Pressure-temperature-time constraints on the evolution of epidote-bearing albite granite from Mt. Medvednica (Croatia): Further evidence of the Middle Triassic opening of the Neotethys Ocean

Albite granite from Mt. Medvednica in northern Croatia is the only known surface appearance of granite in the complex Zagorje-Mid-Transdanubian zone. This granite contains almost pure albite (An(01), similar to 50-55 vol. %), quartz (similar to 20-25 vol. %), epidote (similar to 1 vol. %), phengite (5-12 vol. %), and secondary chlorite (similar to 10-15 vol. %) and calcite (similar to 5 vol. %). Accessories are zircon, apatite, and ilmenite. The granite has a calc-alkaline geochemical signature with a metaluminous and high-Na character. CaO, MgO, and FeO contents are relatively low. Normalised contents of rare-earth elements (REE) show a relatively flat distribution of those that are heavy, suggesting a magma source in the lower continental crust. The modelled empirical relationship for average crustal thickness based on Sr/Y ratio and contents of REE indicates a 34 km thick continental crust. Zircon typology is characterised by the predominance of {100} prisms and {101} bipyramids. This typology, zircon chemistry, zircon saturation temperature (775 degrees C), and Ti-in-zircon temperature (mean 785 degrees C) also suggest a deep-seated magma source. Epidote and perhaps phengite crystallised at a pressure around 1.0 GPa from the melt according to thermodynamic modelling. Temperatures were 650 degrees C or more at this stage. A U-Pb concordia age of 242.9 +/- 4.0 Ma (2 sigma) was determined on zircon coinciding with the Middle Triassic peak of magmatic activity in the Dinarides, but also in the Southern Alps and Western Carpathians. The age is interpreted as marking the beginning of the fragmentation of continental lithosphere and the onset of rifting processes, which was followed by the broadening of the newly-formed Neotethys Ocean.