Podłoże krystaliczne polskiej części południowego Bałtyku

Abstract

Nowe materiały geofizyczne i wyniki badań petrograficznych skał z wierceń, wykonanych na polskim obszarze południowego Bałtyku, umożliwiły zestawienie schematycznej mapy geologicznej powierzchni podłoża krystalicznego. W południowo-zachodniej części krystalinik składa się prawdopodobnie z presvekofenno-karelskich granitoidów, a w północno-wschodniej - z granitoidów rapakiwipodobnych. Na północ od Rozewia- Mierzei Helskiej wychodzi w morze presvekofenno-karelska strefa fałdowa reprezentowana przez enderbity i gnejsy piroksenowe. Stwierdzono, ze polskie granitoidy rapakiwipodobne są przedłużeniem transskandynawskiego łańcucha granitowego południowej Szwecji. Trudne dotychczas uzasadnienie bardzo młodego wieku "gotyjskiej" charnockityzacji, migrnatytyzacji i anateksis tłumaczy się quasi-orogenezą hallandzką. Odnowienie pregotyjskich uskoków południkowych wyjaśnia się natomiast grenvillską rotacją tarczy bałtyckiej i jej kolizją z tarczą laurentyjską.

The crystalline basement of the Polish part of the southern Baltic

Results of petrographic investigations of rocks from boreholes carried out by W. O. Petrobaltic in the Polish part of the Southern Baltic showed some new elements increasing the knowledge of the Precambrian crystalline basement structure. Analysis of geophysical materials allowed to make out the lithological map of the top of crystalline basement. As a result, it has been stated, that to the north of Rozewie - the Hel Bay Bar (Fig. 1) the Presvecofenno-Karelian Kasubian complex come out into the sea. The rocks of this unit are built of enderbites and pyroxene gneisses - the older rocks of the complex - called the granulite group and hornblende gneisses and biotite gneisses – representing the younger unit of the complex the plagiogneiss group.

In the south-western part of the Southern Baltic area occur likely the Presvecofenno-Karelian rocks of granitoide complex, considered as such on the base of similarity of geophysical picture to the Dobrzyń-Pomerania Massif. In the north-eastern part of the said area, it has been stated the Gothian Rapakivi-like granitoides (about 50% area), that are most likely an extension of the Mazury complex expanding from Suwałki to the Vistula Bay Bar and Gdańsk. Migmatites and anatectic granitoides are secondary and the considered as the products of Late Gothian transformation, they were developed in the entire peripheral part of the East-European Platform.

New results from investigations of the Pomerania Precambrian and Polish part of the Southern Baltic allowed to compare with coresponding rocks of Southern Sweden. In the result of these there is a revision of hitherto existing views on evolution of rocks of the Polish part of the East-European crystalline platform in Proterozoic. It has been stated that Polish Rapakivi-like granitoides form the south-eastern extension of the Transscandinavian belt of the Gothian granites and they extended through Southern Lithuania up to tectonic fissures determined the Byelorussian-Baltic boundary of the granulite zone (W. Ryka, 1982b). The age of Rapakivi-like granitoides (1470 MY) is slightly younger than the age of the Transscandinavian granite belt (1800-1500 MY). Vein granitoides cutting charnockites, migmatites and metasomatic granites are of the isochrone age of 1340 MY and up to now were described as final repercussion of the Gothian transformation occurring in the whole border part of the East-European Platform, not only in Poland, but also in Lithuania and Byelorussia. The above processes along with the closing granitoide intrusions are connected with the Hallandian event (F.H. Hubbard, 1975), described in the Varberg area, South-western Sweden. The orogeny marked result (1500-1400 MY) with a strong transformation in the conditions of amphibolite facies, by charnockitization, migmatitization and anatexis. Similar changes took place in the Polish area in 1400-1340 MY, thus they also late, also the Gothian transformations were late in comparison with the similar processes observed in Southern Sweden. Changes in the evolution pattern of the Polish part of the Precambrian basement can be expected in the result of revision of hitherto existing opinions concerning the youngest magmatic processes described in South-western Sweden. The Sveconorwegian (Grenville) Orogeny may explains the age of some granitoides cutting the old Presvecofenno-Karelian complexes dated about 1250 MY. This corresponds to the Grenville Orogeny - 1250-900 MY. Further investigations may solve the problem of age of the Polish platform intrusions, if there are successful correlation tests with the anorogenic pyroxenite-syenite, granitoide intrusions, swarm mafic dykes and basic vulkanites of Southern Sweden, dated about 1200 MY.

The age of the maridian fault regeneration predating the Gothian Orogeny been determined. This process is explained by rotation of the Baltic Shield, and then its collision with the Laurentian Shield (1190-1050 MY) - Fig. 7.