Crustal Evolution in the New England Orogen, Australia: Repeated Igneous Activity and Scale of Magmatism Govern the Composition and Isotopic Character of the Continental Crust
Authors: Siégel C , Bryan S E , Allen C M , Gust D A , Purdy D J 
Published: July 2020

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.