J. metamorphic Geol., 2004, 22, 427–442 doi:10.1111/j.1525-1314.2004.00524.x High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea M. BRO¨ CKER1 ,D.BIELING1 , B. HACKER2 AND P. GANS2 1Institut fu¨r Mineralogie, Zentrallaboratorium fu¨r Geochronologie, Corrensstr. 24, 48149 Mu¨nster, Germany (
[email protected]) 2Department of Geological Sciences and Institute for Crustal Studies, University of California, Santa Barbara, CA 93106-9630, USA ABSTRACT In the lower main unit of the Attic-Cycladic crystalline belt (Greece), white mica geochronology (Rb–Sr, K–Ar, 40Ar–39Ar) has established the timing of at least two metamorphic events: well-preserved high- pressure/low-temperature (HP/LT) rocks yielded Eocene ages (c. 53–40 Ma) and their greenschist facies counterparts provided Oligocene–Miocene dates (c. 25–18 Ma). Marbles from Tinos Island contain high-Si phengite with Rb–Sr (phengite–calcite) and 40Ar–39Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and 40Ar–39Ar total fusion ages generally are 3–6 Ma older than corresponding Rb–Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older 40Ar–39Ar dates. This concept also suggests that the Rb–Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca-rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system.