A lower Devonian point in the geological timescale

The Gocup Granite, in southeastern New South Wales, intrudes low-grade metasediments tentatively assigned to the Ludlovian on fragmentary conodont evidence, and is overlain by a unit rich in volcanic material but containing a good brachiopod fauna of lower to middle Siegenian age. The K-Ar age of four muscovite samples is 409 ± 3m.y.; a Rb-Sr whole-rock isochron on five samples yields a Model 1 age 402 ± 3m.y., initial 87Sr/86Sr 0.7090 ± 0.0013. Preliminary data by X-ray fluorescence and unspiked strontium mass spectrometry illustrate the effects of some sources of experimental error.