Metamorphic zircon formation by solid-state recrystallization of protolithigneous zircon

Protolith zircon in high-grade metagranitoids from Queensland, Australia, p artially recrystallized during granulite-grade metamorphism. We describe th e zircon in detail using integrated cathodoluminescence, U-Pb isotope, trac e element and electron backscatter diffraction pattern (EBSP) analyses. Pri mary igneous oscillatory zoning is partially modified or obliterated in are as within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgre ssive recrystallized zircon usually dominant. Associated with these areas a re recrystallization margins, interpreted to be recrystallization fronts, t hat have conformable boundaries with transgressive recrystallized areas, bu t contrasting cathodoluminescence and trace element chemistry. Trace elemen t analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. By this process, trace ele ments in the protolith zircon are purged during recrystallization and parti tioned between the enriched recrystallization front and depleted recrystall ized areas. However, recrystallization is not always efficient, often leavi ng a 'memory' of the protolith trace element and isotopic composition. This results in the measurement of 'mixed' U-Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystal lization. Recrystallization is shown to probably not significantly affect L u-Hf ages. Recrystallization has been determined by textural and trace elem ent analysis and EBSP data not to have proceeded by sub-grain rotation or l ocal dissolution/re-precipitation, but probably by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state r ecrystallization is probably common to high-grade terranes worldwide. The r ecognition of this process of formation is essential for correct interpreta tion of zircon-derived U-Pb ages and subsequent tectonic models.