THE CYCLADIC BASEMENT AND THE PRE-CENOZOIC TECTONIC HISTORY OF THE SOUTHERN CYCLADES, IOS ISLAND, GREECE

The Cycladic Basement is one of the major units exposed on the Greek Cycladic islands of the southern Aegean Sea. While most of the islands are dominated by the Cycladic Blueschist Unit (CBU), the Cycladic Basement is exposed in the hearts of metamorphic core complexes (MCCs) in the southern Cyclades such as on the island of Ios, ~20 km N of Santorini and the active volcanic arc behind the retreating Hellenic subduction zone. This basement complex has few published age constraints and is composed of multiple granitic, orthogneissic, and paragneissic units. Its pre-Cenozoic history and paleogeography remain poorly understood as it was strongly overprinted by Cenozoic subduction- and extension-related fabrics. Zircon U-Pb crystallization ages for the granitic core of the Ios MCC range from 335 to 250 Ma, suggesting polyphase or protracted emplacement of Carboniferous to Permian plutons into Peri-Gondwanan crust, as evidenced by inherited zircon cores with a distinct Pan-African detrital zircon (DZ) age signature and zircon growth rims with ages of ~310 Ma. DZ signatures of Basement paragneisses and garnet-mica schists are also primarily Pan-African with maximum depositional ages (MDAs) ranging from 354 Ma to 450 Ma, suggesting they indeed represent the host rocks for Carboniferous-Permian intrusives. Interestingly, DZ U-Pb dating revealed a distinct second unit within the Basement paragneisses with significantly younger MDAs than the older paragneisses and most of the granitic intrusives, exhibiting bimodal zircon populations at ~290 Ma and ~335 Ma, suggesting that deposition of these rocks postdates intrusion and exhumation of the classic Basement complex and may have been deposited in localized rifts following the Variscan orogeny. Preliminary apatite U-Pb data record rapid cooling of Basement orthogneisses in the Early Permian, supporting the possible syn-rift nature of the Permian paragneisses. A few samples also contain zircon rims with U-Pb ages in the Late Jurassic, perhaps indicative of high pressure conditions earlier than the Cenozoic. Zircon (U-Th)/He data for both the granitic and all paragneissic units shows that they were rapidly exhumed together between ~9-14 Ma in the footwall of detachment faults coeval with MCC formation in the Naxos and Paros areas.