Identifying accessory mineral saturation during differentiation in granitoid magmas: an integrated approach

Numerical reconstruction of processes that may have operated during igneous petrogenesis often model the behaviour of important trace elements. The ge ochemistry of these trace elements may be controlled by accessory mineral s aturation and fractionation. Determination of the saturation point of acces sory minerals in granitoid rocks is ambiguous because assumptions about cry stal morphology and melt compositions do not always hold. An integrated app roach to identifying accessory mineral saturation involving petrography, wh ole-rock geochemical trends, saturation calculations and mineral chemistry changes is demonstrated here for a compositionally zoned pluton. Within and between whole-rock samples of the Boggy Plain zoned pluton, eastern Austra lia, the rare earth element (REE)-enriched accessory minerals zircon, apati te and titanite exhibit compositional variations that are related to satura tion in the bulk magma, localized saturation in intercumulus melt pools and fractionation of other mineral phases. Apatite is identified as having bee n an early crystallizing phase over nearly the whole duration of magma cool ing, with zircon (and allanite) only saturating in more felsic zones. Titan ite and monazite did not saturate in the bulk magma at any stage of differe ntiation. Although some trace elements (P, Ca, Sc, Nb, Hf, Ta) in zircon ex hibit compositional variation progressing from mafic to more felsic whole-r ock samples, normalized REE patterns and abundances (except Ce) do not vary with progressive differentiation. This is interpreted to be a result of li mitations to both simple 'xenotime' and complex xenotime-type coupled subst itutions. Our data indicate that zircon REE characteristics are not as usef ul as those of other REE-rich accessory minerals as a petrogenetic indicato r.