Crustal and thermal structure of the Thomson Orogen: constraints from the geochemistry, zircon U–Pb age, and Hf and O isotopes of subsurface granitic rocks

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature (ϵ_Hf_(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (ϵ_Hf_(t) = –13 to +15), Thomson (ϵ_Hf_(t) = –14 to +5) and Delamerian (ϵ_Hf_(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.

地下汤姆森造山带（Thomson Orogen）内高地温梯度的成因，以及其下方地壳基底（crustal basement）究竟属于洋壳还是陆壳，至今仍是未解之谜。以往研究表明，要解释这类异常热梯度，需要更高的地壳放射热输入。本研究通过对埋藏花岗岩类开展地球化学、地质年代学以及铪（Hf）和氧（O）同位素分析，并评估变沉积岩的产热潜力，探究了该地壳热输入的性质与时代。低-中等产热的I型和S型花岗岩类的矿物学特征、化学成分，以及其形成于新元古代/寒武纪至泥盆纪的时代属性，与汤姆森造山带西南缘附近被认为是高地温梯度重要成因的大湖岩套（Big Lake Suite）石炭纪A型高热产花岗岩形成鲜明对比。这些差异表明，大湖岩套岩体并未延伸至温度异常区的昆士兰州部分区域。变沉积岩的产热率同样处于低至中等水平。基于具有类地幔氧同位素组成（ε_Hf(t) = –12 至 +2）的锆石铪同位素组成，我们提出该温度异常源于汤姆森造山带下方中元古代和/或古元古代高热产岩体的存在。因此，前寒武纪（Precambrian）地壳远位于塔斯曼线（Tasman line）以东。本研究结果不支持以往提出的新元古代至寒武纪洋壳假说，反而表明汤姆森造山带的基底为大陆地壳。铪同位素数据显示，随着时代推进，古老地壳源区对花岗岩岩浆的贡献逐渐降低，整体呈现出同位素组成更趋于年轻的趋势。拉克伦造山带（Lachlan Orogen，ε_Hf(t) = –13 至 +15）、汤姆森造山带（ε_Hf(t) = –14 至 +5）以及德拉梅里安造山带（Delamerian Orogen，ε_Hf(t) = –7 至 +4）的铪同位素特征存在显著差异，凸显了这些造山带下方地壳基底的年龄结构存在横向变化。