# 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  
(1) Genesis of Granites  
The first modern geochemical classification scheme for granitic rocks was proposed by [PERSON] and [PERSON] [39], distinguishing I-type and S-type granites. I-type granites are metaluminous to weakly paraluminous, with relatively high sodium and a wide range of SiO\({}_{2}\) contents (55-77%), suggesting their formation from mafic metamorphic igneous rocks. In contrast, S-type granites are strongly peraluminous, with relatively high potassium and higher SiO\({}_{2}\) contents (64-77%), indicating their derivation from the partial melting of metasedimentary rocks. This classification assumes that granitic rocks can be easily distinguished by their source materials. However, some granites may form from the partial melting of multiple sources, including both crustal and mantle materials. TheGejiu granites exhibited Nb/Ta ratios similar to those of highly evolved, mineralized granites [33]. A-type granites are a kind of granitic rocks that are rich in alkali (K\({}_{2}\)O + Na\({}_{2}\)O) and have a high Fe/Mg ratio and low water content. The geochemical characteristics of A-type granites are usually characterized by high silicon content: SiO\({}_{2}\) is typically \(>\)70%, but lower than highly differentiated I/S-type granites (such as haplogranite); low aluminum saturation: A/CNK is mostly \(<\)1.1, belonging to sub-aluminous to weakly peraluminous, which is significantly different from S-type (peraluminous) granites; rich in alkali (Na + K): K\({}_{2}\)O + Na\({}_{2}\)O \(>\) 8%, and the K\({}_{2}\)O/Na\({}_{2}\)O ratio is variable (can be \(>\)1 or \(<\)1). The study area is composed of a shoshonite series (gabbro, diorite, syenite, and monzonite) and high-silica granites (with aplitic granite--highly differentiated characteristics, such as the Laocang and Gaosong rock bodies). It should be noted that the major element composition and mineral assemblage of highly differentiated alkali feldspar granites tend to approach haplogranite [40], which may interfere with traditional genetic models. The Gejiu granites exhibited a wide range of acidity (SiO\({}_{2}\): 64.21-77.96 wt%, with an average of 72.94%) and displayed characteristics typical of I-type granites. For instance, in the P\({}_{2}\)O\({}_{5}\) vs. SiO\({}_{2}\) diagram, they show a negative correlation (correlation coeff r = \(-\)0.915), consistent with the evolutionary trend of I-type granites. This is because apatite is a preferentially crystallizing mineral in metaluminous-to-peraluminous and peralkaline granitic magmas, and the P content of the residual magma decreases with apatite fractional crystallization. Consequently, the P\({}_{2}\)O\({}_{5}\) content of I-type granites decreases with increasing SiO\({}_{2}\) content [41; 42; 43]. However, these granites had high total alkali contents (average K\({}_{2}\)O + Na\({}_{2}\)O value of 8.50, greater than 7.5%). The A/CNK values ranged from 0.87 to 1.96 (average: 1.07). They were metaluminous to weakly peraluminous (except for two samples from the Jiasha and Gaosong areas, which were strongly peraluminous). According to the classification (I-type/A-type granites A/CNK \(<\) 1.1, S-type  
Figure 8: (a) Sr/Y. (b) Nb/Ta. (c) Zr/Hf versus SiO\({}_{2}\) plots for the Jiejiu granitoids, with dashed lines indicating the inflection points of the evolution trend. granites A/CNK \(>\) 1.1 [42]), The Gejuń granites mainly exhibit the characteristics of A-type and I-type. Moreover, most Geju granites (except those from Longchahe and Shenshui areas) exhibited low contents of Zr + Nb + Y + Ce (\(<\)350 ppm) and relatively low 10,000 \(\times\) Ga/Al ratios. Therefore, it suggests a type A and type I hybrid composition for the granites of the Geju complex. The TsatZr values of the Laochang and Gaosong area granites are relatively low (with an average of 748.35 degrees), which is related to the highly differentiated characteristics of the granites in the two regions.