The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ~10. km thick quartzarenite-siltstone-pelite-dominated succession, previously constrained to have been deposited between 1450. Ma and 750. Ma. The Rocky Cape Group contains the enigmatic fossil Horodyskia ('string of beads') and has the potential to place Tasmania within supercontinent reconstructions. Detrital zircon and authigenic monazite grains dated via U-Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analysis yield a deposition window between c. 1450. Ma (youngest zircon populations) and c. 1330. Ma (oldest authigenic monazite population) for the ~9. km thick lower-middle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Subgroup which hosts Horodyskia, Detention Subgroup). The upper units (~1. km) include the Irby Siltstone, which is younger than c. 1310. Ma; this unit is likely separated from both the lower-middle units and the overlying c. <1010. Ma Jacob Quartzite by disconformities. The lower-middle Rocky Cape Group is dominated by detrital zircon populations between 1600 and 1900. Ma, with subordinate populations at c. 1450. Ma and a spread of older grains mostly between 2300 and 2900. Ma. The overlying Irby Siltstone has a bimodal detrital zircon distribution with a dominant peak at c. 1340. Ma and a secondary peak at c. 1720. Ma; no >1900. Ma grains were identified.
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) 1358-1292Ma (inclusive of two sigma errors) (Lagoon River Quartzite and Pedder River Siltstone), (2) 1283-1239Ma (Cowrie Siltstone and Balfour Subgroup), and (3) 1085±9Ma (Detention Subgroup). We suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin. 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 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 c. 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) Palaeoproterozoic 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 palaeogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodic post-depositional fluid flow events between c. 1.35 and 1.05. Ga, possible thermotectonic imprints of the subsequent assembly of Rodinia.
- Authigenic monazite
- Detrital zircon
- Proterozoic basin