CRYPTIC TRACE-ELEMENT VARIATION AS AN INDICATOR OF REVERSE ZONING IN A GRANITIC PLUTON - THE RICANY GRANITE, CZECH-REPUBLIC

Cryptic variation recognized on the basis of trace-element patterns sh ows the Ricany granite in the northeastern part of the Hercynian Centr al Bohemian Pluton to be a reversely zoned, high-level intrusion. Unli ke many other reversely zoned plutons, there are no marked differences in modal composition between the margins and the centre of the intrus ion. The granite is generally peraluminous and geochemically evolved, as demonstrated by its restricted and high SiO2 range and low K/Rb rat io, coupled with a c. two-fold variation in Ba and Sr concentrations. Modelling shows that the geochemical variations can be most readily ex plained by K-feldspar-dominated fractionation in a magma chamber below the present level of exposure. The reverse zoning is interpreted as t he product of emplacement of an essentially single pulse of magma from a deeper level magma chamber in which fractionation has led to a vert ical compositional gradient. The least evolved magma was emplaced in t he centre of the high-level pluton with the more evolved magma around it. Recognition of cryptic reverse zoning in granites has major implic ations for granite petrogenesis in that the magmatic evolution of such bodies has to be established before assessing potential mechanisms of emplacement.