The distribution of some major and trace elements in feldspars from the Rosses and Ardara granite complexes, Donegal, Ireland

Abstract

The feldspars in twenty-three samples of rock from the Rosses and Ardara complexes have been analysed for K, Na, Ca, Fe, Ga, Rb and Sr. In the plagioclases the Ca contents are directly related to the compositions of the rocks while the K contents vary irregularly, being due largely to the presence of sericite. In the alkali feldspars the Na contents are not systematically related to the compositions of the rocks; this is probably the result of postmagmatic recrystallization, for which petrographic evidence is given. Postmagmatic recrystallization of the alkali feldspars also provides an explanation for the distribution of Ga, which is more abundant in the plagioclases than in the alkali feldspars. Fe is more abundant in the plagioclases than in the alkali feldspars, and is present mostly in sericite in the former and hematite in the latter. Rb is much more abundant in the alkali feldspars than in the plagioclases, and Sr is slightly more abundant in the plagioclases than in the alkali feldspars. The distributions of Na and Sr between the coexisting feldspars are discussed in relation to their use in geothermometry.