Insights to the evolution of the Thomson Orogen from geochronology, geochemistry and zircon isotopic studies of magmatic rocks

Zircon U-Pb ages, εHf(t) and δ18O isotopic data, geochemistry and limited Sm-Nd results mostly from deep basement drill cores from undercover parts of the Thomson Orogen, provide strong temporal links with outcropping regions of the orogen as well as important clues for its evolution and relationship with the Lachlan Orogen. SHRIMP U–Pb ages from three Early Ordovician volcanic samples and one granite from the undercover, Thomson Orogen shows that magmatism of this age is widespread across the central, undercover regions of the orogen and occurred in a narrow time-window between 480 Ma and 470 Ma. These rocks have evolved, εHf(t)zrn (-6.26 to -12.18), εNd (-7.1 to -11.3), and supracrustal δ18Ozrn (7.01–8.50‰) which is in stark contrast to the Early Ordovician rocks in the Lachlan Orogen, that are isotopically juvenile. Two samples have latest Silurian to earliest Devonian ages (1586685 DIO Ella 1; 425.4 ± 6.6 Ma and 2122055 Hungerford Granite; 419.1 ± 2.5) and coincide with a major period of intrusive magmatism in the southern Thomson and the Eastern and Central Lachlan Orogen. These samples have evolved εHf(t)zrn (-4.62 to -6.42) and supracrustal δ18Ozrn (9.26–10.29‰) which is similar to Lachlan Orogen rocks emplaced during this time. Four samples have mid Early to early Late Devonian ages (408–382 Ma) and appear to have been emplaced in a generally extensional tectonic regime. Two of these are from the Gumbardo Formation (1682891 PPC Carlow 1 and 1682892 PPC Gumbardo 1), the basal unit of the Adavale Basin, and constrain its opening to between 408 Ma and 403 Ma. The other two samples (1585223 AAE Towerhill 1 and 2122056 Currawinya Granite) have ages of ca. 382 Ma. These latter samples generally show a shift towards more juvenile εHf(t)zrn and mantle-like δ18Ozrn values, a trend that is also seen in rocks of this age in the Lachlan Orogen. Collectively, zircon Hf and O isotopes show that magmatism in the central, undercover part of the Thomson Orogen was initially derived from isotopically evolved magma sources but progressed to more juvenile sources during the Devonian. Furthermore, it appears that samples from the Thomson Orogen may fall along two distinct Hf-O isotopic mixing trends. One trend, appears to have incorporated an older (more evolved) supracrustal component and occurs in the northern two-thirds of the Thomson Orogen, while the other trend is generally less evolved and occurs in the southern third of the Thomson Orogen and is geographically continuous with the Lachlan Orogen. Citation: A. J. Cross, D. J. Purdy, D. C. Champion, D. D. Brown, C. Siégel & R. A. Armstrong (2018) Insights into the evolution of the Thomson Orogen from geochronology, geochemistry, and zircon isotopic studies of magmatic rocks, Australian Journal of Earth Sciences, 65:7-8, 987-1008, DOI: 10.1080/08120099.2018.1515791