The role of polybaric crystallization in the construction of the Gangdese continental magmatic arc, South Tibet

The understanding of storage conditions and evolution processes of igneous rocks is pivotal in unraveling the architecture and dynamics of trans-crustal magmatic systems. Here we combine amphibole major and trace elements compositions, geothermobarometers and chemometry and published whole-rock compositions of the Paleocene-Eocene (65–40 Ma) Gangdese batholith from longitude 85° E to 95° E to discuss the nature and formation processes of the batholith, explore the across sectional structure of the large-scale paleo-continental magmatic arc batholith, and reveal the resultant implications for crustal formation and evolution. The pressure results suggest that most of the exposed 65–40 Ma Gangdese batholith was emplaced at 1–8 kbar, corresponding to the middle to upper crust. Specifically, these exposed intrusions were predominantly emplaced at 2–4 kbar, with deeper denudation observed toward the eastern part of the Gangdese arc. The calculated melts in equilibrium with amphiboles are more silicic (mainly rhyolitic) than their host rocks (dioritic to granodioritic), yet share similar geochemical features with those of the coeval Gangdese high-Si granites and Linzizong high-Si rhyolites. This feature suggests that the granitoid batholiths represent crystal mushes that experienced varying degrees of partial melt loss prior to solidification, and high-Si magmas are residual silicic melts extracted from the batholitic magma mushes. The geochemical evolutionary trend of the 65–40 Ma Gangdese batholith parallels the 1–8 kbar isobaric and polybaric experimental liquid lines of descent of hydrous basaltic magmas. The whole-rock geochemical data suggest that most mafic samples of the Gangdese batholith are not primitive basaltic magmas, but rather underwent fractional crystallization at depth prior to emplacement. Some samples even contain minor amphiboles cores derived from magma reservoirs in the deep crust. These observations indicate that most of magmas were mainly mantle-derived and evolved through polybaric crystallization processes. The shallower granitoid batholiths could represent frozen fossilized magma mushes, intricately linked with shallow volcanic eruptions, magmatic processes in the deep crust, or even melting in the mantle source. Despite crustal rock remelting and magma mixing can occur during batholith formation, polybaric differentiation processes in trans-crustal magmatic columns, saturating with distinct mineral assemblages at different depths, crucially shapes the chemically stratified continental crust.