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The nodular granite of Castanheira, north central Portugal: origin of the nodules and evidence for diapiric mobilization of granite

Published online by Cambridge University Press:  01 May 2009

R. J. Reavy ,
D. H. W. Hutton  and
A. A. Finch
R. J. Reavy
Affiliation:
Department of Earth Sciences, Parks Road, Oxford OX1 3PR, U.K.
D. H. W. Hutton
Affiliation:
Department of Geological Sciences, University of Durham, Durham DH1 3LE, U.K.
A. A. Finch
Affiliation:
Department of Chemistry, University of Aberdeen, Aberdeen AB9 2UE, U.K.
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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.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 2 , March 1993 , pp. 145 - 153
Copyright
Copyright © Cambridge University Press 1993

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References

Burnham, C. W. 1967. Hydrothermal fluids at the magmatic stage. In Geochemistry of Hydrothermal Ore Deposits, vol. 1 (ed. Barnes, H. L.), pp. 34–76. New York: John Wiley & Sons.Google Scholar
Burnham, C. W. 1979. The importance of volatile constituents. In The Evolution of Igneous Rocks: Fiftieth Anniversary Perspectives (ed. Yoder, H. S. Jr), pp. 439–82. Princeton University Press.Google Scholar
Deer, W. A., Howie, R. A. & Zussman, J. 1966. An Introduction to the Rock Forming Minerals. London: Longman.Google Scholar
Korzhinski, D. S. 1968. The theory of metasomatic zoning. Mineralium Deposita 3, 222–31.CrossRefGoogle Scholar
Norton, D. L. 1987. Advective metasomatism. In Chemical Transport in Metasomatic Processes (ed. Helgeson, H. C.), pp. 123–32. Dordrecht: Reidel.CrossRefGoogle Scholar
Reavy, R. J. 1989. Structural controls on metamorphism and syn-tectonic magmatism: the Portuguese Hercynian collision belt. Journal of the Geological Society 146, 649–57.CrossRefGoogle Scholar
Reavy, R. J., Stephens, W. E., Fallick, A. E., Halliday, A.N. & Godinho, M. M. 1991. Geochemical and isotopic constraints on petrogenesis: the Serra da Freita pluton, a typical granite body from the Portuguese Hercynian. Geological Society of America Bulletin 103, 392–401.2.3.CO;2>CrossRefGoogle Scholar
Van Diver, B. B. 1970. origin of biotite orbicules in “Bullseye Granite” of Craftsbury, Vermont. American Journal of Science 268, 322–40.CrossRefGoogle Scholar
Webster, J. D. & Holloway, J. R. 1988. Experimental constraints on the partitioning of Cl between topaz rhyolite melt and H2O and H2O+CO2 fluids: new implications for granitic differentiation and ore deposition. Geochimica et Cosmochimica Acta 52, 2091–105.CrossRefGoogle Scholar