New data on the structure of the Eastern Mediterranean basin from seismic reflection

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 excess⁴⁰Ar. This excess⁴⁰Ar is very inhomogeneously distributed through the gabbroic mass. The bulk is stored in the plagioclase, in which mineral the excess⁴⁰Ar attains unusually high values ranging from about 14 × 10^?6 to around 150 × 10^?6cm³ NTP/g. The excess⁴⁰Ar 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 radiogenic⁴⁰Ar 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 excess⁴⁰Ar was acquired by the Avanavero Dolerite during a moderate tectonothermal event 1200 ± 100 Ma ago, when the⁴⁰Ar partial pressure was much higher in the environmental basement complexes than in the basic mass.