Foley, Elliot1, Henderson, Robert1, Roberts, Eric1, Kemp, Tony2, Spandler, Carl3
1James Cook University, Townsville, QLD 4811, 2The University of Western Australia, Perth, WA 6009, 3The University of Adelaide, Adelaide SA 5005
The tectonic setting of the east Gondwana margin during the Jurassic and Early Cretaceous is an enduring geological unknown. Whereas Paleozoic to early Mesozoic (~520 to 220 Ma) accretionary orogenic domains in eastern Australia are considered an exemplary record of convergent margin processes, the Late Triassic to Cretaceous represents an enigmatic gap in this record due to the paucity of exposed igneous and metamorphic rocks. This latter witnessed the deposition of vast quantities (>1.5 x 106 km3) of sediment into the Great Australian Superbasin, including Jurassic silicic tuff horizons and a substantial Cretaceous component identified as volcanogenic.
The nature of magmatism that provided this volcanogenic material is debated, with two principal hypotheses posited. One suggests a continental magmatic arc enduring from the Carboniferous to mid-Cretaceous. The second model favours intraplate, rift-related magmatism unrelated to subduction, exemplified by the early-mid Cretaceous Whitsunday Igneous Province, a silicic large igneous province (SLIP) generated in the prelude to rupture of east Gondwana in the Late Cretaceous. Resolution of this question has been hampered by the sparse Jurassic-Cretaceous igneous record for eastern Australia. To overcome this deficiency, we investigated detrital zircon from the Great Australian Superbasin as a proxy record for subjacent igneous activity, employing U-Pb geochronology and Hf isotopic analysis to evaluate Mesozoic magmatism and clarify this enigmatic episode of east Gondwana crustal evolution.
Detrital zircon ages indicate that magmatism along the east Gondwana margin continued into the mid Cretaceous, with short-lived (~10 Myr) pulses of Mesozoic magmatic activity indicated by peaks at ~160, ~140, and ~100 Ma. A trend of increasing igneous activity, from the Jurassic towards eventual Late Cretaceous continental rupture of east Gondwana, as predicted by the SLIP hypothesis, is not supported by the detrital zircon record. Hf isotopic analysis of dated zircons shows a strongly positive εHf signature (+8 to +12) throughout the Mesozoic to ~95 Ma indicative of juvenile sources for the original igneous parent rocks. Similar positive εHf signatures are characteristic of Permian – Triassic granitic rocks of the New England Orogen for which a continental magmatic arc setting has been long accepted.
A potential Australian igneous source for Cretaceous zircon, the Whitsunday Igneous Province, is of limited aerial extent and a Jurassic source is unknown. Northern Zealandia, now submerged, formed the eastern borderland of east Gondwana prior to the Late Cretaceous, and must have been the main locus of Jurassic and Cretaceous magmatism.
Elliot Foley is a PhD Candidate at James Cook University and a member of the multidisciplinary Jurassic Arc Research Group. His research focuses on the sedimentology, stratigraphy, provenance and hydrocarbon potential of basin fill in the northern sector of the Jurassic-Cretaceous Great Australian Superbasin.