Prekambryjska ewolucja platformy wschodnioeuropejskiej w Polsce

Abstract

Prekambryjski magmatyzm i metamorfizm przedstawiono na tle rozwoju geotektonicznego platformy wschodnioeuropejskiej w Polsce. Obszar ten uformował się głównie w epoce prekambryjskiej, kiedy to powstały „ensialiczne” bloki granitoidowe, a w miejscach subsydencji na ścienionych obszarach prakontynentu „ensimatyczne” strefy fałdowe. Ważną rolę spełniał wówczas wulkanizm bazaltowy i diapirowy magmatyzm granitoidowy. W epoce karelskiej i gotyjskiej skały podłoża uległy znacznym przekształceniom, które nie spowodowały jednak zasadniczej zmiany starego planu strukturalnego.Rozwój platformy prekambryjskiej zakończył magmatyzm anorogeniczny i wulkanizm wendyjski.  PRECAMBRIAN EVOLUTION OF POLISH PART OF THE EAST-EUROPEAN PLATFORM Crystalline basement of Polish part of the East-European Platform has been primarily shaped in the pre-Karelian epoch, in times of formation of “ensialic” granitoid blocks and “ensimatic” fold zones. The former represent repeatedly regenerated substratum of praecontinental crust whereas the latter were originating in places where the crust was subjected to tension, the rate of sedimentation fairly high and basic volcanism predominating. Granodiorite diapirism, somewhat younger, appeared in “ensialic” zones but some portions of magma were reaching surface of “ensimatic” blocks in the form or lava and pyroclastic materials. In result of regional mdall1orphism, tholeiite basalts and accompanying pyroclastic materials have formed granulite group and younger calc-alkaline lavas and related pyroclastic materials – gneiss group. At that time the pre-Karelian rocks were subjected to metamorphism under granulite facies conditions. Pressure was initially close to 0.5 GPa and temperature – to. 750°C (granulite group), thereafter changing to close 0.7 GPa and 600°C respectively (gneiss group), under which conditions the rocks were subjected to isoclinal folding. uplift and erosion.Rocks of the Karelian cycle were presumably widely distributed. However, deep pre-Vendiall peneplanation resulted in removal of the bulk of them so they are nowadays present in area of 12% of the platform only. Older Karelian rocks are represented by amphibolites alternating with hornblende gneisses and the younger ones, in connection with a change in character of volcanism from basic to acid, by products of metamorphism of its products – crystalline schists and phyllites. The metamorphism was developing under conditions typical of the amphibolite and epidote-amphibolite groups: pressure about 0.6 GPa and temperature ranging from 500 to 600°C.Granitization was intensively developing in the Gothian epoch, being especially aggressive in relation to pre-Karelian rocks. Metasomatosis was primarily affecting dynamically alterated rocks and granitizing emanations were moving along faults, mylonitization zones and structural discontinuities, including decollement planes between rocks of the Karelian complex and old basement. The granitizabon was accompanied by strong stress directed from the south or south-west. Such conditions resulted in concentration of granitoids in NE SW oriented zones of tectonic looseness, especially those connected with contact of different structural units. The pressure resulted in break-up of granitoid massifs and fold zones into blocks and their northward shift. Moving in that direction, the blocks were reaching substratum of the Mazury zone, delineated by deep crustal fractures in the south, and sinking in it. The pressure was lower in the Mazury zone than in the south which resulted in large-scale development of metasomatic processes of formation of rapakiwi-like granitoids there. The most northward blocks or fold structures became pressed into the substratum of the Mazury zone and subjected to disintegration.Mobilizate, formed from granulites, gave rise to anorthosite massifs. The Suwałki massif originated at northern boundary of the Podlasie zone, and the Kętrzyn – at contact of the Ciechanów and Mazury zones.The final morphological image of top of the crystalline basement originated after the time of deep peneplanation which was continuing for about 500 m.y. A thin cover of the quasiplatform complex infilling tectonic troughs and morphological depressions in the top of the crystalline basement, has originated in the same time. Deposition of quartz sandstones was accompanied by acid volcanism.The rocks were subjected to metamorphism at temperatures not higher than 500°C and they are flat-laying except for local flexural bends. Pre-Karelian rocks were subsequently subjected to marked alterations in the Karelian and Gothian epochs but this did not change the basic structural pattern. The development of Precambrian basement ended with Vendian platform magmatism and volcanism.