Immobile trace elements in K-bentonites indicate three volcanic stages during the Sandbian Stage of the Late Ordovician: Early Grefsen is characterised by high-Ti source magmas alternating with a few high-Zr layers. Middle Grefsen is characterised by overall low-Ti source magmas with multiple high-Zr layers alternating with less frequent low-Zr layers. Late Grefsen to Early Katian is characterised by a low content of all studied immobile trace elements (Ti, Zr, Nb, and Th). In all K-bentonites, the Nb/Al2O3 ratio indicates subalkaline volcanism dominated by rhyolites. In the Early Grefsen stage, andesites and dacites occur. The TiO2/Al2O3 versus Zr/Al2O3 discrimination diagram suggests low- and high-temperature magmatism. Low-temperature magmatism may have occurred during subduction of water-rich oceanic sediments in the frontal arc of Avalonia and high-temperature magmatism during the overriding of a mid-ocean ridge or hot spot. The coeval end of the volcanic record in Avalonia and Baltica may indicate the collision of Avalonia with a hypothetical continental crust within the Iapetus Ocean.
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