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[SRD41-02] Granite-series and the origin of granitic magma
Keywords:granite, Japan Arc, subduction zone
In the Sr-O isotope diagram, the plots of most granites except for part of high-Sr granites are consistent with the curves of source contamination (convex downward). The SrI-NdI diagram also indicates that most granites were formed by the assimilation-fractional crystallization processes between strongly depleted materials (= mantle materials) and sedimentary rocks. To explain the data consistently, the geological setting of which mantle materials react with sedimentary rocks during magma generation is necessary. The sedimentary rocks should be common and homogeneous materials, and should be drawn into magma source regions. The materials met the above conditions are subducted sediments. Therefore, it can be deduced that the granite-series in Japan Arc depends mainly upon the reaction-rate between primitive magma and subducted sediments. Furthermore, it is also inferable that the main source materials of granitic magma in Japan Arc are not lower crustal materials but subducted sediments reacted with mantle materials.
Reference: Takagi, T (2004) Am. Jour. Sci., 304, 169-202.