## 5 Discussion
### Paleoproterozoiczoic Tectonothermal Event  
Zircon, a ubiquitous accessory mineral in various rock types, is highly valued for its exceptional resistance to alteration, significant U-Th content, and remarkably slow Pb diffusion rates [76; 77]. These characteristics make it an ideal geochronometer for preserving precise records of geological events [78]. Even when subjected to multiple magmatic and high-grade metamorphic events, zircon maintains its primary geochemical and isotopic signatures, rendering it particularly valuable for investigating ancient tectonic evolution [79; 80; 81; 82; 83]. The mineral's extremely low \({}^{176}\)Lu/\({}^{177}\)Hf ratio (resulting from high Hf but negligible Lu content) ensures its measured \({}^{176}\)Hf/\({}^{177}\)Hf ratio accurately reflects the initial Hf isotopic composition at formation [84]. Modern MORB (representing asthenospheric mantle) has \(\epsilon_{\rm Hf}=+13.9\)[85]. During mantle differentiation, crustal materials develop lower Lu/Hf ratios than depleted mantle, causing their \({}^{176}\)Hf/\({}^{177}\)Hf to evolve more slowly. Consequently, crustal \(\epsilon_{\rm Hf}\)(t) values become increasingly negative over time, while mantle values trend positive. Positive zircon \(\epsilon_{\rm Hf}\)(t) values suggest protolith formation involved mantle-derived or ancient crustal components, whereas negative values indicate predominantly crustal sources [86; 87]. Metamorphic zircon can originate through substitution reactions that take place under subsolidus conditions, which [PERSON] and [PERSON] (2000) [88] termed \"solid-state recrystallization\", or alternatively, through crystallization from melts or fluids. Crystallization of zircon has been documented to occur during prograde [89; 90], peak [88; 91], as well as retrograde [92; 93]\(P\)-\(T\) conditions. In metamorphic rocks, various patterns of Hf isotope homogenization have been documented, ranging from partial to complete, which may  
Figure 8: Hypothetical model for the genetic mechanism of the metamorphism and anatexis of biotite schists and granitic greisses in the Sulu orogenic belt (modified after [PERSON] et al., 2024) [75]. (**A**) Subduction–collision stage (\(\sim\)245–215 Ma): Subduction of the Yangtze Craton continental crust beneath the North China Craton; (**B**) Slab break-off stage (\(\sim\)215 Ma): The Yangtze Craton crust reaches maximum depth, undergoing ultrahigh-pressure (UHP) metamorphism; (**C**) Post-collision stage (\(\sim\)215–205 Ma): Exhumation of the deeply subducted and UHP-metamorphosed Yangtze Craton crust, accompanied by retrograde metamorphism and anatexis. result from the dissolution of relic grains and the subsequent formation of new zircon grains [94,95]. Alternatively, significant variability in Hf isotope compositions has also been observed [96,97]. This becomes especially significant in cases where other minerals have undergone extensive retrograde metamorphic overprinting, resulting in the loss of their primary information. The ancient basement of the YC was established during the Archean era, with rock exposures dating back to 3.3-2.5 Ga sporadically distributed across the region [3,98]. Multiple areas within the YC preserve evidence of tectonothermal events at approximately 2.5 Ga. For instance, zircon U-Pb ages of 2493 \(\pm\) 19 Ma have been obtained from TTG gneiss of the Zhangbaling Group in the Nanhuang area of Jiashan [99]. In the Dabie orogenic belt, zircon analyses of Shuanghe gneiss yield an upper intercept age of 2458 \(\pm\) 76 Ma [100]. [PERSON] et al. (2002) [101] similarly reported an upper intercept age of 2489 \(\pm\) 25 Ma from eclogite samples collected from the same region. Detrital zircon age spectra from the northern YC display a significant peak at \(\sim\)2.49 Ga. As demonstrated by [PERSON] et al.