# Genesis and Magmatic Evolution of the Gejiu Complex in Southeastern Yunnan, China
## 4 Results
### Geochemistry
#### 4.2.1 Major Element Geochemistry  
The major element compositions of the samples are presented in Table 1. The Gejiu granites exhibited significant spatial heterogeneity in their major element compositions. In the western sector, the Falang, Jiasha, and Shenxianshui granites displayed relatively homogeneous geochemical characteristics: the Falang granite showed Na\({}_{2}\)O/K\({}_{2}\)O ratios of 0.53-0.67, Rittmann index (\(\sigma=(\)K\({}_{2}\)O + Na\({}_{2}\)O\()^{2}/(\)SiO\({}_{2}\) \(-\) 43)) values of 2.30-2.90, and aluminum saturation indices (A/CNK = Al\({}_{2}\)O\({}_{3}\)/(K\({}_{2}\)O + Na\({}_{2}\)O + CaO)) of 0.96-1.04; the Jiasha granite had a Na\({}_{2}\)O/K\({}_{2}\)O ratio of 0.56, \(\sigma\) values of 1.77-2.43, and A/CNK values of 1.10-1.16; and the Shenxianshui granite was characterized by Na\({}_{2}\)O/K\({}_{2}\)O ratios of 0.64-0.68, \(\sigma\) values of 2.30-2.53, and A/CNK values of 0.89-1.05. These consistent geochemical signatures suggest a common magmatic source and evolution dominated by fractional crystallization. In contrast, the Longchahe granite in the same sector exhibited broader compositional variations, with Na\({}_{2}\)O/K\({}_{2}\)O ratios ranging from 0.27 to 0.54, \(\sigma\) values of 3.36-6.85, and A/CNK values of 0.87-1.01, indicating complex differentiation processes. In the eastern sector, the Laochang and Gaosong granites showed distinct alteration features. The Laochang granite was marked by high loss on ignition (LOI = 1.03-2.29 wt%), Na\({}_{2}\)O/K\({}_{2}\)O ratios of 0.57-0.82, \(\sigma\) values of 1.81-2.19, and A/CNK values of 1.01-1.05. Notably, its low TiO\({}_{2}\) content (0.06-0.07 wt%) may reflect titanium depletion during chloritization. The Gaosong granite, which was affected by intense chloritization, exhibited an extremely low alkali content (total alkalis = 3.76 wt%), with a Na\({}_{2}\)O/K\({}_{2}\)O ratio of 0.05, \(\sigma\) value of 0.4, and a high A/CNK value of 1.96. Its significantly high LOI (3.07 wt%) further corroborates pervasive hydrothermal alteration. These spatial variations highlight the interplay between primary magmatic processes and post-magmatic fluid interactions in shaping the geochemical diversity of the Gejiu granites. In the A/CNK-A/NK diagram, the samples predominantly display characteristics of metaluminous rocks, typically showing a positive correlation with SiO\({}_{2}\) (Figure 4b). On the TAS diagram, these samples are shown to be mainly distributed within the granite field (Figure 4a). In Harker variation diagrams, Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), Fe\({}_{2}\)O\({}_{3}\), MgO, MnO, CaO, and P\({}_{2}\)O\({}_{5}\) can be seen to decrease with increasing SiO\({}_{2}\), while Na\({}_{2}\)O remained relatively constant (Figure 5), indicating the fractional crystallization of ferromagnesian minerals, plagioclase, Ti-Fe oxides, and apatite. Figure 4: (**a**) TAS diagram. (1. Dunite-bearing gabbro. 2a. Alkaline gabbro. 2b. Sub-alkaline gabbro. 3. Gabbro-diorite. 4. Diorite. 5. Granoidrite. 6. Granite. 7. Quartz porphyry. 8 Svenite-bearing gabbro. 9. Svenite-bearing diorite. 10. Syenite. 11. Quartz serine. 12. Monzonite. 13. Alkali feldspar-bearing gabbro. 14. Alkali feldspar-bearing serine-monzonite. 15. Alkali feldspar-bearing serine-monzonite. 16. Alkali feldspar-bearing monzonite. 17. Feldspathoid plutonic rock. 18. Feldspathoid rock). (**b**) A/CNK vs. A/NK diagram. (**c**) K\({}_{2}\)O vs. SiO\({}_{2}\) diagram [31]. The gabbro samples exhibited a wide range of total alkali (ALK = K\({}_{2}\)O + Na\({}_{2}\)O) contents, varying from 2.37 to 9.85 wt%. The Na\({}_{2}\)O/K\({}_{2}\)O ratios ranged from 0.44 to 1.52, with most values below 1. The A/NK values ranged from 1.87 to 5.30, and the aluminum saturation index (A/CNK = Al\({}_{2}\)O\({}_{3}\)/(CaO + Na\({}_{2}\)O + K\({}_{2}\)O)) ranged from 0.51 to 1.33. The Rittmann index (\(\sigma\)) was relatively high, ranging from 1.53 to 20.11. In the A/CNK-A/NK diagram, the samples exhibit characteristics of metaluminous-to-peraluminous rocks (Figure 4b).