Boninite-like volcanic rocks in the 3.7–3.8 Ga Isua greenstone belt, West Greenland: geochemical evidence for intra-oceanic subduction zone processes in the early Earth

The 3.7–3.8 Ga Isua greenstone belt of southwest Greenland is characterized by variably metamorphosed, metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks. The voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units. The sedimentary rocks consist mainly of banded iron formation, cherts, conglomerates and siliciclastic turbidites. The least altered metavolcanic amphibolites (the Garbenschiefer unit) from the Central Tectonic Domain of the Isua greenstone belt are characterized by high Mg-number (0.60–0.80), MgO (7–18 wt.%), Al2O3 (14–20 wt.%), Ni (60–645 ppm) and Cr (60–1920 ppm) contents, but low TiO2 (0.20–0.40 wt.%), Zr (12–30 ppm), Y (6–14 ppm) and rare earth element (REE) concentrations. These compositional features collectively represent a coherent mafic to ultramafic suite. Chondrite-normalized REE patterns are concave upward. On the primitive mantle-normalized extended trace element diagrams, they are characterized by relative depletion of Nb, but with an enrichment of Zr, relative to neighboring REEs. Alteration, deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition. The average initial εNd value of these metavolcanic rocks is +2. Collectively, these geochemical characteristics are comparable to those of Phanerozoic boninites. Given the observation that in the Tertiary, boninites are exclusively associated with intra-oceanic subduction environments (e.g., Izu–Bonin–Mariana subduction system), this suggests that intra-oceanic subduction zone-like geodynamic processes were operating as early as 3.7–3.8 Ga.