GENERAL GEOLOGY AND EVOLUTIONARY HISTORY OF THE EARLY PROTEROZOIC POLMAK-PASVIK-PECHENGA-IMANDRA VARZUGA-USTPONOY GREENSTONE-BELT IN THE NORTHEASTERN BALTIC SHIELD

The early Proterozoic Polmak-Pasvik-Pechenga-Imandra/Varzuga-Ust'Ponoy Greenstone Belt is situated in the northeastern part of the Baltic Shield and is discontinuously developed over a length of ca. 1000km. It comprises five main zones, which are separated by longitudinal and a number of transverse syndepositional faults. Each zone contains a number of lithostratigraphic units at the rank of Groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16,000 m. The sequence comprises a series of sedimentary-volcanic rhythms usually separated by stratigraphic breaks marked by palaeo-weathering. The Groups are separated by major disconformities which may mark/record rift inversion or orogenic episodes. At least seven distinctive lithostratigraphic markers occur in the Belt including two komatiitic volcanic series separated by immature conglomerates, two red-coloured proto-evaporitic formations separated by the alkaline volcanics, and two different ''black shale'' formations. Perhaps the main characteristic of the belt is the apparently long history of sedimentary/volcanic development, at the same site, spanning a time-period of 2500 to 1800 Ma. A three stage rift development model is considered: (1) an intracontinental rift stage (2500-2100 Ma), separated by the major inversion (2400-2300 Ma) and comparable with present-day Afar Triangle and East African Rifts; (2) a transitional from intracontinental to to intercontinental rift stage (2100-1970 Ma) with possible short-lived spreading (about 1990-1970 Ma); (3) a collision related intercontinental rift stage (1970-1800 Ma) followed by the Svecofenian orogeny (1800-1700 Ma). The observed distribution of platinum group elements (PGE)-mineralization is related to layered gabbro-norite complexes (Fedorova and Panskie Tundry, Gora General'skaya intrusions) dated at 2470 Ma which are apparently co-magmatic and coeval with the first komatiitic volcanic series (Seidorechka Formation) found in the eastern part of the Belt only. The major chromite occurences are connected with a layered gabbro-norite intrusion (Imandra Lopoloth) which is co-magmatic and coeval between 2440-2330 Ma with the second komatiitic volcanic series (Rusinga, Polisarka, Akhmalahti and Malbekk Formations). The Ni-Cu deposits of the belt are connected with a differentiated gabbro-werhlite complex which is co-magmatic and coeval (ca. 1970 Ma) with the specific Fe- and Ti-rich ferropicrites with komatiitic affinities.