Geochemistry of a komatiitic, boninitic, and tholeiitic basalt association in the Mesoarchean Koolyanobbing greenstone belt, Southern Cross Domain, Yilgarn craton: Implications for mantle sources and geodynamic setting of banded iron formation

Abstract

Komatiite–basalt magmatism and Algoma-type banded iron formation (BIF) of Archean greenstone terranes have common time series, but there are few constraints on the geodynamic setting in which BIF were precipitated. The ∼3.0Ga Koolyanobbing greenstone belt (KGB), Southern Cross Domain, Yilgarn craton, exhibits an greenschist to lower amphibolite facies metamorphosed lithostratigraphic series, including four predominantly mafic to ultramafic volcanic sequences (S1–S4) with prominent BIF horizons (and local basal pyrite-dominated massive sulfide lenses) in between. Petrography, XRD, high-precision major, trace high field strength, and REE element chemistry of representative ultramafic and mafic rocks from S1, S2, and S3 are consistent with a komatiitic, boninitic, and tholeiitic basalt association. Komatiites are Al-depleted and Al-undepleted (Al2O3/TiO2 12–28 and (Gd/Yb)N, ≥1) endorsing a zoned plume and/or melting over a range of depths. The boninite-series rocks (Al2O3/TiO2 28–45 and (Gd/Yb)N, <1) preserve characteristic U-shaped REE patterns and positive Zr–Hf/MREE anomalies. Komatiites include uncontaminated members and counterparts contaminated by metasomatized continental mantle lithosphere, suggesting eruption proximal to a rifted arc setting, whereas boninite-series were erupted at an intraoceanic arc, collectively consistent with plume eruption into a convergent margin. Al-depleted and Al-undepleted komatiites plot close together to the mantle array on the Th/Yb versus Nb/Yb discrimination plot of Pearce (2008), implying a depleted, mildly heterogeneous, source for the mantle plume from which komatiites erupted. Boninites lie parallel to and above the mantle array, in the field of intraoceanic arcs, in keeping with their inferred oceanic convergent margin setting. Ratios of Nb/Yb extend to lower and higher values than komatiites implying a more heterogeneous upper mantle source of the subarc wedge.The KGB exhibits the first reported boninite-suite rocks in the Southern Cross Domain, Yilgarn craton. Conclusively, the komatiitic, boninitic, and tholeiitic basalt association suggests that BIF and basal massive pyrite lenses of the Koolyanobbing greenstone belt were precipitated from low-temperature submarine discharge of a seawater-dominated hydrothermal system driven by magmatic systems of a mantle plume erupting in a forearc convergent margin suprasubduction ophiolite.