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

Arrays of near-concordant U Pb 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 176 Hf/ 177 Hf i 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) U Pb isotope and trace element spot analyses in selected, representative grains from the meta-gabbro were combined with high-resolution backscatter electron imaging to link U Pb 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 U Pb isotope mobility. Within each studied grain, several U Pb 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.