Early tertiary volcanism in the Skagerrak: New chemical evidence from ash-layers in the mo-clay of northern Denmark

Abstract

Microprobe analyses of glass and/or minerals from 12 selected ash-layers from the lower Tertiary mo-clay (approx. 53–54 m.y.) in northern Denmark are presented. The lower part of the ash-series provides evidence of a variety of rock types including tholeiitic basalts, rhyolite, and highly differentiated peralkaline types characterized by Ti-aegirine. The upper part of the ash-series forms a homogenous population of high Fe-high Ti-low A1 tholeiites, similar to Tertiary and recent basalts from the North Atlantic province. The presence of central volcanoes and a subvolcanic complex in the source area is indicated. This, together with variations in volcanic intensity, suggests an episode of abortive spreading in the Skagerrak-North Sea area simultaneously with the early spreading in the North Atlantic Ocean.