Apatite-supergroup minerals in UK Palaeogene granites: composition and relationship to host-rock composition

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 REE3þþSi4þ Ca2þþP5þ, although the substitution NaþþREE3þ 2Ca2þ occurred in the more peralkaline granites. Zonation is common in apatite and britholite and is ascribed to kinetic effects. Key-words: apatite group, britholite group, Palaeogene granites, composition gap, substitution mechanisms. 1