Unravelling the Formation and Exhumation of the Cycladic Blueschist Unit and Basement in the Southern Aegean, Sikinos Island, Greece

The Cycladic Islands of Greece expose an assemblage of HP/LT metamorphic rocks that record both processes related to subduction and backarc metamorphic core complex formation in the wake of the Hellenic subduction zone retreat. Sikinos Island is located in the southern Cyclades and exposes both the Cycladic Blueschist Unit (CBU) - greenschist-facies marbles and schists with preserved blueschist-facies - and the tectonically underlying Cycladic Basement(CB) - felsic plutonic rocks and metasedimentary country rocks known as the Carapace. Both units experienced Eocene subduction under eclogite/blueschist facies conditions and Miocene large-magnitude back-arc extension under greenschist facies conditions. The contact between the two units has been described as an extensional shear zone or a subduction-related thrust that was reactivated as an extensional top-to-the-N detachment. This study presents new (1) zircon U-Pb data to determine detrital provenance and maximum depositional ages (MDAs) of the CBU intervals on Sikinos, constrain protolith ages for the CB and Carapace, and quantify the timing of HP-LT metamorphism, (2) zircon and apatite (U-Th)/He cooling ages to elucidate the cooling and exhumation history of the CBU and CB in Sikinos and the nature of the contact between the two units. Results from zircon U-Pb analysis from the CBU show a spread of detrital zircon U-Pb ages dominated by Triassic, Permian Variscan, Pan-African and Proterozoic modes and MDAs from Early Cretaceous to Late Jurassic. This is in contrast to ubiquitous Triassic CBU on other Cycladic islands. LA-SS-ICP depth-profile analysis allows for simultaneous trace element analysis and for the identification of metamorphic zircon rims with ages of 26, 50 and 70 Ma. Notably, metamorphic rims of 26 Ma have not been reported in the CBU of the southern Cyclades prior to this study. These rims appear to be restricted a zone along the CBU-Basement contact, suggesting fluid flow as a possible explanation. Interestingly, metamorphic rims at 50 Ma in both CBU and Basement indicate that the two units were already juxtaposed before peak HP/LT metamorphism. Zircon (U-Th)/He ages from CB and CBU are statistically indistinguishable ( 9 to 12 Ma) and record rapid exhumation in response to top-to-the-N detachment faulting.