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

Abstract

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution-reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of 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 sensitive high-resolution ion microprobe (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 meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52·4 ± 0·8 Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength 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 for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syros.