A Rb-Sr study of the lower Silurian stage circle Shale, Canberra, Australia

The radiometric age of the Upper Silurian (Lower Ludlovian) has been measured as equal to or older than 438 ± 4 m.y., using whole-rock and biotite samples from dacitic ash flows. Relative to this, the deposition of the Lower Silurian State Circle Shale is placed at about 445 ± 7 m.y. (Upper Llandoverian). Samples of the Shale have real differences in initial Sr⁸⁷ Sr⁸⁶ for this age and two groups are recognized: one having uniform Sr⁸⁷ Sr⁸⁶ of 0.720, and the other having Sr⁸⁷ Sr⁸⁶ about 0.73, with some scatter. Regression-lines through each group on the isochron-diagram have indistinguishable slopes, the pooled value being equivalent to an age of440 ± 9 m.y. A single regression through all the data gives 455 ± 15 m.y. Neither of these results can be separated from the estimated 445 ± 7 m.y., so that no inheritence in age from the provenance is detected. No difference can be detected between oxidized (weathered, probably in the Tertiary) and unaltered samples. X-ray diffraction analysis shows that the Shale is composed of quartz, chlorite, L M and (principally) 2 M muscovite, small amounts of feldspars, and in the oxidized zone, goethite and kaolinite. The samples having low depositional Sr⁸⁷ Sr⁸⁶ are distinguished by an absence of K-feldspar, presumably reflecting some difference in provenance. The micas may be all detrital or much of the 2 M may have grown from l M during low-grade metamorphism and folding after deposition. At present we favour the latter hypothesis, and interpret the 440 ± 9 m.y. event registered by the Shale as relating to a period of folding in the Lower Ludlovian.