Spandler, Carl1, Henderson Bob2, Foley, Elliot2, Roberts, Eric2, Kemp, Tony3
1The University of Adelaide, Adelaide SA 5005, 2James Cook University, Townsville, QLD 4811, 3The University of Western Australia, Perth, WA 6907
The Phanerozoic tectonic setting of eastern Australia involved two separate regimes. The older setting was a Cambrian to Triassic active convergent margin as registered by a succession of orogenic systems collectively grouped as the Tasmanides. The younger setting was a passive margin, driven by plate divergence that initiated in the Cretaceous, and continues to characterize eastern Australia. The tectonic setting of eastern Australia during the gap between these contrasting tectonic regimes (approx. 130 m.y. from the late Triassic to Cretaceous) has been poorly documented and remains open to question. While extensive continental detritus of this age is preserved in the Great Artesian Basin, recognized igneous activity is restricted to the Whitsunday Igneous Province that formed from 132 Ma to 95 Ma.
Here we investigate a suite of igneous rocks/units that includes the Grahams Creek Formation (>0.25 M km3) and a series of small plutons and volcanic units that are exposed in the region between the Sunshine Coast and Maryborough in SE Queensland. The plutonic rocks range from I–type, hornblende-rich gabbros and diorites, to granodiorites, and quartz syenites, while the Grahams Creek Formation consists of a thick sequence (up to 1200 metres) of volcanic to volcaniclastic rocks of basaltic to dacitic composition. Both plutonic and volcanic components have distinctive subduction-related trace element compositions, including relative depletion in Ti, Nb and Ta, and enrichment in Pb, Sr, K, Rb, Th, U, Ba and Cs. These compositions are typical of hydrous magmatic rocks from continental arc settings. Uranium-Pb dating of magmatic zircons from these samples returned ages between 145 and 140 Ma; an age range that is distinctly older that the Whitsunday Igneous Province. The initial Hf and O isotope composition of these zircons (εHf = +8 to +12.5; δ18O = +5.7 to +6.5) is consistent with a juvenile mantle source for these magmas.
The recognition of this new suite of magmatic rocks, together with new chemical analyses of mafic rocks from the Whitsunday Igneous Province and detrital zircon records of quartzo-feldspathic sedimentary sequences of the Great Artesian Basin (see Foley et al. 2020, this session), allow a re-evaluation of the tectonic setting of eastern Australia across the Mesozoic. We propose that the plate convergence regime along eastern Australia that formed the New England Orogen persisted across the Triassic, Jurassic and into the Cretaceous, with the newly-recognised arc rocks representing the youngest episode of continental arc magmatism recorded on the Australian continent. Eastwards rollback of the slab in the Early Cretaceous led to continental extension and opening of a continental back-arc (analogous to the present-day Okinawa Trough) that formed the Whitsunday Igneous Province. Continued slab rollback and extension of the overlying plate in the late Cretaceous and Cenozoic lead to rifting and fragmentation of the eastern continental margin to form the current configuration of the eastern Australia-Zealandia, where large tracts on thinned continental crust remain submerged.
Carl Spandler is a petrologist/geochemist with interest in broad aspects of geology, particularly crustal growth processes, plate tectonics, mantle evolution and critical minerals ore systems.