Coexisting monazite and allanite in peraluminous granitoids of the Tribec Mountains, Western Carpathians

Monazite, a typical Light rare-earth element (LREE) mineral of S-type grani toids in the Western Carpathians, was found in the peraluminous biotite gra nodiorite-tonalite in the Tribec Mountains commonly containing polyminerali c inclusions. These inclusions are dominated by anhedral allanite, although allanite also occurs rarely as discrete grains not enclosed by monazite. T he monazite studied here is relatively homogeneous and characterized by hig h Th contents with proportions of huttonite (ThSiO4) and brabantite [CaTh(P O4)(2)] up to 14.6 and 9.3% respectively. The discrete allanite grains are highly aluminous with a composition consistent with the peraluminous type o f host rock. However, allanite included in monazite is extremely variable i n LREE, Al, Fe, and Mg contents. This variation is interpreted to result fr om entrapment of allanite (+ melt) in monazite before local equi librium wa s attained. The change from allanite to monazite as the stable LREE-rich ph ase is related to an overall decrease in Ca concentration caused by the ons et of plagioclase crystallization. The early precipitation of allanite was possible because of the high LREE concentrations in the melt. The crystalli zation temperature of allanite must have been higher than monazite saturati on (>856-845 degrees C and 798-790 degrees C for two analyzed samples). The Zr saturation temperature based on zircon solubility and REE thermometry b ased on monazite solubility reflect an increase in temperature from the edg e to the center of the pluton, which coincides with an increase in the hutt onite content in monazite. The primary LREE assemblage is accompanied by sm all grains of late huttonite(?) replacing monazite and brabantite replacing allanite.