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Abstract

Two monazite grains altered during postmagmatic, hydrothermal processes in the Variscan Jawornik granitoid (NE Orlica-Śnieżnik Dome, Sudetes, SW Poland) were investigated. Different assemblages of secondary phases replacing and forming close to altered monazite indicate two different courses of the mineral-fluid reactions. The first monazite is surrounded by two zones comprising rhabdophane group minerals (rhabdophane henceforth) and hematite (inner zone) and allanite (outer zone) that form a concentric corona texture. The growth of secondary phases is interpreted as being related to decreasing temperatures below ca. 500 °C. The inner zone formed due to alteration of the monazite that was driven by fluid-induced coupled dissolution-reprecipitation reactions. This alteration resulted in the progression of partial replacement by submicron layers of rhabdophane along the monazite surface and pseudomorphic replacement of a substantial part of the monazite by rhabdophane and hematite. The presence of the hematite suggests oxygen fugacity above the hematite-magnetite buffer. Alteration of the second investigated monazite is related to dissolution induced by a K-bearing alkali fluid, partial replacement by thorianite (ThO2), and overgrowth by K-feldspar with minor titanite formed during the chloritization of biotite and the alteration of feldspars. These results demonstrate that the local character of mineral-fluid reactions can take an entirely different course at the microscale, within a distance of several hundreds of microns. The assemblages of secondary phases replacing two investigated monazite grains indicate that the alteration processes are related to moderate temperature (<500 °C) hydrothermal processes assigned to upper greenschist facies conditions. The absence of secondary apatite (close to the first monazite) and secondary REE- and P-bearing phases (close to the second monazite) indicates the high mobility of the elements removed by the fluid, in contrast to the low mobility of the Th taken up by secondary rhabdophane or thorianite crystallizing at the monazite surface. The presence of rhabdophane group minerals and thorianite in hydrothermal secondary phases assemblages replacing monazite can be used as indicators of the local character of the environment at the microscale.