Geochemistry, Nd, Pb and Sr isotope systematics, and U–Pb zircon ages of the Neoarchean Bad Vermilion Lake greenstone belt and spatially associated granitic rocks, western Superior Province, Canada

摘要

The Neoarchean (ca. 2720 Ma) Bad Vermilion Lake (BVL) greenstone belt is composed of metamorphosed bimodal tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gabbros, and subordinate sedimentary rocks. All these rocks are intruded by granitic rocks. The HFSE systematics of the volcanic and volcaniclastic rocks and gabbros are consistent with subduction zone geochemical signatures. The BVL greenstone belt is geochemically similar to Neoarchean subduction-related ophiolites. Two rhyolite samples from the belt have been dated as 2724±23 Ma and 2722±11 Ma, representing approximate age of the arc volcanism. The ca. 2664 Ma Ottertail Lake granite marks the end of tectonic accretion in the study area. Both the volcanic rocks and gabbros show large ranges of Nd, Pb and Sr isotopic compositions, reflecting post-magmatic alteration. Pb-Pb regression ages of gabbros (2725±83 Ma) agree with the U-Pb ages of the rhyolites, suggesting that the U-Pb system was the most robust isotopic systems.