Geochemistry and petrogenesis of the K-rich ‘Bongo-type’ granitoids in the Paleoproterozoic Bole–Nangodi greenstone belt of Ghana

• I-type granitoids, generally more akin to the ‘belt-type’ granitoids in Ghana.
• Display typical subduction zone setting signatures.
• Emplaced in a volcanic arc geotectonic environment, with affinity to A2 sub type.
• May be suggestive of a post orogenic; i.e. a possible arc-back arc tectonic setting.
• Share geochemical similarities with high silica adakites (HSA).

The ‘Bongo-type’ granitoids intrude the northern portion (where it cross cuts the Tarkwaian), central and south-central portions of the Bole–Nangodi greenstone belt in Ghana. The non-foliated, porphyritic granitoids are characterised by microcline (5–18 mm), often set in a medium to fine grained matrix of quartz and plagioclase, contains granular quartz and show incipient development of mymerkites under thin section. The major mafic phases are a brownish-green variety of hornblende ± biotite, with sphene, epidote and apatite occurring as accessory minerals. The pink looking ‘Bongo-type’ granitoids are classified as granites with subordinate granodiorites and quartz monzonites. Whole-rock geochemical analyses show that the granitoids are magnesian and metaluminous to weakly peraluminous. On the basis of their Th–Co–K2O and SiO2 contents, the granitoids classified as high-K calc-alkaline and shoshonite series rocks. They show enriched LILE trends, decoupled LILE–HFSE patterns, negative Ta–Nb anomaly, pronounced negative Ti anomaly and variable positive and negative Sr anomalies. The K-rich granitoids from the middle and south-central portions of the Bole–Nangodi belt show the lowest concentrations of LILE and some HFSE with the most prominent negative Ti and Si anomalies. The ‘Bongo-type’ granitoids show I-type characteristics. Based on Ba–U–Th concentrations, they are interpreted to have had some interaction with crustal materials or some influence of subducted sediments. The granitoids unequivocally show subduction-related signatures and mimic high silica adakites (HSA). They show evidence of being emplaced in a volcanic arc geotectonic environment. Their geochemical similarities to A-type granitoids, however, suggest an arc–back-arc tectonic setting.