## 4 Discussion
### Constraints of Oxygen Isotope on Causes of Alteration  
Many studies have identified that the whole-rock \(\delta^{18}\)O values of the granites in the study area are generally stable at 8.02-10.8% after testing the oxygen isotopes of granites of different periods in the northern Tianshan Mountains. In the process of hydrothermal alteration, owing to inhomogeneities of the fluid channel, duration, and water-rock ratio in different parts of the rock, the degree of hydrothermal alteration differed, resulting in a difference in the oxygen isotope composition of the altered rocks at different scales [54]. The oxygen isotope test on the altered granites with different buried depths in the study area revealed that some altered granites have a low \(\delta^{18}\)O value. Previous studies found that clay minerals from hydrothermal alteration have the smallest \(\delta^{18}\)O values, generally 2-14%, whereas clay minerals produced by surface weathering have moderate \(\delta^{18}\)O values, usually 15-19% [55,56]. This finding also verifies that most of the samples with low \(\delta^{18}\)O values were from deeper parts of the study area (i.e., 600-700 m below the surface), while the samples obtained at the surface have normal \(\delta^{18}\)O values. Glacial meltwater,  
Figure 5: Dark enclaves in surface granite near the water conveyance tunnel. groundwater, atmospheric precipitation, and other fluids with low \(\delta^{18}\)O values can infiltrate along tectonic channels. When the conditions required for water-rock reaction are satisfied, fluids with low \(\delta^{18}\)O values react with the surrounding rock mass, thereby reducing the \(\delta^{18}\)O value of the rock within a certain depth range [57; 58]. However, some rock belts with high \(\delta^{18}\)O values appear above the granite belt with low \(\delta^{18}\)O values. This reflects continued exchange between hydrothermal fluids and the upper granite in the subsequent rising process that results in the growth of the \(\delta^{18}\)O value, resulting in the appearance of altered rocks with high \(\delta^{18}\)O values in the upper part. It also indicates that the deep altered rocks are related to atmospheric precipitation, and that they might also be caused by a deep hydrothermal metamorphism [59; 60]. As shown in Figure 6, only the \(\delta^{18}\)O value of the granodiorite in the samples shows a notable trend of change with the LOI, which decreases with increase in the LOI, and some granodiorites have the characteristic of a low \(\delta^{18}\)O value (1.9-3.6%). Analysis of the major elements revealed that the altered granodiorite with deeper burial depth has a larger LOI (2.76-9.23%) (Figure 7), indicating that the altered granodiorite within the study area is mainly caused by meteoric water and magmatic hydrothermal alteration, although some granodiorite caused by meteoric water hydrothermal alteration has low \(\delta^{18}\)O characteristics (1.9-3.6%). Through consideration of the geological history and the \(\delta^{18}\)O value characteristics of the study area, it can be inferred that in the main part of the late Paleozoic early Variscan subcycle, under the influence of plate movement, geosynclinical folding uplift occurred, accompanied by large numbers of magmatic intrusions, which formed large quantities of granodiorite. After the tectonic movement of the middle Variscan subcycle, the ancient Tianshan Mountains were formed, accompanied by frequent intermediate-acidic magmatic activities. The previously formed low-\(\delta^{18}\)O-value granodiorite was remelted, assimilated, and rebalanced by the magmatic hydrothermal fluid to form new granite. The samples of altered monzonitic granite with large buried depth have LOI values that are not high (0.6-2.33%), but they also have low \(\delta^{18}\)O characteristics (2.6-5.2%), and the \(\delta^{18}\)O values of the monzonitic granite, which are higher than the \(\delta^{18}\)O values of the granodiorite, have no obvious trend of change with the LOI values.