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

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 delta O-18 mostly ranging between similar to 4 and 11 parts per thousand. 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 delta O-18 -7 parts per thousand. 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. 40 Ma ages. These new data are the first geochronological constraints that successfully demonstrate an Eocene metamorphic event in the western Cyclades. and new Ar-40/Ar-38 phengitic mica ages of 38-32 Ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter. (C) 2011 Elsevier B.V. All rights reserved.