SUBDUCTION INITIATION, MANTLE FLOW, GENESIS AND RISE OF THE YAKUSHIMA GRANITE PLUTON, SW JAPAN

Middle Miocene (~15 Ma) granites are distributed widely over the length of the Outer Zone of SW Japan in front of the present volcanic front. The Yakushima granite pluton is located at the nearest to the axis of Nankai trough-Ryukyu trench system among them. This paper provides an overview of internal and external structures of the Yakushima pluton that suggest oblique rise of the pluton toward the inner trench wall (Anma, 1997). Newly obtained SHRIMP U-Pb data indicate incorporation of subducted sediments into granite melts. Isotopic data suggests minor influence of an enriched mantle. Accepted model to account the genesis of the Middle Miocene granites in SW Japan invokes subduction of young and hot Philippine Sea plate that started ~ 25 Ma. Analogue modeling showed patterns of mantle flow above subducting rigid plate and mimicked oblique rise of inclusions. Nevertheless, it is still hard to attribute overall Middle Miocene plutonism solely to the subduction of the Philippine Sea plate with NS-ward spreading ridge. Perhaps, subduction of a transform fault system can generate the widely distributed, synchronous plutonism along the margin.