Eocene tectonometamorphism on Serifos (western Cyclades) deduced from zircon depth-profiling geochronology and mica thermochronology

Abstract

Phengite-bearing metasedimentary sequences preserving relict glaucophane from Serifos, western Aegean, are indicative of high-pressure conditions during Cycladic blueschist metamorphism. Depth-profiling and conventional SIMS U–Pb geochronology and O-isotope analysis on zircon from highly strained orthogneisses at the base of the sequence reveal xenocrystic zircon with varying degrees of recrystallization. One end-member population has needle-like or prismatic morphologies, is internally oscillatory-zoned, with little to no rim material created during recrystallization. These zircons yield Triassic to Carboniferous ages, elevated Th/U, and δ18O mostly ranging between ~4 and 11‰. In marked contrast, a separate zircon population has a spongy structure created by complete recrystallization of the pre-existing crystal. These zircons possess low Th/U and flat HREE patterns, and yield Eocene ages with δ18O ~7‰. Chondrite-normalized REE profiles are extremely depleted. Within this spectrum, other zircons show variable states of recrystallization and mixed ages. Regression of U–Pb data from three mylonitic orthogneiss samples that exhibit near complete recrystallization and similar age-depth profiles yields c. 40Ma ages. These new data are the first geochronological constraints that successfully demonstrate an Eocene metamorphic event in the western Cyclades, and new 40Ar/39Ar phengitic mica ages of 38–32Ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter.