Ultramafic–mafic and granitoids supra‐subduction magmatism in the southern Ashanti volcanic belt, Ghana: Evidence from geochemistry and Nd isotopes

Abstract

Geochemical and Nd isotope data are presented for Palaeoproterozoic ultramafic to mafic rocks and granitoids which are associated with volcanic rocks in the southern part of the Ashanti greenstone belts of Ghana. The Prince's Town granitoids display subduction‐zone geochemical features, with some showing signatures similar to high‐SiO 2 adakites (HSA). Their initial ε Nd (2.1 Ga) values range from −1.01 to +2.92, and they have T DM2 of 2.17–2.51 Ga. The gabbros show slightly LREE‐depleted and ‐enriched patterns, Th–U troughs, negative Nb, Zr–Hf and Ti anomalies, spikes in Sr and Pb, have ε Nd (2.1 Ga) values ranging from −1.23 to +5.23, and T DM2 values from 2.08 to 2.57 Ga. The Ahama ultramafic body is characterized by two groups of pyroxenites. Both groups show LREE enrichment patterns, display Th–U and Nb–Ta troughs, minor or pronounced negative Zr–Hf and Ti anomalies. However, the Group I pyroxenites exhibit minor Ce anomalies and lower total REE contents (28.1–32.8 ppm) whereas Group II pyroxenites show significant negative Ce anomalies and relatively higher total REE contents of 57.1–124 ppm. The pyroxenites show negative initial ε Nd (2.1 Ga) values (−0.58 to −5.68) and have T DM2 values of 2.52 to 2.75 Ga. The pyroxenitic and gabbroic rocks were most likely originated from a subduction‐related lithospheric mantle. The Aketakyi ultramafic–mafic complex is made up of mainly cumulate dunite, harzburgite, pyroxenites, and gabbro. The rocks display LREE‐depleted to flat patterns with low total REE contents (1.71 to 22.3 ppm) and demonstrate a supra‐subduction affinity. They show high positive initial ε Nd (2.1 Ga) values (+3.69 to +4.93), and T DM2 values of 1.99 to 2.04 Ga, which suggest that they were derived from depleted mantle magmas and were juvenile at their time of formation. The Nd isotopic data provides evidence for a possible contamination of the juvenile Birimian crust of the southern Ashanti belt by some amount of a pre‐Birimian (or Archaean?) crustal material. The trace element and isotopic signatures as well as the field relations of the studied granitoids and mafic–ultramafic rocks show they were derived through supra‐subduction‐related magmatism during the Palaeoproterozoic.