ZIRCON LA-SS-ICP-MS U-PB ANALYSIS OF THE PETROGENESIS, CHRONOSTRATIGRAPHY, AND PROVENANCE OF THE CYCLADIC BLUESCHIST UNIT AND BASEMENT IN THE SOUTHERN CYCLADES, SIKINOS AND IOS, GREECE

The Greek Cycladic Islands contain an assemblage of metamorphic units that record both early Cenozoic subduction-related HP/LT metamorphism and late Cenozoic back-arc extension and metamorphic core complex formation during slab rollback. Sikinos and Ios islands (Southern Cyclades) are composed of the Cycladic Blueschist Unit (CBU - greenschist to blueschist facies metabasite, metapelite, and marble) and the Cycladic Basement (CB - sheared felsic orthogneisses and metasedimentary paragneisses known as Carapace). This study utilizes detrital zircon U/Pb geochronometry to determine (1) the sedimentary provenance of the CBU on Sikinos and northern Ios, (2) the crystallization and depositional ages of the CB and Carapace on Sikinos to differentiate the two units, and (3) the timing of HP/LT metamorphism on both islands. Laser-ablation split-stream ICP-MS analysis of detrital zircons is used to simultaneously determine U-Pb ages and trace element compositions to distinguish between magmatic and metamorphic zircons and hence determine the age and petrogenesis of metamorphism and the maximum depositional ages (MDAs) for the CBU and Carapace rocks. Preliminary results from zircon U-Pb analyses from Sikinos indicate that both the Carboniferous CB and the Permo-Carboniferous Carapace underwent HP/LT metamorphism at ~50 Ma as evidenced by metamorphic zircon rims. The CBU exhibits a spread of detrital zircon U-Pb ages with age modes at Triassic, Permian Variscan, Pan-African and Proterozoic. Surprisingly, MDAs show that the CBU on Sikinos and Ios is entirely composed of Late Jurassic and Early Cretaceous strata and lacks Triassic MDAs that prevail in the CBU of the western and northern Cyclades. Metamorphic zircon rims of both CBU and CCB exhibit peaks at ~50 and ~70 Ma and strongly suggest that the two units were juxtaposed prior to Eocene metamorphism. Conspicuous Oligocene metamorphic rims from CBU samples within the CBU-CB interface are likely related to preferential hydrothermal fluid circulation. The conclusions of the study have broad implications for the paleogeographic evolution of the Aegean and the northern margin of Gondwana and the subduction interaction with the Eurasian plate, the evolution of the Hellenic subduction zone, and the multi-stage exhumation history of the CBU in the Aegean core complexes.