Late Permian-Early Triassic igneous activity in the Attic Cycladic Belt (Attica): New geochronological data and geodynamic implications

Igneous rocks of the Attic Cycladic Belt (ACB) formed prior to the Alpine orogeny have received relatively little attention, especially regarding their crystallisation age. New U-Pb SHRIMP zircon data from mylonitised metagranitoid rocks within metapelites and metabasites of the Pentelikon mountain, lower tectonic unit (LTU) of Attica, in combination with internal zircon characteristics reveal the following: the metagranitoid precursor was very likely an I-type pluton crystallising in two successive stages, at 255±3Ma and 246±2Ma (weighted mean ²⁰⁶Pb/²³⁸U ages; error at 95% c.l.). These age results are respectively marginally older than and within uncertainty of the 240±4Ma protolith age reported for metagranitoids of the same unit in the area of Lavrion, Attica. They are in line with the protolith ages of several meta-igneous rocks within the Cycladic Blueschist Unit of Aegean islands. One core zircon domain and one single zircon grain yielded ²⁰⁶Pb/²³⁸U ages of 311±3Ma and 318±4Ma (1σ errors), providing evidence for inheritance from a previous Hercynian event. The new data, whilst interpreted to favour a rift-related geotectonic setting of formation (poly-episodic magmatism), do not provide additional strong arguments for a rifting versus a subduction scenario. Assuming that the dated rocks formed indeed in a rift setting, prominent at that time in the broad European area, they probably represent a higher level of the crust, compared to Permo-Triassic metagabbros of the Rhodope or the European Alps, which formed at deeper crustal levels (underplated gabbros). The new data, in combination with earlier petrological data on the type of the metamorphic path, favour the view that the LTU of Attica belongs geotectonically to the Cycladic Blueschist Unit of the ACB. This inference is thus filling an important missing link in the geotectonic configuration of the geology of Attica.