From Nuna to Gondwana : a critical assessment of the early tectonic history of Tasmania

The Proterozoic geology of Tasmania (southeast Australia) preserves evidence of an exotic microcontinent accreted to the Pacific margin of Gondwana in the early Paleozoic. The location of Tasmania during the Proterozoic and the tectonic setting in which its extensive Mesoproterozoic and Neoproterozoic sedimentary sequences formed are poorly understood. The goals of this study are to (1) understand the Proterozoic tectonic setting of Tasmania in the context of supercontinent cycles, (2) use the geological record of Tasmania to test and refine paleogeographic reconstructions proposed for the supercontinents Nuna and Rodinia, and (3) understand the regional tectonic setting of middle Cambrian orogenesis in Tasmania in the context of the development of the Pacific margin of Gondwana. These goals are addressed primarily through refinements to the depositional age, stratigraphy, and provenance of Mesoproterozoic and Neoproterozoic strata in Tasmania and correlative strata on continents adjacent to Tasmania during the Proterozoic. The oldest rocks in Tasmania include an at least 10-km-thick sequence of siliciclastic strata comprising the 1450‚ÄövÑvÆ1300 Ma lower-middle Rocky Cape Group. Detrital zircon U-Pb-Hf isotopic data demonstrate that the lower-middle Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in present-day Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in southwest Laurentia (Proterozoic North America) and East Antarctica show striking similarities to the lower-middle Rocky Cape Group detrital zircon signature. Integrating detrital zircon and paleocurrent data indicate that the majority of sediment in the lower-middle Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania. The lower-middle Rocky Cape Group is interpreted to be the sedimentary fill of a rift basin that developed on thinned continental crust between southwest Laurentia and East Antarctica during the break up Nuna. The lower-middle Rocky Cape Group is disconformably overlain by the late Mesoproterozoic upper Rocky Cape Group, which includes a <1260 Ma carbonate-shale sequence disconformably overlain by <1170 Ma quartz arenite and siltstone. The stratigraphy of the upper Rocky Cape Group is similar to the stratigraphy of late Mesoproterozoic basins in southwest Laurentia, which comprise 1255‚ÄövÑvÆ1230 Ma carbonate-shale sequences disconformably overlain by 1150‚ÄövÑvÆ1100 Ma shallow marine and terrestrial siliciclastic deposits. Detrital zircon, muscovite, and biotite from late Mesoproterozoic strata in southwest Laurentia provide a key link between intracratonic basin formation and the tectonic evolution of the Grenville Orogeny. The 1255‚ÄövÑvÆ1230 Ma carbonate-shale sequences were deposited in a regionally extensive retroarc basin system with clastic sediment sourced in part from an active continental arc along the southern margin of Laurentia. The overlying 1140‚ÄövÑvÆ1100 Ma siliciclastic sequences were deposited in a foreland basin system during the continent-continent collisional phase of the Grenville orogen and were sourced from unroofing thrust nappes from the structurally high parts of the orogen. Correlation of the upper Rocky Cape Group and late Mesoproterozoic strata in southwest Laurentia is supported by similarities in stratigraphy and the U-Pb age distribution and Hf isotope compositions of detrital zircons. This correlation suggests the upper Rocky Cape Group formed part of the extensive syn-orogenic basin system associated with the Grenville Orogeny and supports a late Mesoproterozoic position for Tasmania along the southwest margin of Laurentia. The detrital zircon provenance of late Mesoproterozoic strata in Tasmania and southwest Laurentia is similar to detrital zircon ages from basement terranes in the central Transantarctic Mountains (East Antarctica), which supports a connection between southwest Laurentia and East Antarctica within Rodinia. The late Neoproterozoic geological record of Tasmania records rift-related magmatism and basin formation during the breakup of Rodinia. The Oonah Formation is a >5-km-thick sequence of ca. 730 Ma turbidites and syn-sedimentary alkalic mafic rocks that are widely exposed throughout Tasmania. Detrital zircon and monazite in the Oonah Formation were derived from reworking of Mesoproterozoic successions in Tasmania and Neoproterozoic granites on King Island (~100 km to the northwest of Tasmania). Lithological similarities and a similar detrital zircon provenance support correlation of the Oonah Formation with turbidites and shallow marine conglomerate and sandstone at the base of the late Neoproterozoic Togari Group in western Tasmania. The late Neoproterozoic tectonostratigraphic record of Tasmania is similar to that preserved along the southwest margin of Laurentia and eastern margin of East Antarctica, suggesting that Tasmania occupied a broad riftzone between these continents during the breakup of Rodinia. Mafic magmatism and sedimentation in the Oonah Formation and Togari Group reflects relatively minor lithospheric extension during a failed rifting event. A major rifting episode at ca. 580 Ma isolated Tasmania as a microcontinent in the paleo-Pacific Ocean during the final breakup of Rodinia. The 515‚ÄövÑvÆ505 Ma Tyennan Orogeny records arc-microcontinent collision and resulted in the obduction of an ophiolite over the Proterozoic crust of Tasmania. The high-grade metamorphic sole of the Tasmanian ophiolite includes strongly deformed granulite‚ÄövÑvÆlower amphibolite facies mafic cumulate rocks, dolerite, and basalt. Kinematic indicators in mylonitic amphibolites record the west-directed obduction of the ophiolite and suggest the metamorphic sole formed in an eastdipping subduction zone located to the east of Tasmania. Major and trace element whole rock and relict igneous spinel geochemistry indicates that the protoliths to the metamorphic sole formed at a back arc basin spreading centre. The new data supports a model in which east-dipping subduction in Tasmania was driven by collapse of a back arc basin developed above an earlier west-dipping subduction zone outboard of the pacific margin of Gondwana. The incorporation of the Tasmanian microcontinent into Gondwana occurred in a complex geodynamic environment similar to the modern southwest Pacific.