Melt-assisted coupled dissolution-precipitation of zircon in metagranite, Bohemian Massif

In zircon, transgressive textures across the oscillatory zoning and associated chemical modifications were mostly attributed to the process of fluid-mediated coupled dissolution-precipitation (CDP) over the past c. 20 years. Some works also admitted a possible role of melt in zircon CDP, expressing it for example by “fluid/melt”, but usually without explicit documentation of melt presence. The studies of melt-mediated zircon modification by CDP are thus extremely rare, and show an important but underrated process, with consequences on geological interpretation of zircon ages and chemical composition.

We choose one of the most common crustal rock types, a meta-granite, which undergone migmatization, and focused on modification of zircon textures and chemistry. To unravel primary and secondary zircon textures we use combination of high-resolution cathodoluminescence (CL), back-scattered electron (BSE) and secondary-electron (SE) images, because in CL most of the secondary textures were not visible. To relate zircon texture and date with trace- and rare earth element (REE) composition we use laser ablation–split-stream inductively coupled plasma–mass spectrometry (LASS). We relate the mineral inclusions with zircon textures, and for the secondary metamorphic inclusions we compare their assemblage and mineral chemistry with the rock assemblage and infer P–T conditions of their formation. Because the textures are typical of coupled dissolution-precipitation process and the P–T conditions of the secondary inclusions and matrix assemblage are above the wet solidus, we interpret zircon modification as caused by melt-mediated coupled dissolution-precipitation process.   

Oscillatory zoning is commonly blurred to a variable degree and it is in places truncated by patchy, convolute or structureless embayments. The irregular and relatively sharp boundaries of the embayments may be spatially associated with micro-porosity located ahead, and are interpreted as modification fronts of dissolution-precipitation. Some modification fronts are superimposed and relative timing can be inferred. Micro-porosity and inclusions are arranged in trails along modified BSE-light grey channels and are spatially associated with depressions at the surface, indicating more pronounced dissolution over precipitation. Larger inclusions tend to be located at the joints of the channels. The metamorphic zircon domains are characterized by an overall decrease of HREE with large variation in Yb/Gd, increase or decrease in LREE, increase in U, and decrease of Th and Th/U, compatible with presence of melt, garnet and titanite. Inclusions of Ph−Grt−Ttn are compatible with the matrix assemblage of Grt−Ph−Bt−Ttn−Kfs−Pl−Qz±Rt±Ilm, and equilibrated at eclogite-facies, at 15−17 kbar and 690–740 °C. The melt-mediated zircon modification resulted in a smear of mostly concordant dates from protolith oscillatory zoned domains with Cambro-Ordovician age to c. 330 Ma.