Early Eocene leucogranite dikes in the Gangdese batholith, southern Tibet: Constraints on transcrustal magmatic systems

High-silica granites are important constituents of the continental crust, yet their intracrustal evolutionary processes, which are critical to understanding continental crustal growth and evolution, remain poorly understood. To contribute to this topic, we present a comprehensive study integrating field, petrological, mineralogical, and geochemical analyses, with thermodynamic simulations, focusing on the early Eocene leucogranite dikes of the Nymo intrusive complex of the Gangdese batholith, southern Tibet. These high-K, high-silica (SiO2 = 67.73–76.89 wt%) rocks crystallized at 50–45 Ma. Their juvenile isotopic signatures [zircon εHf(t) = +3.74 to +14.57; whole-rock εNd(t) = +4.88 to +6.68] closely match those of the coeval Nymo intermediate-mafic intrusive complex [zircon εHf(t) = +5.03 to +13.14; whole-rock εNd(t) = +2.74 to +6.49], suggesting a potential genetic relationship. Petrography and backscattered electron imaging reveal complex plagioclase zonation, indicative of prolonged magmatic evolution. Geochemical trends and thermodynamic modeling indicate that both mafic and felsic rocks of the Nymo intrusive complex originated from a shared magma reservoir via successive melt extraction. The leucogranites represent the terminal products of extensive fractional crystallization from mantle-derived parental magmas. We propose that the Nymo leucogranite dikes likely represent the shallow expression of a fully developed transcrustal magmatic system. In this preferred model, their formation can be plausibly attributed to crystal mush dynamics involving (1) recurrent magmatic recharge, (2) vertically focused melt transport, and (3) incremental melt segregation. The final emplacement as dike swarms was likely facilitated by crustal shortening and fracturing during the early Eocene India-Asia collision. This study highlights the potential role of transcrustal magmatic systems in generating evolved granites and contributing to continental crustal growth and stratification during orogeny.