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

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 438Ma). 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-florencitemineral. 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. [ABSTRACT FROM AUTHOR]