THE LOWER SILURIAN OSMUNDSBERG K-BENTONITE - PART II - MINERALOGY, GEOCHEMISTRY, CHEMOSTRATIGRAPHY AND TECTONOMAGMATIC SIGNIFICANCE

The Lower Silurian Osmundsberg K-bentonite is a widespread ash bed tha t occurs throughout Baltoscandia and parts of northern Europe. This pa per describes its characteristics at its type locality in the Province of Dalarna, Sweden. It contains mineralogical and chemical characteri stics that permit its regional correlation in sections elsewhere in Sw eden as well as Norway, Estonia, Denmark and Great Britain. The <2 mu m clay fraction of the Osmundsberg bed contains abundant kaolinite in addition to randomly ordered (RO) illite/smectite (I/S). Modelling of the X-ray diffraction tracings showed the I/S consists of 18% illite a nd 82% smectite. The high smectite and kaolinite content is indicative of a history with minimal burial temperatures. Analytical data from b oth pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composi tion and the probable tectonic setting of the source volcanoes. The pa rental ash was dacitic to rhyolitic in composition and originated in a tectonically active collision margin setting. Whole rock chemical fin gerprinting of coeval beds elsewhere in Baltoscandia produced a pronou nced clustering of these samples in the Osmundsberg field of the discr iminant analysis diagram. This, together with well-constrained biostra tigraphic and lithostratigraphic data, provides the basis for regional correlation and supports the conclusion that the Osmundsberg K-benton ite is one of the most extensive fallout ash beds in the early Phanero zoic. The source volcano probably lay to the west of Baltica as part o f the subduction complex associated with the closure of Iapetus.