Excess radiogenic argon in the Precambrian Avanavero Dolerite in western Suriname (South America)

Abstract

The Avanavero Dolerite intruding the Guiana Shield in western Suriname is a quartz gabbroic mass yielding a Rb-Sr isochron age of 1603 ± 27 Ma, while the mass has invaded basement with a Rb-Sr isochron age of 1810 ± 40 Ma ( λ = 1.47 × 10 −11/a ; errors with 95% confidence level). From 61 K-Ar measurements on whole-rocks, sieve fractions of whole-rocks, density fractions and separated minerals it is evident that nearly all investigated samples, fractions and minerals (except microcline which suffered some argon loss) contain excess 40Ar. This excess 40Ar is very inhomogeneously distributed through the gabbroic mass. The bulk is stored in the plagioclase, in which mineral the excess 40Ar attains unusually high values ranging from about 14 × 10 −6 to around 150 × 10 −6cm 3 NTP/g. The excess 40Ar contents of biotite, hornblende and the pyroxenes are comparable to published data. Two samples from a core drilling were studied in some detail. In graphs of radiogenic 40Ar versus K, four mineral separates from one sample display a fairly good, but geochronologically meaningless linear arrangement roughly passing through the origin. Most density fractions from the other sample show a tendency towards an array parallel to the 1603 Ma reference isochron. It is assumed that the excess 40Ar was acquired by the Avanavero Dolerite during a moderate tectonothermal event 1200 ± 100 Ma ago, when the 40Ar partial pressure was much higher in the environmental basement complexes than in the basic mass.