Geochemical analysis of two distinctive bentonites and their constituent apatite crystals from the Late Llandovery of Gotland, has provided unique chemical signatures for each bentonite. These data have allowed the beds to be correlated across the island. The chemical composition of constituent apatite crystals indicates that apatite crystallized in a transitional alkaline environment. Chemical and mineralogical evidence suggests that the earlier Lusklint Bentonite originated from a potassic magma generated in a waning subduction zone, while the later Ireviken Bentonite was not consanguinous with its predecessor, but was derived from a medium-K calc-alkaline magma produced in a continental margin environment. The volcanoes which gave rise to these bentonites probably lay in the Tornquist Sea, during its late stages of closure, somewhere between southern Poland and Denmark. These chemically distinct bentonites could act as sensitive chemostratigraphic tools for correlation of Late Llandovery horizons elsewhere in Northern Europe.

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