UPPER ORDOVICIAN PALEOGEOGRAPHY OF THE BOHEMIAN MASSIF - IMPLICATIONSFOR ARMORICA

The palaeogeographic position of Armorica in the upper Ordovician is s till 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 cent ral Bohemian Massif (Czech Republic) has been carried out. A total of 29 sites were sampled, including four from a diabase sill intruded int o 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 p resent-day direction of the local geomagnetic field (D/I = 360 degrees /67 degrees). Component A (southerly and shallow before bedding correc tion) is only sporadically present, and corresponds to a remagnetizati on event of Permo-Carboniferous age which is well known throughout Her cynian Europe. Component B, present as stable endpoints in the sill, a nd as a secondary component in the host-rock sandstones, yields an ove rall mean direction of 360 degrees/39 degrees, alpha(95) = 9.3 degrees , k = 52.3 in situ, and 355 degrees/21 degrees, alpha(95) = 9.3 degree s, k = 52.3 after bedding correction. Component C, identified as stabl e endpoints in sedimentary and volcanic rocks, passes both a fold test and a baked contact test and is interpreted as being the primary dire ction of magnetization. The overall mean direction for C is 183 degree s/-59 degrees, alpha(95) = 9.5 degrees, k = 30.3 (nine sites) after be dding correction which corresponds to a palaeopole position of 80 degr ees S; 360 degrees E. This translates into palaeolatitudes of 40 degre es S for the area studied in latest Ordovician times, but indicates la rge amounts of rotation (up to 170 degrees) of the Bohemian Massif bef ore final consolidation of Hercynian Europe. Although such large amoun ts of rotation are difficult to accommodate, they are consistent with palaeomagnetic results obtained from lower Ordovician and upper Siluri an rocks elsewhere in the Barrandian Basin. If the Bohemian Massif is interpreted as being an integral part of the Armorican microplate, the n these results indicate that prior to Caradoc times, Armorica rifted away from the northern margin of Gondwana, and that by Ashgillian time s it was located at intermediate palaeolatitudes.