Geological Magazine
- Geological Magazine / Volume 130 / Issue 02 / March 1993, pp 145-153
- Copyright © Cambridge University Press 1993
- DOI: http://dx.doi.org/10.1017/S001675680000981X (About DOI), Published online: 01 May 2009
Articles
The nodular granite of Castanheira, north central Portugal: origin of the nodules and evidence for diapiric mobilization of granite
R. J. Reavya1, D. H. W. Huttona2 and A. A. Fincha3
a1 Department of Earth Sciences, Parks Road, Oxford OX1 3PR, U.K.
a2 Department of Geological Sciences, University of Durham, Durham DH1 3LE, U.K.
a3 Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE, U.K.
Abstract
The Castanheira pluton in north-central Portugal is a small (1000 m × 600 m) granite body of Hercynian age which contains a remarkable abundance of granite-cored, biotite-rimmed nodules. The nodules are interpreted as representing original bubbles in the uppermost volatile-rich zone of a granitic pluton. Strong depletion in K and Rb in the host granite around the nodules suggests that the biotite is magmatic in origin. The nodules may have formed 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 the vapour bubble stabilized a gradient in chemical potential with respect to the host granite, giving rise to the nodules. Chemical, petrological and structural data suggest that the pluton was part of a larger granite body, which was forcefully emplaced during synchronous transcurrent shearing. The inferred presence of volatiles, in addition to the pervasive tourmalinization of the roof zone, suggest that the magma was halogen-rich; this may imply that the magma had low viscosity.
(Received June 15 1992)
(Accepted October 01 1992)
