The Early to Middle Triassic (250–240 Ma) onset of rifting in the Attica-Cyclades realm: A-type rhyolites of NE Attica, Greece

The NE Attica (meta)volcanics in the Lower Attica unit of the Attic-Cycladic Crystalline Complex (Greece) comprise an Early- to Middle- Triassic (250–240 Ma) thick succession (~ 300 m) dominated by tuffs, porphyritic rhyolite lava flows with subordinate intercalations of mafic pyroclastics and rare basaltic lavas. Detailed new U-Pb zircon data from the (meta)rhyolites reveal two dominant age groups at 251.9±0.9 Ma (37%) and 237.5±1.1 Ma (37%). The two age groups most likely relate to multiple magmatic pulses that grew over marginally older resorbed zircon, which is supported by many resorption features in the cathodoluminescence images. The rhyolites have a potassic mildly alkaline affinity and peraluminous character. They display many of the typical features of A-type magmas, including enrichment of incompatible elements, such as Zr, Nb, Y, Ga, Zn and Ce, as well as high FeO*/(FeO* + MgO) and 10,000*Ga/Al₂O₃ ratios. The A-type rhyolites have LREE-enriched patterns with pronounced negative Eu anomalies comparable with typical REE profiles for “hot-dry reduced rhyolites”. The investigated trace element patterns indicate that the NE Attica rhyolites were most likely to have evolved through simple fractional crystallization of a parental magma derived from an enriched mantle source, supplemented by a crustal component through assimilation of continental crust. The NE Attica rhyolites probably erupted in a within-plate setting in the back-arc region of the Cycladic realm.

The eruption of these rhyolites marks the onset of the anorogenic period during which the continental plate of the External Hellenides was subjected to extension and intra-plate rifting, which led to the opening of the Pindos-Cyclades back-arc basin.