High-Si phengite records the time of greenschist facies overprinting:: implications for models suggesting mega-detachments in the Aegean Sea

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. On Tinos, the lowermost part of the metamorphic succession has experienced a pervasive greenschist facies overprint. Si-rich phengite from marbles representing this lithostratigraphic level yielded Rb-Sr ages of c. 24 Ma. If the earlier metamorphic history is not taken into account, such data sets may lead to the erroneous conclusion of Miocene HP metamorphism. This study indicates that this phengite experienced pervasive rejuvenation of the Rb-Sr isotope system during overprinting, without significant changes in Si content, due to bulk-compositional constraints. This leads to the conclusion that in the absence of critical mineral assemblages the Si value of phengite is not a reliable indicator for metamorphic pressures in impure marbles. Recent studies have reported large displacements (>100 km) for detachment faults in the Aegean Sea. A critical parameter for such models is the age of HP metamorphism as deduced from white mica dating in the basal units of the Cyclades. We question the underlying idea of Miocene HP metamorphism and suggest, instead, that this age constrains the timing of the greenschist facies overprint and that the existence of mega-detachments in the study area requires further investigation.