Formation of authigenic grey monazite: a marker of palaeo-thermal anomaly in very-low grade metamorphic rocks?

The reassessment of the rare earth element (REE) potential of France led us to investigate REE behaviour in black-shales belonging to the Middle Ordovician Angers-Traveusot formation from central Britany (France), with an emphasis on the formation of nodular grey monazite during the diagenetic and low-grade metamorphic evolution. Temperatures conditions and mass transfer underwent in the black shales were first characterized using rock geochemistry, rock-eval pyrolysis and by gradual changes of clay mineral crystallinity. Then, monazite texture, composition and U–Pb in-situ dating were determined and correlated with the diagenetic/anchimetamorphic conditions. In the Ordovician black shales, nodular monazite appears at the transition between the upper diagenesis and the anchizone metamorphic facies, at conditions between 140 and 250°C, in response to processes controlled by different proxies of competing influences such as the organic matter maturation, Fe oxide/hydroxide and clay transformation with fluid releasing. Monazite occurs mainly as elongated nodules, up to 2 mm in diameter that are mostly characterised by their grey colour due to abundance in host-rock mineral inclusions. Monazite nodules compositions are systematically low in Th and U contents but are zoned with Nd and middle REE rich cores surrounded by light REE-rich rims, with no evidence of inherited domains. More rarely, small grains of LREE-rich monazite are observed in late stage fractures. Monazite nodules were dated at ca. 405-400 Ma, which is proposed to record high heat flux responsible of the anchimetamorphic conditions recorded at the base of Angers-Traveusot formation. Late monazite-Ce were dated at ca. 385 and 350 Ma at the onset of the Variscan deformation.