Age constraints from the Rocky Cape Group: putting Tasmania on the Mesoproterozoic map

The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ~10 km thick quartzarenite–siltstone–pelite-dominated succession, which was previously constrained to have been deposited between 1450 Ma and 750 Ma. New detrital zircon and authigenic monazite ages dated via U–Pb Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) sampled from sub-greenschist facies sandstones and siltstones throughout the Rocky Cape Group allow us to: (1) vastly improve on depositional age constraints, including constraining the age of the ‘string of beads’ fossil Horodyskia-bearing strata; (2) make regional basin-scale correlations; and (3) speculate on the tectonic correlations between proto-Australia and Laurentia at ca 1.45 Ga.

Detrital zircon and authigenic monazite grains ages analysis yield a deposition window between ca 1450 Ma (youngest zircon populations) and ca 1330 Ma (oldest authigenic monazite population) for the ~9 km thick lower– middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Sub-group, which hosts Horodyskia, Detention Sub-group). The upper units (~1 km) include the Irby Siltstone, which is younger than ca 1310 Ma; this unit is likely separated from both the lower–middle units and the overlying < ca 1010 Ma Jacob Quartzite by disconformities. Authigenic monazite age distributions are complex, with multiple age domains within most samples. The common Pb corrected ²⁰⁶U/²³⁸Pb ages, defined by oldest grains in each sample, identify three statistically significant groups: (1) ca 1330 Ma (Lagoon River Quartzite and Pedder River Siltstone), (2) ca 1260 Ma (Cowrie Siltstone and Balfour Subgroup), and (3) 1085 ± 9 Ma (Detention Sub-group). We suggest monazite was precipitated during episodic fluid flow events at these three stages. The original source for REE-bearing fluids could be detrital monazite, which is rarely preserved, and/or organic matter from the interbedded carbonaceous shales.

The lower–middle Rocky Cape Group has a shared provenance with the higher-grade metasediments (Surprise Bay and Fraser formations) of nearby King Island; the newly derived depositional ages also overlap and support the correlation of these rock associations. On the basis of current datasets, there are no obvious correlations that can be made with Mesoproterozoic basins preserved in mainland Australia. Instead, an overlap in the timing of deposition, similarities in detrital zircon signatures and analogous depositional environment suggests the ca 1.45–1.37 Ga upper Belt-Purcell Supergroup (Missoula and Lemhi groups) of western North America constitutes a plausible correlation with the Tasmanian Mesoproterozoic succession. If the (unexposed) Paleoproterozoic basement of Tasmania correlates with the Transantarctic Mountains region of East Antarctica as previously proposed, we suggest that the overlying Mesoproterozoic sequences were deposited during rifting of the supercontinent Nuna, between proto- Australia (including the Mawson craton of Antarctica) and Laurentia as predicted by the most recent paleogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodic post-depositional fluid flow events between ca 1.35–1.05 Ga, possible thermotectonic imprints of the subsequent assembly of Rodinia.