Eocene tectonometamorphism on Serifos (western Cyclades) deduced from zircon depth-profiling geochronology and mica thermochronology D.A. Schneider a, ⁎, C. Senkowski a , H. Vogel a , B. Grasemann b , Ch. Iglseder b , A.K. Schmitt c a Department of Earth Sciences, University of Ottawa, 140 Louis Pasteur, Ottawa, K1N 6N5, Canada b Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria c Department of Earth and Space Sciences, University of California-Los Angeles, 595 Charles Young Dr E, Los Angeles 90095-1567, USA abstract article info Article history: Received 12 October 2010 Accepted 12 February 2011 Available online 19 February 2011 Keywords: Cycladic islands SIMS depth-profile Zircon U–Pb Oxygen isotopes 40 Ar/ 39 Ar 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 δ 18 O 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 δ 18 O ~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. 40 Ma ages. These new data are the first geochronological constraints that successfully demonstrate an Eocene metamorphic event in the western Cyclades, and new 40 Ar/ 39 Ar phengitic mica ages of 38–32 Ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Tectonites are rocks whose fabric reflects their deformation history and that have experienced dynamic recrystallization under solid-state ductile conditions during intense strain. Timing constraints on these types of rocks, such as protolith crystallization ages or cooling histories, can be critical for helping elucidate the mechanisms by which high-grade metamorphic rocks were buried and exhumed (de Sigoyer et al., 2004; Gessner et al., 2001; Reddy et al., 1999; Ring and Glodny, 2010; Ring et al., 2007; Schneider et al., 2001). In the quest for such constraints, a number of geochronologic techniques have been utilized with varying degrees of success (e.g. Beltrando et al., 2009; Dunlap, 1997; Getty and Gromet, 1992; Resnor et al., 1996; Teufel and Heinrich, 1997). When high strain is achieved only under moderate to low temperature conditions, the systematics become fraught with incomplete chemical equilibrium, partial resetting, and a lack of suitable chronometers. The situation becomes even more complex when an area of interest has a polymetamorphic history. In situ and micro-analytical techniques have been the most reliable and effective means by which to decipher temporal riddles. Further, the convergence of data from independent chronometers strengthens the tectonic interpretation. In the Aegean region, a south- directed extrusion wedge has been proposed for the exhumation of remarkably preserved high-pressure rocks in the Cycladic islands (e.g. Syros, Sifnos) known as the Cycladic Blueschist Unit (CBU). In the extrusion wedge model for the eastern Aegean, Eocene exhumation of deep-seated parts of the Hellenides orogen was accomplished via two main shear zones: the Selçuk normal shear zone to the north and the coeval Cycladic–Menderes thrust to the south operating between 42 and 32 Ma (Rb–Sr and Ar–Ar constraints; Ring et al., 2007; Ring and Glodny, 2010). Similar models have been proposed for the central portions of the Aegean Sea, notably between Evia and Ios, where the CBU wedge was exhumed between shear zones with opposing kinematics around 35–30 Ma (e.g. Huet et al., 2009). Exhumation in these models is shortly following Eocene high-pressure metamorphic conditions (Altherr et al., 1979; Jolivet and Brun, 2008; Maluski et al., 1987; Putlitz et al., 2005; Ring et al., 2007; Ring et al., 2009; Wijbrans et al., 1993). Current exhumation models for the high-pressure crystalline wedge are based on a compilation of geological, geochronological, and geophysical data derived predominantly from the central and eastern Lithos 125 (2011) 151–172 ⁎ Corresponding author. E-mail addresses: david.schneider@uottawa.ca (D.A. Schneider), bernhard.grasemann@univie.ac.at (B. Grasemann), christoph.iglseder@univie.ac.at (C. Iglseder), axel@oro.ess.ucla.edu (A.K. Schmitt). 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.02.005 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos