# Genesis and Magmatic Evolution of the Gejiu Complex in Southeastern Yunnan, China
## 5 Discussion
### Genesis of the Gejiu Complex
#### 5.2.1. Genesis of Granites and Granitic Enclaves
The average values of FeOt/MgO and (Na\({}_{2}\)O + K\({}_{2}\)O)/CaO in the two regions are 5.24, 2.65, and 8.14, 2.98, respectively. It is worth noting that, when viewed comprehensively, the 13-10 sample from the Jiasha area (with an A/CNK value of 1.16, a FeOT/(FeOT + MgO) value of 0.86, and a \(\delta\)Eu value of 0.61) and the sample from the Gaosong area (with an A/CNK value of 1.16, a FeOT/(FeOT + MgO) value of 0.73, and a \(\delta\)Eu value of 0.06), respectively, also exhibit characteristics of S-type and strongly altered S-type granites. As mentioned earlier, the contemporaneous occurrence of abundant mantle-derived magmas highlights the significant contribution of mantle materials to the Geju granules [8]. However, high concentrations of trace elements such as Th (43.92, \(>\)10 ppm), Pb (78.72, \(>\)20 ppm), and U (12.37, \(>\)2 ppm) suggest that the formation of these granites involved crustal components, indicating a crustal signature [44]. Previous studies have used the Ce/Yb vs. Eu/Yb discrimination diagram to identify the genetic types of granites, proposing that a linear relationship indicates significant mixing characteristics [45]. The Geju granites exhibited an excellent linear relationship in this diagram (Figure 9a). Additionally, the presence of numerous mafic enclaves in the granites further supports the occurrence of magma mixing during their evolution [46]. [PERSON] (2005) [47] proposed three petrogenetic models for I-type granites: (1) the mixing of mantle-derived and crust-derived magmas; (2) the contamination of mantle-derived magmas by crustal materials [48]; and (3) the simultaneous melting of pre-existing amphibites (with basaltic protoliths) and metasedimentary rocks in the basement [43]. Considering the evidence for the occurrence of magma mixing during the evolution of the Geju granites, this study suggests that their formation involved the mixing of mantle-derived and crust-derived magmas. The formation of supergiant granitic bodies requires not only a substantial supply of materials but also significant heat. The additional heat generated by the melting of the lithospheric mantle is sufficient to cause partial melting of crustal rocks [10]. Studies [49; 50; 51] have shown that the initial magma temperatures of the Late Cretaceous granites in the Geju area ranged from 806 \({}^{\circ}\)C to 835 \({}^{\circ}\)C, indicating that the primary magmas of the Geju granules were products of high-temperature partial melting. The dehydration melting of biotite, i.e., the decomposition reaction of biotite, often generates substantial heat (-800 \({}^{\circ}\)C or higher). In the Pb-Ba discrimination diagram (Figure 9b), Pb is an order of magnitude lower than Ba, reflecting melts derived from biotite decomposition rather than low-temperature muscovite decomposition, as Pb is more likely to enter muscovite than biotite [52; 53]. Therefore, the primary magmas of the Geju granules were likely generated by the decomposition reaction of biotite. (2) Genesis of Granitic Enclaves  
The composition of the granitic enclaves in the Gejuń granites ranged between gabbro and granite (Figure 10). Various trends suggest that these enclaves result from the mixing of two magma components: gabbro-related mafic magmas and granitic magmas represented by other granite samples. Specifically, the Longchahe and Jiasha granitic enclaves were formed by the mixing of mantle-derived mafic magmas and peralkaline granitic magmas. The granitic enclaves exhibited relative enrichment of incompatible trace elements, such as Nb, which was present in higher concentrations in the enclaves compared to gabbro samples (Figure 10). This enrichment may be attributed to the lower degree of partial melting of the mantle-derived mafic magmas within the enclaves. Figure 10: SiO\({}_{2}\) versus trace elements of gabbro–granitic enclaves-granite in the Gejiu area. Figure 9: (**a**) Ce/YB-EU/Yb diagram of Gejiu [10,54]. (**b**) Whole-rock Ba-Pb diagram of Gejiu (The legend is consistent with that of Figure 8).