Two metabentonite suites occur within the Wenlock limestones and marls of Gotland, one
within the Slite Formation (M. belophorus to the C. ellesae biozones) and the other in the Mulde
Formation (G. nassa–M. dubius Biozone). Their geochemical characteristics based on rare-earth element
(REE) distributions in apatite crystals separated from the metabentonites suggest origins from three
separate volcanic sources. One of these sources has an alkaline affinity, reflected in relatively high
levels of Th, Nb and Zr, which suggests that it may have lain over thickening continental crust during
the waning stages of the closure of the Tornquist Sea, while the other two represent calc-alkaline
magmas. The source of the volcanic ash is placed some 400 km to the southwest of Gotland, on the
Tornquist–Teysseyre Zone, which is the northernmost expression of the Trans-European Suture Zone.
Distinctive differences in REE distribution in apatite from these metabentonites should assist in future
correlation studies in Wenlock stratigraphy, both within the Baltic region and further afield.