U-Pb dating of silicic lavas, sills and syneruptive resedimented volcaniclastic depositis of the Lower Devonian Crudine Group, Hill End Trough, New South Wales

The Hill End Trough of central-western New South Wales was an elongate deep marine basin that existed in the Lachlan Fold Belt from the early Late Silurian to late Early Devonian. It is represented by a regionally extensive, unfossiliferous sequence of interbedded turbidites and hemipelagites of substantially silicic volcanic derivation, which passes laterally into contemporaneous shallow-water sedimentary rocks. The Turondale and Merrions Formations of the Lower Devonian Crudine Group are two prominent volcanogenic formations in the predominantly sedimentary trough sequence. They contain a range of primary and resedimented volcanic facies suitable for U-Pb dating. These include widespread subaqueous silicic lavas and/or lava cryptodomes, and thick sequences of crystal-rich volcaniclastic sandstone emplaced by a succession of mass-flows that were generated by interaction between contemporaneous subaerial pyroclastic flows and the sea. ion microprobe dating of the two volcanogenic formations by means of the commonly used SL 13 zircon standard yields ages ranging between 411.3 +/- 5.1 and 404.8 +/- 4.8 Ma. Normalising the data against a different zircon standard (QGNG) yields preferred slightly older mean ages that range between 413.4 +/- 6.6 and 407.1 +/- 6.9 Ma, These ages broadly approximate the Early Devonian age that has been historically associated with the Crudine Group. However, the biostratigraphically inferred late Lochkovian-early Emsian (mid-Early Devonian) age for the Merrions Formation is inconsistent with the current Australian Phanerozoic Timescale, which assigns an age of 410 Ma to the Silurian-Devonian boundary, and ages of 404.5 Ma and 395.5 Ma to the base and top of the Pragian, respectively. There is, however, good agreement if the new ages are compared with the most recently published revision of the Devonian time-scale. This suggests that the Early Devonian stage boundaries of the Australian Phanerozoic Timescale need to be revised downward. The new ages for the Merrions Formation could also provide a time point on this time-scale for the Pragian to early Emsian, for which no data are presently available.