CONSTRAINTS ON THE ORIGIN OF ZONATION OF THE GRANITE COMPLEXES IN THEFICHTELGEBIRGE (GERMANY AND CZECH-REPUBLIC) - EVIDENCE FROM A GRAVITYAND GEOCHEMICAL STUDY

Compositional zoning of granitic plutons is a common feature, but the spatial variation of zoning is rarely studied in detail. It is suggest ed that important additional information can emerge if the composition al zoning is regarded with relation to the root zones, i.e. with the g ranite shape at depth. A combined geochemical and gravity investigatio n was performed on the Hercynian Fichtelgebirge granites, which are co mposed of an older intrusive complex (OIC) and younger intrusive compl ex (YIC). The OIC and YIC are distinct from each other with respect to magma origin, differentiation pattern, shape of the granite complexes at depth and the zonation pattern in relation to the root zone. The O IC shows normal compositional zoning, whereas the YIC both reverse (YI C-1) and normal zoning (YIC-2). The general zonation pattern of the OI C and the YIC is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplace ment. In addition to the general zonation pattern, local zonation in i ndividual granite bodies of the YIC was caused by sidewall crystallisa tion producing a less-differentiated marginal facies (G2) and a more-d ifferentiated core facies (G3 and G4). It is suggested that the comple x zonation patterns of the Fichtelgebirge granite complexes reflect in teractions between the rate of magma output which is partly governed b y their rheological properties and the rate of deformation which provi des the room for the magma to be emplaced.