Formation and deformation of Triassic skarn (Uppermost Unit, Crete/Greece)

The nappes of the Uppermost Unit of Crete are counted among the most enigmatic constituents of the Eastern Mediterranean. U-Pb dating of zircons separated from felsic metavolcanics of the deepest Preveli nappe yielded Triassic emplacement ages at 237.3 ±1.8, 241.5 ±1.2 and 242.1 ±1.2 Ma. U-Pb dating of andradite-grossular of subvolcanic skarn yielded the same age at 239.3 ±2.3 Ma. The skarn is dominated by hydrous phases (ferri-actinolite, hydro-grandite, epidote), which developed at T = 400 – 450 °C in a rift-related setting due to fluid-assisted metasomatic reactions between limestone and mafic veins. Thus, U-Pb dating of skarn grandite allows dating the complementary record of hydrothermal activity related to the cooling of the causative melt. Early Cretaceous subduction caused deformation under epidote-blueschist facies conditions at T = 360 ±40 °C and P >1.0 GPa. An 39Ar-40Ar age of 125 ±10 Ma obtained from synkinematic ferri-winchite is interpreted to reflect the time of Eohellenic HP-LT metamorphism. During subduction, the differential stress was >300 MPa. Under these conditions, the garnet strain was accommodated not only by fracturing, but also by strain-induced grain boundary migration and subgrain formation. Thus, crystal plastic deformation of garnet is possible at temperatures far below the threshold temperature (ca. 500 °C) commonly reported for dislocation creep.

The new ages obtained from the Preveli nappe suggest the Uppermost Unit of Crete to be derived either from the peri-Rhodope domain of the Internal Hellenides or from the Pontides of northern Turkey.

Funding by DFG (Zu 73-34) is acknowledged.