Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U–rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.

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