Eastern margin of the Vaasa Migmatite Complex, Kauhava, western Finland: preliminary petrography and geochemistry of the diatexites

H. Mäkitie
2001 Bulletin of the Geological Society of Finland  
MÄKITIE, HANNU 2001. Eastern margin of the Vaasa Migmatite Complex, Kauhava, western Finland: preliminary petrography and geochemistry of the diatexites. Bulletin of the Geological Society of Finland 73, Parts 1-2, 35-46. Extensive biotite diatexites resembling S-type granitoids occur in the eastern part of the Vaasa Migmatite Complex (VMC, 1.89 Ga, 6000 km 2 ) at Kauhava, western Finland. These rocks have undergone a textural homogenization that has destroyed the primary bedding.
more » ... ding. Pre-migmatization structures are preserved only in inclusions. The diatexites are clearly peraluminous and have relatively high concentrations of TiO 2 (0.7-0.9 wt%), CaO (2.4-3.0 wt%) and total of Fe 2 O 3 tot + MgO (6.6-8.8 wt%). The contact of the diatexites against pelitic metatexites is usually a gradual zone, 1-200 m wide, but locally a boundary limiting places of higher melt fraction. The contacts indicate that the anatectic melt formed, with residual solids, has not ascended greatly; the VMC forms the most voluminous in situ melted rocks with granitoid appearance in Finland. The study area also includes peraluminous orthopyroxene-bearing rocks, granitic in texture, which are hornblende free and have relatively high TiO 2 (1.0-1.2 wt%) and MgO (2.1-2.5 wt%) contents. They gradually change to biotite diatexites and also suggest fusion of metasedimentary rocks at reduced a H2O . The chemical composition of minerals in the orthopyroxene-biotite diatexite indicates crystallization conditions of high-grade metamorphism; X Mg in garnet is 0.23, orthopyroxene has 3 wt% Al 2 O 3 and biotite contains over 5 wt% TiO 2 .
doi:10.17741/bgsf/73.1-2.003 fatcat:kwvh62fs3nhk7nspr3y52e2jze