Hostname: page-component-78c5997874-s2hrs Total loading time: 0 Render date: 2024-11-05T05:04:24.113Z Has data issue: false hasContentIssue false

The crystallization history and mechanism of emplacement of the western part of the Galway Granite, Connemara, Western Ireland

Published online by Cambridge University Press:  05 July 2018

Bernard E. Leake*
Affiliation:
Department of Geology, University of Bristol, Bristol 8, England

Summary

The Galway Granite often has a highly siliceous aphyric alkali granite at the margin, followed inwards by a K-feldspar phenocrystic adamellite and then a steeply layered granodiorite. This previously puzzling arrangement, which seemed to conflict with the necessity to cool from the periphery and crystallize the most basic granite at the margin, is examined by a traverse of chemically analysed rocks and biotites. The Mg/(Mg+Fe+Mn) values indicate that the granodiorite probably crystallized from the outside inwards. The marginal aphyric granite crystallized from the acid residuum in the nearly solid adamellite. This was drawn out of the adamellite by blocks of country rock falling into the adamellite and creating a zone of rarefaction behind them. This also explains the absence of a chilled margin and the aplitic texture and mineralogy of the aphyric granite. The adamellite largely accumulated by crystal settling with gravity grading but the granodiorite crystallized in vertical layers during upward vertical movements of the magma that tipped up the gravity layering in the adamellite. Scattered microdioritic xenoliths are postulated to be disrupted dykes. Eighteen granite and twenty-seven biotite analyses are tabulated.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1974

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Aucott, (J. W.), 1965. Nature, 207, 929.CrossRefGoogle Scholar
Aucott, (J. W.), 1966. The petrology, structure and geochemistry of the Galway Granite in the Crook Moithan area, Connemara, Co. Galway, Life. Ph.D.thesis, University of Bristol.Google Scholar
Aucott, (J. W.), 1968. XXllI lnternat. Geol. Congress (Prague), 6, 185.Google Scholar
Claxton, (C. W.), 1968. Geol. Mag., 105, 149.CrossRefGoogle Scholar
Claxton, (C. W.) 1971. Proc. Roy. Irish Acad., 71, B, 155.Google Scholar
Coats, (J. S.) and Wilson, (J. R.), 1971. Min. Mag., 38, 138.CrossRefGoogle Scholar
Emeleus, (C. H.), 1963. Min. Soc. Amer. Spec. Paper, 1, 22.Google Scholar
Farah, (A.), 1960. Gravity observations on the northwestern border of the Galway Granite. M.Sc. thesis, University of Birmingham.Google Scholar
Ferguson, (J.) and Pulvertaft, (Z. C. R.), 1963. Min. Soc. Amer. Spec. Paper, 1, 10.Google Scholar
Lawrence, (G.), 1968. The geochemistry of the Galway Granite of Lettermullan, Co. Galway, Life. Ph.D. thesis, University of Bristol.Google Scholar
Leake, (B. E.), 1968. Earth Planet. Sci. Letters,, 3, 311.CrossRefGoogle Scholar
Leake, (B. E.), Hendry, (G. L.), Kemp, (A.), Plant, (A. G.), Harvey, (P. K.), Wilson, (J. R.), Coats, (J. S.), Lunel, (T.), and Howarth, (R. J.), 1969. Chemical Geology,, 5, 7.CrossRefGoogle Scholar
Leggo, (P. J.), Compston, (W.), and Leake, (B. E.), 1966. Quart. Journ. Geol. Soe., 122,91.Google Scholar
Wilshire, (H. G.), 1969. Mineral layering in the Twin Lakes Granodiorite, Colorado. Geol. Soc. Amer. Mere., 115, 235.Google Scholar
Wright, (P. C.), 1964. Proc. Roy. Irish Acad., 63, B, 239.Google Scholar