TECTONIC JUXTAPOSITION OF BLUESCHISTS AND GREENSCHISTS IN SIFNOS ISLAND (AEGEAN SEA) - IMPLICATIONS FOR THE STRUCTURE OF THE CYCLADIC BLUESCHIST BELT

In the Cyclades, kilometre-thick high-pressure rock sequences, display ing blueschist- and eclogite facies mineral assemblages, overlie a roc k sequence which was thoroughly overprinted in the greenschist and amp hibolite facies during its exhumation. Both rock sequences, subducted and metamorphosed at high-pressures in Eocene times, have been conside red by previous workers to have been exhumed as a coherent rock unit. In contrast, it is suggested here that the preserved high-pressure roc k sequences were exhumed more rapidly and prior to the underlying gree nschists. Significant metamorphic, structural and geochronological dis continuities exist across the blueschist-greenschist contact and field evidence suggests that the high-pressure metamorphic rocks in Sifnos are tectonically juxtaposed above the greenschists. These two rock seq uences were juxtaposed by a low-angle fault subsequent to the Oligocen e-Miocene greenschist-facies overprint. Published geochronological dat a and petrological criteria are used to show that the high-pressure se quence cooled below 350 degrees C when the rocks now immediately under lying it suffered a greenschist-facies overprint at temperatures of ca 450 degrees C. The section inferred to absorb this temperature differ ence is now missing and it is suggested that it has been cut out by th e low-angle fault.