Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

K Waltenberg,S Bodorkos,R Armstrong,B Fu

doi:10.1080/08120099.2018.1463928

K Waltenberg, S Bodorkos + Show 2 more

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https://doi.org/10.1080/08120099.2018.1463928

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Abstract

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ18O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ϵHft range from −8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ϵHft (∼−7 to −4) and elevated δ18O values (∼9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ϵHft (∼−9 to −6) but more subdued δ18O values (∼7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ϵHf and mantle-like δ18O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (ϵHft as high as +8.5, δ18O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (ϵHft as high as −0.5, δ18O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two.

Highlights

The Tasmanides constitute the eastern third of the Australian continent, and broadly record early Paleozoic to early Mesozoic crustal addition to the eastern margin of Precambrian Australia, via arc-related accretion and associated sedimentation (e.g. Cawood & Buchan, 2007; Champion, 2016; Collins, 2002; Glen, 2005; Gray & Foster, 2004).There exist numerous, but variable, models to explain the current geological configuration of the Tasmanides
The isotopic character of all samples in this study can be best described by three-component mixing of variable amounts of high d18O–unradiogenic eHf source component, a high d18O–unradiogenic eHf source component and one or more low d18O–radiogenic eHf source components
Data coverage remains sparse owing to the paucity of appropriate available samples, especially within the Thomson Orogen; we have not yet identified any contrast in Lu–Hf or O-isotopic character between the northern Lachlan Orogen Albury-Bega Terrane and the southern Thomson Orogen

Summary

Introduction

The Tasmanides constitute the eastern third of the Australian continent, and broadly record early Paleozoic (ca 500 Ma) to early Mesozoic (ca 250 Ma) crustal addition to the eastern margin of Precambrian Australia, via arc-related accretion and associated sedimentation (e.g. Cawood & Buchan, 2007; Champion, 2016; Collins, 2002; Glen, 2005; Gray & Foster, 2004).There exist numerous, but variable, models to explain the current geological configuration of the Tasmanides. Burton, 2010; Fergusson & Henderson, 2013; Glen et al, 2013; Gray & Foster, 2004; Kirkegaard, 1974; Moresi, Betts, Miller, & Cayley, 2014; Murray & Kirkegaard, 1978; Scheibner, 1973). Current understanding of the Tasmanides is based largely on relatively well-exposed geological relationships in the southern and eastern Tasmanides: the Delamerian Orogen in South Australia, the Lachlan Orogen in Victoria and New South Wales (NSW) and the New England Orogen in northern NSW and southern Queensland (Figure 1). A major blind spot in understanding the Tasmanides is the Thomson Orogen: a very large region of Queensland and northern NSW. Its location largely coincides with the region covered by the Mesozoic Eromanga Basin. Limited information indicates that Paleozoic metasedimentary and igneous rocks comprise the bulk of geology beneath the Eromanga Basin (Brown, Carr, & Purdy, 2012; Murray, 1994)

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