Early Palaeozoic bimodal rift‐related magmatism is widespread throughout much of the Variscides of Europe. It is traceable from the Polish Sudetes to NW Iberia. Granitic plutonism generally predates Cambro–Ordovician bimodal magmatism. In the N Bohemian Massif this early Palaeozoic granitic plutonism was generated by partial melting of Cadomian basement, whereas contemporaneous alkali granites with a mantle component are typical of the NW Iberian Massif. Silurian‐Devonian mafic magmatism in the N Bohemian Massif, Massif Central and NW Iberian Massif is partly preserved as obducted ophiolites. Compositional diversity displayed by Cambro‐Ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume. This indicates that combined tensional forces and mantle plume convection assisted the early Palaeozoic dispersal of terranes from the N Gondwana margin. Continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust.

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SUMMARY: Within the Ordovician sequence of the Southern Uplands of Scotland numerous basaltic units occur, forming local basement to sequences of chert, graptolitic mudstone and greywacke. The basalts are fault-bounded at their base. Although they have suffered secondary alteration to varying degrees, the least mobile elements, Ti, Zr, Y and Nb, show a correlation between relative chemical abundances and supposed tectonic level. The most northern tectonic slice contains alkali basalts; the central slice contains 11 examples of what are apparently ocean-floor tholeiites and 4 mildly alkaline basalts; a third slice, found at one locality only, also contains tholeiites whose chemistry is comparable with the most primitive basalts. Samples of Ordovician extrusives from Bail Hill have calc-alkaline to alkaline affinities, while extrusives from Wrae Hill are quartz-keratophyres. The geochemical pattern is consistent with a model of an opening Iapetus Ocean in which rifting pre-dated spreading such that the basement to the Lower Palaeozoic sequence of the Southern Uplands was oceanic.

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