Petrology And Geochemistry Of The K-Rich Granitoids In The Bole-Nangodi Belt Of The Paleoproterozoic Birimian Terrane Of Ghana

ABSTRACT

The Paleoproterozoic Birimian province of Ghana is made up basically of parallel and equally spaced NE-SW trending greenstone belts (with the exception of one which trends N-S), one of which is the Bole-Nangodi belt which outcrops in the northern parts of Ghana. These belts petrologically consists of metasedimentary and metavolcanic rocks, both of which are intruded by granitoids of various ages. These granitoids can broadly be grouped into four; the Cape Coast, Winneba, Dixcove and Bongo (and Banso) granitoids. Of interest to this study is the Bongo granitoids (2097±3 Ma) in the Bole-Nangodi belt. Petrology and geochemical studies of the granitoids of the Bole-Nangodi belt help unravel the petrogenesis and tectonic settings of these rocks, aiding in the generation of a robust model for the entire evolution of the Paleoproterozoic Birimian province of Ghana. Petrographically, these rocks are classified broadly as granite, pegmatite, aplite, granodiorite and sheared granodiorite. However, geochemical classifications categorizes these rocks into granite, granodiorite, mozonite and quartz mozonite. The rocks are mineralogical composed of K-feldspar, quartz, ±plagioclase, ±hornblende, ±pyroxene, ±biotite and ±muscovite. Accessories include, opaque oxides/sulphides, titanite, zircon, epiodote, allanite, carbonate, fluorite and apatite. The granitoids are K-rich (K2O, 3.11-5.49 wt.%) with the exception of some granitic gneisses (K2O, 1.31-2.80 wt.%), some granodiorite (K2O, 1.92 wt.%), pegmatite (K2O, 2.26 wt.%) and aplite (K2O, 2.26 wt.%). They have high SiO2 (61.78-81.89 wt.%), relatively low Na2O (3.28-5.07 wt.%) and low MgO (0.10-2.57 wt.%) contents with characteristically general high concentrations of Rb, Ba, Sr, Cr and Ni (with the exception of some granite, granitic gneiss and granodiorite). They can be classified as calc-alkaline metaluminous to slightly peraluminous highly fractionated I-type granitoids. They are however, highly evolved. They are mostly shoshonitic and

iii

magnesian, and are enriched in LILEs and LRREs and depleted in HFSEs and HREEs, with almost negligible negative Eu anomalies to slightly positive Eu anomaly. The 87Sr/86Sr and 143Nd/144Nd range between 0.7090-1.8622 and 0.5109-0.5129 respectively with initial 87Sr/86Sr and initial 143Nd/144Nd ratios calculated for a crystallization age of 2.1Ga in the range of 0.5987-1.8115 and 0.5099-0.5101 respectively. ԐNdt(2.1 Ga) values range from -1.0 to +8.3 with Nd(TDM2) model age of 2.2-1.7 Ga. The granitoids show affinity for volcanic arc and syn-collisional granites types. They are mostly late to post-orogenic and emplaced at temperature of approximately 680-750°C and pressures of 3.4 kbar for the granites and 17.1 kbar for the adamelites. The geochemical characteristics of these rocks indicate magma mixing and assimilation fractional crystallization of mafic melts from magma sources and crustal source melts (mostly felsic). Hence, initial thickening of crust by arc-collision (continental), followed by slab subduction under high pressure and temperature conditions resulting in underplating of mafic magmas in shallow crustal levels is likely to have generated these K-rich granitoids. However, there might have been a shift in tectonic regime from compressional to tensional as evidenced by crustal thinning in late-post collisional extension regimes as a result of slab roll-back and back-arc tectonic settings associated with the granitoids.