The 265‒263 Ma Broughton Formation (Sydney Basin, Australia): Intense back-arc volcanism behind a quiescent peri-Gondwanan arc

The Broughton Formation in the Permian-Triassic Sydney Basin (eastern Australia) is dominated by latite flows interlayered with volcanogenic sandstones, both of which have geochemical signatures suggesting a back-arc setting. Previously reported are a 263.51 ± 0.05 Ma U-Pb zircon age (internal error) of an ash layer above the top of the formation and an Ar-Ar feldspar age of 264.15 ± 0.35 Ma for the Bumbo Latite Member in the middle of the formation. New SHRIMP U-Pb analysis of sparse detrital zircons in the sandstones throughout most of the formation shows predominantly ∼275–270 Ma and ∼265–263 Ma grains, with generally high Th/U indicating mafic-intermediate igneous sources. One exception documented here is the Flagstaff Point Sandstone Member near the top of the formation (Wollongong harbour), where pre-Permian grains dominate. CA-ID-TIMS analysis on ‘SHRIMPed’ ∼265–263 Ma grains from the Wesley Park Sandstone Member towards the base of the formation (Black Head locality) have an age of 265.6 ± 0.12 Ma. This indicates the entire formation developed over 2 Myr or less in the Wordian − early Capitanian, with the lower part of the succession up to the Bumbo Latite Member accumulated rapidly within ∼1 Myr. The Milton Monzonite emplaced into strata underlying the Broughton Formation has an age of 263.99 ± 0.07 Ma (CA-ID-TIMS U-Pb zircon and baddeleyite). This represents a hypabyssal intrusion temporally equivalent to the volcanism in the Broughton Formation. The Wordian − early Capitanian was a quiescent period for magmatism in the peri-Gondwanan arc that laid to the east of the Sydney Basin. We propose that the Broughton Formation reflects a pulse of vigorous back-arc igneous activity, perhaps triggered by slab roll-back in the palaeo-Pacific Ocean or less likely by slab break-off. Following this event, arc magmatism intensified in the Late Permian and Early Triassic.