The tectonic significance of a porphyroblastic blueschist facies overprint during Alpine orogenesis: Sifnos, Aegean Sea, Greece

The MacArgon program^∗ has been used to model published apparent age spectra for white micas from the island of Sifnos, Cyclades, Greece. These micas formed during a period of porphyroblastic mineral growth (M₂) in the epidote-blueschist facies, at > ~ 460 °C and < ~ 14 kbar. M₂ marked the onset of a major period of deformation (D₃) during which kilometre-scale ductile shear zones formed and widespread recumbent folding took place. The modelling experiments suggest that flat apparent age spectra observed in the eclogite-blueschist domain (EBD) can only be obtained if the rocks cooled rapidly below ~ 350 °C after M₂ at ~ 42 Ma. Cooling rates must be > 50 °C/m.y., and since M₂ is followed by D₃ we infer that these are the cooling rates during D₃. Modelling experiments for micas from the greenschist domain (GSD) structurally underneath the EBD suggest that the GSD cooled rapidly, but at ~ 32 Ma. One tectonic model that might explain the rapidity of cooling rates inferred for the EBD model supposes that ambient temperatures were at ~ 500 °C when M₂ took place, but thereafter the high pressure rocks were thrust to the north and rapidly cooled as the result of their juxtaposition against cooler, shallower levels of the crust.