Silurian K-bentonites in the Iapetus Region: A preliminary event-stratigraphic and tectonomagmatic assessment

More than 150K-bentonite beds are known from the pre-Pridolian Silurian successions in Balto-scandia and the British Isles but only seven such beds have been reported from this stratigraphic interval in eastern North America. A review of their regional biostratigraphic and geographic distribution shows that single distinctive beds, or complexes of beds, are concentrated in certain graptolite zones in both Baltoscandia and the British Isles, and some of these beds appear to have event-stratigraphic potential. The few K-bentonite beds known from the Silurian of North America also occur at approximately the same stratigraphic levels as some widespread K-bentonite beds in northwestern Europe. In Europe, the trace element composition of the Wenlockian K-bentonites appears to differ from those of the Llandoverian and Ludlovian beds. Judging from immobile trace element geochemistry, the Silurian K-bentonite beds in the Iapetus Region had a calc-alkaline source, and were derived from volcanoes in a destructive plate margin tectonic setting. The geographic location of the source volcanoes remains enigmatic but the distribution patterns of the Silurian K-bentonites suggest that it was different from that of the numerous Ordovician K-bentonites in northwestern Europe and eastern North America.