Partial melting of subduction zones

俯冲带是地幔对流环的下沉翼，是地球内部的重要物理与化学系统。俯冲带具有比周围地幔更低的温度，因此，一般认为俯冲板片并不会发生部分熔融，而是脱水导致上覆地幔楔发生部分熔融。但是，也有研究认为，在水化的洋壳俯冲过程中可以发生部分熔融。特别是在下列情况下，俯冲洋壳的部分熔融是俯冲带岩浆作用的重要方式。年轻的大洋岩石圈发生低角度缓慢俯冲时，洋壳物质可以发生饱和水或脱水熔融，基性岩部分熔融形成埃达克岩。太古代的俯冲带很可能具有与年轻大洋岩石圈俯冲带类似的热结构，俯冲的洋壳板片部分熔融可以形成英云闪长岩-奥长花岗岩-花岗闪长岩。平俯冲大洋高原中的基性岩可以发生部分熔融产生埃达克岩。扩张洋中脊俯冲可以导致板片窗边缘的洋壳部分熔融形成埃达克岩。与俯冲洋壳相比，俯冲的大陆地壳具有很低的水含量，较难发生部分熔融，但在超高压变质陆壳岩石的折返过程中可以经历广泛的脱水熔融。超高压变质岩在地幔深部熔融形成的熔体与地幔相互作用是碰撞造山带富钾岩浆岩的可能成因机制。碰撞造山带的加厚下地壳可经历长期的高温与高压变质和脱水熔融，形成S型花岗岩和埃达克质岩石。;Subduction zones are descending limbs of mantle convection cells, and the major physical and chemical system of Earth's interior. Subduction zones have lower temperature than the adjacent mantle, and therefore it is proposed that subducted materials including oceanic crust and continental crust do not undergo partial melting, whereas dehydration of the subducted oceanic crust and underlying mantle causes partial melting of the overriding mantle wedge to generate arc magmatic rocks. However, some studies concluded that the hydrated oceanic crust could be partly molten during the subduction. In particular, partial melting of subducted oceanic slabs is probably main mechanism of arc magmatism in the following circumstances. The water-saturated and dehydration melting of subducted oceanic materials can generate the adakitic rocks during the shallow and slow subduction of the younger oceanic lithosphere. The Archean subduction zones have similar features to those of the younger oceanic lithosphere, and therefore the partial melting of the subducted Archean oceanic crust resulted in the formation of tonalite-trondhjemite-granodiorite (TTG). The partial melting of mafic rocks in the shallowly subducted oceanic plateau generated the adakitic rocks. The partial melting of oceanic crust near the margins of slab window during the subduction of spreading mid-oceanic ridge produced the adakitic rocks. Contrary to the subducted oceanic crust, partial melting of deeply subducted continental crusts should be rare due to very low water content. However, the ultrahigh-pressure rocks underwent intensive dehydration melting during the exhumation from the mantle to crust. Interaction between the melt derived from the ultrahigh-pressure metamorphic rocks and the mantle is a potential formation mechanism of the potassic or ultrapotassic magmatic rocks in the collisional orogens. The thickened lower crust of the collisional orogens experienced prolonged high-temperature and high-pressure metamorphism and associated partial melting, and in turn resulted in the formation of S-type granitoids and adakitic rocks.