Hostname: page-component-848d4c4894-sjtt6 Total loading time: 0 Render date: 2024-06-30T02:56:37.710Z Has data issue: false hasContentIssue false
  • English
  • Français

Formation of atoll garnets from the aureole of the Ardara pluton, Co. Donegal, Ireland

Published online by Cambridge University Press:  05 July 2018

J. A. T. Smellie
J. A. T. Smellie*
Affiliation:
Department of Geology, Queen's University, Belfast
Get access Rights & Permissions [Opens in a new window]

Summary

A detailed petrographic and microprobe study was carried out on atoll-shaped garnets from the aureole of the Ardara pluton, in which regionally metamorphosed rocks have been affected by thermal metamorphism. Textural evidence showed that the atoll garnets are formed by replacement mainly by biotite, some plagioclase, quartz, and opaques, and may proceed from the garnet interior towards the margins, or from the margins into the core. Core replacement occurs mainly in those garnets enclosed by quartz or quartz-rich matrix.

Microprobe analyses are presented of garnets in different stages of atoll formation. Compositional profiles at the garnet margins show abnormal (reversed) zoning with increase in MnO and Fe2O32, marked decrease of CaO, and decrease of MgO towards the extreme edge. With decreasing width of the atoll rims, there is generally an increase in MnO and to a lesser extent Fe2O3, a decrease in CaO; MgO is variable. The microprobe data support the textural observations and a model for atoll formation involving prograde resorption (i.e. garnet replacement during temperature increase) is presented.

Type
Research Article
Information
Mineralogical Magazine , Volume 39 , Issue 308 , December 1974 , pp. 878 - 888
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.)

Footnotes

1

Present address: Institute of Geological Sciences, Gray’s Inn Road, London WC1.

References

Akaad, (M. K.), 1956a. Quart. Journ. Geol. Soc., 112, 263–88.CrossRefGoogle Scholar
Akaad, (M. K.) 1956b. Geol. Mag., 93, 377–92.CrossRefGoogle Scholar
Atherton, (M. P.) and Edmunds, (W. M.), 1966. Earth Planet. Sci. Letters, 1, 185–93.CrossRefGoogle Scholar
Atherton, (M. P.) 1968. Contr. Min. Petr., 18, 347–71.Google Scholar
Cooper, (A. F.), 1972. Journ. Petrology, 13, 457–92.CrossRefGoogle Scholar
Edmunds, (W. M.) and Atherton, (M. P.), 1971. Lithos, 4, 147–61.CrossRefGoogle Scholar
Edwards, (A. B.), 1954. Textures of the ore minerals. Australas. Inst. Min. Metall.Google Scholar
Engel, (A. E.) and Engel, (C. G.), i960. Pt. II, Min. Bull. Geol. Soc. Amer., 71, 1–57.Google Scholar
Forestier, (F. H.) and Lasnier, (B.), 1969. Contr. Min. Petr., 23, 194–235.CrossRefGoogle Scholar
Grant, (J. A.) and Weiblen, (P. W.), 1971. Amer. Journ. Sci., 270, 281–96.CrossRefGoogle Scholar
Green, (J. F. N.), 1915. Min. Mag., 17, 207–41.Google Scholar
Hollister, (L. S.), 1966. Science, 154, 1647–51.CrossRefGoogle Scholar
Hollister, (L. S.) 1969. Geol. Soc. Amer. Bull., 80, 2465–94.Google Scholar
Kurat, (G.) and Scharbert, (H. G.), 1972. Earth Planet. Sci. Letters, 16, 379–87.CrossRefGoogle Scholar
Naggar, (M. H.) and Atherton, (M. P.), 1970. Journ. Petrology, 11, 549–89.CrossRefGoogle Scholar
Pitcher, (W. S.) and Sinha, (R. C.), 1957. Quart. Journ. Geol. Soc., 113, 393–408.CrossRefGoogle Scholar
Pitcher, (W. S.) and Read, (H. H.), 1960. Ibid. 116, 1-36.CrossRefGoogle Scholar
Rast, (N.), 1963. The British Caledonides. Oliver and Boyd (Edinburgh and London), 123-42.Google Scholar
Rast, (N.) 1965. The controls of metamorphism. Oliver and Boyd (Edinburgh and London), 73–102.Google Scholar
Schouten, (C.), 1934. Earn. Geol., 29, 653–82.Google Scholar
Schouten, (C.) 1946. Ibid. 41, 348-61.CrossRefGoogle Scholar
Smellie, (J. A. T.), 1972. Min. Mag., 38, 614–17.CrossRefGoogle Scholar
Smellie, (J. A. T.) 1972. Unpubl. Ph.D. thesis, Queen's University, Belfast.Google Scholar
Smellie, (J. A. T.) 1974. Min. Mag., 39, 672–83.Google Scholar
Smith, (C. S.), 1952. Imperfections in nearly perfect crystals. John Wiley and Sons, Inc. (New York), 377–401.Google Scholar
Williamson, (W. C.), 1935. Quart. Journ. Geol. Soc., 91, 382–405.CrossRefGoogle Scholar