Genesis of strongly peraluminous granites facilitates nitrogen retention in the crust

Abstract

Sediments/sedimentary rocks are a major sink of atmospheric N₂, which is mainly fixed into diazotrophic biomass and recycled into other organic matters or diagenetically transferred (in the form of NH₄⁺) into the crystal structures of phyllosilicate minerals in sediments. Driven by tectonic activities, sedimentary rocks may experience various degrees of metamorphism with some eventually being melted to form S-type granitoids. The destiny of sedimentary N during these processes, i.e., retained in the crust or returned to the atmosphere, determines the long-term N evolution in the crust and the atmosphere, which profoundly impacts not only volatile properties in the lithosphere but also the surface climatic and environmental conditions. Previous studies have reported various extents of N loss (from very subtle to up to 70 %) from regional meta-sedimentary rocks. However, detailed studies on the N-losing mechanism during crustal anatexis and S-type granitoid formation are rare. Here, we examined the Permian–Triassic strongly peraluminous granites (SPGs) in the Diancangshan-Ailaoshan area in SW China, which were formed by extensional decompression melting of Precambrian pelitic sediments in the upper–middle crust within a back-arc basin triggered by slab rollback of the subducted Paleo-Tethyan oceanic crust. The results show that the SPGs have a N-content range of 6.8 ppm to 58.2 ppm (mean = 37.9 ± 11.3 ppm; 1σ; n = 19). The δ¹⁵N values of the samples mostly cluster between 0.0 ‰ and + 3.1 ‰ with 3 samples showing higher values of + 4.2 ‰, +5.2 ‰ and + 7.5 ‰ (mean = +2.4 ± 1.7 ‰; 1σ; n = 19). For comparison, one altered sample has much higher N content of 128.7 ppm with a δ¹⁵N value of + 0.1 ‰. Compared with Precambrian sedimentary rocks, these SPGs contain statistically at least an order of magnitude less N. Data modeling suggests that decompression-induced magmatic N₂ degassing preponderates metamorphic N devolatilization for the major N loss from the Precambrian sedimentary rocks, which were also observed to control the N signatures of sediment-derived PGs in subduction-zone settings. Further compilation of the N contents of global granitoids show that SPGs contain 2–4 times higher N than non-SPGs, suggesting better retention of crustal N in SPGs. This can be attributed to the crystallization of hydrous peraluminous minerals (micas in particular, which have higher N-hosting capacities) in the H₂O-rich strongly peraluminous melts. We further estimated the N inventory in global SPGs as 0.9^{± 0.5} × 10¹⁷ kg N, which accounts for 4–9 % of the N budget in the upper continental crust.