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

Abstract

Thorianite (ThO 2 ) 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 continentcontinent collision.

Multiple origins of UHP eclogites in a garnet peridotite block (Nové Dvory, Czech Republic) and short duration of heating

“…The age of high-pressure granulite metamorphism is dated at ~340 Ma from U-Pb zircon dating (e.g., Aftalion et al, 1989;Kröner et al, 2000;Sláma et al, 2008) in the southern Bohemian Massif, but older zircon ages of ~360 Ma have also been reported and interpreted as protolith formation ages (e.g., Janoušek et al, 2006). U-Th-Pb ages obtained from garnet peridotite in the Gföhl Unit have yielded a weighted mean age of 333.8 ± 4.5 Ma (Naemura et al, 2008). Eclogites from Nové Dvory (the study area) have yielded Sm-Nd ages for garnet-omphacite pairs of ~340 Ma, which is very similar to the metamorphic age of the granulites, although eclogites from other areas, such as Biskupice and Bečváry, have yielded different Sm-Nd ages of around 320 and 380 Ma, respectively (Beard et al, 1992;Medaris et al, 1995a).…”