Cryptic trace-element variation as an indicator of reverse zoning in a granitic pluton: the Ricany granite, Czech Republic

Abstract

Cryptic variation recognized on the basis of trace-element patterns shows the Øíèany granite in the northeastern part of the Hercynian Central Bohemian Pluton to be a reversely zoned, high-level intrusion. Unlike many other reversely zoned plutons, there are no marked differences in modal composition between the margins and the centre of the intrusion. The granite is generally peraluminous and geochemically evolved, as demonstrated by its restricted and high SiO 2 range and low K/Rb ratio, coupled with a c. two-fold variation in Ba and Sr concentrations. Modelling shows that the geochemical variations can be most readily explained by K-feldspar-dominated fractionation in a magma chamber below the present level of exposure. The reverse zoning is interpreted as the product of emplacement of an essentially single pulse of magma from a deeper level magma chamber in which fractionation has led to a vertical compositional gradient. The least evolved magma was emplaced in the centre of the high-level pluton with the more evolved magma around it. Recognition of cryptic reverse zoning in granites has major implications for granite petrogenesis in that the magmatic evolution of such bodies has to be established before assessing potential mechanisms of emplacement.