Geochemistry of Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt, Ghana: Petrogenetic and tectonic setting implications

Abstract

Geochemical data are presented for Paleoproterozoic metavolcanic rocks from the southern Ashanti volcanic belt with the aim of inferring their petrogenesis and tectonic setting in which they were formed. The metavolcanic rocks, which are predominantly basalts/basaltic andesites and andesites, have high Cr and Ni contents, indicating that they have not undergone significant fractional crystallization from mantle-derived melts. Two types of basalts/basaltic andesites, Type I and Type II, were identified. The Type I basalts show flat to slightly LREE-depleted patterns with (La/Sm) N = 0.69–1.03, (La/Yb) N = 0.57–0.71 and minor negative and positive Eu anomalies (Eu/Eu* = 0.92–1.24). The Type II basalts/basaltic andesites show fractionated REE patterns with (La/Sm) N = 1.34–2.31, (La/Yb) N = 2.08–4.25 and minor positive Eu anomalies (Eu/Eu* = 1.09–1.13). The andesites also show fractionated REE patterns, (La/Sm) N = 1.97–2.78 and (La/Yb) N = 4.11–8.48 with minor positive to non-existent Eu anomalies (Eu/Eu* = 0.99–1.15). N-MORB-normalized, trace element patterns show that the Type II basalts/basaltic andesites and the andesites have geochemical patterns characterized by enrichment in LILE relative to HFSE and in LREE relative to HREE. The andesites and the Type II basalts/basaltic andesites exhibit characteristics of subduction zone-related magmas; i.e., the former displays strong Nb and Ti anomalies and relatively high Th/Nb ratios (0.69–95) whereas the latter displays strong negative Nb anomalies but relatively smaller Ti anomalies and lower Th/Nb ratios (0.35–0.37). The Type I basalts/basaltic andesites are generally moderately enriched in LILEs and depleted in HFSEs and HREEs relative to N-MORB. Like the andesites and the Type II basalts/basaltic andesites, the Type I basalts/basaltic andesites display subduction-related trace element characteristics of positive Ba and Sr anomalies together with slightly negative Nb–Ta anomalies. The low Th/Nb ratios of 0.06–0.11 coupled with their MORB-like REE patterns indicate no or negligible amount of sedimentary component in the Type I basalts/basaltic andesites. Crustal contamination may be ruled out as the cause of the negative Th and HFSE anomalies observed in the basaltic and andesitic rocks. Rather the anomalies are interpreted, on the basis of Th–Nb–La–Ce, to reflect a recycled slab-derived lithosphere component. The LREE-depleted tholeiitic Type I basalts/basaltic andesites exhibit back-arc basin geochemical signatures. The calc-alkaline andesites and the Type II basalts/basaltic andesites show intra-oceanic island arc signatures. The high Mg basaltic/andesitic rocks may, however, have evolved in a forearc setting. We, therefore, infer an intra-oceanic island arc-forearc-backarc setting for the Paleoproterozoic Birimian metavolcanic rocks from the southern Ashanti volcanic belt. REE modeling indicates that the magma from which the Type I basalts were formed was generated by about 17–30% partial melting of a depleted mantle (DMM) composed of spinel lherzolite in an extensional marginal basin. The parental magmas of the high Mg calc-alkaline basaltic/andesitic rocks were produced in the forearc by about 20–30% partial melting of the metasomatized mantle, and the Type II basalts/basaltic andesites and the andesites are related by fractional crystallization.