Geochemistry of subduction-related mafic to felsic volcanic rocks of the late Archean Wawa greenstone belts, Superior Province, Canada

Abstract

The late Archean (2.7 Ga) Schreiber-Hemlo and White River-Dayohessarah greenstone belts are characterized by tectonically juxtaposed volcanic and siliciclastic sedimentary rocks, which were collectively intruded by the syn- to post-kinematic, high-Al, high La/Ybn tonalite-trondhjemite-granodiorite (TTG) plutons. The volcanic rocks are composed of two distinct types of sequences: (1) komatiitic to tholeiitic basalts ocean plateau sequences and (2) mafic to felsic volcanic arc sequences. Based on major and trace element abundances, mafic to felsic, arcrelated volcanic rocks of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts are divided into three major suites: (1) mafic to intermediate tholeiitic flows, (2) mafic to intermediate calc-alkaline flows, and (3) felsic calc-alkaline flows. All these suites share positively fractionated REE patterns, and negative anomalies of Nb and Ti. However, some intermediate and felsic volcanic rocks have positive Zr and Hf anomalies. Fractionated REE patterns, and negative Nb and Ti anomalies with respect to the neighbouring REE are all consistent with a juvenile oceanic island arc origins a metasomatized mantle wedge source for the tholeiitic suite. In comparison to tholeiites, the mafic to intermediate calc-alkaline suite is distinct in terms of: (1) enrichment in MgO, Th and LREE, Cr, and Ni; (2) higher Al2O3/TiO2 ratios; (3) more fractionated REE patterns; (4) more pronounced negative Nb and Ti anomalies; and (5) less fractionation between HREE and Sc-V, suggesting deeper and more primitive mantle source compositions for the calc-alkaline suite than the tholeiitic counterpart. Felsic rocks are defined by the enrichment of Zr and Hf, with respect to MREE, and correlate negatively with Sc abundances, suggesting garnet + amphibole ± clinopyroxene residual in the source. The inferred presence of garnet and clinopyroxene in the source is consistent with an independent argument for ecologitic residues in the slab. The existence of overlapping fields between mafic, intermediate, and felsic suites on trace element ratio diagrams suggest that processes controlling the production of these volcanic rocks were more complex than simple slab and/or wedge melting. This complexity may have resulted from a mixture of slab and wedge melts, second stage melting, magma mixing, fractional crystallization, partial equilibration with sub-arc wedge peridotite, crustal contamination, or some combination. Higher MgO, Ni, and Cr contents at a given Mg# in the basalts and andesites are consistent with higher geothermal gradients at Archean subduction zones than Phanerozoic counterparts.