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Southwestern Japan during Paleogene time was affected by subduction of the Kula and Pacific plates beneath the eastern margin of Eurasia, followed by a brief episode of transform faulting to allow the Shikoku Basin to open at 27-15 Ma. Subsequent opening of the Japan Sea back-arc basin in middle Miocene time broke Japan away from Eurasia, and SW Japan rotated clockwise ∼45° as it drifted south (17-15 Ma). Southward drift required subduction of the Philippine Sea plate beneath SW Japan, leading to the formation of a magmatic arc. Subduction of the hot Shikoku Basin lithosphere and spreading ridge resulted in distinctive volcanism in the SW Japan forearc between 17 and 12 Ma. This volcanism included midoceanic-ridge basalt (MORB)-like intrusions and oceanic-island basalt (OIB)-type alkali basalts, felsic plutons in the accretionary prism, and high-magnesium andesites along the Setouchi forearc basin. Mafic magmas in the outermost forearc originated from the subducted Shikoku Basin spreading ridge, whereas Setouchi high-magnesium andesites (HMA) may have resulted from the interaction of melts of the subducted Philippine Sea plate with the overlying mantle. Felsic magmas in the forearc resulted from melting of Shimanto Belt sediments caused by intrusion of HMA magmas. Persistent rear-arc volcanism was caused by upwelling astheno sphere associated with opening of the Sea of Japan. Rear-arc volcanism began ca. 25 Ma as rift-fill low-alkali tholeiite volcanism and was replaced gradually after 12 Ma by alkali basalt volcanism. Upwelling-related alkali volcanism continues up to the present, whereas forearc volcanism ceased at 12 Ma. The volcanic arc narrowed with time as the Philippine Sea slab descended and slowly cooled. Adakitic dacites erupted after 1.7 Ma above the 100-km-depth contour of the subducted Philippine Sea plate, suggesting that melting resulted from interaction of the slab with upwelling asthenosphere. Interactions between upwelled rear-arc asthenosphere and subduction of the hot Philippine Sea slab appear to have been the main controls of magmatism for the Neogene SW Japan arc. [ABSTRACT FROM AUTHOR]

Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt-like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc. [ABSTRACT FROM AUTHOR]