Constraints on the origin of zonation of the granite complexes in the Fichtelgebirge (Germany and Czech Republic): Evidence from a gravity and geochemical study

Compositional zoning of granitic plutons is a common feature, but the spatial variation of zoning is rarely studied in detail. It is suggested that important additional information can emerge if the compositional zoning is regarded with relation to the root zones, i.e. with the granite shape at depth. A combined geochemical and gravity investigation was performed on the Hercynian Fichtelgebirge granites, which are composed of an older intrusive complex (OIC) and younger intrusive complex (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 OIC shows normal compositional zoning, whereas the YIC both reverse (YIC-1) and normal zoning (YIC-2). The general zonation pattern of the OIC and the YIC is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplacement. In addition to the general zonation pattern, local zonation in individual granite bodies of the YIC was caused by sidewall crystallisation producing a less-differentiated marginal facies (G2) and a more-differentiated core facies (G3 and G4). It is suggested that the complex zonation patterns of the Fichtelgebirge granite complexes reflect interactions between the rate of magma output which is partly governed by their rheological properties and the rate of deformation which provides the room for the magma to be emplaced.