Ridge Subduction and Origin of Sanbagawa-Ryoke Metamorphic Rocks and Associated Granitic Rocks: Link to Regional Dynamics and Material Cycling

Abstract

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 metamor-phism 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 Sanbaga-wa（high-P type）and Ryoke（high-T type）metamorphic rocks associated with granitic batho-liths, their similar metamorphic ages, and a common mode of syn-metamorphic deformation （pro-late 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）exer