engleski

P-T-t constraints on the evolution of albite granite from the Medvednica Mts. (Croatia)

The Medvednica Mts. near the Croatian capital city Zagreb belong to the Zagorje-Mid-Transdanubian Zone (ZMTDZ). The geological complexity of the ZMTDZ emerges from its location at the junction of the Alps, the Dinarides, and the Tisia Mega-Unit. An albite granite is the only known granitoid in the ZMTDZ. This rock comprises predominantly albite (~50‒55 vol.%) and quartz (~20‒25 vol.%) with subordinate amounts of phengite (5‒12 vol.%), magmatic epidote (~1 vol.%), secondary chlorite (~10‒15 vol.%) and calcite (~5 vol.%), as well as accessory zircon, apatite and opaque minerals. The albite granite has a calc-alkaline geochemical signature characterized by a metaluminous composition together with high Na and moderate to low CaO (after subtraction of secondary calcite), MgO and FeO contents. Trace element contents and relatively flat distribution of whole-rock heavy rare-earth elements suggest a magma source in the lower continental crust. The modelled empirical relationship for average crustal thickness based on the Sr/Y ratio and the REE content revealed a continental crust up to ~34 km thick. The external morphology of zircon is defined by {;100}; prisms and {;101}; bipyramids, which, accompanied by zircon geochemistry, zircon saturation (775 °C) and Ti-in-zircon temperatures (785 °C), also suggest a deep magma source in the lower continental crust. The 206Pb/238U versus 207Pb/235U concordia age of 242.9±4.0 Ma (2σ) determined on zircon coincides with the Middle Triassic peak of magmatic activity in the Dinarides. This age represents the beginning of the continental lithosphere fragmentation and the onset of rifting processes followed by the broadening of the Neotethys Ocean.

albite granite ; epidote ; zircon ; Triassic ; Neotethys

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