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

Abstract

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 10–20 μm) from the same samples show a progressive increase in age with grainsize (<2 μm = 413.1 ± 8.3–404.7 ± 8.1 Ma; 3–5 μm = 417.1 ± 8.3 Ma; 10–20 μm = 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. Kübler 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°C and P = 4 kb suggests a depth of burial of approximately 15 km was reached by these rocks and that a geothermal gradient of 20°C/km operated during subduction.