International Geologiical Congress - Oslo 2008

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MPI-03 Granite classification - a never-ending problem

 

Paleoproterozoic oxidized A-type granites in northern Finland and vicinity

 

Tapani Rämö, University of Helsinki (Finland)
Jaana Halla, Finnish Museum of Natural History (Finland)
Esa Heilimo, University of Helsinki (Finland)
Hannu Huhma, Geological Survey of Finland (Finland)
Asko Kapyaho, Geological Survey of Finland (Finland)
Matti Kurhila, University of Helsinki (Finland)
Kai Front, Technical Research Centre of Finland (Finland)
 

 

The Paleoproterozoic Nattanen-type granites in northern Finland and adjacent Russia are found as highly discordant, post-orogenic, high-level multiple granite plutons in the far north of the Fennoscandian shield. These plutons (the Tepasto, Pomovaara, and Nattanen stocks and the Riestovaara and Vainospää batholiths on the Finnish side; the Juvoaivi stock and the Litsa-Aragub complexes on the Russian side) are markedly younger (∼1.80-1.77 Ga) than their immediate country rocks and are found along a ∼400-km-long NE-striking zone that transects both Paleoproterozoic and Archean metamorphic crust. The Nattanen-type granites are characterized by miarolitic cavities, evolved (biotite leucomonzogranitic) compositions, marginal peraluminosity, and an oxidized redox budget (titanite and magnetite as conspicuous minor phases). They are also associated with evolved rhyolitic dike rocks and minor molybdenite mineralization. Available isotope data (initial epsilon-Nd and epsilon-Hf values on the order of -6 to -9 and -10 to -12, respectively; unradiogenic uranogenic feldspar Pb isotope ratios) imply a major, yet varying, Archean source component. In spite of this, these granites barely show evidence for xenocrystic zircons. The Nattanen-type granites have traditionally been considered I-type in general. In the new classification scheme of Dall'Agnol & Oliveira (2007; Lithos 93, 215-233), designed to separate calc-alkaline granites from A-type granites and oxidized A-type granites from reduced A-type granites, the Nattanen-type granites show, however, a consistent oxidized A-type character. They are, in this respect, similar to, for instance, the slightly older (1.88 Ga) oxidized anorogenic A-type granites of the eastern Amazonian craton. The 1.80-1.77 Ga Nattanen-type granites probably represent partial melting of ancient lower crust during post-collisional thermal relaxation in the aftermath of convergent tectonics at >1.80 Ga.

 

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