## 2 Geological Setting and Field Relationships  
The Tibetan Plateau consists of several northward accretionary blocks, from north to south: the Qaidam, Songpan-Ganze, Qiangtang, Lhasa, and Himalaya terranes [33]. The northern part of the Lhasa terrane is bounded by the Bangong-Nujiang Suture Zone (BNSZ), and the IYZSZ is to the south (Figure 1a). The Lhasa terrane is divided into northern, central, and southern parts from north to south by the Shiquan River-Nam Tso Melange Zone (SNMZ) and the Luobadui-Milashan Fault (LMF) (Figure 1a) based on the different sedimentary sequences and basements [34].  
The IYZSZ, extending approximately 2000 km from east to west, is the remnant of the Neo-Tethys oceanic crust and mantle, formed during the India-Eurasia collision. It represents the final suture zone between the Indian plate and Eurasia, marking the closure of the Neo-Tethys in the Paleogene [35; 36; 37]. The Lhasa terrane to the north is mainly composed of Paleozoic to Cretaceous marine sedimentary strata. Subsequently, it was intruded and transformed by large-scale calc-alkaline intrusive and volcanic rocks, known as the Gangdese arc.  
Abundant zircon U-Pb geochronological data have shown that the age of the Gangdese magmatic belt ranges from Late Triassic (220 Ma) to late Miocene (10 Ma) [32; 38], due to the northward subduction of the Paleo- to Neo-Tethys oceanic lithosphere [39; 40; 41].  
The study area is located in western Zedong, southern Tibet, which is part of the eastern section of the IYZSZ. It is mainly composed of a suite of island arc rock assemblage (K\({}_{2}\)) and ophiolite. The ophiolite extends approximately 20 km from east to west, with Late Triassic (T\({}_{3}\)) flysch sedimentary facies to the south, representing the passive continental margin of the northern Indian plate (Figure 1b). The Sanggeda mafic intrusion consists  
Figure 1: (**a**) Simplified tectonic map of the Tibetan Plateau (modified from [34]), showing the location of the Lhasa terrane and the study area. (**b**) Detailed geological map of the Zedong ophiolite at the eastern of the IYZSZ (modified from [35]). (**c**) Geological cross-section AA\({}^{\prime}\) shows the contact relationships between the Sanggeda gabbro and the Middle Cretaceous volcanic rocks. Abbreviations are as followings: JSSZ = Jinshajiang Suture Zone, BNSZ = Bangong–Nujiang Suture Zone, SNMZ = Shiquan River–Nam Iso Melange Zone, LMF = Luobadui–Milashan Fault, IYZSZ = Indus–Yarlung Zangbo Suture Zone, SL = southern Lhasa subterrane, CL = central Lhasa subterrane, and NL = northern Lhasa subterrane.  
of fine-grained hornblende gabbro and porphyritic hornblende gabbro, both containing centimeter-sized clots of amphiboles or hornblendite enclaves. The country rocks surrounding the gabbros are Middle Cretaceous volcanic rocks and Zedong ophiolite, while the main body of the intrusion remains unexposed (Figure 1c). Based on the outcrops, it is inferred that the intrusion extends several kilometers from Sanggeda Village in the south to the southern foothill of jiasha in the north, and approximately 1 km from east to west (Figure 1b). The gabbros display a faulted contact with the Middle Cretaceous andesites (Figure 2a) but contain andesitic xenoliths and roof pendants, indicating an intrusive contact with the volcanic rocks, primarily andesite. Additionally, clear intrusive contact relationships with Zedong ophiolite can be observed (Figure 2b,g,h). The contact margin along the intrusion side exhibits a fine-grained texture, while the ophiolite side shows strong serpentinization and schistosity (Figure 2b,g). The contact boundary between the fine-grained hornblende gabbro and porphyritic hornblende gabbro is sinuous (Figure 2c). Hornblendite cumulate enclaves are observed in both fine-grained and porphyritic gabbros (Figure 2d-f).  
Figure 2: _Cont._