## 5 Discussion
Geochemical analyses of arc basalts show that partial melts with acidic components form at approximately 800 \({}^{\circ}\)C, while those with neutral components appear at around 1300 \({}^{\circ}\)C [40]. On the other hand, lower overriding plate movement rates (Figure 6d) cause mantle wedge temperatures near the subduction slope to be more influenced by the subduction process itself. This results in a reduction in the high-temperature zone above 800 \({}^{\circ}\)C, an increase in the depth at which partial melts form, and a decrease in the volume of melt, thereby diminishing its contribution to arc magmatism. Figure 6e,f depict the temperature structures and magmatism depths for the same overriding plate movement rate \(U_{0}\) but with different subduction rates (\(V_{1}<V_{2}\)). At a lower subduction rate (Figure 6e), temperatures 50-85 km above the subduction zone range from 800 to 1000 \({}^{\circ}\)C, affected by shear friction heating. Hornblende minerals in the subducted oceanic crust dehydrate at depths below 100 km, where temperatures reach 900-1100 \({}^{\circ}\)C [35]. The water that is released subsequently migrates to the overriding shear heating zone, causing partial melting of the mantle wedge and promoting arc magmatism. [38; 39]. Conversely, at a higher subduction rate (Figure 6f), the temperature in the mantle wedge decreases, which reduces the extent of dehydration at the same depth and its contribution to partial melting. Consequently, there is less mantle melting and diminished arc magmatism due to a reduced high-temperature zone from shear friction heating. Thus, it can be concluded that a higher overriding plate movement rate or a lower subduction rate facilitates the generation of higher temperatures in the mantle wedge. Furthermore, the duration of the high-temperature anomaly in the mantle wedge is extended, which further facilitates the partial melting of mantle material and the subsequent development of arc magmatic activity. The plate tectonic reconstruction of the Halmahera arc and its periphery (Figure 7) reveals that the northward-drifting Australian plate collided with the southern part of the westward-drifting Halmahera arc approximately during the Early Miocene (20 Ma) to the Middle Miocene (15 Ma). Subsequently, from the Late Miocene (10 Ma) to the Early Pleistocene (2 Ma), the Australian plate persisted in drifting northward and rotating clockwise, and continued to push the southern part of the Halmahera arc (Obi Island) through the left-lateral strike-slip faults [41]. In the early Pleistocene (2 Ma), the Halmahera arc initiated a collision with the Sangihe forearc, leading to a deceleration in its westward migration [6; 8]. The evolution of plate tectonics in Halmahera and its vicinity since 20 Ma (Figure 7b) illustrates that the southern part of the Halmahera arc (Obi Island) has been accelerated due to the persistent northwestward pushing of the Australian plate since the Middle Miocene, increasing the movement rate of the overriding microplate in the south. Videlicet, the movement rate of Obi Island microplate in the south is greater than that of the \"K shape\" Halmahera Island microplate in the north, corresponding to Case 1 and Case 2, as depicted in the simulation results above. Thus, the dynamic cause of volcanic age disparity between the northern and southern parts of the Halmahera arc can be concluded. Since the Early Miocene (20 Ma), the Australian plate has persistently drifted northward. Around the Middle Miocene (15 Ma), the Halmahera subduction initiated, and the Halmahera subduction zone retreated westward with the subduction. At this juncture, the Australian plate kept drifting northward and continuously pushed the Obi Island microplate in the southern part of the Halmahera arc via the left-lateral strike-slip faults (Figure 8a).