SHRIMP U–Pb detrital zircon ages from Proterozoic and Early Palaeozoic sandstones and their bearing on the early geological evolution of Tasmania

Abstract

Detrital zircons from 13 Late Mesoproterozoic to Early Neoproterozoic sandstones and two Palaeozoic sandstones from Tasmania were dated in order to improve constraints on depositional ages, to test correlation between Proterozoic inliers, and to characterise source regions. These include successions considered to be the oldest presently exposed in Tasmania. Typical features of the age distributions of the Proterozoic rocks are prominent data concentrations at 1800–1650 Ma and 1450–1400 Ma, and a minor spread of Archaean ages. Statistical testing of the similarity of the age profiles shows that widespread quartzarenaceous samples from the Detention Subgroup, Needles Quartzite and from the Tyennan region are strongly similar, consistent with broad correlation. Relatively large differences are seen between the Detention Subgroup and the conformable, stratigraphically higher Jacob Quartzite, which contains an additional spread of 1300–1000 Ma zircons suggestive of a Grenvillian source. Age profiles of the quartzarenites and quartzwacke turbidites (Oonah Formation and correlatives) cannot be readily differentiated. The Oonah Formation likewise includes samples with and without Grenvillian ages, and there is no 750 Ma zircon population that would be expected if the turbidites were genetically related to the Wickham Orogeny. The simplest interpretation is that the quartzarenites (Rocky Cape Group and correlatives) and the turbidites (Oonah Formation and correlates) are lateral equivalents, although a younger (post‐Wickham Orogeny) age for the Oonah Formation cannot be discounted. A maximum age of ca 1000 Ma is inferred for the Oonah Formation, Rocky Cape Group and correlatives. A minimum age of ca 750 Ma is provided by the basal age of the overlying Togari Group and correlatives. In a metasediment from western King Island, the youngest detrital zircons are ca 1350 Ma, allowing a pre‐Grenvillian depositional age as suggested by previous dating of metamorphic monazite. However, the age profile of this sample is not dissimilar to the other Tasmanian successions that are inferred to be 1000–750 Ma. The Wings Sandstone, of southern Tasmania, contains an unusual profile dominated by Grenvillian ages, consistent with an allochthonous origin. Basement ages that broadly match the age spectra of the Tasmanian Proterozoic sediments are found in southwestern Laurentia, consistent with mutual proximity in Rodinia reconstructions. The Palaeozoic sandstones, from the turbiditic Mathinna Supergroup of northeastern Tasmania, have zircon age profiles typical of the Lachlan Fold Belt, with a predominant latest Neoproterozoic–Early Cambrian component and a lesser, broad Proterozoic data concentration at ca 1000 Ma. Western Tasmania was not a significant part of the source area for these rocks.

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

“…Albeit only a small island in Bass Strait, King Island predominately comprises Precambrian rocks and is situated in a key position with major implications for tectonic reconstruction in SE Australia. The oldest rocks of King Island are Mesoproterozoic metasediments that crop out at the southern tip of the island (Black et al, 2004;Berry et al, 2005;Calver, 2007). It is important to note here that no corresponding rocks are known in Tasmania and Victoria (Calver, 2007).…”

“…The oldest outcropping rocks in western Tasmania are predominantly metaquartzites and siltstones of the Rocky Cape Group in the Rocky Cape Block (ranging from~1000 Ma to a maximum of 1430 Ma) (Black et al, 2004). The Rocky Cape Group is unconformably overlain by the Late Neoproterozoic to Cambrian Togari Group, which has been dated by Calver (1998) at ca 750 Ma.…”

Evolution of a Cambrian active continental margin: The Delamerian–Lachlan connection in southeastern Australia from a zircon perspective

“…The ophiolite complex was emplaced in an island arc -passive margin collision along an east-dipping subduction zone west of the island arc (Berry and Crawford, 1988). The basement Neoproterozoic quartzose siliciclastic successions contain abundant detrital zircons of 1800-1650 Ma with less abundant 1450-1400 Ma ones and some samples containing 1300-1000 Ma ages indicating a likely early Neoproterozoic age for the succession (Berry et al, 2001;Black et al, 2004). The position of Tasmania in the Tasmanides prior to the Ordovician is problematic with some authors arguing for its initial location along the East Antarctic passive margin prior to the Ross Orogeny (Berry et al, 2008;Gibson et al, 2011).…”

Early Palaeozoic continental growth in the Tasmanides of northeast Gondwana and its implications for Rodinia assembly and rifting

“…They are the deformed fill of a huge sedimentary basin (Fig. 12) (Ireland et al, 1998;Fergusson and Fanning, 2002;Black et al, 2004;Buick et al, 2005;Fergusson et al, 2005bFergusson et al, , 2007aMaidment et al, 2007;Greenfield et al, 2010;Glen et al, 2013;Maidment et al, 2013). Pb data is used for age estimates >1Ga.…”

“…They have been found in the Amadeus and Georgina basins and their metasedimentary equivalents in the adjacent Harts Range representing exhumed and tectonised deep basinal assemblages (Maidment et al 2007(Maidment et al , 2013 as well as successions of the Delamerian and Lachlan orogens (Ireland et al, 1997; and Early Paleozoic strata of Tasmania (Black et al, 2004). Sediment produced during Pacific-Gondwana and Grenville times was a significant contributor to continental growth within the Tasmanides, and to central Australian cratonic basins.…”