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Abstract

This article describes the occurrence, bulk geochemistry and phenocryst compositions of Caledonian volcanic ash beds (bentonites) in the sedimentary sections of the Palaeozoic Baltic sedimentary basin. Four periods of volcanism are recognized in the eastern Baltic region: (a) Late Sandbian with sources derived from the convergent margin between Avalonia and Baltica; (b) Late Katian with sources from the margin of the Iapetus Ocean (Norwegian Caledonides); (c) Aeronian (with extension into Telychian and Sheinwoodian) with sources in the Central European Caledonides; and (d) Telychian to Early Ludlow with sources derived from the convergent margin between Laurentia and Baltica (Norwegian Caledonides). Trace element compositions in bentonites indicate mostly evolved source magmas of rhyolitic and dacitic composition. The volcanism in the Aeronian is characterized by less evolved basaltic and trachyandesitic compositions. Sanidine compositions indicate the existence of potassium-dominated (over sodium) source magmas in Late Sandbian and from the late Homerian to Early Ludlow. During other periods both potassium- and sodium-dominated source magmas occur. The presence of sodium-rich sanidine in many bentonites combined with the scarcity of biotite suggests that the source magmas were water-undersaturated. Biotite phenocrysts are mostly Mg-rich, but Fe-rich varieties occur in the Late Sandbian and Early Telychian.

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