Insights into 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 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 δ¹⁸O_zrn (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 δ¹⁸O_zrn (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 δ¹⁸O_zrn (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.

本数据集涵盖采自汤姆森造山带（Thomson Orogen）覆盖区深部基底钻井岩芯的岩浆岩样品的锆石U-Pb年龄、εHf(t)、δ¹⁸O同位素数据，以及地球化学特征与有限的Sm-Nd同位素测试结果，可为该造山带出露区与覆盖区建立紧密的时空关联，并为其演化过程及与拉克伦造山带（Lachlan Orogen）的成因联系提供关键科学线索。SHRIMP锆石U-Pb定年结果显示，早奥陶世岩浆作用广泛分布于汤姆森造山带中部覆盖区，且形成于480~470 Ma的狭窄时限范围内。该套岩石具有演化型锆石εHf(t)值（-12.18~-6.26）与εNd值（-11.3~-7.1），以及壳源锆石δ¹⁸O值（7.01~8.50‰），这与拉克伦造山带内具有亏损型同位素特征的早奥陶世岩浆岩形成显著反差。另有两件样品形成于晚志留世（425~420 Ma），四件样品形成于泥盆纪（408~382 Ma）。晚志留世岩石具有演化型锆石εHf(t)值（-6.42~-4.62）与壳源δ¹⁸O值（9.26~10.29‰）；而时代更年轻的泥盆纪岩石则表现出εHf(t)向更具亏损型特征偏移的趋势，这一趋势同样见于拉克伦造山带同期岩浆岩中。值得注意的是，两件晚泥盆世早期样品具有亏损型锆石εHf(t)值（0.01~1.92），但同时具备壳源δ¹⁸O值（7.45~8.77‰），指示新生幔源物质发生了快速循环再造。汤姆森造山带岩浆岩存在两条截然不同的Hf-O同位素混合演化趋势：其中一条趋势体现了更强烈的壳源组分混入，对应汤姆森造山带北三分之二区域的采样样品；另一条趋势则整体演化程度更低，对应汤姆森造山带南三分之一区域的采样样品，其同位素特征与拉克伦造山带岩浆岩完全匹配。早奥陶世岩浆作用与更强演化程度的变质沉积特征在空间上的耦合关系表明，汤姆森造山带至少北部区域的基底为更古老的德拉梅里造山运动前变质沉积岩。