ORDOVICIAN PALEOGEOGRAPHY OF SIBERIA AND ADJACENT CONTINENTS

Ordovician palaeomagnetic data from the upper reaches of the Lena Rive r, Southern Siberia, confirm and refine the earlier reported data sets . Ordovician palaeomagnetic poles from Siberia define a systematic sou thwesterly apparent polar wander (APW) trend during Ordovician times ( mean south poles: 500 Ma: 42 degrees N, 310 degrees E; 467 Ma: 27 degr ees N, 314 degrees E; 460 Ma: 23 degrees N, 313 degrees E; 448 Ma: 22 degrees N, 301 degrees E and 437 Ma: O N, 290 degrees E). A primary or early magnetization age is verified by the reversal stratigraphy. Sib eria was geographically inverted at low southerly latitudes during the Latest Cambrian and Early Ordovician and drifted slowly northward and across the equator at an average palaeo-latitudinal velocity of c. 5c m/year. In Early Ordovician times, Avalonia and the European Massifs ( e.g. Armorica and Bohemia) were located together with Gondwana in high southerly latitudes, Laurentia was positioned in equatorial latitudes whereas Baltica was located at intermediate southerly latitudes. Sibe ria was probably located north of Baltica in latest Cambrian-Early Ord ovician times. Subduction-related, eclogite-facies metamorphism in lat est Cambrian-Early Ordovician time in the Scandinavian Caledonides occ urred in an ocean-continent transition zone marginal to Baltica but fa cing northern Siberia, and thus throws doubt on traditional Baltic-Lau rentian correlations during this particular time period. With Baltica rotating counterclockwise during the Ordovician, the plate scenario al lows for a Siberian source for Late Ordovician sedimentation in some a reas of maritime Laurentia and perhaps even northern Norway. It also h elps to explain the imposition of a deep-seated, sinistral strike-slip , fault regime between the obliquely converging Baltica and Laurentia, a transcurrent system which may have led to the permissive ascent of carc-alkaline granitoid magmas in favourable sites prior to the main s tages of Scandinavian orogenic deformation. Recent proposals that Laur entia formed a conjugate margin to the South American part of Gondwana during Ordovician times are permissible from palaeomagnetic data, but a tight continental fit during the entire Ordovician is contradicted by biogeographic data. The tight palaeomagnetic fit could perhaps be a n artefact of inaccuracies in the palaeomagnetic record for Gondwana.