Zircon Typology: a Synthesis. Applications to Magmatic, Metamorphic and Sedimentary Rocks

Abstract

Abstract The study of more than 101 000 zircon crystals from 1105 different magmatic rocks (anatectic crustal, calc-alkaline, alkaline or tholeiitic) enables a general synthesis to be established which is useful for arguments in petrology. Populations are defined in typological distributions, mean points and typological evolutionary trends according to genetic groups. The overall logic obtained confirms the essential factors (temperature, chemistry) responsible for typological variations. The Hf contents of zircons, in substitution for Zr, make it possible to refine their origins (crustal, hybrid or mantle-derived), in particular by considering the early phase of crystallization, in equilibrium with the initial magma. They support the typological arguments and underline the role of the fluid phase in the magmatic evolution. Zircon crystals first formed in a given environment and subsequently immersed in a new medium of different chemical composition, temperature and fluid content show a spectacular readjustment of their typology. All the data obtained from the magmatic rocks are directly usable on ortho-derived metamorphic rocks with preserved zircon populations. For migmatites affected by partial melting, inherited zircon nuclei provide evidence (morphology, geochemistry notably Hf, Y) to constrain the nature of the protolith. Synthetic tables are proposed to help in the search for the origin of zircons found in sedimentary or volcano-sedimentary rocks, with a special mention for the most significant types and subtypes.