Geochemical systematics of 2.7 Ga Kinojevis Group (Abitibi), and Manitouwadge and Winston Lake (Wawa) Fe-rich basalt–rhyolite associations: Backarc rift oceanic crust?

We present new geochemical data for Neoarchean Kinojevis and Manitouwadge–Winston Lake volcanic rocks from the formerly contiguous Abitibi and Wawa subprovinces. The 2702–2701 Ma Kinojevis Group (KG), Abitibi greenstone belt, is dominated by Fe- to Mg-rich basalt flows (Mg-number = 30–56) having a tholeiitic fractionation trend, and prominent magnetic striping. REE patterns are near-flat at 4 to 15 times chondrite. Normalized La/Sm and Gd/Yb ratios span 0.70 to 1.17 and 0.96 to 1.34 respectively, the latter indicative of a variable involvement of garnet in the residue. Basalts do not plot on mixing lines with average Archean upper crustal composition, and accordingly were likely erupted in an oceanic setting. Tholeiitic high-Si rhyolites also feature near-flat REE patterns, with pronounced negative Eu anomalies, consistent with extreme fractionation products of basaltic melts.Niobium contents and Zr/Nb ratios (1.7–6.4 ppm; 10–24) of basalts are mostly intermediate between respective values for Archean primitive oceanic arc tholeiites (PAT), and ocean plateau tholeiitic basalts (OPB: 2–9 ppm; 12–20) associated with komatiites, likely representing ocean plateaus erupted from mantle plumes. Similarly, Nb/Th (4–8, versus 8 for primitive mantle) ratios in KG basalts are intermediate between ranges for Archean PAT (2–6) and OPB (8–20). Compositionally, these basalts are commensurate with recent backarc basalts (BAB) that are intermediate between mid-ocean ridge basalts (MORB) and PAT: they are interpreted as oceanic crust in a backarc rift, from mildly hydrous melting of depleted upper mantle asthenosphere. Kinojevis basalts plot mostly on the MORB–ocean island basalt (OIB) array in log Nb/Y versus Zr/Y coordinates, whereas structurally overlying Blake River basalts cross that array from depleted plume asthenosphere to adakite “end-members”. Consequently, their asthenosphere sources and geodynamic settings may have differed.Tholeiitic basalts and FIII-type rhyolites in the 2722–2680 Ma Manitouwadge and Winston greenstone belts, Wawa subprovince, are compositionally akin to their Kinojevis counterparts. Basalts and rhyolites in both belts are spatially and temporarily associated with volcanogenic massive sulphide deposits (VMS), indicative of the potential of the KG for similar deposits. The KG is interpreted as intra-oceanic backarc crust. Similarly, the Wawa volcanic and metallogenic associations are interpreted to have been deposited in a backarc setting, to the north-northwest of the Schreiber-Hemlo subduction–accretion complex.