Cooling during the exhumation of a blueschist terrane: Sifnos (Cyclades), Greece

Abstract

Alpine high-pressure metamorphic rocks exposed in the Cyclades were overpinted during their exhumation. We studied the metamorphic evolution of a pervasively overprinted rock sequence, the greenschist unit, exposed in the island of Sifnos. We show that the metamorphic evolution in the Greenschist unit is characterized by the progressive transformation of eclogite-facies rocks through albite-bearing epidote blueschists into greenschists. The evolution of the mineral assemblages during exhumation is shown by a series of chemographic diagrams which are referenced to specific P-T fields in the petrogenetic grid proposed by Evans (1990). P-T conditions corresponding to the major paragenetic groups are estimated also on the basis of mineral chemistry, Fe/Mg exchange thermometry and the use of GEOCALC-PTX and THERMOCALC softwares. A P-T path for the Greenschist unit is presented indicating eclogite-facies conditions of around 480-520 ° C and 12-15 kbar, followed by overprinting in the albite-bearing epidote-blueschist facies at P-T conditions of 8-10 kbar and ca. 480-500 ° C. Pressures of 5.5 kbar and temperatures around 450 ° C are estmated for the subsequent overprint in the greenschist facies. The decompression part of the P-T path is characterized by cooling with no indication of a temperature rise. Thus, the high-pressure metamorphic rocks of Sifnos appears not to follow the thermal evolution path predicted by models which assume whole crust uplift and erosion. On a regional scale the exhumation of the Cycladic blueschist belt is characterized by cooling in the west, but broad clockwise P-T loops with significant heating during decompression are found in the east. We note that cooling affected rocks situated at the deepest levels of the Alpine orogenic wedge (Sifnos, Syros, Tinos) whereas heating has affected rocks exhumed from shallower structural levels (Naxos, Ios)