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Abstract

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist-/greenschist-facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P–T) method of THERMOCALC coupled with detailed textural work, we separate an early blueschist-facies event at 576 ± 16 to 619 ± 32 °C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist-facies overprint at 384 ± 30 °C and 3.8 ± 1.1 kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist-facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist-facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist-facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high-pressure rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km Ma⁻¹, completing a full blueschist-/greenschist-facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist-facies overprint of the blueschist-facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic reequilibration, and is unrelated to the strong high-temperature metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr-isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.