Formation of authigenic xenotime-(Y) during very low-grade metamorphism in the graywackes and shales of the Moravo-Silesian Culm

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Acta Mus. Moraviae, Sci. Geol. 97 (2012), issue 1, pages 55–69
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Abstract:

Formation of authigenic xenotime-(Y) during very low-grade metamorphism in the graywackes and shales of the Moravo-Silesian Culm

Authigenic xenotime-(Y) was found in graywackes and shales of the Protivanov Formation (Drahany Upland) and Moravice Formation (Nízký Jeseník Mts.), Moravo-Silesian Culm, Bohemian Massif. It occurs as minute (typically <30 μm) irregular aggregates, which commonly enclose authigenic minerals (quartz, anatase). Textural evidences suggest an authigenic origin of xenotime-(Y). In each sample where authigenic xenotime was formed, authigenic monazite was present as well, and both Y+REE phosphates can intergrowth. Xenotime displays a similar habit as authigenic monazite, but is far less common. Authigenic monazite and xenotime growth was almost synchronous and they formed in relation to deep burial and very low-grade metamorphism of basin sediments at a maximum temperature ~ 200–300 °C in the Drahany Basin and ~ 200–350 °C in the Nízký Jeseník Basin. At this temperature range an intensit Y+REE remobilization in sediments was observed in relation to alteration of many Y+REE bearing detrital minerals. Very low-grade xenotime and monazite was observed only in graywackes and shales with low Ca content (< 0.5 wt. % CaO) in the whole-rock compositions. In the Ca-rich shales and graywackes, which underwent the same low-temperature metamorphic overprint, authigenic synchysite-(Ce) or parisite-(Ce) are common instead. Chemical composition of low-grade authigenic xenotime differs from that of higher temperature magmatic or metamorphic xenotime. Very low-grade xenotime is enriched in MREE and HREE (0.18–0.28 apfu) and shows flat chondrite normalized REE pattern from Gd tu Lu with high GdN/YbN ratio (avg 1.4) and a weak negative Eu anomaly. Authigenic xenotime typically has low content of Th and U with relatively high Th/U ratio (0.4–2).

Contact:

Renata Čopjaková, Czech Geological Survey, Leitnerova 22, 658 69 Brno, Czech Republic, e-mail: copjakova@sci.muni.cz
Radek Škoda, Institut of Geological Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic, e-mail: rskoda@sci.muni.cz

Citation:
Čopjaková, R., Škoda, R., 2012: Vznik autigenního xenotimu-(Y) v podmínkách velmi slabé metamorfózy drob a břidlic moravsko-slezského kulmu. – Acta Mus. Morav., Sci. Geol., 97, 1, 55–69 (with English summary)
ISSN: 1211–8796