Apatite-supergroup minerals in UK Palaeogene granites: composition and relationship to host-rock composition
Macdonald, Ray; Bagiński, Bogusław; Dzierżanowski, Piotr; Jokubauskas, Petras
European Journal of Mineralogy Volume 25 Number 3 (2013), p. 461 - 471
publié: Sep 13, 2013
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 REE 3+ + 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.
Mots-clefs
apatite group • britholite group • composition gap • palaeogene granites • substitution mechanisms