Origin of the granodiorite in the forearc region of southwest Japan: Melting of the Shimanto accretionary prism

In the forearc region of southwest Japan, felsic to intermediate volcano-plutonic complexes of Middle Miocene age intruded into the Shimanto accretionary complex, possibly in association with megatectonic events such as the opening of the Japan Sea and resultant underthrusting of the newly-borne Shikoku Basin (Philippine Sea plate) beneath the southwest Japan arc. Activities of mantle-derived magmas are also distinct in the forearc of Middle Miocene southwest Japan: tholeftic basalts are closely related to igneous activity of the Shikoku Basin, and high-Mg andesites probably formed under anomalously high-temperature conditions caused by the opening of Japan Sea. Two plutons (Uwajima and Miuchi) were chosen for understanding the origin of felsic magmas in the forearc region of southwest Japan. All granitic rocks of the plutons show enriched. isotopic characteristics with higher 87Sr86Sr and lower 143Nd144Nd tha bulk Earth, which suggests that the contribution of crustal materials has a magma source. However, they have lower 87Sr86Sr and higher 143Nd144Nd than Shimanto sediments, and hence an additional component with lower 87Sr86Sr and higher 143Nd144Nd than granitic rocks must be required. Possible candidates for such components are coeval mantle-derived magmas. Mass-balance modelling between mantle-derived magmas and Shimanto sediments on the basis of their SrNd isotopic compositions suggested that mixing of anatectic melt of the Shimanto sediments and porphyritic andesite magma evolved from high-Mg andesite as a possible mechanism of the origin of the granodiorites of the Uwajima pluton. Injection of high-Mg andesite into the Shimanto accretionary prism may triggered the melting of sediments. Calculated composition of anatectic melt of the Shimanto sediment and P-T condition of the gneissose enclave in the granodiorite suggest that the melting of sediments may have occurred at higher-temperature conditions than those of the melting reactions involving biotite breakdown.