## 4 Results
### Zircon U-Pb Ages  
Table 3 lists the zircon U-Pb isotopic results. Among the light gray-white, fresh, unaltered granites and diorites (P22-39 TW01, P20-25 TW01, P25-08 TW01, P25-12 TW, P21-32 TW01, and P7-65 TW01), zircons are predominantly elongate (100-300 \(\upmu\)m in length with aspect ratios concentrated between 2:1 and 3:1) and transparent to light yellow in color. The crystal surfaces are bright and clear, with straight edges and pronounced rhythmic bands, consistent with the characteristics of magmatic zircons.  
The muscovite monzogranite (P22-39 TW01) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.0070-0.4316 (average = 0.0741; \(<\)0.1), with an abnormal Th/U ratio, indicating that these magmatic zircons crystallized in a magma with a special composition [19; 20; 21]. The weighted average age obtained from 14 points is 214.0 \(\pm\) 1.2 Ma (MSWD = 0.79, n = 14; Figure 6a).  
The mica monzogranite (P20-25 TW01) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.0042-1.0226 (average = 0.4486; \(>\)0.4). A strong positive correlation is present between Th and U, indicating that they are typical magmatic zircons [19; 20; 21]. The weighted average age obtained from nine points is 216.6 \(\pm\) 1.6 Ma (MSWD = 0.47, n = 9; Figure 6b).  
The biotite granodiorite (P25-08 TW01) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.2413-0.5179 (average = 0.2913), indicating that the Th/U ratios are related to the Th and U contents of the magma and the distribution coefficient between the zircon and magma [19; 20; 21]. CL images show clearly developed oscillating bands (Figure 7a), which are characteristic of magmatic zircons. The weighted average age obtained from 19 points is 212.71 \(\pm\) 0.89 Ma (MSWD = 0.93, n = 19; Figure 6c).  
The biotite monzogranite (P25-12 TW) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.1930-0.6078 (average = 0.3095), indicating that the Th/U ratios are related to the Th and U contents of the magma and the distribution coefficient between the zircon and magma [19; 20; 21]. CL images show oscillating bands (Figure 7b) typical of magmatic zircons. The weighted average age obtained from 19 points is 214.7 \(\pm\) 1.0 Ma (MSWD = 1.10, n = 19; Figure 6d).  
The tonalite (P21-32 TW01) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.4416-0.8648 (average = 0.5938; \(>\)0.4), with a strong positive correlation between Th and U, indicating that they reflect magmatic crystallization [19; 20; 21]. The weighted average age obtained from eight points is 209.3 \(\pm\) 2.8 Ma (MSWD = 0.45, n = 8; Figure 6e).  
The mica granite (P7-65 TW01) has \({}^{232}\)Th/\({}^{238}\)U ratios of 0.0043-0.1461 (average = 0.0897; \(<\)0.1), with an abnormal Th/U ratio, indicating that these zircons crystallized in a magma with a special composition [19; 20; 21]. The weighted average ages obtained from 19 points is 205.99 \(\pm\) 0.79 Ma (MSWD = 0.36, n = 19; Figure 6f).  
Figure 6: Concordia diagrams for granite samples from the Dahongliutan pluton. (**a**) P22-39 TW01, (**b**) P20-25 TW01, (**c**) P25-08 TW01, (**d**) P25-12 TW, (**e**) P21-32 TW01, and (**f**) P7-65 TW01.  
Figure 7: Cathodoluminescence images of selected (**a**) biotite granodiorite (sample P25-08 TW01) and (**b**) biotite monzogranite zircons (sample P25-12 TW). Solid circles indicate analytical spots used for U–Pb dating.