Abstract

The Palaeogene granites of Scotland and Northern Ireland range mineralogically from biotite ± amphibole – through fayalite-hedenbergite– to riebeckite-bearing types and compositionally from metaluminous to peralkaline. Five members of the apatite supergroup of minerals are reported from the granites. Apatite is a relatively early-crystallizing phase in all rocks, whilst britholite crystallized during later magmatic stages in about half the samples, independent of granite type. There is a major composition gap between apatite and britholite, ranging from ~10 to 48 % of the britholite component. The dominant substitution mechanism was REE3++Si4+ = Ca2++P5+, although the substitution Na++REE3+ = 2Ca2+ occurred in the more peralkaline granites. Zonation is common in apatite and britholite and is ascribed to kinetic effects.

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