HYDROTHERMAL ALTERATION OF MONAZITE-(CE) FROM THE SANTA-MARIA DE ITABIRA PEGMATITE DISTRICT (MINAS-GERAIS, BRAZIL)

Monazite-(Ce) is found in granitic pegmatites in the Santa il Maria de Itabira pegmatite district (Minas Gerais, Brazil). During the magmati c stage, monazite-(Ce) seems to have had higher contents of cheralite and huttonite in the solid solution. The Th content in primary monazit e-(Ce) is high and characteristic for each pegmatite body. During the late stage (albitization), the mean LREE content in the altered zone i s slightly higher and Tn content is very low. The accessory mineral as semblages changed; huttonite and cheralite crystallize together with T h-poor and La-rich monazite-(Ce) at the border of altered crystals, Nd /Sm and U/Pb ratios are also changed during the hydrothermal stage. (C ) Academie des sciences/Elsevier, Paris.