Geochronology and thermochronology of mafic volcanic rocks in the offshore Roebuck Basin, North West Shelf, Australia

Zircon and apatite U–Pb geochronology and (U–Th)/He thermochronology is presented for seven core samples from three wells (Poissonnier-1, Bedout-1, Apus-1) drilled in the Bedout Sub-basin, Roebuck Basin, offshore northwestern Australia. All samples are from underexplored fine-grained mafic volcanic rocks that underlie hydrocarbon-hosting Triassic sandstones and shales. Various apatite U–Pb age components and a zircon U–Pb age imply that the Poissonnier-1 samples belong to Archean (meta)volcanic units from the offshore extension of the Pilbara Craton. In contrast, Bedout-1 apatite yields a well-defined U–Pb age of 274 ± 17 Ma, consistent with the assumed Permian emplacement age of the extensive Bedout Volcanics. Apatite U–Pb results from the Apus-1 well yield a less well-defined linear regression intercept age of 526 ± 36 Ma, hinting at multi-cycle apatite in the Bedout Volcanics, possibly inherited from underlying Permian sedimentary successions. Thermal history modelling of (U–Th)/He analyses from the same grains points to relatively recent heating to the apatite He partial retention zone (40–90 °C) during the late Neogene in all wells. This thermal pulse could represent dynamic topography effects in the area, indicating that mafic volcanic rocks can act as a thermal buffer and facilitate hydrocarbon preservation.