A regional survey of the geochemical composition of Lower Palaeozoic shales deposited on Baltica indicates that Llanvirn (Lower/Middle Ordovician) to Lower Silurian shales have higher concentrations of Na, Mg, Cr, Ni and Fe and lower concentrations of K, Rb and Ti compared to Arenig shales. This geochemical signature can be traced from Scania to the Oslo Region, i.e. in areas approximately 500 km apart, but is not present in Middle Ordovician sediments from Avalonia. The geochemical signature matches island arc tholeiites such as those in the Fundsjø Group within the Upper Allochthon of the Norwegian-Swedish Caledonides. Hence, these sediments were probably predominantly derived from island arcs formed during the end phase of closure of the Iapetus Ocean. Simple two component mixing calculations between oceanic and continental sediment sources suggest that the oceanic component diminishes towards the south where modifications related to longer sediment transport distances can be recognised. The introduction of sediment derived from island arcs coincides with increases in subsidence rates in the Oslo Region and may reflect an early stage in foreland basin development. The presence of the geochemical signature in Scania implies that island arcs systems were geographically widespread. The combined evidence indicates that the Arenig /Llanvirn boundary marks an important change in the continuing closure of the Iapetus Ocean. The data suggest that island arcs were obducted onto the outer margins of Baltica presumably during the Arenig. Continued obduction of island arcs in the Mid Ordovician and younger intervals is likely.
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