Near-concordant zircon Pb-loss array caused by melt-mediated coupled dissolution-precipitation replacement reactions: Signatures and geological significance

Arrays of near-concordant Usingle bondPb zircon ages are interpreted to develop during (i) long-lived geological events involving protracted periods of zircon growth or (ii) mobilization and escape of radiogenic Pb during a secondary event. However, distinguishing between either interpretation remains challenging. In this contribution we investigate the complex geochronology of a meta-gabbro from the Mawson Charnockite plutonic complex in East Antarctica. Previously published work on this sample produced a near concordant Pb-loss age array spanning 400 million years despite showing a narrow range of 176Hf/177Hfi values consistent with a single population of zircons. Zircons from the meta-gabbro are characterised by relative homogenous internal textures with no evidence of crystal plastic deformation nor significant radiation damage that would otherwise be consistent with diffusion related Pb-loss. In this study, in-situ laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) Usingle bondPb isotope and trace element spot analyses in selected, representative grains from the meta-gabbro were combined with high-resolution backscatter electron imaging to link Usingle bondPb isotopic and trace element variability to microstructures. Microstructures indicative of former coupled-dissolution precipitation replacement reactions such as abundant micro-scale porosity transgressing the interior of grains and sharp compositional reaction interfaces are common and are interpreted to be directly linked to enhanced trace element and Usingle bondPb isotope mobility. Within each studied grain, several Usingle bondPb analyses within a defined textural domain produced arrays of near-concordant dates spanning hundreds of millions of years between 500 Ma and 1000 Ma, where trace element concentrations are progressively depleted with increasingly younger dates. Our observations demonstrate that Pb-loss arrays, where only the oldest and youngest ages may be geologically significant, can result from variable trace element mobility and redistribution during melt-mediated coupled dissolution-precipitation replacement reactions within zircon.