U–Pb geochronology of Archaean volcanic-sedimentary sequences in the Kuhmo greenstone belt, Karelia Province – Multiphase volcanism from Meso- to Neoarchaean and a Neoarchaean depositional basin?

The Kuhmo greenstone belt in eastern Finland is one of the most studied areas of the Karelia Province. A single-grain zircon U–Pb study of andesitic, trachyandesitic and dacitic volcanic rocks from the belt demonstrates multiple widely spaced ages of the rocks. The results show that the volcanic activity took place as two major episodes at ca. 2847–2836 Ma and ca. 2799–2792 Ma, both containing komatiitic members as well as volcanic-sedimentary units. Based on the geochronological results combined with previously published data, we propose a chronostratigraphic interpretation for the Kuhmo greenstone belt, dividing it into three units: the Nuolikangas and Siivikkovaara volcanic units, and the sedimentary Ronkaperä unit. The results support the previous interpretation that age period of 2.80–2.79 Ga was an important crustal forming episode in the Kuhmo area. The zircon populations in the uppermost sedimentary rocks contain >3.0 Ga zircon grains that are older than the volcanic rocks found in the greenstone belt. The youngest detrital zircon populations, ca. 2.73 Ga and 2.70 Ga in a conglomerate and a quartz sandstone, respectively, imply that they were deposited at least ca. 60–90 Ma after the last volcanic phase. Based on the detrital zircon record, it can be interpreted that during the deposition of the detritus of the sedimentary rocks belonging to the Ronkaperä unit, the Karelia subprovinces were likely juxtaposed together and acted as a source provenance for the detritus.