Two-stage breakdown of monazite by post-magmatic and metamorphic fluids: An example from the Veporic orthogneiss, Western Carpathians, Slovakia

The initial to advanced stage of monazite breakdown was identified in a granitic orthogneiss from the pre-Alpine basement in the Veľký Zelený Potok Valley (the Veporic Unit, Western Carpathians, central Slovakia). Monazite-(Ce) formed during Variscan metamorphism of the original Cambrian to Ordovician granitic rock. Two younger, Permian post-magmatic hydrothermal, and Cretaceous metamorphic-hydrothermal events caused a breakdown of the monazite to secondary egg-shaped coronal structures (100 to 500 μm in diameter) with concentric newly-formed mineral phases. Two principal breakdown stages and newly formed mineral assemblages are recognizable: (1) partial to complete replacement of primary monazite with an internal apatite + ThSiO4 (huttonite or thorite) zone and an external allanite-(Ce) to clinozoisite zone; (2) hydroxylbastnäsite-(Ce) partly replacing apatite + ThSiO4 and allanite to clinozoisite aggregates. The monazite breakdown was initiated by fluid sources differing in composition. Stage (1) originated due to post-magmatic hydrothermal fluids, whereas stage (2) indicates an input of younger, CO2-bearing metamorphic-hydrothermal fluids.