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Variscan high-T and Cretaceous low-T monazite in granitoid pebbles of the Permotriassic sequence, Papuk, Croatia

The Variscan and pre-Variscan crystalline basement of the Slavonian Mts. is covered in places by the Permotriassic sequence. The thickness of this Permotriassic cover, in which some well defined groups can be distinguished. At the base of the sequence, coarse-grained clastic rocks occur, represented by conglomerates and in some parts by gravelly sandstones. Several types of clasts can be observed (schists, quartz and granitoid pebbles), and are presumably derived from the surrounding basement complexes of the Papuk Mountain. Three macroscopically different granitoid pebbles, sampled on the locality near Velika, contained accessory monazite grains, which were investigated by means of the electron microprobe in order to determine their U-Th-Pb age. In all samples, Carboniferous monazite ages were measured, implying a Variscan formation of the granitoids. Interestingly, a newly formed Alpine (Cretaceous) monazite generation was found in one pebble, demonstrating that the Permotriassic sequence must have experienced a significant low grade metamorphic event at that time. Beside their obvious discrepancy in age, the two monazite generations show significant chemical differences with respect to their REE patterns and yttrium values. Yttrium contents of the Alpine monazite are uniformingly low, consistent with a low-grade metamorphic formation. Yttrium-in-monazite thermometry indicates formation temperatures of around 350-400°C. Some of the Variscan monazite grains have high yttrium contents of up to 3 wt. % Y2O3, consistent with a high-T igneous origin. Furthermore, Alpine monazite is characterized by low La/Nd ratios and relatively high Eu contents, suggesting that the formation conditions during Alpine metamorphism were oxidizing.

Slavonian Mts.; Permotriassic sequence; granitoid pebbles; monazite; U-Th-Pb

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