Origem e idade u-th-pb do distrito pegmatítico de Santa Maria de Itabira (MG) reveladas pela monazita

Abstract

Backscattered electron images and chemical microanalysis by electron microprobe were obtained from monazite crystals belonging to the Pegmatitic District of Santa Maria de Itabira in the mining of Morro Escuro, Ponte da Raíz and Euxenita. These mining are located in the municipalities of Santa Maria de Itabira, Ferros, Guanhães and Sabinópolis in the State of Minas Gerais, Brazil. The monazite crystals are situated in the Eastern Brazilian Pegmatitic Province (EBPP) in the post-collisional context of the Araçuaí Orogen, which correspond to the end of the Brasiliano event. The samples are compositionally homogeneous, with no internal domains/zoning and their U, Th and Pb contents allowed obtaining an average chemical age of 474 Ma for the Santa Maria de Itabira Pegmatite District. Their chondritenormalized rare earth element patterns show positive Sm anomalies, an element with very high partition coefficient in amphibole. This anomaly suggests that in the post-collisional context of the Araçuaí Orogen there was partial melting of the host rocks of the Guanhães Group (biotite-hornblende gneiss) during the regional decompression process linked with the collapse of the orogen. This contributed to formation of a granitic melt, which was hydrated enough to allow the large crystal growth of the Itambé pegmatites, including monazite. Therefore, this mineral represents not only a geochronometer, but also a petrogenetic indicator of the studied pegmatites.