Abstract
The growth and evolution of crustal-scale magmatic systems play a key role in the generation of the continental crust, the largest eruptions on Earth, and the formation of metal resources vital to our society. However, such systems are rarely exposed on the Earth’s surface, limiting our knowledge about the magmatic processes occurring throughout the crust to indirect geochemical and petrographic data obtained from the shallowest part of the system. The Hf isotopic composition of accessory zircon is widely used to quantify crust-mantle evolution and mass transfers to and within the crust. Here we combine single-grain zircon Hf isotopic analysis by LA-MC-ICP-MS with thermal modelling to one of the best-studied crustal-scale igneous systems (Sesia Magmatic System, northern Italy), to quantify the relative contribution of crustal- and mantle-derived magmas in the entire system. Zircons from the deep gabbroic units define a tight range of εHf (−2.5 ± 1.5). Granites and rhyolites overlap with this range but tail towards significantly more negative values (down to −9.5). This confirms that the entire system consists of hybrid magmas that stem from both differentiation of mantle-derived magmas and melting of the crust. Thermal modelling suggests that crustal melting and assimilation predominantly occurs during emplacement and evolution of magmas in the lower crust, although melt production is heterogeneous within the bodies both spatially and temporally. The spatial and temporal heterogeneity resolved by the thermal model is consistent with the observed Hf isotope variations within and between samples, and in agreement with published bulk-rock Sr–Nd isotopic data. On average, the crustal contribution to the entire system determined by mixing calculations based on Hf isotopic data range between 10 and 40%, even with conservative assumptions, whereas the thermal model suggests that this space- and time-averaged contribution does not exceed 20%. However, spatial and temporal variations in the crustal melt proportion (from 0 up to 80% as observed in the thermal model) may impart significant isotopic variability to different batches of magma observed on the outcrop scale, emphasizing the need to consider a magmatic system as a whole, i.e., by integrating all spatial and temporal scales, to more precisely quantify crustal growth vs. reworking.
Highlights
Silicic magmas in continental igneous systems chemically evolve through differentiation and/or contamination before they solidify to form plutons at different crustal levels and/ or erupt on the Earth’s surface (e.g., Hildreth 2004; Bachmann et al 2007; Cashman et al 2017)
In addition to the coherent set of Hf isotopes in zircon presented here, we argue that the second possibility is more likely because of (1) the close overlap of U–Pb ages of the pyroxenite body compared to other lower crustal units (Karakas et al 2019); and (2) the occurrence of distinctively more radiogenic isotopic composition of possible Permian mantle components in the area compared to the pyroxenite zircons
Igneous rocks of the Sesia Magmatic System (SMS) show a remarkable coherence of zircon Hf isotopic compositions when compared to local mantle and crustal components, consistent with the consanguineous relationships of the different units proposed based on single grain ID-TIMS age determinations (Karakas et al 2019)
Summary
Silicic magmas in continental igneous systems chemically evolve through differentiation and/or contamination before they solidify to form plutons at different crustal levels and/ or erupt on the Earth’s surface (e.g., Hildreth 2004; Bachmann et al 2007; Cashman et al 2017). The geochemistry of the resulting plutonic and volcanic rocks integrates a whole range of igneous processes that can occur from beneath the Moho to the surface. Constraining parameters such as the physicochemical state of magma bodies at different. The source of silicic magmas (whether dominantly reworked crustal material or fractionated mantle-derived mafic melts) has fundamentally different implications on the heat and mass transfer in the crust (e.g., Moyen et al 2021), and on the eruptive behaviour and style
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