P-T-t evolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region

Eclogite and blueschist facies rocks occurring as a tectonic unit between the underlying Menderes Massif (MM) and the overlying Afyon Zone/Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit (CBU) in Turkey. This high-P unit is attributed to the closure of the Pindos Ocean and consists of (a) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (b) the tectonically overlying Upper Cretaceous Selcuk melange with eclogite blocks embedded in a pelitic epidote-blueschist matrix. The coherent series has experienced epidote-blueschist facies metamorphism (490 +/- 25 degrees C/11.5 +/- 1.5 kbar; 38 km depth). Ar-40/Ar-39 white mica and Pb-206/U-238 monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 +/- 0.3 and 40.1 +/- 3.1 Ma for epidote-blueschist facies metamorphism, respectively. The epidote-blueschist facies metamorphism of the matrix of the Selcuk melange culminates at 520 +/- 15 degrees C/13 +/- 1.5 kbar, 43 km depth, and is dated at 57.5 +/- 0.3-54.5 +/- 0.1 Ma (Ar-40/Ar-39 phengite). Eclogite facies metamorphism of the blocks (570 +/- 30 degrees C/18 +/- 2 kbar, 60 km depth) is early Eocene and dated at 56.2 +/- 1.5 Ma by Pb-206/U-238 zircon. Eclogites experienced a nearly isothermal retrogression (490 +/- 40 degrees C/~6 to 7 kbar) during their incorporation into the Selcuk melange. The retrograde overprints of the coherent series (410 +/- 15 degrees C/7 +/- 1.5 kbar from Dilek Peninsula and 485 +/- 33 degrees C/~6 to 7 kbar from Selcuk-Tire area) and the Selcuk melange (510 +/- 15 degrees C/6 +/- 1 kbar) are dated at 35.8 +/- 0.5-34.3 +/- 0.1 Ma by Ar-40/Ar-39 white mica and 31.6 +/- 6.6 Ma by Pb-206/U-238 allanite dating methods, respectively. Regional geological constrains reveal that the contact between the MM and the CBU originally formed a lithosphere-scale transform fault zone. Ar-40/Ar-39 white mica age from the contact indicates that the CBU and the MM were tectonically juxtaposed under greenschist facies conditions during late Eocene, 35.1 +/- 0.3 Ma