## 2 Regional Geological Background  
The Lhasa Terrane, also referred to as the Gangdese Belt, is a major magmatic-tectonic belt bounded to the south by the Indus-Yarlung Zangbo Suture Zone (IYS) and to the north by the Bangong-Nujiang Suture Zone (BNS). It extends approximately 2500 km in an east-west direction, with a width of 100-300 km from north to south. The Southern Lhasa Terrane is bounded to the north by the Luobadui-Milashan Fault (LMF) and to the south by the Yarlung Zangbo Suture Zone (YZSZ) [16]. It represents the region of most intense magmatic activity, characterized by the east-west-trending Gangdese batholiths, which extend for over 1500 km. The Southern Lhasa Terrane, as the southernmost tectonic unit of the Eurasian continent, has been interpreted as a Mesozoic Andean-type active continental margin, formed by the prolonged northward subduction of the Neo-Tethys Ocean. Ithas also been regarded as the leading edge of the Cenozoic India-Eurasia collisional orogen [17; 18]. The Early-Middle Jurassic Yeba Volcanic Arc investigated in this study is primarily situated within the southern Gangdese magmatic belt (Figure 1a). The study area lies within the Southern Lhasa Terrane, part of the Lhasa-Chayu Stratographic Subzone [16]. The stratigraphic sequence spans from the Jurassic to the Paleogene systems, with the Jurassic and Cretaceous strata being predominant. The exposed stratigraphic units include the Lower-Middle Jurassic Yeba Group (J\({}_{1}\)-2y), the Upper Jurassic Duodigou Formation (J\({}_{3}\)d), the Upper Jurassic-Lower Cretaceous Linbuzong Formation (J\({}_{3}\)K\({}_{1}\)l), the Lower Cretaceous Chumulong Formation (K\({}_{1}\)ch), the Lower Cretaceous Taken Formation (K\({}_{1}\)t), the Upper Cretaceous Shexing Formation (K\({}_{2}\)s), the Paleocene Dianzhong Formation (E\({}_{1}\)d), and the Pana Formation (E\({}_{2}\)p). These stratigraphic units exhibit pervasive deformation, characterized by a dominant NWW-trending structural orientation (Figure 1b). The volcanic rocks of the Lower-Middle Jurassic Yeba Formation (J\({}_{1}\)-2y) are mainly distributed in the area between Dagze, Maizhokunggar, and Gongbo'gyamda [19; 20]. The Yeba Formation (J\({}_{1}\)-2y) comprises a polygenic volcanic-sedimentary-metamorphic complex, dominated by intermediate-acid volcanic rocks including dacite, andesite, and tufaceous to basaltic volcanic breccia lavas. Tuff interlayers are widely developed, with localized occurrences of mafic (basalt) to felsic (rhyolite) volcanic rocks. The alternating pyroclastic rocks (e.g., volcanic breccia) and lavas reflect cyclic alternations between explosive and effusive phases of volcanism. Additionally, the formation is intercalated with sedimentary-metamorphic strata, including limestone, clastic rocks, and tectonically overprinted phyllite/schist. Notably, limestone blocks are mainly concentrated in the central part of the formation [21]. Duodigou Formation (J\({}_{3}\)d): Predominantly composed of shallow-marine platform carbonates interbedded with minor clastic rocks, this formation exhibits well-developed superposed folds. It is tectonically juxtaposed against the underlying Yeba Formation along a thrust detachment fault system. Linbuzong Formation (J\({}_{3}\)K\({}_{1}\)l): Characterized by a lithological assemblage of silty slate, carbonaceous slate intercalated with quartz sandstone, and calcareous sandstone. Similarly, superposed folds are prevalent within this formation. It maintains conformable contact with the underlying Duodigou Formation. Chumulong Formation (K\({}_{1}\)c): Composed primarily of shoreface-facies clastic rocks that have undergone low-grade metamorphism. Takena Formation (K\({}_{1}\)t): Dominated by carbonate rocks such as limestone and marl, interbedded with sandstone and mudstone/shale. It maintains conformable contacts with the underlying Chumulong Formation and the overlying Shexing Formation. Shexing Formation (K\({}_{2}\)s): Consists of purplish red continental siliciclastic rocks, including quartz sandstone, siltstone, and mudstone.