Identifying Accessory Mineral Saturation during Differentiation in Granitoid Magmas: an Integrated Approach

Abstract

Numerical reconstructions of processes that may have operated during igneous petrogenesis often model the behaviour of important trace elements. The geochemistry of these trace elements may be controlled by accessory mineral saturation and fractionation. Determination of the saturation point of accessory minerals in granitoid rocks is ambiguous because assumptions about crystal morphology and melt compositions do not always hold. An integrated approach to identifying accessory mineral saturation involving petrography, whole-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 Australia, the rare earth element (REE)-enriched accessory minerals zircon, apatite and titanite exhibit compositional variations that are related to saturation in the bulk magma, localized saturation in intercumulus melt pools and fractionation of other mineral phases. Apatite is identified as having been an early crystallizing phase over nearly the whole duration of magma cooling, with zircon (and allanite) only saturating in more felsic zones. Titanite and monazite did not saturate in the bulk magma at any stage of differentiation. Although some trace elements (P, Ca, Sc, Nb, Hf, Ta) in zircon exhibit compositional variation progressing from mafic to more felsic whole-rock samples, normalized REE patterns and abundances (except Ce) do not vary with progressive differentiation. This is interpreted to be a result of limitations to both simple ‘xenotime’ and complex xenotime-type coupled substitutions. Our data indicate that zircon REE characteristics are not as useful as those of other REE-rich accessory minerals as a petrogenetic indicator.