THE EARLY PROTEROZOIC PECHENGA-VARZUGA BELT - A CASE OF PRECAMBRIAN BACK-ARC SPREADING

The Pechenga-Varzuga Belt (PVB) in the Kola Peninsula, Russia, compris es the western Pechenga and the eastern Imandra-Varzuga portions. Thes e are two major asymmetric synform structures infilled with Proterozoi c volcanogenic and sedimentary rocks dated between 2.45 and 1.9 Ga. Tw o main stages of tectonic-magmatic activity took place: (I) a similar to 2.45-2.3 Ga basal siliceous high-Mg volcanic series, typical for th e Sumian-Sariolian episodes; and (2) the Jutuli, Ludia and Kalevian ep isodes, which began similar to 2.2 Ga ago. The lower level of the seco nd sequence is made up of subaerial alkali basalts. The upper, 2.1-1.9 5 Ga old portions of the sequence consist of tholeiitic pillow-lavas, hyaloclastites and ferropicrite basalt flows associated with deep-wate r sediments such as phyllites, silicites and turbidites. The tholeiite s exhibit REE distribution patterns similar to those of MORE, while th e ferropicrite basalts resemble intraplate olivine basalts of continen tal and oceanic environments. Calc-alkaline subaerial volcanism took p lace along the south-western flank of the Pechenga zone. A final igneo us stage is characterized by the emplacement of potassium granitic int rusions. The Main Lapland Thrust (MLT) occurs along the southern borde r of the Paleoproterozoic Lapland-Umba Granulite Belt, and is a narrow zone of compression, thrusting and folding accompanied by high-pressu re granulite metamorphism. It is approximately coeval with the upper p art of the Pechenga-Varzuga Belt, and is interpreted as representing a complementary structure, i.e. the zone of subduction of the Karelian- Belomorian craton beneath the Kola block. Thus, the upper part of the Pechenga-Varzuga Belt is considered to have originated as a back-are b asin in the rear of a zone of north-directed subduction and continent collision. It was eventually closed, with the appearance of igneous su ites typical of such events. The Pechenga-Varzuga Belt-Main Lapland Th rust system thus provides an instance of an ancient (similar to 2.0-1. 9 Ga ago) collision of lithospheric plates accompanied by back-are spr eading and then closure of the back-are basin. The geodynamic developm ent is similar that which occurred more recently within the Alpine-Him alayan Belt. (C) 1997 Elsevier Science B.V.