Permskie skały wylewne z nadbałtyckiej części Pomorza Zachodniego

Abstract

W otworach wiertniczych na Wolinie, koło Kołobrzegu i Koszalina stwierdzono autuńskie utwory wulkaniczne, reprezentowane przez dacyty, ryodacyty (ignimbryty) i ryolity starszego cyklu wulkanicznego oraz latyty i dacyty (perlity) młodszego cyklu wulkanicznego. Pochodzą one z magmy, która utworzyła się w skorupie ziemskiej na głębokości 15–20 km pod wpływem wzrostu temperatury powyżej 635°C. Badanie produktów albityzaicji powulkanicznej ujawniło istnienie w przeszłości gorącej plamy z maksimum przeobrażeń zaznaczonym w skałach z otworu wiertniczego Kamień Pomorski 13.

PERMIAN VOLCANIC ROCKS IN BALTIC PART OF THE WESTERN POMERANIA

In the Baltic part of the western Pomerania, Autunian volcanic rocks were discovered in 18 drillings (Tab. 1) mainly situated in the Wolin Island (north of Szczecin) and the vicinities of Koszalin and Kołobrzeg (120 and 85 km east of the state boundary, respectively). These rocks, eroded and covered by Autunian conglomerates and tuffs, overlay Carboniferous sedimentary rocks.

The studies of the volcanic rocks showed a marked variability from intratelluric to extratelluric phase (Fig. 1), phenocrysts (Fig. 2), groundmass fabric (Fig. 3), sodium-to-potassium ratio (Fig. 11), silica content (Fig. 7) and heteromorphism of chemical composition (Fig. 8). Petrographic, mineralogical, geochemical and chemical studies of volcanic covers showed the following stratigraphic succession: 5 – dacites (perlites) from Kamień Pomorski, 4 – latites from Warnowo, 3 – rhyolites from Międzyzdroje, 2 – rhyodacites (ignimbrites) from Dargobądz, and 1 – dacites from Przytór.

The above mentioned covers presumably represent two volcanic cycles: older, comprising dacites, rhyodacites and rhyolites, and younger, comprising latites and dacites. The lowermost links of volcanic cycles, represented by andesites – basalts or latite andesites – basalts known from the Fore-Sudetic Monocline and Sudety Mts, are lacking here.

In the Wolin and Uznam Islands and eastern shores of the Szczecin Embayment, where drillings were most numerous, there were traced some lateral changes in the volcanic covers. These changes are shown on maps of isolines of felsitic and mafic indices (Figs. 9 and 10, respectively). Values of felsitic index increase from the north southwards along with steady decrease in content of calcium in relation to sodium and potassium. The course of isolines of mafic index is similar, indicating decrease in content of magnesium in relation to iron from the north southwards and from the east westwards.

In the western Pomerania, volcanic rocks underwent regional albitization which is marked with varying intensity in chemical and mineral composition. Albitization usually effected basic plagioclases and feldspar groundmass. Directions of migration of hydrothermal solutions are not clearly marked in particular borehole sections. Map of albitization index (Fig. 12), showing distribution of sodium in relation to potassium, was made in order to explain this question. The arrangement of isolines on this map fails to show connections with felsitic nor mafic indices. Maximum values of albitization index were found in the vicinities of Kamień Pomorski, a presumed center of hydrothermal alterations of Autunian rocks and a former hot spot.

The studies of volcanic rocks of the western Pomerania showed marked differences in their mineral composition, common changes in texture and structure, and marked changes in chemical composition which are traceable even at short sections of cores (less than a meter long). Moreover, the ultramicroscopic studies carried out by Z. Waleńczak showed the presence of relic structures of sedimentary rocks as well as algae in their original cement. The above mentioned data suggest that Autunian volcanic rocks represents products of crystallization of magma which originated at the depth of 15–20 km at temperatures rising to over 635°C in the Earth crust. The increase in temperature was related to a high geothermal degree after the Asturian phase, and formation of rhyolitic melt was initiated by andesite – basalt intrusions which were responsible for extra heating.

Regional albitization is a post-volcanic process. Mass migration of sodium may be explained by migration of hydrothermal solutions rich in sodium salts derived from the Variscan Orogen. These solutions reached volcanic rocks resulting in their direct alteration or marine basin, leading to a rapid increase in its salinity.