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1 Acta Geophysica Polonica

1 1998

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Paleomagnetism and rock-magnetism of ordovician gneisses from the Leszczyniec unit (Rudawy Janowickie, Sudetes) : study of remagnetization effects

Kądziołko-Hofmokl M., Jeleńska M., Szałamacha M., Szałamacha J.

Acta Geophysica Polonica

1998

Vol. 46, nr 2

187-215

Research performed for Leszczyniec hornblende gneisses comprises paleomagnetic and rock magnetic study. Thermomagnetic and microscopic investigations indicate that magnetic properties of the gneisses are carried mainly by minerals formed or altered and/or magnetized due to deformational stages. The most important changes are related to the stage D1 that took place in the amphibolite facies conditions and ended at the Devonian-Carboniferous boundary. Low field magnetic susceptibility and its anisotropy are in major part related to paramagnetic hornblende. The directions of magnetic lineation before tectonic correction remain in agreement with directions of stretching lineation found in the Leszczyniec (LE) unit. Paleomagnetic study reveals presence of nine components of characteristic remanence. Five of them, labeled T, A, A1, A2 and CP may be interpreted in terms of the geomagetic field or geomagnetic field and tectonics. Component T (in situ) represents Mesozoic overprint, components A and A1 (both in situ) represent Early Permian and Middle Carboniferous overprints, respectively. Components A2 and CP represent geomagnetic field from the pre-tectonic times. In order to interpret them we assumed anticlockwise rotation of the LE unit around the vertical intraplate axis by 80 remaining in agreement with its sinistral shearing connected with the Dl deformational stage stated by Mazur (1995). This procedure shifted the tectonically corrected A2 pole position to the Devonian segment of the reference APW path for Baltica and the CP pole to its Ordovician segment. These results suggest that the LE unit was close to the Baltica perhaps even since the Ordovician and became rotated anticlockwise by about 80 at the final phase of the D1 stage (Upper Devonian-Lower Carboniferous). Despite the apparent logic of the presented results, there is always a possibility that the A2 and CP components of NRM are artefacts as are probably the remaining four isolated components CN, BW, BE1 and BE2.