The Eastern End of the Galway Granite

J. S. Coats1 and J. R. Wilson2
Department of Geology, University of Bristol, Bristol BS8 1TR
1Present address: Institute of Geological Sciences, 64-78 Grays Inn Road, London, W.C.I.
2Present address: Geologisk Institut, Aarhus Universitet, Aarhus, Denmark.

Summary: An account of the eastern part of the Galway Granite is presented together with the first detailed geological map. An aphyric medium-grained alkali granite occurs at the margin of the pluton followed by a coarse porphyritic granite, which becomes more basic towards the granite centre, consistent with findings in the western and northern parts of the batholith. Westwards the coarse porphyritic granite develops a foliation in which xenoliths, and occasionally potash feldspar phenocrysts, are aligned. Chemical fractionation trends illustrate the consanguinity of the granites and aplites. Field, petrographic, and chemical evidence suggest that most of the xenoliths are cognate.

The zonation of the granite is thought to have resulted from crystallization of a magma in which compositional gradients were set up during the early crystallization period. A temperature gradient, decreasing from the centre of the magma chamber outwards into the country rocks, resulted in the migration of water, accompanied by alkalis and other volatiles, towards the granite margin; also the early-crystallizing minerals displaced the residual magma outwards. Crystallization was followed by shearing in the deeper parts of the pluton to produce the granite foliation. A total of 166 rocks have been chemically analysed for 31 constituents.

Mineralogical Magazine; June 1971 v. 38; no. 294; p. 138-151; DOI: 10.1180/minmag.1971.038.294.02
© 1971, The Mineralogical Society
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