Mineral inclusion assemblage and detrital zircon provenance

Abstract

Mineral inclusions are common in magmatic zircon and a potentially rich source of petrologic information. Controls on the relative proportions of inclusion phases, specifically early-crystallizing minerals such as apatite and late-crystallizing phases such as quartz, K-feldspar, and muscovite, have not been systematically studied. For instance, apatite dominates many magmatic zircon inclusion suites, and selective replacement of apatite over other phases has been proposed as a mechanism for generating apatite-poor inclusion assemblages in detrital zircons. However, the extent to which apatite inclusion abundance is influenced by source rock composition has not been established. The preservation of characteristic minerals in granite series, such as differences in magnetite and ilmenite abundances due to varying redox, have also not been systematically explored as inclusion phases in zircon. We surveyed zircon inclusion assemblages in Phanerozoic granitoids of a range of compositions and found a broadly inverse relationship between the presence of apatite in the inclusion suite and whole-rock SiO2 content. Selective loss of apatite is evident from deficits in apatite content among inclusions in contact with cracks in both detrital zircons and some granitoid zircons with independent evidence for fluid ingress (i.e., secondary phases filling open cracks). In cases where microstructural observations can identify primary inclusion assemblages (versus those impacted by fluid ingress), the relative occurrence of apatite can be used to broadly predict source rock SiO2 content. There is little relationship between whole-rock chemistry and the abundance of late-crystallizing phases or the relative proportions of quartz, plagioclase, and alkali feldspar. The presence of ilmenite not in contact with cracks in the host zircon likely points to origins in ilmenite-series (i.e., reduced) magmas, but the presence of magnetite is less diagnostic. Using the apatite abundance metric, detrital Hadean zircons from Jack Hills (Western Australia), appear to derive from felsic rather than intermediate or mafic granitoids and detrital Eoarchean zircons from Nuvvuagittuq Supracrustal Belt (Quebec) may derive from intermediate to felsic granitoids. No Fe-Ti oxides have been identified in Nuvvuagittuq zircons, but rare ilmenite in Jack Hills zircons points to origins of at least some part of the population in ilmenite-series magmas.