Geochemistry of the Abiete-Toko komatiite-like ultrabasites: Petrogenesis, evolution and geodynamic implications

Abstract

The Greenstone Belt of the Paleoproterozoic Nyong Unit, in the northwestern Congo Craton, consists of metasediments and basic-to ultrabasic magmatic rocks. Ultrabasic rocks are of two types in the Abiete-Toko portion of the Nyong Unit. Type 1 has a mesh texture and consists of olivine (∼40 vol %), serpentine (∼30 vol %), orthopyroxene (∼15 vol %), clinopyroxene (∼5 vol %), brucite (∼5 vol %) and magnetite (∼5 vol %), while type 2 typified by spinifex texture is composed of serpentine (∼30 vol %), clinopyroxene (∼25 vol %), magnetite (∼20 vol %), hornblende (∼10 vol %), plagioclase (∼10 vol %) and pyrite (∼10 vol %). These petrographic features indicate that Abiete-Toko ultrabasic rocks have experienced both alteration of the main petrogenic minerals (olivine and pyroxene) and greenschist facies metamorphism. The behaviour of incompatible elements (HFSE and LREE) combined with chemical index of alteration values (35.6 = CIA ≤75.9), suggest that the investigated rocks have retained much of their original chemistry. Olivine is highly forsteritic (Mg# between 89.73 and 90.65), and contains high FeO (9.14–9.89 wt %), low NiO (0.45–0.47 wt %) and CaO (˂ 0.1 wt %), thus suggesting a mantle source. Based on bulk-rock chemical composition, the ultrabasites of the Paleoproterozoic Nyong Unit are hyper-magnesian [(MgO)adj ˃ 39 wt %] and have the Al-undepleted komatiites signature (Munro type). The low (0.17–0.70) (CaO/Al2O3)adj, low (0.60–0.94) (Gd/Yb)N and high (18.18–62.5) (Al2O3/TiO2)adj ratios, associated with the high ƩHREE (0.38–2.03 ppm), Y (0.87–3.57 ppm) and Zr (2–55 ppm) contents, suggest a shallow upper mantle source. The observed chemical characters including, (MgO)adj, (FeO)adj, (Al2O3/TiO2)adj, (CaO/Al2O3)adj, and Na, Ni, Cr, Y, Zr, Hf and REE contents reveal that the parent magma of these rocks was generated through partial melting of a heterogeneous and depleted mantle, at temperatures close to 1550 °C. Multi-element discrimination diagrams are consistent with crystallisation of these rocks in the arc setting, thus suggesting a Paleo subduction zone in the north-western part of the Congo Craton. These results are in favour of plate tectonic dynamics already operating in a hotter and ancient Earth environment point of view.