Tectono‐magmatic division of the Late Ordovician (Sandbian) volcanism at the south‐western margin of Baltica using immobile trace elements: Relations to the plate movements in the Iapetus Palaeo‐Ocean

Abstract

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.