Bulletin of the Geological Society of Finland (Dec 1989)
The early Proterozoic Saari-Kiekki greenstone belt: A representative of the Sariola Group at Kuhmo, Eastern Finland
Abstract
The early Proterozoic Saari-Kiekki greenstone belt in Kuhmo, eastern Finland, is the deformed remnant of a NW‒SE-trending elongate structural basin c. 15 km long and up to 3 km wide. It is correlated with the Sariola Group in eastern and northeastern Finland as well as with the Sumi-Sariola Group in Soviet Karelia, and is thus one representative of this oldest unit of the Karelia Supergroup preserved in Central Finland. The volcano-sedimentary sequence of the belt was deposited in terrestial to fluvial environments on late Archaean granitoids and migmatites, in a fault-bounded basin. The basin-margin faults belong to a regional system of dextral strike-slip faults, which were active both prior to (= late Archaean time), during and after the basin formation (= early Proterozoic time). Locally the marginal zones of the basin are characterized by tectonic‒eluvial granitoid breccia hundreds of metres thick, which contain some arkosic layers (= Latvalampi Formation). Subaereal or partly subaqueous basalts, basaltic andesites and andesites were deposited longitudinally in the centre of the basin (= Kaita-Kiekki Formation). In many places the volcanic suite is overlain by the sublitharenites, arkosic sandstones‒conglomerates and polymictic conglomerates (= Pötsölampi Formation). The whole belt is cut by NW‒SE- or NNW‒SSE-trending early Proterozoic metadiabase dykes. U-Pb zircon and titanite isotopic data for the metadiabase dyke indicate that the Saari-Kiekki greenstone belt was deposited before 2209 + 11 Ma. The problematic calc-alkaline trend of the Kaita-Kiekki intermediate volcanites associated with the intracratonic strike-slip fault system is interpreted to be a consequence of late Archaean crustal contamination of basaltic magma at depth during its uprise. Regional dextral E‒SE-trending strike-slip faulting, overthrusting from west to east, open to tight folding, a strong west plunging lineation and the development of the Saari-Kiekki greenstone belt, record an important stage of early Proterozoic reactivation in the late Archaean basement. The early Proterozoic structural patterns can be divided into three deformational phases and the metamorphism during that time seldom exceeded the lower amphibolite facies.
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