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

• Main Authors: Halpin, JA, Jensen, T, McGoldrick, P, Meffre, S, Berry, RF, Everard, JL, Calver, CR, Thompson, J, Goemann, K, Whittaker, JM
• Format: Conference Object
• Language: English
• Published: Geological Society of Australia 2014
• Subjects: Earth Sciences; Geology; Tectonics; East Antarctica; Transantarctic Mountains; King Island; Balfour; Antarc*; Antarctica
• Online Access: http://ecite.utas.edu.au/100955

The oldest known rocks in Tasmania occur in the Proterozoic Rocky Cape Group, a ~10 km thick quartzarenitesiltstonepelite-dominated succession, which was previously constrained to have been deposited between 1450 Maand 750 Ma. New detrital zircon and authigenic monazite ages dated via UPb Laser Ablation-Inductively CoupledPlasma Mass Spectrometry (LA-ICPMS) sampled from sub-greenschist facies sandstones and siltstones throughoutthe Rocky Cape Group allow us to: (1) vastly improve on depositional age constraints, including constraining the ageof the string of beads fossil Horodyskia -bearing strata; (2) make regional basin-scale correlations; and (3) speculateon 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 lowermiddle units (Pedder River Siltstone, Lagoon River Quartzite, Balfour Sub-group, which hosts Horodyskia , DetentionSub-group). The upper units (~1 km) include the Irby Siltstone, which is younger than ca 1310 Ma; this unit is likelyseparated from both the lowermiddle 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 Pbcorrected 206 U/ 238 Pb 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 flowevents at these three stages. The original source for REE-bearing fluids could be detrital monazite, which is rarelypreserved, and/or organic matter from the interbedded carbonaceous shales. The lowermiddle Rocky Cape Group has a shared provenance with the higher-grade metasediments (Surprise Bayand Fraser formations) of nearby King Island; the newly derived depositional ages also overlap and support thecorrelation of these rock associations. On the basis of current datasets, there are no obvious correlations that can bemade 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 1.451.37 Ga upperBelt-Purcell Supergroup (Missoula and Lemhi groups) of western North America constitutes a plausible correlationwith the Tasmanian Mesoproterozoic succession. If the (unexposed) Paleoproterozoic basement of Tasmaniacorrelates with the Transantarctic Mountains region of East Antarctica as previously proposed, we suggest that theoverlying 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 recentpaleogeographic reconstructions. Both the Tasmanian and western Laurentian packages were affected by episodicpost-depositional fluid flow events between ca 1.351.05 Ga, possible thermotectonic imprints of the subsequentassembly of Rodinia.