Petrogênese e idade química U-Th-Pb dos pegmatitos de Itambé (BA) registradas pela monazita

Abstract

Backscattered electrons images and chemical microanalysis by electron microprobe were obtained from monazite crystals belonging to the Bananeira, Itambé, Coqueiro, Cavada and Paraíso pegmatites in district of Itambé-BA,which are situated in the Eastern Brazilian Pegmatitic Province (EBPP). The samples are compositionally homogeneous, with no internal domains/zoning and their U, Th and Pb contents allowed obtaining an average chemical age of 502 Ma for the Itambé Pegmatite District. Their chondrite-normalized rare earth element patterns show Sm positive anomaly, an element with very high partition coefficient in amphibole. This anomaly suggests that in the post-collisional context of Araçuaí Orogen the partial melting of biotite-hornblende gneiss wall rock would have occurred during the regional decompression process linked to the collapse of the orogen. This contributed to formation of a granitic melt, which was enough hydrated 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.