Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield |
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| Authors: | Hugo Tickyj Lo A Hartmann Marcos A Z Vasconcellos Ruy P Philipp Marcus V D Remus |
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| Institution: | a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil;b Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91501-970 Porto Alegre, Rio Grande do Sul, Brazil |
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| Abstract: | To obtain the chemical Th*–Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th–U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899±43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the São Gabriel block, an age of 643±129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510±68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Várzea do Capivarita complex (552±90 Ma), the sillimanite-garnet gneisses of Camboriú complex (565±77 Ma), the Três Figueiras granite (558±57 Ma), and the Plaza Itapema granite (545±55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units. |
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| Keywords: | Brasiliano cycle Electron microprobe Monazite Southern Brazilian Shield Th– U-total Pb dating Trans-Amazonian cycle |
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