DOI: 10.5176/2251-3353_GEOS15.38
Authors: Emmanuel K. Abitty, Samuel B. Dampare and Daniel K. Asiedu
Abstract: The K-rich “Banso-type” granitoids intruding Palaeoproterozoic (Birimian) terrane of southern Ghana, part of the West African craton (WAC), have been studied for their major and trace elements compositions in order to deduce their petrogenesis and the tectonic setting in which they were formed. Unlike other granitoids from the Birimian terrane of Ghana, these late K-rich granitoids and their counterpart, the “Bongo-type” granitoids, have received little attention. Meanwhile, these granitoids are important for attempts to fully understand the overall geodynamic significance of the much debated ‘Birimian’ terrane of the WAC. In this study, we juxtapose whole-rock geochemical data from ‘Banso-type’ granitoids with existing data from ‘Bongo-type’ granitoids to fully represent the late K-rich granitoids in the Birimian of Ghana. Both suites display identical geochemical characteristics, consistent with magmas melted under water-saturated conditions. They are both calc-alkaline, high-K to shoshonite series rocks, and may have been generated by similar processes from a common source. The ‘Banso-type’ granitoids are enriched in LREE (La/Sm)N = 1.37–5.83, (La/Yb)N = 2.12–39.82, and a relatively flat HREE pattern. The late K-rich melts in the ‘Birimian’ of Ghana may have had a lesser amount of mantle interaction. They both possess decoupled LILE–HFSE signatures, a negative Ta–La anomaly and a pronounced negative Ti anomaly. The granitoids show typical subduction-related geochemical signatures, with evidence of influence of crustal materials/sediments and/or an altered oceanic crust. The K-rich granitoids are, thus, classified as granitoids emplaced in a volcanic arc geotectonic environment, consistent with previous studies on other Birimian granitoids from Ghana.
Keywords: petrogenesis; geochemistry; Ghana; Birimian; K-rich granitoids
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