Fundament krystaliczny Suwalszczyzny

Wacław Ryka




Crystalline basement of Suwalszczyzna (NE Poland) is a part of the northern slope of the Mazury-Suwałki High. The basement is covered by the older (in the north) and middle (in the south) complex of sedimentary cover which is 500 m thick in the east and over 1500 m in the west (Fig. 1). In the Precambrian of Suwalszczyzna three major structural-tectonic units are jointed together and are cut in turn by great Precambrian and Paleozoic intrusions. Rocks of the Karelian complex are absent (Fig. 2) and post-Gothian molasse-like formations occur locally and only over pre-Karelian rocks. The Mazowsze Granitoid Massif of the south Suwalszczyzna (Fig. 3) is the northern fragment of the Mazowsze Block which due to rotation was broken up into small wedge-shaped blocks and mixed with non-competent rocks of the Podlasie Complex. The oldest rocks in Poland (ca. 2.700 Ma K - Ar age; Table) were recorded in the Mazowsze Complex. In the Podlasie Complex the older rocks that are included into granulite group are relatively rare. These are granulites, enderbites and charnockites, locally cordierite gneisses. K - Ar (rejuvenated) age of these rocks indicates 2.050 - 1.950 Ma. The presence of organic ghosts of colonial biocoenoses, saprolites and fungi indicates the organic life in the near-bottom photic zone at that time. Common are younger rocks of the Podlasie Complex that are included into plagiogneiss group. They were formed mainly due to transformation of granodiorite intrusion and their tuffs. The northern part of Suwalszczyzna is composed of Gothian Mazury Complex that is built of rapakiwi granites and Suwałki anorthosites. South of the Mazowsze Complex the Gothian rocks occur in form migmatites that are developed in broad zones, mostly along the Mazowsze Massif - Podlasie Complex border. Rapakiwi granites are south-eastern fragment of the Trans-Scandinavian zone of porphyries and rapakiwi granites that extends from northern Sweden through Baltic Sea, Mierzeja Wiślana and Warmia to Suwalszczyzna. The layer of rapakiwi granites is probably less than 2 km thick and therefore high magnetic gradients were recorded here; these gradients are reflections of transform fault zones and deeply-rooted old structures below the rapakiwi granites. The occurrence of Suwałki anorthosites coincides with the extension of axis of Podlasie Complex within rapakiwi granites. The anorthosites were formed due to pressing of blocks of the granulite group below rapakiwi granites and next their disintegration under conditions of differential anatexis. Paleozoic platform intrusions are related to the period of disintegration of the Precambrian Platform at the end of Paleozoic. At that time originated in Suwalszczyzna: Tajno alkaline-ultrabasic intrusion and Elk syenite intrusion.