako’ozam-Njabilobe area, consisting of horblende-biotite, quartzofeldspathic, and garnet gneisses. They are strongly peraluminous, with low Yb<SUB>N</SUB>, Rb/Sr, K₂O/Na₂O, and enriched in large ion lithophile elements (LILE). The gneisses within the Ako’ozam-Njabilobe area have a trachy-andesite parentage. The negative anomalies in high field strength elements (HFSE) and enrichments in LILE and light rare earth elements (LREE) suggest a subduction setting for this magmatism. The geochemical signatures including, low K₂O/Na₂O ratio, silica content (67.18 to 73.65 wt%), Yb and Y contents less than 1 and 10 ppm respectively, and low heavy REE values of the studied gneisses are akin to chemical signatures of archean crustal rocks. The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination. The similarity in whole rock data of the three lithotypes of gneisses within the Ako’ozam-Njabilobe area suggests a possible mechanical mixing during emplacement. The chemistry of the gneisses indicates a possible Archean inheritance during the emplacement of their protoliths, as such could provide insights into the petrogenetic processes controlling crustal growth and aid in understanding the Archean-Proterozoic transition within the Nyong group.