Paleogene volcanic ash layers in the Danish Basin: compositions and source areas in the North Atlantic Igneous Province

Abstract

The Paleogene mo clay basin in Denmark contains ca. 200 layers of mostly well-preserved volcanic ash. Of these, ca. 77 layers have been analysed for major and trace elements by X-ray fluorescence spectrometry (XRF), 22 for rare-earth elements (REE), and 11 for Sr and Nd isotopes. The ‘negative’ ash series (layers −39 to −1) comprise tholeiitic basalt, crustally contaminated trachyte, and rhyolite; alkaline basalt, trachybasalt, trachyte, and rhyolite; and Ti-rich nephelinite and phonolite. The ‘positive’ ash series (layers +1 to +140) comprise enriched tholeiitic ferrobasalt and two rhyolite layers. The ferrobasalts form one comagmatic group; however, oscillations are seen up-section between less-enriched and more-enriched compositions, as indicated by variations in Zr/Nb, REE contents, and isotope compositions, suggesting heterogeneities in the mantle source. Two samples of Eocene ash from Greifswalder Oie in northern Germany are identical to the positive series ashes. By comparison of the ash compositions with other rocks from the North Atlantic Igneous Province, probable source areas can be identified. Four stages of deposition can be distinguished. In stage 1 (layers −39 to −22), basalts and rhyolites were sourced from centres on the NW European shelf. In stage 2 (layers −21b to −15), trachytes and rhyolites were still sourced from centres on the shelf, whereas the strongly alkaline layers could all have originated in the Gardiner Complex in East Greenland. In stage 3, alkali basalts (layers −13 to −11) may be the products of a failed or propagating part of the opening oceanic rift. In stage 4 (layers +1 to +140), we suggest that the comagmatic suite of voluminous ferrobasalts were sourced from a gigantic volcanic system representing the nascent Proto-Iceland within the opening ocean. The cataclysmic character of stage 4 can be understood if the areas of extremely high magma production associated with the Proto-Icelandic mantle plume, which had until then produced the large subaerial flood basalts in Greenland, at this time moved away from the continent and into the sea-covered opening rift, thus switching the bulk of volcanism from effusive to explosive. When Proto-Iceland finally emerged, the explosive activity abated again.