Boninite series: low Ti-tholeiite associations from the 2.7 Ga Abitibi greenstone belt

Abstract

Boninite series volcanic flows, interfingered with komatiites and tholeiitic basalts, occur at several localities in the ∼2.7 Ga Abitibi greenstone belt. Flows from Whitney Township, Ontario, have a compositional range of SiO 2 44–60 wt%, MgO 24–7.4, Mg# 83–69, and Ni 930–200 ppm. Low TiO 2 (0.14–0.31 wt%) but high Al 2O 3 (13–25 wt%) contents yield variably high Al 2O 3/TiO 2 ratios of 48–100. These boninite series volcanics are characterized by fractionated HREE where Gd/Yb n 0.3–0.7; positive normalized Zr(Hf)/MREE anomalies, and Zr/Hf > 36; generally negative normalized Nb anomalies; and LREE depletion to enrichment (La/Sm n 0.72–1.4). Flows with similar compositional affinities occur in the neighbouring Kidd Volcanic Complex and Tisdale volcanic group. Alteration, and/or contamination by continental crust can be ruled out as the cause of the distinctive and coherent compositions. If the areally extensive komatiite–tholeiite association represents an ocean plateau derived from a mantle plume and the boninite series formed in a convergent margin, then the interfingering of komatiite and boninite series flows may represent interaction of a plume with a subduction zone.