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

The MacArgon program∗∗To obtain the most recent version of MacArgon go to WWW address: http://artemis.earth.monash.edu.au/MacArgon/. een used to model published 40Ar39Ar 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.