Bulletin of the Geological Society of Finland (Jun 1986)
The Proterozoic granitoids of Finland: Granite types, metallogeny and relation to crustal evolution
Abstract
The granitoids are divided into four main groups: synkinematic (1860‒1930 Ma, most commonly 1870‒1890 Ma), late-kinematic (1800‒1850 Ma) and postkinematic (ca. 1800 Ma) Svecokarelian granitoids, and anorogenic rapakivi granites (1540‒1700 Ma). The synkinematic granitoids are the predominant component of the Proterozoic crust in Finland. They occur as composite granodiorite and tonalite plutons in schist belts, and form the large granitoid complex of central Finland, in which granites are common. The granitoids show I-type characteristics, Andino-type plate tectonics being the most viable mechanism to account for their genesis. Newly mantle-derived material was the predominat source of the granitoids, following fractional crystallization or more probably recycling within the crust. Subeconomic, porphyry-type Mo, Cu, Mo-Cu, Cu-Au and Cu-W occurrences are associated with late phases of the synkinematic granitoids. Minor higher temperature, vein-like mineralization containing Cu, As, Mo, Zn, Pb, W and Sn is related to the complex of central Finland. The late-kinematic granites comprise diapiric intrusions and migmatizing granites mainly in high-grade metamorphosed schist belts. These S-type granites were formed after partial melting of Svecokarelian metasediments when metamorphism was at its peak in the thickened crust. Some rare-element pegmatites are associated with the granites. The post-kinematic granitoids consist of small roundish composite stocks in southern Finland, and of multiple granite intrusions up to batholithic size, which were emplaced in the Presvecokarelian crust, in northern Finland. The geochemical and isotope characteristics of the former suggest mantle source, whereas the latter derived from the Archean crust. The late aplite granite phases of the granites in northern Finland contain local Mo mineralization. Rapakivi granites, which exhibit A-type characteristics, occur as large multiple batholiths and stocks in southern Finland. Some of them are associated with gabbros, anorthosites and olivine-tholeiite diabases. The anorogenic rocks are indicative of the beginning of intracontinental rifting of the uplifted Svecokarelian crust. The basaltic mantle magmas evidently initiated major remelting in the granulitic lower crust, giving rise to rapakivi granite magmas. The late topaz-bearing phases of the rapakivi granites, with their marked geochemical specialization and characteristic petrographic features, contain greisen-type Sn-Be-W and Zn-Cu-Pb sulphide mineralization.
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