New structural and U–Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U–Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/−0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U–Pb ages and published K–Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/−0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late Campanian within a magmatic-arc setting and coeval shift of the magmatic arc towards the south; (3) Maastrichtian intrusion of dykes; (4) Palaeocene greenschist facies metamorphism and coeval ductile top-to-the SE thrusting of the Anafi Amphibolite Group on top of the Anafi Greenschist; (5) post-Eocene brittle top-to-the SE thrusting of the Anafi Greenschist and the Anafi Amphibolite Group on top of flysch sediments; (6) clockwise rotation of the system and (Early) Oligocene brittle top-to-the SE thrusting of the Chalepa Group on top of all units mentioned above; and (7) ongoing clockwise rotation of the Aegean block during the Oligocene to Early Miocene and change in stress field (NE–SW compression).