Exhumation of Attica high-pressure rocks in a subduction channel: new metamorphic PT constraints from Attica, NW Cyclades, Greece

The Hymettus area in East Attica contains predominantly greenschist facies schists and marbles; blueschist-facies key minerals are absent in the matrix. However, blueschist facies P-T conditions are recorded in the phyllosilicate mineral chemistry. We provide new integrated thermodynamic data from a lower tectonic unit indicating that the metamorphic path for both prograde and retrograde metamorphism was similar to that in other parts of the Cycladic Blueschist Unit (CBU). New P-T data from the Hymettus area show that a peak pressure of ∼1.4 ± 0.05 GPa was reached at ∼300 ± 25 ºC, followed by exhumation, first with a significant temperature increase to ∼470 ± 20 °C at a pressure of ∼0.8 ± 0.1 GPa, followed by major cooling. The mechanism that exhumed the Hymettus rocks is interpreted to be wedge extrusion in a subduction channel. Accordingly, the 15 km difference in pressure (>0.5 GPa) between the lower and upper tectonic units was eliminated by their juxtaposition during their exhumation in the extruded wedge, which had a shallow-dipping, extensional fault at the top and a parallel thrust at the base. Retrogression of high-to low-pressure assemblages was a result of water infiltration from both sides of the wedge. Both these processes led to a typically symmetrical wedge structure with the relict HP rocks of the LTU in the centre, and the UTU lower pressure rocks in the upper margin.