Zircon Coupled Dissolution–Precipitation Replacement During Melt–Rock Interaction Modifies Chemical Signatures Resulting in Misleading Ages

Abstract

Melt migration through Earth's crust drives well-documented melt–rock reactions, locally changing rock assemblage and geochemistry. However, melt–zircon interaction remains understudied. We report on three zircon-melt interaction events from the Pembroke Granulite, New Zealand. Primary zircon from gabbroic gneiss which was subject to minor post-emplacement melt migration and primary zircon from younger dykes exhibit straightforward microstructures, microchemistry, and age data. In contrast, zircon from melt-mediated reaction halos adjacent to the dykes and from melt-fluxed high-strain zones display dissolution modification of grains, micro-porosity and blurred or truncated internal zoning typical of replacement by coupled dissolution-precipitation. Replaced zircon domains show changed rare earth element patterns and redistributed or lost radiogenic Pb that generates ambiguous apparent spot date arrays, smeared over tens of millions of years. We conclude that the metamorphism and three melt–rock interaction events were brief, and the arrays misrepresent the true age and duration of the metamorphism. Pb-loss persisted beyond the metamorphism, with porosity and inclusions formed during coupled dissolution-precipitation making replaced zircon domains more susceptible to subsequent Pb-loss compared to the structurally intact, primary magmatic zircon in the host gabbroic gneiss or dykes. We recommend conducting high-resolution microstructural investigations upon recognition of spot date arrays observed in single samples to rule out the possibility of spurious arrays resulting from coupled dissolution-precipitation.