海嶺の沈み込みと三波川-領家変成岩および花崗岩の成因 : 広域ダイナミクス, 物質循環との関連性

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The origin of the Sanbagawa-Ryoke metamorphic rocks and associated granitic rocks is discussed with respect to thermal structure and water cycling in subduction zones. First, the relationship between thermal structure and water cycling is summarized based on previous studies of numerical modeling for three different settings along the Japan arcs. This shows that water cycling becomes shallower in a warmer environment, which has been demonstrated or supported by studies on seismic structures and distribution of volcanoes. In a very hot environment associated with a young (<10 Ma) plate, including ridge subduction, arc magmatism is shut down to switch to granitic magmatism and regional metamorphism in the forearc region, as both heat and water are supplied to the region. Numerical modeling studies show that a paired metamorphism can occur associated with ridge subduction in a single forearc domain within a relatively short period (a few tens of million of years), which explains the spatial association of Sanbagawa (high-P type) and Ryoke (high-T type) metamorphic rocks associated with granitic batholiths, their similar metamorphic ages, and a common mode of syn-metamorphic deformation (prolate strain). Therefore, in terms of thermal structure and water circulation, arc magmatism and regional paired metamorphism associated with granitic magmatism are regarded as representing different stages of a series of thermal and fluid processes in subduction zones. At any stage of these processes, even after major dehydration of the subducting slab and the overlying mantle wedge, nominally anhydrous minerals, such as olivine, pyroxene, and garnet, can carry a significant amount of water (up to several 1000 ppm) to the deep mantle. Recent geochemical and statistical studies have revealed that such signatures of deeply subducted fluid and associated elements are indeed identified in oceanic basalts as exits of global material circulation. Based on an understanding of the thermal structure and water circulation in subduction zones, it is argued that the fluid processes and slab configuration (e.g., cold hard slab vs. hot soft slab) exercise great control on large-scale dynamics in subduction zones, such as formation of backarc basins (e.g., Japan Sea), and wet region representing extensive magmatism with abundant volatile components over eastern Asia. Regional metamorphism, arc magmatism, large-scale dynamics in Eastern Asia, and global material circulation are potentially all connected with each other, irrespective of qualification or disqualification of the model or working hypothesis discussed in this paper.
2008-02-25

海嶺の沈み込みと三波川-領家変成岩および花崗岩の成因 : 広域ダイナミクス, 物質循環との関連性

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