Zircons from Syros, Cyclades, Greece - Recrystallization and mobilization of zircon during high pressure metamorphism

Zircons were studied from high-pressure / low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to eachother by dissolution-reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents in trivalent cations, and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios (( 0.1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y-HREE-Th phases and fluid inclusions, showing variable progress of replacement-recrystallization. Both end-members yield distinct SHRIMP U-Pb ages. The pristine generation has an age of 80.2 ± 1.6 Ma from a metagabbro, and 76.4 ± 2.1 Ma from a metaplagiogranite dyke. The skeletal, low Th/U zircon generation yields an age of 52.4 ± 0.8 Ma. The Upper Cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The Eocene age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of HFSE-elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U-Pb ages with white mica Ar-Ar ages of the same samples support, that Eocene is the true age of high-pressure metamorphism on Syros