Palaeomagnetic and Rock Magnetic Study of Lower Devonian Sediments from Podolia, SW Ukraine: Remagnetization Problems

rachmanto surahmat;

The Early Devonian segment of the Apparent Polar Wander Path (APWP) for Baltica was defined by seven poles almost all from British Islands. Among them only one was obtained for Devonian sediments from Ukraine. The lack of data from other parts of Baltica than Britain makes this segment of the path dubious. To improve this situation we carried out palaeomagnetic study of the Devonian sedimentary sequence in Podolia (Ukraine). The studied rocks represent grey limestone and red beds and were sampled in seven outcrops in the basin of the D niester River. The standard palaeo magnetic experiments reveal Permian remagnetization corresponding to the poleposition :47◦S/351.5◦ E forred beds and 45◦S/340◦ E for grey lime stone. The second component corresponding to the nearly Devonian pole position (2.3◦S/338.4◦E for red beds and 0◦S/329◦E for grey limestone) was determined as primary based on the fact that they were recognized in different sediments—limestone and sandstone and carried by different magnetic minerals: multidomain (MD) magnetite in grey limestone, and detritic grains composed of primary hematite with ilmenite intergrowths in red beds. The fabric of anisotropy of magnetic susceptibility of red sandstone and these samples of grey limestone which preserved Devonian magnetization are typical for sedimentary structure. The preservation of primary component of natural remanence was supported by analysis of mechanism of remagnetization and acquisition of secondary components. Hysteresis properties indicate that red beds underwent chemical remagnetization whereas grey limestone were not altered. Variscan orogeny caused the elevated heat and fluid flows along the edge of the Trans European Suture Zone and deep burial of Podolia. We suppose that red beds underwent chemical remagnetization caused by fluid flows, whereas grey limestone could acquire thermoviscous remanence due toelevated temperature caused by burial.Hot fluids couldeasilypenetrate into sandstone and cause chemical alteration and creation of new iron oxides. As detrital hematite is chemically very stableit could preserve original magnetization. As grey lime stone cemented at the beginning of digenesis was not penetrable by fluids during burial their remagnetization was caused mainly by increase of temperature and was not accompanied by alteration of rock. MD magnetite present in grey limestone could easily acquire thermoviscous magnetization. The remagnetization pattern is in agreement with the thermal history of Lower Palaeozoic rocks from Podolia. Maximum palaeotemperatures during burial reached 200 ◦C. The age of maximum palaeotemperature is about 300 Ma in excellent agreement with the position of poles calculated from remagnetization on APWP.