RADIOMETRIC DATING OF THE TERTIARY VOLCANICS IN LOWER SILESIA, POLAND. IV. FURTHER K-Ar AND PALAEOMAGNETIC DATA FROM LATE OLIGOCENE TO EARLY MIOCENE BASALTIC ROCKS OF THE FORE-SUDETIC BLOCK

Krzysztof BIRKENMAJER (1), Zoltán PÉCSKAY (2), Jacek GRABOWSKI (3), Marek W. LORENC (4) & Pawe³ P. ZAGO¯D¯ON (5)

1) Institute of Geological Sciences, Polish Academy of Sciences, Cracow Research Centre, Senacka 1, 31-002 Kraków, Poland; e-mail: ndbirken@cyf-kr.edu.pl
2) Institute of Nuclear Research, Hungarian Academy of Sciences, Bem tér. 18c, 4001 Debrecen, Hungary; e-mail: pecskay@moon.atomki.hu
3) Polish Geological Institute, Rakowiecka 4, 00-975 Warszawa, Poland; e-mail: jgra@pgi.waw.pl
4) Institute of Geological Sciences, Polish Academy of Sciences, Sudetic Geology Department, Podwale 75, 50-449 Wroc³aw; e-mail: mwlorenc@twarda.pan.pl
5) Institute of Mining, Wroc³aw University of Technology, pl. Teatralny 2, 50-051 Wroc³aw, Poland e-mail: Pawel.Zagozdzon@ig.pwr.wroc.pl

Birkenmajer, K., Pécskay, Z., Grabowski, J., Lorenc, M. W. & Zago¿d¿on, P. P., 2004. Radiometric dating of the Tertiary volcanics in Lower Silesia, Poland. IV. K-Ar and palaeomagnetic data from Late Oligocene to Early Miocene basaltic volcanics of the Fore-Sudetic Block. Annales Societatis Geologorum Poloniae, 74: 1-19.

Key words: K-Ar dating, basaltic rocks, palaeomagnetism, Late Oligocene, Early Miocene, Lower Silesia, Poland.
Abstract: The Tertiary volcanic rocks of the Fore-Sudetic Block (FSB), Lower Silesia (Poland), exposed between Strzelin in the east and Legnica in the west, typically represent within-plate basalts. Petrologically, they consist mainly of alkali basalts, basanites, tephrites and ankaratrites. 16 new K-Ar dates are recorded, spanning the Oligocene (31 Ma) through Early Miocene (Burdigalian c. 18 Ma) time. The majority of these K-Ar dates plot around two significantly different ages: 27±1.5 Ma, and 20±1.5 Ma. They indicate the presence in the FSB of two distinct separate phases of Tertiary vulcanicity: (i) the first phase, mainly Late Oligocene (Chattian), with a peak at c. 27; (ii) the second phase, Early Miocene (Aquitanian-Burdigalian), with a peak at about 20 Ma. These phases seem to be separated by a gap in vulcanicity about 3 Ma long at the Oligocene/Miocene boundary.

Correlation of K-Ar-dated volcanic activity in the FSB with specific radiometrically-dated polarity intervals, poses some problems, and cannot be regarded definite at this stage of investigations. Considering the whole set of K-Ar and palaeomagnetic data from 40 sites, between the Opole area in the east and the Legnica area in the west, we suggest that volcanism of the first phase (Oligocene), although significantly spread out in time across multiple reversals, took place mostly during two well-defined, previously recognized events: (i) an older, reversed Odra event (within the C9r chron: 28.1±1.2 Ma); and (ii) a younger, normal Gracze event (within the C8n chron: 26.5±1.1 Ma). The second phase (Early Miocene) volcanism includes mainly a continuous set of reversely magnetized sites (mostly a single reversed C6r chron: 20.5±0.87 Ma).

Annales
Societatis
Geologorum
Poloniae

 

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