Summary. Early Wenlock (c. 430 Ma) lavas from the East Mendips Inlier, Southern Briain, are characterized by either single-component magnetizations (Type 1) or

New late Tremadoc—early Arenig palaeomagnetic results from SW Wales imply that S Britain (part of Eastern Avalonia) occupied a southerly latitude of c . 60°S in early Ordovician times. When combined with Scottish Ordoviaan palaneomagnetic data, which indicate a 15°S latitude, the results indicate that the British sector of the Iapetus Ocean reached n latitudinal width of c . 5000 km in Tremadoc—Arenig times, which was reduced to c . 3000 km by Llanvirn—Llandeilo (mid-Ordovician) times. The new data resolve two previous controversies in Palaeozoic palaeogeography. First, the high southerly palaeolatitude links Avdonia to Gondwana, marginal to W Africa, thus reconciling codlicting reconstructions based upon either palaeomagnetic or faunal/facies evidence alone. Second, reliable Llanvim palaeomagnetic data imply that Avlonia had rifted northwards by Arenig time, whereas Armorica remained proximal to northern Africa throughout the Ordovician. The combined data therefore establish that Avdonia and Armorica formed separate micro-continents when rifting from Gondwana.

Early Wenlock volcanic rocks from the East Mendips inlier, SW England, yield primary palaeomagnetic directions which indicate that southern Britain (Eastern Avalonia) occupied a palaeolatitude of 13 ± 5°S in Mid-Silurian (430 Ma) times. When combined with coeval data from Laurentia, the southern Laurentian margin (Scottish terranes), and Baltica, the collective data indicate that: (1) the British and Scandinavian sectors of the Iapetus Ocean were closed, to within the limits of the palaeomagnetic resolution, by the Early Wenlock; (2) Acadian deformation across Britain, which culminated in Early Devonian time, post-dated initial docking of Eastern Avalonia and Laurentia; (3) a previous palaeomagnetic requirement for the Tornquist Sea to remain open into Mid-Silurian times is removed, thereby reconciling palaeomagnetic and biogeographical constraints upon the convergence of southern Britain and Baltica. Recently-acquired palaeomagnetic data from southern Britain and Baltica have facilitated more reliable reconstructions of the early Palaeozoic palaeogeography of Northern Europe (Fig. 1). Palaeomagnetic data now record the progressive drift of southern Britain across the Iapetus Ocean during Ordovician times (Torsvik & Trench 1991; Channell et al. 1992 ) and a predominant northward drift and counterclockwise rotation of Baltica from Cambrian to Devonian times ( Torsvik et al. 1991 a Torsvik et al. 1992 a; Perroud et al. 1992 ). Nevertheless, details on the closure history of the Iapetus Ocean and Tornquist Sea (Fig. 1) have remained sketchy due to a sparseness of coeval Late Ordovician–Mid-Silurian data from the bordering continents. These palaeomagnetic uncertainties permit competing geological models. In this contribution, we address two outstanding problems relating to the closure