Paleoproterozoic Svecofennian orogenic belt in the surroundings of the Gulf of Finland

A major part of the crystalline basement in the area of the Gulf of Finland and its surroundings formed during the Paleoproterozoic Svecofennian orogeny. Older Archean basement rocks only occur in the northeast of the map area. In places, rapakivi bodies and Jotnian sediments disturb the continuity of the Svecotennian-age basement. The oldest Paleoproterozoic supracrustal rocks in the area, over 2 Ga old occur on deeply eroded Archean basement gneiss in the northeast. All rocks in the Svecofennian domain, which covers the major part of the area, are younger. These range from preorogenic to postorogenic phases 1.93-1.8 Ga old. In the southwestern corner of the map, the data are poorest. Metamorphic rocks form the most extensive group. In general, their composition and preserved primary structures clearly indicate that they are former volcanic, plutonic and sedimentary lithologies. Their origin is only strongly obscure in relatively restricted subareas or linear zones. The southern part of the Central Finland Granitoid Complex extends to the map area. The younger Potassium Granite Migmatite Zone of southern Finland extends to, or has limited occurrences in, Russia and Estonia. Migmatite zones, which are characteristic of a deep erosional level, are widespread. Several crustal scale features have a strong, nearly E - W trend For example, the primary distribution of major supracrustal and igneous rock provinces, major structures and metamorphic zones.