Acta Musei Moraviae

Detrital and authigennic REE minerals from Culm sediments of Drahany Uplands, their significance for provenance study of clastic material and diagenetic processes

Electron microprobe study of mineral chemistry, chemical age and alteration of detrital monazite as well as composition and textural position of authigenic REE minerals – rhabdophane-(Ce), synchysite-(Ce) and thorogummite – have been carried out on the samples of clastic Culm sediments from the Drahany Uplands. Detrital monazite is commonly homogeneous in BSE, only it exceptionally shows an irregular zoning. Some monazite grains are replaced by apatite and hydrated REE-minerals propagating from rim inwards and along fractures. Three different stages of detrital monazite alteration were observed: the first stage is characterized by minor depletion of heavy REE (Gd, Dy, Er), Y, Th and Pb, but it does not show any significant deviations in the ideal stochiometry. The second stage is accompanied by strong depletion of Gd, Dy, Er, Y, Th, U and Pb, the stochiometry is disrupted and totals decrease up to 98–96 wt. % of oxides. The last stage is characterized by depletion of light REE (La, Ce) and all above mentioned elements. Grade of alteration positively correlates with intensity of diagenesis from the Myslejovice to the Protivanov Formation. Unaltered detrital monazite from all stratigraphic units and monazite from pebbles in the Račice conglomerates were used for U–Th–Pb electron microprobe dating. Majority of detrital monazite from Protivanov, Rozstání and lower part of Myslejovice Formation displays Viséan age (339 ± 15, 331 ± 19 and 335 ± 14 Ma, respectively). Monazite from clasts of mica-schist, orthogneiss and aplite yield similar ages: 330 ± 21 Ma, 332 ± 22 Ma and 331 ± 17 Ma, respectively. Three small grains of monazite gave late Devonian age 373 ± 49 Ma. One detrital monazite from lower part of Myslejovice Formation yielded Precambrian age 603 ± 65 Ma, which is similar to the Moravo-Silesian Zone (Brno Batholith or orthogneisses protholite). Monazite dating does not support beginning of sedimentation at the border Tournai/Viséan and diachronous deposition of the flysch formations. It also suggests similar stratigraphic range of the Protivanov, Rozstání and Lower part of Myslejovice Formation. Common authigennic minerals – rhabdophane-(Ce), thorogummite and OH analogue of synchysite-(Ce) show similar chemical composition despite they occur either as replacement of monazites or crystallize in the matrix.