MICROPROBE MONAZITE DATING AND THE AGES OF SOME GRANITIC AND METAMORPHIC ROCKS FROM SOUTHEASTERN BRAZIL

Abstract

Electron microprobe monazite crystallization ages are presented for selected granite and metamorphic rock samples from the Socorro and Guaxupé Nappes and the Alto Rio Grande and Ribeira Belts, southeastern Brazil. Results are consistent with ages close to 625 Ma of the main metamorphic episode in the nappe structures. Anatetic granite magmatic events in these terranes and in the Alto Rio Grande Belt were roughly coeval, most samples sharing ages between 610-625 (± 15) Ma. A crust-derived granite and a granite contact aureole from the Ribeira Belt have ages of 600-608 (± 15) Ma., suggesting that the main granite magmatism in this belt was somewhat younger, probably contemporaneous with the late orogenic phase in the nappe domains. Some regional and granite samples from all these terranes point to possible inheritance or incompletely homogenized monazite grains and intra-grain domains giving older ages, up to 690 Ma. The microprobe age results agree very well with independent isotopic data, reinforcing the applicability of the method to highlight an overall picture of main geochronological trends within the continental crust. The high spatial resolution of the probe should play an important role in understanding geochronological patterns of metasedimentary rocks and related migmatites and granites, as these rocks and their minerals often present chemical and isotopic domains related to contrasting geological events, not always recognized by conventional dating schemes.