GEOCHEMISTRY AND TECTONIC SETTING OF METAVOLCANICS AND ASSOCIATED AMPHIBOLITES IN THE PALEOPROTEROZOIC WINNEBA-KIBI VOLCANIC BELT OF GHANA

Geochemical data are presented for metavolcanic rocks and associated amphibolites in the NE–SW trending Paleoproterozoic Winneba–Kibi greenstone belt of Ghana, part of the West African Craton. On the basis of the geochemical data, we discuss the tectonic setting of the belt during the Paleoproterozoic.

The analyzed metabasalts show LREE-enriched patterns, and display Nb–Ta troughs, minor negative Zr–Hf and Ti anomalies. The metadacites show fractionated REE patterns, and exhibit negative Nb–Ta, Zr and Ti anomalies. The metadolerites display LREE-enriched patterns, enrichment in LILE relative to HFSE and enrichment in LREE relative to HREE with slightly positive Nb and Ti anomalies. Three compositional groups of amphibolites are recognized. Group I is characterized by low TiO₂ contents, high Mg# (72–84), LREE-enriched patterns, and they display pronounced negative Nb–Ta anomalies, minor or large negative Zr–Hf and Ti anomalies. Group II has moderate TiO₂ contents, Mg# of 50–64, nearly flat REE patterns, and they display large negative Nb–Ta anomalies, slightly negative Zr and Ti anomalies. Group III (high-Ti) amphibolites with low Mg# (35–40), are enriched in high-field-strength incompatible elements (Zr, Nb, Ti, Y), especially Ti (TiO₂ = 2.67–4.37 wt.%), and are characterized by high REE abundances and LREE-enriched patterns, slightly negative and positive Nb–Ta and Ti anomalies.

The metabasalts, metadacites and Groups I and II amphibolites exhibit characteristics of subduction zone-related magmas, such as negative Nb–Ta, Zr–Hf and Ti anomalies in the N-MORB/primitive mantle-normalized trace element diagrams. The metadolerites and Group III amphibolites mostly demonstrate evidence of an OIB component (e.g., high Nb, Ta and Ti) in their source region. Also, the metadacites fall mainly in the field of within-plate basalt/OIB whereas the metabasalts and metadacites plot in the fields of volcanic-arc and back-arc on a number of trace element discrimination diagrams, and trace element ratio plots, including Th/Yb–Nb/Yb and Ce/Yb–Th/Nb.

In conclusion, the geochemical characteristics indicate an island-arc and back-arc basin affinity for most of the metavolcanic–plutonic rocks in the Winneba–Kibi greenstone belt, but OIB signatures are evident in some of the rocks.