Volcanic ash intercalations in the Kilpisjärvi Caledonian nappes, Finland

Y.J. Systra (1) and T. Kiipli (2)

(1) Department of Mining, Tallinn University of Technology, Ehitajate tee 5, EE-19086 Tallinn, Estonia (systra@staff.ttu.ee) (2) Geological Survey of Estonia, Kadaka tee 82, EE-12618 Tallinn, Estonia

In the Kilpisjärvi area, Caledonian nappes have been thrust onto the autochthonous foreland, which consists of the deeply eroded Archean basement and a thin Neoproterozoic-Cambrian sedimentary cover. The Jerta Nappe was only transported for a relatively short distance and is characterized by clayey slate, bluish quartzite and micritic dolomite. The Nalganas Nappe was transported much further, and represents the lowest thrust sheet of the Middle Allochton, consisting of quartzite with narrow biotite- and chlorite-rich intercalations. The lower parts of the overlying Nabar Nappe include amphibolites, which locally preserve amygdales, commonly with Cu-minerals, indicating a volcanic flow origin. The uppermost part of this nappe is represented by garnet-bearing mica gneiss. The uppermost unit in the Finnish Caledonides is the Vaddas Nappe, which is composed of mafic-ultramafic magmatic rocks (Lehtovaara, 1995). All these nappes belong to the Middle Allochthon (Gee & Sturt, 1985).
Volcanic ash was found in weakly metamorphosed quartzites of the Nalganas Nappe, sometimes with up to 5–6 intercalations of thickness 1–10 cm in the 2,5 m of sequence. When volcanic ash forms thicker (20–25 cm) horizons, distances between them also corre¬spondingly increase. All ash layers can be distinguished from quartzite by their dark colour, mineralogy and chemical composition.
Analyses (n=21) fall into two main groups, with SiO2 contents 43–50 wt.% and 56–68 wt.%, and Na2O+K2O contents 1.5–5.3 wt.% and 4.6–8.0 wt.% respectively. Members of both groups can be found in all analyzed series, although one of them usually dominates.
Field work was done with financial support of the LAPBIAT grant HPRI-CT-2001-00132 and ongoing research is supported by Estonian Science Foundation grant No 5921.

Lehtovaara, J.J. (1995). Geol. Survey of Finland, Espoo, 1–64.
Gee, D.G. & Sturt, B.A. (1985). Scandinavian Caledonides tectonostratigraphic map. Sveriges Geol. Undersökning ser. Ba No 35.