Formation of authigenic monazite-(Ce) to monazite-(Nd) from Upper Carboniferous graywackes of the Drahany Upland: Roles of the chemical composition of host rock and burial temperature

Irregular aggregates of monazite-(Ce) to monazite-(Nd), up to 150 μm in diameter, have been found in graywackes from the Protivanov Formation, Drahany Upland, Czech Republic. Monazite aggregates enclose a variety of detrital and authigenic minerals. Textural evidences suggest that they are of authigenic origin and grew during very low-grade metamorphism at a maximum temperature ~ 200–300 °C in relation to a deep burial. Rarely, they enclose relics of detrital monazite with a chemical composition typical for high-grade monazite. Intensive fluid-related REE-remobilization occurred in relation to this very low-grade metamorphism. Authigenic monazite appears to form only in Ca-poor rocks. In Ca-rich graywackes, which underwent the same low-temperature metamorphic overprint, authigenic synchysite-(Ce) or parisite-(Ce) are common instead. In the overlaying Rozstání and Myslejovice Formations (T ~ 150–200 °C), remobilization of REEs was low and authigenic monazite did not form even in the Ca-poor graywackes. Authigenic monazite shows low concentrations of Th, U, Ca, Y + HREE, and a low Eu anomaly. The central parts of the aggregates are enriched in Nd + MREE, whereas the outer parts are enriched in La and Ce. Zoning pattern suggests an evolution in fluid chemistry during diagenesis/very low-grade metamorphism or it relates to different solubility of the individual REE-phosphates. The major source of REEs in the pore fluids are probably detrital minerals mainly monazite. Other minor sources of REEs probably include Fe-oxide/hydroxide and/or clay minerals, which could have contributed to the Nd + MREE-rich cores in the authigenic monazite.