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 Ar-40/Ar-39 app arent age spectra for white micas from the island of Sifnos, Cyclades, Greece. These micas formed during a period of porphyroblastic mineral growth (M(2)) in the epidote-blueschist facies, at > similar to 460 d egrees C and < similar to 14 kbar. M(2) marked the onset of a major pe riod of deformation (D-3) during which kilometre-scale ductile shear z ones formed and widespread recumbent folding took place. The modelling experiments suggest that dat apparent age spectra observed in the ecl ogite-blueschist domain (EBD) can only be obtained if the rocks cooled rapidly below similar to 350 degrees C after M(2) at similar to 42Ma. Cooling rates must be > 50 degrees C/m.y., and since M(2) is followed by D-3 we infer that these are the cooling rates during D-3. Modellin g experiments for micas from the greenschist domain (GSD) structurally underneath the EBD suggest that the GSD cooled rapidly, but at simila r to 32 Ma. One tectonic model that might explain the rapidity of cool ing rates inferred for the EBD model supposes that ambient temperature s were at similar to 500 degrees C when M(2) took place, but thereafte r 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. Copyright (C) 1996 Elsevier Science Ltd