Multiple Episodes of Zircon Growth during Anatectic Metamorphism of Metasedimentary Rocks in Collisional Orogens: Constraints from Felsic Granulites in the Bohemian Massif

Zircon is a key accessary mineral for metamorphic geochronology and geochemical tracing, but it has been a challenge to interpret its complex chemical zoning and age record acquired during multiple episodes of anatectic metamorphism in collisional orogens. This is illustrated by a combined study of petrography, phase equilibrium modeling and metamorphic P-T-t determination for granulites from the Bohemian Massif in the Variscan Orogen. These rocks record multiple episodes of zircon growth during anatectic metamorphism. They started from the compressional heating for prograde metamorphism to high-pressure (HP) to ultrahigh-pressure (UHP) eclogite facies with low degrees of partial melting. Afterwards, they underwent a decompressional stage from UHP eclogite facies to HP granulite facies for dehydration melting. These were followed by a further decompressional stage either to kyanite granulite facies or to sillimanite granulite facies at ultrahigh-temperature (UHT) conditions. Episodes of zircon growth are linked to specific metamorphic conditions for peritectic reactions on the basis of zoning patterns, trace element signatures, index mineral inclusions in dated domains and textural relationships to coexisting minerals. The results indicate that relict zircon domains are preserved even at UHT granulite facies conditions. A few zircon domains in the kyanite granulite grew during the prograde to peak UHP metamorphism, possibly corresponding to consumption of biotite and plagioclase but growth of garnet. During the decompressional exhumation to the HP granulite-facies, relict or prograde zircon domains were mostly dissolved into anatectic melts produced by muscovite breakdown. Most zircon grains grew during this transition to the HP granulite-facies in the kyanite granulite and are chemically related to continuous growth of garnet, whereas abundant zircon grains grew subsequently at the UHT granulite facies in the sillimanite granulite and are chemically related to garnet breakdown reactions. Another peak of zircon growth occurred at the final crystallization of anatectic melts in the sillimanite granulite rather than in the kyanite granulite, and these zircon grains mostly show oscillatory zoning, low HREE + Y contents and significantly negative Eu anomalies. In terms of the inference for protolith nature, it appears that zircon in metasedimentary rocks can grow at a short timescale in different stages of anatectic metamorphism, and its dissolution and growth are mainly dictated by anatectic conditions and extent, the property of peritectic reactions, and the stability of Ti-rich minerals.