## 5 Discussion  
### Mechanisms of Whole-Rock Element Migration  
The field identification of skarn types mainly relies on their structures that are similar to their protooliths. The banded structures in the outer skarn are closely related to the layered metamorphic sedimentary rock protolith, while the weakly altered inner skarn usually shows residual igneous rock patches [3]. However, some skarn have a spotty structure, which may affect the accuracy of visual identification. Therefore, distinguishing skarn types based on geochemical methods is more reliable [8]. Based on the whole-rock data in Section 4.1, it can be seen that most elements undergo significant changes during the evolution of skarn. From the inner skarn to the outer skarn, Dy, Ho, Er, Tm, Yb, Lu, Y, Zn and Sc show the same evolutionary trend, while Na, Al, K, Ti and Sr show a decreasing trend, and V, Ni, Fe, Mg and Zn show an increasing trend. Based on the major and trace element analysis of the granite, inner skarn, outer skarn and marble on the TS-10 skarn profile, we have discussed the migration regularities of the constant elements and trace elements in this profile. [PERSON] et al. (2009) proposed an isocon diagram based on [PERSON]'s (1986) method to reflect the migration of elements after the opening of a closed system [38; 39]. The mathematical derivation process can be found in [PERSON] et al. (2009, 2013) [38; 40]. We first assume that the granite (TS-10-14) and marble (TS-10-1) represents the initial state, and the inner or outer skarn represents the state after the system has been opened. Next, it is necessary to perform mass balance calculations on the geological samples before and after the system is opened to eliminate the influence of changes in the total mass of the samples [39; 41; 42; 43]. According to the discussion results of [PERSON] (2005) [44], the selection of the invariant component needs to be combined with the actual geological processes studied to choose the most inactive component. In the granite-inner skarn system, TiO\({}_{2}\) is a suitable invariant component. However, in the marble-outer skarn system, due to the significant migration of elements, we cannot find a consistent invariant component. Therefore, we assume that the total mass is constant under various conditions in this system, in order to draw diagrams that reflect the relative migration of elements. The isocon diagram indicates that the inner skarn stage may result in the enrichment of Ca and Mn, accompanied by the loss of K, Rb and Ba. The inner skarn gradually shows enrichment in light REEs (such as Ce, Pr, Nd, etc.) towards the boundary, while heavy REEs are generally depleted. The element migration in the outer skarn is very intense. In the process from TS-10-2 to TS-10-7, the contents of most elements show a downward trend, but are still enriched by 10-100 times compared to marble, which is consistent with the feature that these elements were originally not present in the surrounding marble. In addition, Ca, Sr and Cs in the outer skarn show depletion characteristics, indicating that these elements may have migrated from marble to the outer skarn. In the inner skarn, K, Rb and Ba are almost completely depleted (Figure 11c), but in the outer skarn, except at the boundary with marble, they are still in a depleted state (Figure 12c), indicating that these elements may have migrated to a further location than marble. In the outer skarn, in addition to Sr and Cs from marble, most of the elements from the inner skarn are enriched at the boundary between marble and the outer skarn, such as Cr, Th, Nb, Y, etc. (Figure 11c,d and Figure 12c,d). At the boundary between the inner skarn and outer skarn, sample TS-10-7 in the granite-inner karn system and sample TS-10-8 in the marble-outer skarn system each exhibit significant differences from the other samples. At the inner skarn boundary, major elements such as Ca, Fe and Mg are significantly enriched in sample TS-10-8 (Figure 11a).