Isotopic ages in the high-grade metamorphic Coeroeni group, Southwestern

Abstract

Six whole-rock samples from the high-grade metamorphic Coeroeni Group in southwestern Suriname define a Rb-Sr isochron age of 2042 ± 97 Ma with initial 87Sr/86Sr = 0.7027 ± 0.0039 (λ87 Rb = 1.39 x 10-11 a-1; errors at 95% confidence level).This isochron relationship dates the event of high-grade metamorphism during the Trans-Amazonian Orogenic Cycle. The initial 87Sr/86Sr value does not record a substantial pre-metamorphic crustal history. K-Ar and Rb-Sr analyses of separated minerals (4 hornblendes, 2 muscovites and 6 biotites) yield a highly discordant age pattern, ranging from about 2000 to 1200 Ma (omitting one hornblende containing excess radiogenic argon). This pattern is interpreted as reflecting various degrees of resetting of Trans-Amazonian ages due to a low-grade metamorphic event 1200 ± 100 Ma ago (the Nickerie Metamorphic Episode). The order of mineral dates fits into the common order of radiogenic nuclide retentivities, corresponding to a temperature of about 350°C during the low-grade metamorphism.