THE JORMUA OPHIOLITE: A MAFIC-ULTRAMAFIC COMPLEX FROM AN ANCIENT OCEAN-CONTINENT TRANSITION ZONE

Abstract

The Jormua Ophiolite is an allochtonous mafic-ultramafic rock complex, thrusted onto the Karelian Craton margin that formed within a passive margin environment about 100 km southwest from its present position. Since transitional contacts between all main ophiolite units can be demonstrated, the Jormua Ophiolite Complex is interpreted to represent a practically unbroken sample of seafloor from an ancient ocean–continent transition (OCT) zone, strikingly similar to that reported from younger similar tectonic settings such as the Cretaceous West Iberia nonvolcanic continental margin. The Jormua Ophiolite Complex is the northernmost and the most completely preserved example of the ophiolite fragments within the Paleoproterozoic North Karelia Schist Belt and the Kainuu Schist Belt in the central part of the Fennoscandian Shield. The Kainuu Schist Belt that encloses the Jormua Ophiolite occupies a structural depression between two rigid blocks of the basement structure: the Eastern Finland Complex and Pudasjärvi–Iisalmi Complexes. The earliest stages of the Svecofennian deformation, related to the early tectonic processes that contributed to the detachment of the Jormua Ophiolite Complex from oceanic environment and its subsequent thrusting across the foreland, involved tectonic disruption of the original ophiolite assemblage.