The Geniai Tuff in the southern East Baltic area - a new correlation tool near the Aeronian/Telychian stage boundary, Llandovery, Silurian

 

Authors: Kiipli T, Radzievičius S, Kallaste T, Kiipli E, Siir S, Soesoo A, Voolma M

Published in: Bulletin of Geosciences, volume 87, issue 4; pages: 695 - 704; Received 25 August 2011; Accepted in revised form 8 December 2011; Online 23 May 2012

Keywords: REE, goyazite, florencite, K-bentonite, Silurian, East Baltic,

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Abstract

In three drill cores in Latvia and Lithuania a thin (1 cm thick) altered volcanic ash bed with high concentrations of phosphorus (up to 3%), cerium (1%), lanthanum (0.5%) and strontium (2.5%), has been found close to the Aeronian/Telychian boundary (Llandovery, Silurian, ca 438 Ma). Small, millimetre-thick lenses within the ash bed contain up to 12% P and Sr, up to 6% Ce and 3% La. These elements occur as a solid solution of goyazite-florencite mineral. Additionally, authigenic K-feldspar and kaolinite occur in this ash bed. The high concentrations of REE elements, strontium and phosphorus suggest a carbonatite source magma, but the strontium isotope ratio contradicts that origin. Such a unique composition in a volcanic ash layer is a good basis for using it as a marker horizon for correlation of sections.