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Provenance and tectonometamorphic evolution of the Uppermost Unit (External Hellenides, Cete).

Subject Area Palaeontology
Term from 2019 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 423521329
 
Final Report Year 2023

Final Report Abstract

The results of the present project shed light on the protolith age, provenance and tectonometamorphic evolution of the Uppermost Unit of Crete. U-Pb dating of zircons separated from felsic metavolcanic rocks of the Preveli nappe (Lefkogia) yielded Triassic emplacement ages at 237.3 ±1.8, 241.5 ±1.2 and 242.1 ±1.2 Ma. U-Pb grandite dating of adjacent skarn yielded the same age at 239.3 ±2.3 Ma. The skarn is dominated by hydrous phases (ferriactinolite, hydro-grandite, epidote), which developed at T = 400 – 450 °C in a marine extensional setting due to fluid-assisted metasomatic reactions between chert-bearing limestone and igneous veins. Although the development of skarn is regarded as a short-lived process, the skarn formation lasted for more than 20 m.y. as is indicated by additional U-Pb grandite ages obtained from Preveli skarn exposed near Kerames (232.7 ±1.5 Ma) and Gerakari (218.0 ±3.5 Ma). The structural data and deformation microfabrics of quartz, coupled with petrological constraints, suggest that subduction of the Preveli rocks occurred under epidote-blueschistfacies conditions (T = 360 ±40 °C, P >1.0 GPa) and differential stresses >70 MPa. Localized deformation of fine-grained monomineralic grandite of the skarn led to discrete shear zones along which ferri-winchite developed. A 125 ±10 Ma 39Ar-40Ar age of ferri-winchite, together with U-Pb ages of rutile (132 ±12 and 135 ±10 Ma) are interpreted to reflect the time of Early Cretaceous (Eohellenic) HP-LT metamorphism. Brittle-viscous deformation of grandite in high-strain domains was accommodated by cataclasis, incongruent dissolution precipitation creep (IDPC), and incipient dislocation creep (DC). Deformation of grandite occurred at high differential stress (> 250 MPa) and was significantly controlled by externally derived fluids, which contributed to hydraulic fracturing and the redistribution and addition of mobile elements. The detrital zircons of the Uppermost Unit show Variscan ages and are characterized by a Minoan-type age spectrum. As those of the high-grade Asteroussia crystalline complex (ACC) continue until the Cretaceous, the ACC is a likely equivalent of the low-grade metamorphic trench and fore-arc deposits of the Vatos nappe pointing to Late Cretaceous slab roll back. The presence of (1) Triassic rift-related volcanics, (2) late Permian detrital zircons and (3) Late Cretaceous arc-type granitoids suggest that the Uppermost Unit of Crete is derived from the late Permian/Late Cretaceous magmatic belt situated north of the Sava-Vardar-Izmir-Ankara Suture in the Strandja-Rhodope area. To achieve their recent position on Crete, the nappes had to travel more than 500 km. The traveling path is well tracked by rocks of the Upper Cycladic Unit. The large displacement of the Cretan nappes was controlled by dextral strike-slip along the North Anatolian Fault Zone and related counterclockwise rotation of the Anatolian block since the Eocene. Top-to-the west nappe transport of the Uppermost on top of the Pindos Unit occurred in late Eocene/early Oligocene times (31 ±9 Ma, U-Pb on calcite of pressure shadow), whereas the emplacement of the Pindos on top of the Tripolitza Unit occurred later in the early Miocene (19 ±2.5 Ma, Rb-Sr on synkinematic white mica of a basal Tripolitza mylonite).

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