Acta Musei Moraviae

Authigenic monazite-(Ce) to monazite-(Nd) in the greywackes of the Drahany Upland: formation related to the chemical composition of the greywackes and diagenetic temperature

Poikilitic monazite-(Ce) to monazite-(Nd), irregular in shape, enclosing variety of detrital and authigenic minerals, together with rounded, partly altered detrital monazite, were found in greywackes of the Protivanov Formation, Drahany Upland. Textural evidences suggest authigenic origin of poikilitic monazite. The most likely explanation is that the main source of REE (mainly LREE) in fluids was detrital monazite. Detrital monazite in greywackes of the Protivanov Formation is unstable and underwent a variety of replacement reactions during deep burial with maximal temperature close to 250 °C corresponding to very low-grade metamorphism. The products of the detrital monazite alteration reflect the whole-rock composition of greywackes. Newly-formed poikilitic monazite was observed only in greywackes with low Ca content in the whole-rock compositions. The CHIME dating revealed the precipitation of poikilitic monazite at 324 Ma (± 28 Ma), thus during high-temperature diagenesis. Chemical composition of lowgrade poikilitic monazite significantly differs from that of higher temperature magmatic or metamorphic detrital monazite. Poikilitic monazite shows low concentrations of Th, U, Ca, Y +HREE and weak Eu anomaly. Distribution of the REE in the poikilitic monazite grains shows typical trend. The central part is enriched in Nd, Sm and Gd, whereas the outer part is enriched in La and Ce. Some analyses from the central part yield Nd as dominant REE, therefore it corresponds to monazite-(Nd). Rarely, small core devoid of mineral inclusions with compositional characteristic typical of a high-temperature origin (high Th, U and Y content), interpreted as relict of detrital monazite, was found in central part of poikilitic monazite.