CASE ISLAND OF SOISALO, EASTERN FINLAND: FROM RIFT RELATED VOLCANISM TO SILICICLASTIC CONTINENTAL MARGIN

Palaeoproterozoic metasedimentary strata of Eastern Finland, Fennoscandian Shield, provide direct geochemical and mineralogical evidence of ancient processes. These processes were active during the extension, rifting, closure, and deformation of the sedimentary basin(s) presently located on the boundary zone between the Svecofennian and Karelian domains, zone whose exact location is still under discussion. To distinguish between otherwise similar-looking schistose metagraywackes 62 drill core samples obtained from 5 drill holes 150 to 300 m deep were analysed by standard whole-rock methods. The drill cores consist mainly of schistose metagraywackes, with 0.5–2 m wide interlayers of meta-arenites and metaconglomerates. The main minerals of the metagraywackes are quartz, plagioclase, biotite ±K-feldspar ±pyrrhotite. Total C content varied from 0.1 to 5.4 wt-%, which is indicative of presence of graphite precipitated after primary organic carbon in the sediments. Total S content, 0.6–16.6 wt-%, indicates presence of pyrrhotite, pyrite, and chalcopyrite. Interlayers of black schists within the metagreywackes display high total C and S (4.1-9.1 wt-% ; 8.4–15.7 wt-%). The modest Ni and Cu (b.d.l.–714 ppm; 2–1060 ppm) and low Co (b.d.l.–67 ppm) of all samples show positive correlation with the total S. Some of the sulphides and graphite occur in veins indicating mobilization after the sediment deposition.The non-migmatized though locally intensely veined Soisalo metagraywackes are variable in SiO₂ (56.7-73.6 wt-%) and show negative correlation with MgO (1.8–7.1 wt-%). Molar Na/K (0.13–2.91) ratio is lower and Ca/Al (0.11–0.97) consistently higher than Pirkanmaa, Central Finland Granitoid Complex and Kuopio migmatized paragneiss compositions. Anomalously high Ca/Al (n=4) record calc-silicate alteration. The source rocks of the Soisalo metagraywackes may have contained higher amount of calcic plagioclase relative to the Central Finland region. Also the metamorphic grade may have been higher on Soisalo Island relative to the Kuopio region in the northwest.