Geochemistry of basaltic ash beds from the Fur Formation, Island of Fur, Denmark

The major, trace, rare earth and strontium isotopic compositions of five ash layers of Early Eocene age from Knudeklint, Fur (Denmark) have been determined in order to establish the geochemical nature of the magmas and, particularly, to provide geochemical constraints on the source. The sample set includes some layers reported to show systematic local grain size and thickness variations ( +51 and+ 114) as well as layers that apparently lack such variations ( + 1, +35 and +60). Despite these differences, the five layers have remarkably similar compositions, apart from showing a limited degree of fractionation, and are therefore believed to have a common source. The magmas sourcing the eruptions were of continental tholeiite type, comparable to the early Tertiary basalts of the Faeroes-East Greenland province. Var­iations in initial 87Sri86Sr ratio appear to correlate with SiO2, suggesting that small amounts of continental crustal material may have assimilated by the tholeiitic magma prior to eruption.