Pressure-temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high-pressure rocks in the Central Aegean

摘要

The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high-pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550-600 degreesC and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P-T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O-CaO-FCO-MgO-Al2O3-SiO2-H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10-12 degreesC km(-1) for both units. The retrograde overprint can be assigned to low-pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian-type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression-cooling paths, which can be explained by exhumation in a fore-arc setting during on-going subduction and associated crustal shortening. Back-arc extension is only responsible for the final stage of exhumation of the HP units. [References: 54]