Upper Ordovician palaeogeography of the Bohemian Massif: implications for Armorica

Abstract

SUMMARY The palaeogeographic position of Armorica in the upper Ordovician is still rather unclear, due mainly to the lack of reliable palaeomagnetic data. To help resolve this, a palaeomagnetic study of Caradocian and Ashgillian sediments and volcanics of the Barrandian Basin in the central Bohemian Massif (Czech Republic) has been carried out. A total of 29 sites were sampled, including four from a diabase sill intruded into Hirnantian-age sediments, thus providing a baked contact test. Three components of magnetization, labelled A, B and C, have been resolved, in addition to a low-temperature component which corresponds to the present-day direction of the local geomagnetic field (D/I= 360°/67°). Component A (southerly and shallow before bedding correction) is only sporadically present, and corresponds to a remagnetization event of Permo-Carboniferous age which is well known throughout Hercynian Europe. Component B, present as stable endpoints in the sill, and as a secondary component in the host-rock sandstones, yields an overall mean direction of 360°/39°, α95= 9.3°, k= 52.3 in situ, and 355°/21°, α95= 9.3°, k= 52.3 after bedding correction. Component C, identified as stable endpoints in sedimentary and volcanic rocks, passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization. The overall mean direction for C is 183°/−59°, α95= 9.5°, k= 30.3 (nine sites) after bedding correction which corresponds to a palaeopole position of 80°S; 360°E. This translates into palaeolatitudes of 40°S for the area studied in latest Ordovician times, but indicates large amounts of rotation (up to 170°) of the Bohemian Massif before final consolidation of Hercynian Europe. Although such large amounts of rotation are difficult to accommodate, they are consistent with palaeomagnetic results obtained from lower Ordovician and upper Silurian rocks elsewhere in the Barrandian Basin. If the Bohemian Massif is interpreted as being an integral part of the Armorican microplate, then these results indicate that prior to Caradoc times, Armorica rifted away from the northern margin of Gondwana, and that by Ashgillian times it was located at intermediate palaeolatitudes.