Geochemistry of boninite-type volcanic rocks in the Frotet-Evans greenstone belt, Opatica subprovince, Quebec: implications for the evolution of Archaean greenstone belts

Low titanium (TiO2=0.35–0.67 wt.%), high silica (SiO2=49.0–59.8 wt.%) and magnesium-rich (MgO=4.5–13.1 wt.%) volcanic rocks in the Frotet-Evans greenstone belt (FEGB) of the Archaean Superior Province display U-shaped REE profiles, based on La/SmN (1.4–2.1) and Gd/LuN (0.67–1.1) ratios, that are inherent to Cenozoic boninites. These Archaean volcanic rocks, although not boninites in sensu stricto, show strong Nb and Ta depletions, enrichment in LREE and Th as well as distinct positive Zr and Hf anomalies in primordial mantle-normalised spidergrams. The Zr and Hf anomalies are characteristic of many Western Pacific boninite suites. The stratigraphic position of the Frotet-Evans boninite-type rocks is of significance because it differs from other known greenstone belts. The boninite-type rocks are contained within a predominantly tholeiitic basalt assemblage composed of transitional tholeiites, magnesian tholeiites, and ferrotholeiites. The studied sequence may either overlie adakitic-type pyroclastic rocks or be interstratified with andesitic to rhyodacitic calc-alkaline volcanic formations that compare favourably with oceanic arcs. This stratigraphic association of the FEGB differs from the Abitibi greenstone belt in which boninite-type volcanic rocks are interstratified with komatiites that formed a proto-arc sequence and were controlled by a combination of subduction- and plume-generated processes. In contrast, the proposed setting for the subduction-generated boninitic rocks of the FEGB is a marginal oceanic basin following forearc splitting and extension caused by hinge rollback.