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 Sil urian to late Early Devonian. It is represented by a regionally extensive, unfossiliferous sequence of interbedded turbidites and hemipelagites of sub stantially silicic volcanic derivation, which passes laterally into contemp oraneous shallow-water sedimentary rocks. The Turondale and Merrions Format ions of the Lower Devonian Crudine Group are two prominent volcanogenic for mations in the predominantly sedimentary trough sequence. They contain a ra nge of primary and resedimented volcanic facies suitable for U-Pb dating. T hese include widespread subaqueous silicic lavas and/or lava cryptodomes, a nd thick sequences of crystal-rich volcaniclastic sandstone emplaced by a s uccession of mass-flows that were generated by interaction between contempo raneous subaerial pyroclastic flows and the sea. ion microprobe dating of t he two volcanogenic formations by means of the commonly used SL 13 zircon s tandard yields ages ranging between 411.3 +/- 5.1 and 404.8 +/- 4.8 Ma. Nor malising the data against a different zircon standard (QGNG) yields preferr ed 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 biostratigrap hically inferred late Lochkovian-early Emsian (mid-Early Devonian) age for the Merrions Formation is inconsistent with the current Australian Phaneroz oic Timescale, which assigns an age of 410 Ma to the Silurian-Devonian boun dary, 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 compar ed with the most recently published revision of the Devonian time-scale. Th is suggests that the Early Devonian stage boundaries of the Australian Phan erozoic Timescale need to be revised downward. The new ages for the Merrion s Formation could also provide a time point on this time-scale for the Prag ian to early Emsian, for which no data are presently available.