U-TH-TOTAL PB DATING OF PRIMARY AND SECONDARY MONAZITE FORMATION IN GNEISS AND GRANULITE CLASTS FROM THE SILESIAN UNIT (WESTERN OUTER CARPATHIANS, POLAND)

Gneiss and granulite clasts from the Carpathian flysch rocks in the Silesian Unit (S Poland) preserved various stages of primary monazite breakdown and formation of secondary one. These rocks are considered to be derived from the Silesian Ridge – one of the hypothetical source areas that supplied Carpathian basins with clastic material. SEM-EDS and EMPA were applied for structure observations, monazite U-Th-total Pb dating, and garnet-biotite (GB), Ti-in-biotite (TB), and Zr-in-rutile (ZR) thermometry.

Monazite in one gneiss sample yielded ages of 628±6 Ma (n=40) and in the second one 413±20 Ma (n=16), that are related to igneous protolith. Monazite ages of 367±11 Ma (n=34) and 322±11 Ma (n=30) in three gneisses reflect metamorphic events in temperatures of up to ca. 550°C (TB). Monazite breakdown resulted in its replacement by coronas of apatite ± thorianite ± uraninite ± allanite ± epidote. Secondary monazite (196±26 Ma; n=10) formed in outermost parts of corona was also observed. Secondary monazite formation processes probably took place in greenschist-facies conditions.

Two kyanite-bearing granulites were metamorphosed in temperatures of ca. 890°C (ZR) followed by amphibolite-facies conditions metamorphic event (670 down to 550°C; TB and GB, respectively). Apatite is commonly overgrown by necklace-like monazite grains. However, these are not suitable for dating because of trace Th and U contents. Monazite formation at the grain boundaries probably took place in hydrothermal conditions. Third kyanite-bearing granulite preserved two possible detrital monazite grains (inclusion in garnet and in the rock matrix) that yielded age 332±6 Ma (n=13). Uniform age is probably caused by resetting of monazite in upper granulite-facies conditions.

Protolith of the gneiss clasts from the Silesian Unit was formed at Precambrian and Ordovician to Silurian times. Clastic material was metamorphosed in amphibolite- or granulite- to amphibolite-facies conditions during Variscan orogeny. Formation of secondary monazite was probably caused by thermal event related to rifting during Upper-Triassic – Early-Jurassic.