关键词: 热模拟, 洋壳俯冲, 部分熔融, 埃达克岩

Abstract: Adakitic rocks play an important role in the evolution of continental crust, and recently have attracted great attention of many scientists all over the world. Cenozoic adakites are exclusively distributed in circum-Pacific regions, and are mostly produced by partial melting of young and hot subducting oceanic crust at 70～90km depths. Previous kinematical model of Subducting oceanic plate shows that ridge would approach to trench and ridge subduction may occur if the velocity of ridge outspreading is significantly smaller than subducting velocity in actual geological process. In this condition, dehydration of some hydrous minerals in oceanic crust and melting in subduction zone will become crucial geodynamic condition for the generation of adakitic rocks. In this paper, we quantitatively model the thermal evolution of ridge subduction with the finite element method, and discuss the effects of dehydration and melting on arc volcanism during the process of active ridge subduction. The result of finite element modeling shows there is a distinctive cooler zone along the suducted slab. Dehydration of amphibolites occurs in the subducted oceanic crust at temperature range of 825～1 000℃. While the ridge arrived at trench and subducted with the oceanic plate, the dehydration temperature zone ascended to the depth of 75～85km. And, the eclogite in the dehydrated oceanic crust would melt and produce adakite. After ridge subduction, the dehydration temperature range of 825～1 000℃ would drop to the depth of 100km under the Arc. At this depth, oceanic crust could not melt due to high pressure. However, andesitic magmas might be formed when the released free water moves up to the mantle wedge. Thermal modeling reveals that the dehydration in the subducted oceanic crust may cause two possible cases of partial melting. One possible case is the melting of the subducted oceanic crust itself, which will produce adakitic magma activity in fore arc region. Another case is the partial melting of overriding mantle wedge due to dehydration of subducted slab, resulting in the formation of andesitic magma in the arc. Because of the differences in dehydration and melting depths, the formation places of adakite and andesite are different. Adakitic magma generates at the fore arc region, whereas andesitic magma forms in the Arc. Besides, the occurrence time of adakite or andesite is mutually exclusive. Andesitic magma would be produced after ridge subduction, but adakitic magma would be active only in the period of ridge subduction. In other words, the andesitic magma would not occur during Adakitic activity. Oppositely, the adakitic magma would disappear when andesite magmas are formed.

Key words: thermal modeling, oceanic crust subduction, partial melting, Adakite