Pre-Alpine basement within the Northern Cycladic Blueschist Unit on Syros Island, Greece

The main lithological sequence on Syros Island forms part of the Blueschist Unit within the Attic-Cycladic Crystalline Complex in the Hellenides. The structurally lowermost outcrops in SW Syros expose minor occurrences of intensely deformed K-feldspar megacryst augen gneisses. Ion microprobe U-Pb zircon geochronology yielded a range of single spot ages between similar to 331 to 299 Ma. Based on cathodoluminescence criteria, an age of 3 15 3 Ma is interpreted to approximate the magmatic crystallization of the granitic protolith. The magmatic protolith age correlates with the age of granitoid intrusions well documented from pre-Alpine basement units of the southern Cycladic Islands. The studied orthogneiss provides the first evidence that remnants of a Variscan consolidated crust are preserved within the Blueschist Unit of the northern Cyclades.