Petrochronological study of chloritoid schist from Medvednica Mountain (Zagorje Mid-Transdanubian zone, Croatia)

The metamorphic conditions and evolution of the Palaeozoic-Mesozoic metamorphic complex of Medvednica Mountain (Zagorje-Mid-Transdanubian zone, Croatia) are still a matter of debate. The results of the investigation of five samples of metapelitic schists with the mineral association of quartz, white mica and chlorite are presented. The studied schists are part of the continental margin of Adria and were metamorphosed under upper greenschist- to amphibolite-facies conditions. The focus of this study is a sample representing the highest metamorphic grade that additionally contains chloritoid blasts. Pressure-temperature pseudosection modelling together with classical geothermobarometric calculations yielded peak metamorphic conditions of 0.94 ± 0.05 GPa and 550 ± 20 °C for chloritoid schist. Monazite in-situ U-Th-total Pb electron microprobe dating indicates two metamorphic events at 167 ± 2 Ma and 143 ± 2 Ma, which are interpreted as the time of monazite growth during two distinct metamorphic phases. The formation of the chloritoid paragenesis is related to the older event (around 167 Ma) and linked with the Middle Jurassic subduction-accretion processes of Neotethys-derived ophiolitic lithologies. The younger metamorphic event (around 143 Ma) is related to the obduction of ophiolites onto the continental margin of Adria.