Scott E. Bryan, Alex. G. Cook, Charlotte M. Allen, Coralie Siegel, David J. Purdy, James S. Greentree and I. Tonguc Uysal. Episodes 2012;35:142-52. https://doi.org/10.18814/epiiugs/2012/v35i1/013

The generation of continental crust, its bulk composition and temporal evolution provide important records of plate tectonics and associated magma-generating processes. However, the long-term integrated effects of repeated magmatic events on crustal growth, composition and differentiation and, therefore, on crustal evolution are rarely considered. Here, we examine long-term (∼350 Myr) temporal compositional trends of granitic magmatism within a limited (∼200 km × 100 km) area in the Northern New England Orogen of Queensland, Australia to avoid lateral crustal variations in order to understand how temporal–compositional variations of silicic igneous rocks record crustal evolution. Long-term temporal compositional variations are tracked using whole-rock chemistry, zircon chronochemistry and zircon Hf isotopic compositions. We particularly focus on whole-rock U, Th and K abundances and calculated heat-production values as proxies for crustal evolution, and tracking crustal sources involved in granitic magmatism. We identified two major compositional groupings within the study area that were repeatedly produced over time: compositional Group 1 comprises voluminous I-type igneous rocks emplaced during the Permo-Carboniferous and Early Cretaceous; Group 2 represents mainly lower volume A-type igneous rocks of Triassic, Middle Cretaceous and Tertiary age. Importantly, these compositional groupings alternate over the 350 Myr history of granitic magmatism within the study area. Heat-production values over time exhibit a zigzag pattern and mirror zircon Hf isotopic signatures where rocks with elevated heat-production values exhibit unradiogenic (crustal) Hf isotopic compositions. We identify the composition of crustal sources, level of the crust undergoing partial melting, scale of magmatism and source crustal volume as important factors in understanding the compositional diversity of silicic igneous rocks. We interpret the two chemical groupings to reflect the following magma-generating conditions: Group 1 igneous rocks record large-scale magmatic systems triggered by extensive crustal melting of multiple lower to middle crustal sources, which produce more compositionally and isotopically uniform magma compositions that approach bulk crustal compositions. In contrast, Group 2 igneous rocks reflect smaller-scale magmatic systems generated from smaller-scale partial melting events of the middle to upper crust that produced A-type magmas. Over the long term, the successive large-scale magmatic events (recorded by Group 1 igneous rocks) through their concomitant basaltic underplating make the Hf composition of the lower crust more radiogenic, and tend to homogenize the isotopic composition of the continental crust. We consider three important coupled controls: (1) promotion of extensive crustal melting by large-scale magmatic systems, potentially blending multiple crustal sources that can also include a significant juvenile source contribution; (2) melt depletion, whereby older, and potentially more unradiogenic crustal materials become more refractory; (3) ‘crustal jacking’, where mantle-derived magmas are added as underplate to the crust (i.e. basification) and can shift older crustal materials to more shallow levels (potentially in concert with erosion and exhumation) and away from zones of crustal melting. Our findings highlight the importance of integrating the geological and intrusive history with whole-rock geochemical data and isotopic information, and have direct implications for continental regions that exhibit protracted igneous histories and where isotopic compositions may trend towards more juvenile compositions such as circum-Pacific or retreating accretionary orogens.

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.

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.