Pressure and temperature (P-T) conditions of formation of peak metamorphic parageneses and mylonitic mineral assemblages were calculated using the average PT method of Powell and Holland (1994). For this, an updated version (5/2001 data) of the thermodynamic data set of Holland and Powell (1998) was used. This was augmented by the application of the jadeite-albite-quartz barometer to omphacite-bearing samples. In addition, phase petrological modelling in the basic model system NCFMASH (Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O) was successfully applied to some rocks by Will et al. (1998). The data are given in Table 3 and shown in Figure 8.
P-T conditions for high-pressure metamorphism (Table 3) reveal mildly blueschist-facies conditions of ~8-10 kbar and 350-400°C in the Kerketas nappe at 24-21 Ma (Ring et al. 2001). Will et al. (1998) showed that the P-T conditions inferred for high-pressure metamorphism in the overlying Agios Nikolaos nappe of the Cycladic blueschist unit are ∼18-19 kbar and ∼510-530°C. Age data for the Cycladic blueschist unit for a number of islands (e.g. Sifnos, Naxos, Ios, Syros, Tinos, Ikaria) across the entire Aegean are remarkably similar and are interpreted to date the peak of high-pressure metamorphism at 55-50 Ma (Wijbrans et al. 1990; Tomaschek et al. 2003). 40Ar/39Ar dating of phengite by Ring and Layer (2003) showed that a similar age range has to be envisaged for the peak of high-pressure metamorphism in the Cycladic blueschist unit on Samos.
The temperatures inferred for the strongly foliated chloritoid-kyanite-white mica schists from the Ampelos nappe are rather consistent with 500-540°C but pressures ranging from c. 15 to 5 kbar. A possible interpretation of these data is that the rocks experienced a near isothermal decompression from eclogite to epidote-amphibolite and greenschist facies conditions related to tectonic extrusion of the Ampelos nappe between 42-32 Ma (Ring et al. 2007).
In metagabbro from the basal Selçuk nappe, calculations were carried out on mineral assemblages containing barroisitic hornblende, epidote/zoisite, plagioclase, chlorite and ± quartz. Clearly, this is an assemblage transitional between the middle/upper greenschist- and lower blueschist-facies. Furthermore, the jadeite-barometer was used for the omphacite-bearing massive metagabbros. A garnet-amphibolite from the Selçuk nappe in western Turkey yielded well-constrained P-T conditions of 550±18°C and 12.4±1.2 kbar, which is considered to reflect maximum P-T in the Selçuk nappe. The conditions inferred correspond with estimates on undeformed metagabbros from Samos: 8-12 kbar and 400-500°C and are transitional between epidote-amphibolite and eclogite facies conditions. In contrast, strongly foliated, mylonitized Selçuk nappe metagabbros in the Selçuk normal shear zone consistently yielded P-T values of 4±1.5 kbar and 450±40°C. Similar greenschist facies P-T estimates of c. 3-4 kbar and 420-440 °C were also obtained for Selçuk nappe metagabbros that occur as lenses in the underlying Ampelos nappe.
Evidence for high-pressure metamorphism in the Selçuk nappe is only preserved in the unfoliated samples, but is not longer present in the mylonitized metagabbros from the Selçuk normal shear zone. Presumably, this is the case because fluid ingress hydrated the mylonitic metagabbros in the Selçuk shear zone and caused retrogression of the rocks under greenschist facies conditions of 3-5 kbar. According to the age data reported in Ring et al. (2007) the deformation-related greenschist facies overprint in the Selçuk nappe occurred before 32 Ma.
The Eocene eclogite facies metamorphism (10-12 kbar) of the Selçuk nappe was followed by a shearing-related greenschist facies overprint at 3-4 kbar. These data imply that the Selçuk nappe must have been exhumed by c. 20-30 km between the high-pressure metamorphism and the end of the mylonitization event. This decompression was accompanied by only slight to moderate cooling from 500°C to 420-440 °C and is ascribed to Eocene normal shearing in the Selçuk normal shear zone (Ring et al. 2007).
The P-T data reveal a pronounced metamorphic break (up to 10 kbar) towards higher pressures and temperatures between the Kerketas and Agios Nikolaos nappes. An inverse break in metamorphic pressure of c. 3-5 kbar occurs above the Agios Nikolaos nappe.