Fluid-mineral interactions and constraints on monazite alteration during metamorphism

Clasts of metamorphosed Cadomian granites from the ∼50-60 Ma Carpathian flysch in Gródek near the Rożnowskie Lake (Silesian Unit, SE Poland) are studied. They are considered to represent the Silesian Ridge, one of the hypothetical, currently unexposed source areas that supplied Carpathian sedimentary basins with clastic material. The gneisses preserve several examples of corona textures that include cores of primary monazite surrounded by polygonal grains of secondary apatite with thorianite inclusions, with intermediate zones of lamellar grains of secondary monazite and outermost rims of clay minerals, or various combinations thereof. Preservation of the complete textures is rare with polygonal apatite with thorianite inclusions, lamellar grains of monazite and clay minerals being particularly prevalent. Locally, polygonal apatite with thorianite inclusions surrounded by allanite and REE-epidote corona with a bastnäsite-synchysite phase occurs also. The textures observed developed during primary monazite breakdown and replacement by secondary minerals. The variation in reaction products indicates that alteration was strictly dependent on the local chemical system.