Magnesian andesites, Nb-enriched basalt-andesites, and adakites from late-Archean 2.7 Ga Wawa greenstone belts, Superior Province, Canada: implications for late Archean subduction zone petrogenetic processes

Abstract.

Magnesian andesites (MA) occur with 'normal' tholeiitic to calc-alkaline basalt-andesite suites in four greenstone belts of the 2.7 Ga Wawa subprovince, Canada. Collectively, the magnesian andesites span ranges of SiO₂=56–64 wt%, Mg-number=0.64–0.50, with Cr and Ni contents of 531–106 and 230–21 ppm, respectively. Relative to 'normal' andesites, the magnesian andesites form distinct trends on variation diagrams, with relatively high Th and LREE contents, uniform Yb over a range of MgO, more fractionated HREE, and lower Nb/Th_pm and Nb/La_pm ratios. Niobium-enriched basalts and andesites (NEBA; Nb=7–16 ppm), and an Al-enriched rhyolite (adakite) suite are associated in space and time with the magnesian andesites. Nb-enriched basalts and andesites are characterized by high TiO₂, P₂O₅, Th, and Zr contents, variably high Zr/Hf (36–44) ratios, and more fractionated HREE (Gd/Yb_cn=1.3–4.1) compared to the 'normal' tholeiitic to calc-alkaline basalt-andesite suites. The adakite suite has the high Al (Al₂O₃=16–18 wt%), high La/Yb_cn (21–43), and low Yb (0.4–1.2 ppm) of Archean tonalite-trondhjemite-granodiorite (TTG) suites and Cenozoic adakites, indicative of liquids derived mainly from slab melting. The basalt-andesite suites are not characterized by normal tholeiitic or calc-alkaline fractionation trends of major or trace elements. Rather, compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and/or andesites by adakitic liquids. The occurrence of magnesian andesites, Nb-enriched basalts/andesites, and adakites has been described from certain Phanerozoic arcs featuring shallow subduction of young and/or hot oceanic lithosphere. Adakites likely represent slab melts, magnesian andesites the product of hybridization of adakite liquids with mantle peridotite, and Nb-enriched basalts/andesites melts of the residue from hybridization. Geological similarities between the late-Archean Wawa greenstone belts and certain Cenozoic transpressional orogens with the MA-NEBA-adakite association suggest that subduction of young, hot oceanic lithosphere may have played an important role in the production of this arc-related association in the late Archean.