Age determination of thorianite in phlogopite-bearing spinel-garnet peridotite in the Gföhl Unit, Moldanubian Zone of the Bohemian Massif

Abstract

Thorianite (ThO₂) was found in the phlogopite-bearing spinel-garnet peridotite (Plešovice peridotite) occurring as decimeter-size lenticular body in the Gföhl granulite that forms the metamorphic core of the Variscan orogenic belt. Thorianite occurs as a member of multiphase solid inclusions, consisting of phlogopite + carbonates + apatite + graphite + rutile + monazite + thorianite, in chromian spinel. U-Th-Pb ages of the thorianite give a good concentration at 333.8 ± 4.5 Ma, which overlaps the U-Pb ages of zircon extracted from the host Gföhl granulite, probably reflecting the Variscan high-pressure high-temperature metamorphism during the continent-continent collision.