New constraints for the source characteristics, deposition and age of the 2.1–1.9Ga metasedimentary cover at the western margin of the Karelian Province

7.6 Enhanced Accumulation of Organic Matter: The Shunga Event

“…Originally the black shales were organic-rich mud deposited on the sea-floor under apparent anoxic conditions, and were later enriched in metals by hydrothermal input (Loukola-Ruskeeniemi and Heino 1996). Detrital zircon and Sm-Nd iotope data are compatible with a major Archaean provenance (Lahtinen et al 2010). Other black shales assigned to the Lower Kaleva include those forming part of the turbidite sequences of the Martimo Formation in the Per€ apohja Belt (Perttunen and Hanski 2003) and the Haukipudas Formation in the Kiiminki Belt (Fig.…”

“…Kontinen 1987;Lahtinen et al 2010). The depositional ages of the Kalevian sedimentary rocks are still poorly constrained; Lower Kalevian sediments were likely deposited later than c. 1.98 Ga, the age of the youngest mafic dyke generation in the Archaean basement, while detrital zircon ages of c. 1.95-1.92 Ga determine the maximum age of sedimentation for Upper Kalevian rocks (Lahtinen et al 2010). The latter ages were interpreted as indicating potential sedimentary sources in the Kola Orogen for the Upper Kaleva, while the Lower Kaleva had mainly an Archaean provenance (Lahtinen et al 2010).…”

3.2 Litho- and Chronostratigraphy of the Palaeoproterozoic Karelian Formations

“…Ward (1987) applied the same style of informal division to the North Karelia Belt , but it was shown later that the two units differ in their lithology, age, and provenance (Lahtinen et al 2010). In North Karelia, the Lower Kaleva grades southwestward without a clear boundary into the migmatitic Savo foldbelt of the Svecofennian domain.…”

“…According to Lahtinen et al (2010), a typical feature is the absence of volcanic rocks within the pelitic metasediments. An important observation is that the occurrence of tectonically emplaced slivers of ophiolitic rocks is confined to the Upper Kaleva both in Kainuu (Jormua ophiolite complex) and North Karelia (Outokumpu ophiolite complex) (Kontinen 1987;Peltonen et al 2008).…”

“…This provides a minimum age for the Kalevian deposition in the northern lake Ladoga area. In-situ dating of detrital zircons recovered from the Lower Kaleva in Finland has provided dominantly Archaean ages, but Nd isotopic characteristics suggest that the rocks have a detrital component from Jatulian mafic lavas and dykes (Lahtinen et al 2010). In contrast to the Lower Kaleva, only c. 25 % of the zircon populations in the Upper Kaleva are Archaean in age with the rest falling mainly in the approximate range of 2160 to 1920 Ma.…”