Zircon U-Pb chronostratigraphy and provenance of the Cycladic Basement and CBU on Sikinos and Ios Islands, Greece

Sikinos and Ios Islands, located in the Southern Cyclades of the Aegean, are part of a Cenozoic metamorphic core complex system that exposes subduction-related rocks in the highly-extended back-arc region of the Hellenic subduction zone. The HP-LT metamorphic units exposed on the Islands are the Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and Paleozoic ortho- and paragneisses of the Crystalline Basement Unit. While these units appear to share a common subduction metamorphic history, the magmatic and stratigraphic evolution of these two units and their relationship remain poorly understood. In particular, the nature of the contact between the CBU and Basement has been variably described as either an extensional shear zone or a subduction-related thrust that was potentially reactivated as an extensional top-to-the-north detachment. This study employed zircon U-Pb geochronometry on 59 samples to constrain (1) the crystallization ages of the Cycladic Basement on Sikinos Island, (2) the Maximum Deposition Ages (MDA) and the detrital provenance of the metasedimentary units. These new data allow to establish of a (chrono-) stratigraphic framework for the CBU and Cycladic Basement, elucidate the nature of the contact, and shed light on the subduction processes and pre-subduction paleogeography. The results reveal that (1) the Basement is composed of early Paleozoic metasedimentary rocks intruded by Carboniferous granites, (2) the Cycladic Basement is unconformably overlain by Permian and Mesozoic metasedimentary rocks of the CBU, (3) the CBU stratigraphy is clarified and its deposition spans from Permo-Triassic to Late Cretaceous times. This chronostratigraphic framework is also supported by the detrital zircon provenance U-Pb record that provides an additional stratigraphic fingerprint. These relationships in southern Sikinos demonstrate a nearly continuous stratigraphic record from the Cycladic Basement into the CBU and argue for a depositional contact and against both a subduction or extension-related structural juxtaposition of the CBU and the Cycladic Basement. On the other hand, the same stratigraphic constraints exhibit clear old-over-young relationships in NE Sikinos and Ios Islands, supporting structural repetition by thrust imbrication of the CBU slivers as a result of underplating during subduction in the Paleogene.