## 4 Discussion
### Constraints of Microstructure and Rock Chemical Composition on the Genesis of Pseudotachylyte
The comparative study of major elements (e.g., Na\({}_{2}\)O\({}_{2}\), K\({}_{2}\)O, Al\({}_{2}\)O\({}_{3}\), CaO, FeO, and MgO) and SiO\({}_{2}\) content demonstrates that amphibole, biotite, and feldspar were preferentially melted under transient high-temperature/pressure conditions, while quartz remained largely unmelted and was preserved as clasts within the pseudotachylyte matrix. This selective melting pattern indicates non-equilibrium melting, where thermodynamically unstable phases (e.g., hydrous minerals like hornblende and biotite) and alkali feldspars decomposed rapidly, whereas quartz resisted melting. The low SiO\({}_{2}\) content in the pseudotachyylte melt (e.g., 49.92-57.63 wt.% in AF19-43D samples) further supports limited quartz participation in melting. Instead, quartz fragments were mechanically incorporated into the melt as relics, consistent with the observed clast-rich microstructure. Notably, the pseudotachylyte matrix exhibits chemical complementarity with the host rock residues: elevated FeO and MgO (derived from mafic minerals like hornblende) and enriched K\({}_{2}\)O (from K-feldspar breakdown) in the melt contrast with residual Na-Al-Ca enrichment in the restite (e.g., AF19-43D2: CaO = 9.17 wt.%, Na\({}_{2}\)O = 4.70 wt.%). This dichotomy aligns with partial melting processes, where melt and residue compositions diverge but retain genetic links to the parent rock. The inheritance of plagioclase chemistry (An33-41 in pseudotachylyte vs. An20-54 in host rocks) and the overlapping trends in Harker diagrams (e.g., Na\({}_{2}\)O-SiO\({}_{2}\): positive correlation; CaO-SiO\({}_{2}\): negative correlation) underscore a local origin for the melt. Moreover, the spatial distribution of pseudotachylyte veins--intimately associated with fractured granite and showing sharp, non-reacted contacts--rules out an external injection mechanism. The homogenization of plagioclase compositions (e.g., An12-47 in AF19-43D) and the ternary feldspar system (Ab-An-Or) equilibration further attest to rapid melting-quenching cycles, preserving disequilibrium signatures. In summary, the geochemical coherence between pseudotachylyte and host rocks, coupled with microstructural evidence (e.g., clast-melt relationships), conclusively supports an in situ melting genesis within seismogenic faults. The pseudotachylyte formed via frictional heating-induced selective melting of hydrous and alkali-rich phases, while refractory minerals (e.g., quartz) survived as clasts, leaving a distinct chemical imprint of coseismic slip dynamics. Model of Genesis Pseudotachylyte and Deformation During Seismic Period Cycle in the Anning River Fault Zone  
Through the study of the microstructures, deformation fabrics, and chemical compositions of pseudotachylyte and its associated fractured rocks and mylonites in the Anning River fault zone, the process of pseudotachylyte formation can be summarized in the following stages (Figure 14):  
1. Pre-peak Friction Strength of the Fault: Before the fault reaches its peak friction strength, the friction within the fault zone is relatively high, resulting in the occurrence of numerous coseismic micro-fractures. Multiple sets of high-angle brittle fractures extend along the fault plane, forming fractured rocks associated with co-seismic rupture. During this phase, quartz in the parent rock begins to fragment. The temperature rise induced by coseismic sliding causes the quartz particles in the parent rock to develop high dislocation densities, which leads to dislocation creep in the quartz (manifested by undulose extinction in quartz grains). 2. Fault Reaching Peak Friction Strength: After the fault reaches its peak friction strength, rapid sliding along the fault plane occurs. Due to the frictional shear heating during coseismic movement, the temperature continuously increases, causing the rock to melt. The molten material is distributed along the fault plane and injected into the fractures of the host rocks (e.g., sample AF19-43D).