⁴⁰Ar/³⁹Ar and K-Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW

White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrix-melanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40Ar/39Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 +/- 13 Ma and 476 +/- 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 +/- 8.6 Ma and 449.2 +/- 9.0 Ma. Fractions (2, 3-5 and 10-20m) from the same samples show a progressive increase in age with grainsize (2m=413.1 +/- 8.3-404.7 +/- 8.1 Ma; 3-5m=417.1 +/- 8.3 Ma; 10-20m=434 +/- 8.6-437.5 +/- 8.8 Ma 2 standard deviations). The older whole rock 40Ar/39Ar ages are attributed to recoil loss of 39Ar. The cleavage forming event is believed to have occurred at 460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kubler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b0 lattice parameter values (x=9.033; n=21) are characteristic of the intermediate pressure facies series. Assuming a temperature of approximately 300 degrees C and P=4 kb suggests a depth of burial of approximately 15km was reached by these rocks and that a geothermal gradient of 20 degrees C/km operated during subduction.