Unique aspects of the geological heritage of the Republic of Karelia, NW Russia

Y.J. Systra (1, 2), V.V. Makarikhin (2), P.V. Medvedev (2) and D.V. Rychanchik (2)

(1) Department of Mining, Tallinn University of Technology, Ehitajate tee 5, EE-19086 Tallinn, Estonia (systra@staff.ttu.ee),
(2) Institute of Geology, Karelian Research Centre RAS, Pushkinskaya St.11, Petrozavodsk, RU-185910 Karelia, Russia

The Republic of Karelia is located on the Archean part of the Fennoscandian Shield (Systra, 1999). Several geological sites have a unique value for understanding the evolution of the oldest crust. The eastern part of the shield comprises 3 major NW-SE trending crustal provinces, which record a geological history of Karelia.
The Karelian Craton forms the oldest (3.5–2.7 Ga) and most stable nucleus of the shield. It consists of Late Archean (3.0–2.8 Ga) greenstone belts and granitic gneiss terrains with Early Archean relicts, overlain unconformably in synclinal folds by Early Proterozoic (2.5–1.9 Ga) greenstone facies volcano-sedimentary sequence with numerous examples of stromatolites. Near the village Hirvas, 90 km N of Petrozavodsk, three volcanic centres of a different age (3.0, 2.4 and 2.1 Ga) occur within a ring 3 km in diameter. This small area records three unconformities: one between Archean and Proterozoic, two between regional Early Proterozoic stratigraphic units. The Early Proterozoic Kukasozero and Paanajärvi synclines relate to the for¬mation of shear zones, duplexes and continental rift along province boundaries.
The Karelian Craton is flanked to the NE by the Belomorian Fold Belt, which represents a Late Archean collision zone, composed of strongly (high pressure granulites and eclogites) and multiply metamorphosed migmatic gneisses (3.0 Ga) and amphibolites, partly of ultramafic composition. Sites near the villages Pongo¬ma and Gridino show exhumated multiple folded and reworked in the deep level up to eclogites continental crust.
The Early Proterozoic Svecofennian Fold Province flanks the craton to the SW and represents the progres¬sively metamorphosed margin of the collision zone with granitic gneiss domes, low pressure granulites, multiple folded volcano-sedimentary rocks, cut by mafic, acid and alkaline intrusions and dykes.

Systra, Y.J. Mem. Descr. Carta Geol.d’It. LIV (1999), 361–366.