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Geological Survey of Finland, 02150 Espoo, Finland
The Proterozoic bedrock of eastern Finland consists of Karelidic sediments, which were formed under cratonic conditions, and of the Svecofennidic volcanic-sedimentary complex, which is located west of the Karelides and is considered to be a subduction-related island-arc system. The marginal area between the Svecofennidic and Karelidic formations has been interpreted as a suture zone, along which the Svecofennides were accreted to the Archaean continent (Koistinen 1981). The details of the Svecofennian orogeny have been rather unclear; it has been generally supposed that the main phase of orogeny took place about 1900–1860 Ma (e.g. Papunen & Vorma 1985, Patchett & Kouvo 1986). Metamorphic studies in the Svecofennidic belt near the Archaean craton margin in central Finland and in the migmatite belt of southern Finland have clarified our concept of the early Proterozoic orogeny. According to Korsman et al. (1984), the Svecofennidic orogeny and metamorphism can be divided into at least three main episodes. The development of regional metamorphism in the northern part of the northwest-southeast-trending Savo schist belt near the craton margin took place considerably earlier than in its southern part in south-western Finland. In the northern part of the Savo schist belt, granulite metamorphism is associated with magmatic activity, whereas in the southern part of the belt it is connected with a tectonically thickened crust, although the intense rise in the thermal gradient is not easily attributable to tectonic thickening alone. This study summarizes mineral assemblages and metamorphic conditions in the Pielavesi area near the Archaean craton margin in the
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