Abstract
Zircon U–Pb ages, εHf(t), and δ18O isotopic data together with geochemistry and limited Sm–Nd results from magmatic rocks sampled in deep-basement drill cores from undercover parts of the Thomson Orogen provide strong temporal links with outcropping regions of the orogen and important clues to its evolution and relationship with the Lachlan Orogen. SHRIMP U–Pb zircon ages show that magmatism of Early Ordovician age is widespread across the central, undercover regions of the Thomson Orogen and occurred in a narrow time-window between 480 and 470 Ma. These rocks have evolved εHf(t)zrn (−12.18 to −6.26) and εNd (−11.3 to −7.1), and supracrustal δ18Ozrn (7.01–8.50‰), which is in stark contrast to Early Ordovician magmatic rocks in the Lachlan Orogen that are isotopically juvenile. Two samples have late Silurian ages (425–420 Ma), and four have Devonian ages (408–382 Ma). The late Silurian rocks have evolved εHf(t)zrn (−6.42 to −4.62) and supracrustal δ18Ozrn (9.26–10.29‰) values, while the younger Devonian rocks show a shift toward more juvenile εHf(t)zrn, a trend that is also seen in rocks of this age in the Lachlan Orogen. Interestingly, two early Late Devonian samples have juvenile εHf(t)zrn (0.01–1.92) but supracrustal δ18Ozrn (7.45–8.77‰) indicating rapid recycling of juvenile material. Two distinct Hf–O isotopic mixing trends are observed for magmatic rocks of the Thomson Orogen. One trend appears to have incorporated a more evolved supracrustal component and is defined by samples from the northern two-thirds of the Thomson Orogen, while the other trend is generally less evolved and from samples in the southern third of the Thomson Orogen and matches the isotopic character of rocks from the Lachlan Orogen. The spatial association of the Early Ordovician magmatism with the more evolved metasedimentary signature suggests that at least the northern part of the Thomson Orogen is underlain by older pre-Delamerian metasedimentary rocks.
Highlights
The Thomson Orogen of eastern Australia covers $1,000,000 km2 but is poorly understood and underexplored due to extensive sedimentary cover that in places reaches thicknesses of 4 km
We fully report the SHRIMP U–Pb geochronology of the nine samples mentioned by Draper (2006) and supplement this work with (1) new SHRIMP U–Pb geochronology from other basement cores and remote outcrops, (2) new zircon Hf–O isotopic data, and (3) whole-rock geochemistry and Nd isotopic data
Further work is required to discriminate between the different possible scenarios for the Trend 2 metasediments, we suggest that the spatial distribution of Trend 1 metasediments, where associated with Early Ordovician magmatism, can be taken as the minimum distribution of pre-Delamerian Orogeny basement in the Thomson Orogen
Summary
The Thomson Orogen of eastern Australia (as defined by Fergusson, Henderson, Hutton, & Withnall, 2013; Glen et al, 2013) covers $1,000,000 km but is poorly understood and underexplored due to extensive sedimentary cover that in places reaches thicknesses of 4 km. Outcropping examples occur in the Charters Towers Province (Fergusson et al, 2007a) and Anakie Province (Fergusson et al, 2001), while undercover probable correlatives have been grouped into the Machattie beds by Carr, Purdy, and Brown (2014) and occur near the western margin of the Thomson Orogen (Purdy et al, 2016b; Figure 2). Overlying these rocks is a widespread package of dominantly low-grade, quartz-rich turbiditic metasedimentary rocks deposited from the middle–late Cambrian to Early Ordovician. Outcropping units occur in the Charters Towers and Anakie provinces (Fergusson et al, 2001, 2007a) as well as in the undercover parts of the Thomson Orogen, where they are represented by the Thomson beds (Carr et al, 2014; Purdy et al, 2016b; Figure 2)
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