Zircon geochronology of basement rocks from the Pelagonian Zone, Greece: constraints on the pre-Alpine evolution of the westernmost Internal Hellenides

Abstract

The Pelagonian Zone of Greece is the westernmost segment of the Internal Hellenides comprising widespread crystalline basement exposures of granites and orthogneisses. We dated these basement rocks in order to identify the major crust-forming episodes and to understand the evolutionary history of the area. In our study we investigated granites, gneisses, meta-rhyolites and mylonites from the major occurrences of the Pelagonian Zone. We applied single-zircon dating techniques such as Pb–Pb evaporation, conventional U–Pb and SHRIMP. The majority of the basement rocks gave Permo-Carboniferous intrusion ages, thus emphasizing the importance of this crust-forming event for the Internal Hellenides of Greece. Triassic intrusion ages were obtained, however, for a meta-rhyolite from the western Pelagonian Zone and two mylonites from the eastern Pelagonian Zone. These ages are interpreted to reflect magmatism accompanying early rifting that led to the subsequent opening of the Pindos Ocean to the west and the Meliata Ocean to the east of the Pelagonian Zone. The geochronological results demonstrate that the magmatic episodes during which most of the Pelagonian Zone crystalline basement formed are predominantly pre-Alpine in age.

Appendix: Analytical methods

For geochronology sample weights were between 8 and 12 kg. The rocks were crushed and sieved to a grain size smaller than 500 μm using standard procedures, followed by mineral separation using a Wilfley table, a Frantz magnetic separator and heavy liquids. In a final step, the zircons were handpicked under a binocular to avoid grains with visible inclusions.

Three different methods were used for zircon geochronology, namely the Pb–Pb evaporation method (Kober 1986, 1987), the conventional U–Pb method and SHRIMP (e.g. Compston et al. 1984; Compston 1999).

For zircon grains that were analysed using the Pb–Pb evaporation method, ages were calculated from the ²⁰⁷Pb/²⁰⁶Pb values after correction for common Pb using the values of Stacey and Kramers (1975). Sample ages were calculated as weighted averages, using Isoplot (Ludwig 2003).

The application of the single-zircon conventional U–Pb method is based on the low contamination method of Krogh (1973). Before dissolution, the zircons were washed in 7 N HNO₃ and a mixed ²⁰⁵Pb–²³⁵U spike was added. The zircon grains were dissolved with HF in Teflon bombs at 200°C. Chemical separation of U and Pb with HBr chemistry followed, using 20 μl columns filled with anion-exchange resin. Some zircon grains of sample V7 were analysed following the vapour digestion method of Wendt and Todt (1991), in which Pb and U are not chemically separated after dissolution but measured from the same Re filament. Pb and U were loaded on single Re filaments with silica gel. Thermal ionisation mass spectrometer (TIMS) measurements were performed on a Finnigan MAT 261 equipped with a secondary electron multiplier at the Max-Planck-Institut für Chemie, Mainz, Germany. After correction for fractionation, blank and common Pb (using the values of Stacey and Kramers 1975), ages were calculated using Isoplot (Ludwig 2003). Procedure blanks were  < 40 pg Pb, the fractionation factor (3‰ per ΔAMU) was determined by repeated measurements of NBS 981 under the same conditions as the samples.

Three samples (Pl54, Pl16 and Ev4) were dated by sensitive high-resolution ion microprobe (SHRIMP) at the ANU, Canberra, Australia. For calibration of the Pb–U ratios the zircon standard FC1 (age 1,099 Ma; Paces and Miller 1993) was used. U concentrations were calibrated using the SL13 zircon standard. Two samples (Pl37 and Pl40) were dated by SHRIMP at the Centre of Isotopic Research, St. Petersburg, Russia. The TEMORA reference zircon (age 416.75 Ma; Black et al. 2003) was used for calibration of the Pb–U ratios and zircon standard 91500 for U concentration calibration (Wiedenbeck et al. 1995). Data reduction and age calculations were based on SQUID (Ludwig 2001). Concordia diagrams were drawn with Isoplot (Ludwig 2003). All ages are given either at 2σ or 95% confidence level.