2.8 GA BONINITE-HOSTING PARTIAL SUPRASUBDUCTION ZONE OPHIOLITE SEQUENCES FROM THE NORTH KARELIAN GREENSTONE BELT, NE BALTIC SHIELD, RUSSIA

Abstract

Neoarchean subduction-related assemblages of the North Karelian greenstone belt, in the NE part of the Baltic Shield, Russia, contain the world's oldest known boninite series, occurring in at least in two areas of the belt. The second area, named the Iringora structure, preserves distinctive features of an ophiolite pseudostratigraphy, including not only gabbro and lava units, but also remnants of a sheeted dike complex. Many ophiolites with arc geochemical signatures are thought to have formed as oceanictype crust above subduction zones. These have been termed suprasubduction zone (SSZ) ophiolites. One possible explanation for lack of Archean boninitic volcanism is that higher heat flow in the Archean may have caused subducted oceanic crust to have partially melted, rather than forming dehydrating reactions characteristic of modern subduction zones. The North Karelian greenstone belt extends for 300 km along the boundary between the Karelian granite–greenstone terrain and the Belomorian mobile belt. It comprises several greenstone belts isolated from each other by granitoids or unexposed areas.