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 Sh ield and is discontinuously developed over a length of ca. 1000km. It comprises five main zones, which are separated by longitudinal and a n umber of transverse syndepositional faults. Each zone contains a numbe r of lithostratigraphic units at the rank of Groups that are subdivide d into several sedimentary and volcanic formations with a total thickn ess over 16,000 m. The sequence comprises a series of sedimentary-volc anic rhythms usually separated by stratigraphic breaks marked by palae o-weathering. The Groups are separated by major disconformities which may mark/record rift inversion or orogenic episodes. At least seven di stinctive lithostratigraphic markers occur in the Belt including two k omatiitic volcanic series separated by immature conglomerates, two red -coloured proto-evaporitic formations separated by the alkaline volcan ics, and two different ''black shale'' formations. Perhaps the main ch aracteristic of the belt is the apparently long history of sedimentary /volcanic development, at the same site, spanning a time-period of 250 0 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 t o intercontinental rift stage (2100-1970 Ma) with possible short-lived spreading (about 1990-1970 Ma); (3) a collision related intercontinen tal rift stage (1970-1800 Ma) followed by the Svecofenian orogeny (180 0-1700 Ma). The observed distribution of platinum group elements (PGE) -mineralization is related to layered gabbro-norite complexes (Fedorov a 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 t he Belt only. The major chromite occurences are connected with a layer ed gabbro-norite intrusion (Imandra Lopoloth) which is co-magmatic and coeval between 2440-2330 Ma with the second komatiitic volcanic serie s (Rusinga, Polisarka, Akhmalahti and Malbekk Formations). The Ni-Cu d eposits of the belt are connected with a differentiated gabbro-werhlit e complex which is co-magmatic and coeval (ca. 1970 Ma) with the speci fic Fe- and Ti-rich ferropicrites with komatiitic affinities.