6533b7d9fe1ef96bd126cae7

RESEARCH PRODUCT

Resorption, growth, solid state recrystallisation, and annealing of granulite facies zircon—a case study from the Central Erzgebirge, Bohemian Massif

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Zircon crystals have been investigated from a gneiss area in the Central Erzgebirge, Bohemian Massif, where small occurrences/lenses of granulites (and sometimes eclogites) are located within amphibolite facies gneisses. Geological relationships indicate that leucocratic quartzofeldspathic rocks within the granulite boudins represent melts, whereas garnet-rich melanocratic rocks are considered restites, derived through multiple extraction of the leucocratic melts. The morphology of zircon crystals is quite different in these two types of granulites, with rounding pointing to higher zircon resorption in garnet-rich rocks due to multiple interaction with melts. Extensive new zircon growth can be demonstrated within granulitic boudins on the basis of zircon morphology, internal zircon textures, and age data. The newly formed zircon grains and rims are non-cathodoluminescent and yield an age of about 340 Ma (secondary ionisation mass spectrometry (SIMS) and zircon evaporation). Raman microprobe investigations revealed heterogeneous metamictisation within zircon grains and pronounced damage of the zircon structure in U-rich non-cathodoluminescent zircon rims. The common reverse discordance of such non-cathodoluminescent zircon rims may be explained as a result of preferential Pb sputtering during SIMS analysis leading to apparent Pb excess, which seems to occur principally in the most damaged zircon domains. It is proposed from field relationships that surrounding gneisses, although equilibrated under amphibolite facies conditions, experienced the same metamorphic PT history as the granulites. These amphibolite facies rocks contain zircon distinct from the granulite zircon populations, with morphologies and internal textures typical for their protolith rocks implying only slight zircon changes due to HP overprint. New zircon growth is here mostly limited to the appearance of very rare new grains or, more commonly, to formation of thin overgrowths. The internal texture of these zircon crystals shows clear signs of solid state recrystallisation leading in most cases to a partial resetting of their U–Pb ages mainly due to Pb loss. In addition, U contents are slightly higher in CL-dark “rims” that we interpret to have formed mainly due to solid state recrystallisation. Most evaporation and SIMS ages vary between protolith rock age and 340 Ma, so that the interpretation of the 340 Ma age is not unambiguous. It could be explained as the age of zircon crystallisation from a U-rich and Th-depleted melt or alternatively as a resetting age due to solid state recrystallisation resulting from melt or fluid interaction with old zircon. It is difficult to determine at which stage of metamorphic evolution new zircon growth occurred in the granulitic boudins, and remains unclear that zircon growth occurred at peak metamorphic conditions.