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

Johann Tuduri, Olivier Pourret , Eric Gloaguen, Philippe Lach, Emilie Janots, Sébastien Colin, Jérôme Gouin, Matthieu Chevillard, Laurent Bailly

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.