New palaeomagnetic 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 controversy a palaeomagnetic study of Arenig and Llanvirn volcanic and sedimentary rocks from the Barrandian Basin, Central Bohemia has been carried out. This basin comprises a sequence of unmetamorphosed Lower Palaeozoic rocks, the main folding of which occurred in Late Devonian to Early Carboniferous times. The samples collected (17 sites, 153 samples) 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 analyses. Up to three directions of magnetization can be identified in these rocks. The first, termed A, is generally removed below 200°C, although in some cases it persists up to 450°C in the volcanic rocks. It is approximately parallel to the present-day Earth's magnetic field in the study area (Dec/Inc 360°/67°) and is thus thought to be of recent origin. The second direction identified, termed B, is isolated at intermediate blocking temperatures (150–350°C), and yields an overall in situ mean direction of 195°/8°, k = 22.3, α95 = 13.1° (seven sites). These directions fail the fold test of McFadden (1990), and correspond to a palaeopole position of 34°S; 356°E. This coincides with the Late Carboniferous sector of the European apparent polar wander path, and thus the B direction is interpreted as being a secondary overprint of this age. The highest blocking temperature direction, termed C, is identified at temperatures between 350° and 450–600°C. It passes the fold test, resulting in an overall mean direction of 312°/83°, k = 21.9, α95 = 14.6° (six sites) after bedding correction. This direction is interpreted as being representative of the Early Ordovician palaeomagnetic field direction in the Bohemian Massif and yields a palaeopole position of 58°N; 355°E. This translates into palaeolatitudes of 76°S for Bohemia, thus demonstrating that the Bohemian Massif was at similar peri-polar latitudes to the rest of Armorica during Early Mid-Ordovician times, and formed part of the northern margin of Gondwana.