Mercury isotope trace magma mixing and crust-mantle interactions in the Yidun arc, eastern Tibetan Plateau

Magma mixing between mafic and felsic melts is widespread in open magmatic process. However, tracing the magma sources of different endmembers is challenging, because elemental and isotopic information of different endmembers commonly achieved equilibrium during magma interactions. Mantle and crustal reservoirs show distinct signatures of mercury (Hg) isotope mass-independent fractionation, making Hg isotope an emerging tool to trace mantle- and crustal-derived magmas. Here we report the Hg isotope data of two types (Type-I and Type-II) of mafic microgranular enclaves (MMEs) and their host granitoids, which have similar whole-rock Sr-Nd and zircon Hf isotope composition, from the Daocheng-Cuojiaoma batholith, Eastern Tibetan Plateau, SW China. Zircon U-Pb dating indicates both the host granitoids and two types of MMEs formed coevally at ca. 216 – 217 Ma, coherent to the subduction of Garzê–Litang ocean (a branch of Paleo-Tethys ocean). The host granitoids are metaluminous to weakly peraluminous characteristics (A/CNK = 0.98 – 1.05) and exhibit negative to slightly positive ∆¹⁹⁹Hg values (-0.2 to 0.02 ‰), indicating their source magma was a mixture of terrestrial sediments- and mantle-derived melts. Type-I MMEs display arc-like trace element patterns, low SiO₂ (53.8 to 55.0 wt%) and positive ∆¹⁹⁹Hg values (0.00 to 0.10 ‰), indicating their derivation from a subduction-related fluid/melt metasomatized mantle source. Type-II MMEs show intervening concentrations of major/trace elements, and intermediate ∆¹⁹⁹Hg values (-0.18 to 0.02), suggesting they were generated via mixing between the temporally and spatially coexisting first two magmas (i.e., type-I MMEs and granitoid). This study demonstrates the powerful use of Hg isotope for understanding magma sources and crustal-mantle interactions.