NEW PALEOMAGNETIC CONSTRAINTS ON THE POSITION OF CENTRAL BOHEMIA DURING EARLY ORDOVICIAN TIMES

Apparently conflicting data within the palaeomagnetic database suggest that Bohemia was at much shallower palaeolatitudes than the Armorican and Iberian massifs in Ordovician times. In order to resolve this con troversy a palaeomagnetic study of Arenig and Llanvirn volcanic and se dimentary rocks from the Barrandian Basin, Central Bohemia has been ca rried out. This basin comprises a sequence of unmetamorphosed Lower Pa laeozoic rocks, the main folding of which occurred in Late Devonian to Early Carboniferous times. The samples collected (17 sites, 153 sampl es) were subjected to stepwise thermal and alternating field treatment . Eleven sites yielded reliable magnetic directions, with analysis of the results using standard principal component and great circle analys es. Up to three directions of magnetization can be identified in these rocks. The first, termed A, is generally removed below 200 degrees C, although in some cases it persists up to 450 degrees C in the volcani c rocks. It is approximately parallel to the present-day Earth's magne tic field in the study area (Dec/Inc 360 degrees/67 degrees) and is th us thought to be of recent origin. The second direction identified, te rmed B, is isolated at intermediate blocking temperatures (150-350 deg rees C), and yields an overall in situ mean direction of 195 degrees/8 degrees, k = 22.3, alpha(95) = 13.1 degrees (seven sites). These dire ctions fail the fold test of McFadden (1990), and correspond to a pala eopole position of 34 degrees S; 356 degrees E. This coincides with th e Late Carboniferous sector of the European apparent polar wander path , and thus the B direction is interpreted as being a secondary overpri nt of this age. The highest blocking temperature direction, termed C, is identified at temperatures between 350 degrees and 450-600 degrees C. It passes the fold test, resulting in an overall mean direction of 312 degrees/83 degrees, k = 21.9, alpha(95) = 14. 6 degrees (six sites ) after bedding correction. This direction is interpreted as being rep resentative of the Early Ordovician palaeomagnetic field direction in the Bohemian Massif and yields a palaeopole position of 58 degrees N; 355 degrees E. This translates into palaeolatitudes of 76 degrees S fo r Bohemia, thus demonstrating that the Bohemian Massif was at similar peri-polar latitudes to the rest of Armorica during Early Mid-Ordovici an times, and formed part of the northern margin of Gondwana.