The Influence of Secondary Alteration on the Ammonium Content of Granites, Exemplified by the Rosses Complex of Donegal

A. Hall
Department of Geology, Royal Holloway College, Egham, Surrey TW20 0EX

Abstract: Ammonium analyses of representative rocks from the Rosses granite complex (Ireland) and their constituent mineral show that any variation of magmatic origin has been completely obscured by pervasive hydrothermal alteration. The altered rocks are strongly enriched in the ammonium ion, most of which is held by chloritized biotite and sericitized plagioclase. The present ammonium content of the Rosses granites averages 13 ppm, but the original ammonium content of the magmas is estimated to have been less than 5 ppm. Potassium feldspars may preserve a better record of the original magmatic ammonium content than the whole rocks or the biotites.

Keywords: ammonium • Rosses complex • Ireland • granite • secondary alteration

Mineralogical Magazine; December 1993 v. 57; no. 389; p. 591-598; DOI: 10.1180/minmag.1993.057.389.03
© 1993, The Mineralogical Society
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