Mäkitie, Hannu (ed.) 2001. Svecofennian granitic pegmatites (1.86-1.79 Ga) and quartz monzonite (1.87 Ga), and their metamorphic environment in the Seinäjoki region, western Finland. Geological Survey of Finland, Special Paper 30. 98 pages, 41 figures, 18 tables and 8 appendices.

This volume describes Palaeoproterozoic Svecofennian granitic rocks which postdate the main stage (1.89-1.88 Ga) of crustal thickening of the Svecofennian orogeny (1.9-1.8 Ga) in the Seinäjoki region, western Finland. The studied rocks are rare-element (RE) pegmatite dykes (1.86-1.79 Ga), associated pegmatitic granites, and an older (1.87 Ga) quartz monzonite intrusion. They represent the youngest igneous rocks of the region. The petrography, chemical and mineralogical  composition, structure, metamorphic environment and U-Pb ages of the rocks are described. The Seinäjoki region is located in the Bothnia Belt, with a southern part extending into the Central Finland Granitoid Complex.

About 250 RE-pegmatites were studied and most of these can be classed as either beryl pegmatites, complex pegmatites or albite-spodumene pegmatites, which are all of the LCT(lithium-caesium-tantalum) type. Beryl-columbite-cassiterite pegmatites (ca. 1.80 Ga, U-Pb tapiolite age) are most common near the town of Seinäjoki. Locally there are a few topaz-bearing pegmatites (ca. 1.80 Ga, U-Pb columbite age) that contain accessory columbite and fluorite, and form a unique subtype among the beryl pegmatites. Of the pegmatites studied, the complex pegmatites of Haapaluoma and Kaatiala (ca. 1.80 Ga, U-Pb columbite ages) are the richest in rare minerals such as columbite, cassiterite, spodumene and lepidolite. Albite-spodumene pegmatites (ca. 1.79 Ga, U-Pb columbite age) occur in the Kruunupyy-Ullava area, which lies 40 km NE of Seinäjoki. The columbites dated differ in chemical composition from ferrocolumbite to mangano-columbite, and there is minor variation in the contents of Ti and W. Older rare-earth-element (REE)-pegmatites (ca. 1.86 Ga, U-Pb monazite age) occur in the Alavus area, the southeastern part of the Seinäjoki region that is part of the Central Finland Granitoid Complex. Except for these REE pegmatites, all the other pegmatites are essentially in the rocks of the Bothnia Belt. There are about 12 RE-pegmatite groups in the study region. These are usually associated with pegmatite granites that are 10-100 m thick and 50-300 m long.

There is a clear relationship between the regional distribution of the granitic pegmatites and the metamorphic grade of the country rocks. Most of the RE-pegmatites occur in low-pressure areas containing low to medium amphibolite grade rocks, such as andalusite mica schists or sillimanite-muscovite-biotite gneisses. The characteristic rare minerals in the pegmatites disappear indicating that the metamorphic grade of the country rocks exceeded those P-T conditions that occurred between the first and second sillimanite isograd. The whole rock chemical composition of the pegmatites also changes relative to the metamorphic grade of the country rocks: the highest Li, Ta, Cs, Rb/Sr contents usually occur in those RE-pegmatites hosted by the andalusite mica schists while, in contrast, high Fe, Th and K/Rb characterise those RE-pegmatites that lie in rocks metamorphosed in the muscovite-sillimanite facies or at a higher metamorphic grade. Local minor variations in chemical composition are the result of differences in the degree of fractionation within the pegmatites. Additionally, the country rocks of the LCT-type pegmatites are enriched in Li (up to 500 ppm), Cs (up to 300 ppm) and Rb (up to 300 ppm). The topaz-bearing pegmatites are exceptional because they lie in high-grade metamorphic mica gneisses near the diatexitic Vaasa Migmatite Complex (1.89-1.88 Ga), which is part of the Bothnia Belt.

The LCT-type pegmatites are clearly younger than the peak of regional metamorphism (1.89-1.88 Ga). The studied pegmatites also cut the main schistosity (S2) seen as a growth of metamorphic muscovite and biotite on the F2 axial plane. Many of these pegmatites are, however, deformed by younger folding. Due to the absence of granite plutons near most of the LCT-type pegmatites, it is suggested that the pegmatites represent either (1) a distinct tectonomagmatic episode that is notably younger than the culmination of the Svecofennian orogeny, or (2) the upward movement of low-solidus anatectic melts that initially formed in the neighbouring high-grade metamorphic migmatite terranes and migrated into the relatively low-grade metamorphic areas.

The porphyritic Luopa quartz monzonite (1.87 Ga, U-Pb zircon age) is located at the boundary of the Central Finland Granitoid Complex and the Bothnia Belt. The intrusion is hosted by migmatitic garnet-cordierite-sillimanite mica gneiss, pyroxene granite and foliated tonalite. The Luopa quartz monzonite is composed of three different rock varieties: a dark green fayalite-augite-hornblende quartz monzonite, a dark green augite-hornblende-biotite quartz monzonite and a light-coloured hornblende-biotite quartz monzonite. The REE distribution spectrum of the light-coloured variety is steeper than in the dark-green varieties: the aforementioned variety is more LREE enriched and HREE depleted. The chemical composition of the intrusion is characterized by low Rb/Sr (0.15-0.25), high XFe (0.9) and low A/CNK (0.91-0.96). The intrusion has the following specific features: an irregular form, a relatively high fayalite content and an intrusive relationship with the surrounding pyroxene granite. The quartz monzonite was emplaced at a relatively high crustal level and it cuts the main regional deformation phase (D2).

The emplacement of the Luopa intrusion resulted in the formation of a contact metamorphic aureole, which overprints the slightly older upper amphibolite to lower granulite facies regional metamorphism. The contact aureole can be divided into a narrow granulite-grade inner aureole (2 m wide) and a wider outer aureole (30 m wide) of slightly lower metamorphic grade. Garnet-orthopyroxene and cordierite-orthopyroxene pairs, as well as symplectites containing hercynite, sillimanite and cordierite, occur in the inner aureole. Texturally, the metapelites in the contact aureole are either diatexites or strongly deformed, tightly banded mica gneisses (due to ballooning of the intrusion). The contact metamorphic temperature is estimated to have been about 750 oC.

One notable feature of the Seinäjoki bedrock is that there are two different high-grade metamorphic terranes. The southern one is composed of migmatitic garnet-cordierite-sillimanite mica gneisses, which contain minor kyanite. Foliated I-type tonalites and the Luopa quartz monzonite are located in this terrane. The other high-grade terrane is situated in the northwestern part of the study region and contains migmatitic garnet-cordierite mica gneisses (with minor andalusite) and granodioritic diatexites belonging to the Vaasa Migmatite Complex. There is a gradual transition from these diatexites, via metatexites, to the mica schists of the Bothnia Belt. One peculiarity of this terrane is the occurrence of topaz-bearing pegmatites.

Hannu Mäkitie
Geological Survey of Finland
P.O. Box 96
FIN-02150 Espoo, Finland

ISBN 951-690-776-8
ISSN 0782-8535