Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Compositional shifts caused by melting of nepheline tephrite of the Vladař-volcano at Žlutice (W-Bohemia)
Abstract
Blocks on the top of the W-Bohemian Cenozoic Vladař-volcano, consisting of fine-grained holokrystalline (analcite) nepheline tephrite, are in some places covered by crusts, up to several cm thick, which show evident features of melting: scoriaceous texture, fluidal phenomena on the surface, very fine afanitic character of the mesostasis around minute vugs. These crusts are evidently the melting product of the holocrystalline basaltic rock. The apparently slow annealing of the melted material was almost not accompanied by the origin of glass. But the question arose, if the rock chemistry remained without changes during the melting. The comparison of the bulk chemical compositions of the holocrystalline tephrite and the melted crust proved some significant shifts, teoretically expected, namely the loss in alkalis compensated by the relative increase of silica content. This slight change of chemistry could be explained by weathering and hydration prior to melting.
