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

Abstract

In zircon, transgressive textures cutting oscillatory zoning and associated chemical modifications are usually attributed to fluid-mediated coupled dissolution-precipitation (CDP). Here, we show an example of melt-mediated CDP in zircon from metagranite, with consequences on calculated ages and chemical composition. Zircon oscillatory zoning is commonly blurred and truncated by embayments and channels of replaced zircon with irregular and sharp boundaries. These are interpreted as replacement fronts and may be spatially associated with micro-porosity. Some superimposed replacement fronts indicate repeated CDP and porosity healing. Micro-porosity and inclusions are associated with replaced domains and thus are epigenetic. We highlight the necessity of combined high-resolution back-scattered electron imaging, otherwise the CDP textures may be overlooked in cathodoluminescence images. Inclusions of Ph–Grt–Ttn–Ap–Qz–Bt are compatible with the matrix assemblage Grt−Ph−Bt−Ttn−Kfs−Pl−Qz ± Rt ± Ilm, which equilibrated at c. 15−17 kbar, 690–740 °C with partial re-equilibration to c. 12 kbar, 680 °C. Since the conditions were above the wet solidus, the zircon changes happened through melt-mediated CDP. The replaced domains show an overall decrease of REE, Y, Th and Th/U, P, a large variation in Yb/Gd, shallower Eu anomaly, and increased U, and Hf. The chemical changes indicate a tendency of zircon purging trace elements during reequilibration with migrating melt. Spots with high LREE are probably due to the effect of micro-inclusions. Based on textures, inherited zircons are c. 540–600 Ma, protolith granite is c. 507–496 Ma, and CDP replacement occurred down to c. 335 Ma. But, the melt-mediated CDP resulted in a smear of mostly concordant, but spurious spot dates between inherited, protolith and modified domains, spanning > 200 Myr.

Bibliographic Information