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Sericitization of plagioclase in the Rosses Granite Complex, Co. Donegal, Ireland

Published online by Cambridge University Press:  05 July 2018

Meideno Que  and
Alistair R. Allen
Meideno Que
Affiliation:
Department of Geology, University College, Cork, Ireland
Alistair R. Allen
Affiliation:
Department of Geology, University College, Cork, Ireland
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Abstract

Sericitization in three separate pulses of the Rosses Granite Ring Complex, Co. Donegal, Ireland, has been investigated texturally and chemically using electron microscopy, electron microprobe and fluid inclusion thermometry. The sericitization, which is restricted to the cores of plagioclase, is associated with pores which are abundant in the cores, but absent in the margins. Alkali feldspar, although porous, is unaltered. Associated with the sericitization is alteration of the adjacent primary plagioclase within the cores of grains to a more sodic composition.

It is postulated that the sericitization resulted from the action of externally derived secondary hydrothermal fluids, which gained access to the pores in the plagioclase via now sealed microfractures, formed either by contraction during cooling of the Rosses Complex, or more likely by hydraulic fracturing by the fluids themselves. Limited fluid/rock ratios restricted the degree of sericitization within the host plagioclase, whilst an absence of alteration in alkali feldspar may have been due to the inaccessibility of pores in the alkali feldspar to the hydrothermal fluids at the time of alteration. Fluid inclusion data suggest that the fluids were of low salinity, and that the sericitization took place at an early stage in the cooling history of the Rosses Complex at temperatures between 400 and 600°C. It is further contended that greisenization in the Rosses Complex predated the sericitization and that the greisenization may have been due solely to volatile-rich late-stage magmatic fluids.

Keywords

sericitizationplagioclaseRosses Granite ComplexIreland
Type
Mineralogy
Information
Mineralogical Magazine , Volume 60 , Issue 403 , December 1996 , pp. 927 - 936
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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