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

TLDR

Numerical reconstructions of igneous petrogenesis often model trace element behavior, but the geochemistry of these elements can be controlled by accessory mineral saturation and fractionation, making determination of saturation points in granitoid rocks ambiguous due to variable crystal morphology and melt composition assumptions. The study demonstrates an integrated approach combining petrography, whole‑rock geochemistry, saturation calculations, and mineral chemistry to identify accessory mineral saturation in a compositionally zoned pluton. The method integrates petrography, whole‑rock geochemical trends, saturation calculations, and mineral chemistry changes to assess accessory mineral saturation. In the Boggy Plain pluton, zircon, apatite, and titanite show compositional variations linked to bulk magma saturation, intercumulus melt saturation, and fractionation, with apatite crystallizing early throughout cooling, zircon saturating only in felsic zones, titanite and monazite never saturating, and zircon REE patterns largely invariant to differentiation, indicating zircon REE characteristics are less useful as petrogenetic indicators.

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.