The Pelagonian Zone forms part of the westernmost side of the Internal Hellenides of mainland Greece. The zone consists of metamorphic nappes which overlay unmetamorphosed units. The metamorphic sequence consists of a Paleozoic basement, ophiolitic rocks, limestones and felsic schists, which have been subjected to blueschist and/or greenschist facies conditions during the Alpine Orogeny.
A geochronological study based on detailed structural and petrographic work has been undertaken in order to date various metamorphic events and to constrain the timing of deformation during these events. The data obtained so far give insights in the timing of the different events, and in the uplift mechanisms and exhumation rates of the units that are currently exposed. Ultimately they will put constraints on transport and/or exhumation rates of blueschist facies rocks in an accretionary wedge setting.
Previous geochronological work using Rb-Sr and 40Ar/39Ar techniques yielded ages of 300 Ma, and in the range of 100 Ma to 20 Ma. An Early Cretaceous blueschist facies event was proposed by Schermer et al. (1990) for the Olympos region, just north of the research area of this study, based on 40Ar/39Ar data by incremental step heating on bulk samples (ca. 100 mg). Petrographic analysis has demonstrated that the mica population in many rocks is not homogeneous. Therefore we have chosen to utilize small sample techniques, using an argon laserprobe. The sensitivity of the laserprobe allows stepwise heating of single grain crystals and spot fusion experiments on minerals in thin section. The first 40Ar/39Ar ages of this study confirm the presence of a Paleozoic basement in the tectonostratigraphic pile. White micas from various localities in the Pelagonian give plateau ages between 280 to 300 Ma, which can be readily interpreted as cooling ages of white mica after crystallization from a granitic melt. Another set of white mica plateau ages cluster around 105 Ma. This age is accompanied by a 81 Ma plateau age on biotite and is interpreted as cooling age of a Cretaceous blueschist facies event. The age spectra often show some 30 to 100 Ma ages in the lower temperature steps. Electron microprobe back-scattered images show a white mica generation that crystallized around Hercynian micas. We suggest that the Alpine ages observed in the age spectra reflect degassing of these micas. A crystallization age of 45 Ma has been derived from greenschist facies white mica in the same tectonostratigraphic pile, which points to a tectonic event during Middle Eocene times. The data obtained so far show that, due to its sensitivity, the argon laserprobe is a powerful tool to distinguish between mica generations that formed at different times under blueschist and/or greenschist facies conditions. Spotwise age dating experiments of various micas in thin section will be carried out to give further insights in the timing and cause of white mica crystallization
Schermer, E. R., Lux, D. R. & Burchfiel, B. C. Tectonics 9, 1165-1195 (1990).