THE NODULAR GRANITE OF CASTANHEIRA, NORTH CENTRAL PORTUGAL - ORIGIN OF THE NODULES AND EVIDENCE FOR DIAPIRIC MOBILIZATION OF GRANITE

The Castanheira pluton in north-central Portugal is a small (1000 m x 600 m) granite body of Hercynian age which contains a remarkable abund ance of granite-cored, biotite-rimmed nodules. The nodules are interpr eted as representing original bubbles in the uppermost volatile-rich z one of a granitic pluton. Strong depletion in K and Rb in the host gra nite around the nodules suggests that the biotite is magmatic in origi n. The nodules may have for-med by reaction between chloroferrate(II) complexes in the vapour phase and silicate melt, possibly followed by condensation of the vapour phase to a small granitic core. Motion of t he vapour bubble stabilized a gradient in chemical potential with resp ect to the host granite, giving rise to the nodules. Chemical, petrolo gical and structural data suggest that the pluton was part of a larger granite body, which was forcefully emplaced during synchronous transc urrent shearing. The inferred presence of volatiles, in addition to th e pervasive tourmalinization of the roof zone, suggest that the magma was halogen-rich; this may imply that the magma had low viscosity.