Eocene Izanagi-Pacific Ridge Subduction Along the NE Eurasian Plate Recorded by the Adakitic Magmatism

The interaction between the Izanagi-Pacific ridge and the northeastern Eurasian Plate during the Eocene remains controversial. We focus on oceanic crust-derived adakitic rocks due to their potential for reconstructing the slab window associated with spreading ridges. New and previously published age data suggest that these adakitic rocks can be divided into three main episodes: early Ypresian (ca. 55 Ma)， Lutetian (ca. 46 Ma)， and Bartonian (ca. 36-38 Ma). These rocks， which range from andesitic to dacitic in composition， are characterized by pronounced depletions in heavy rare earth elements (HREEs)， low Y， high Sr， and high Sr/Y ratios. Variations in Mg， Mg#， Cr， and Ni values indicate partial melting of the subducted slab， followed by interaction with the surrounding mantle during ascent through the mantle wedge. Spatially， the first stage is localized as a ‘spot’ in NE China， suggesting a limited ridge-trench intersection. The subsequent two stages occur in two narrow belts， indicating large-scale ridge-trench intersections subparallel to the NE Eurasian margin. The temporal and spatial variations in adakitic rocks related to slab windows are best explained by changes in the scale of the Pacific-Izanagi ridge subduction， possibly reflecting the variations in Pacific Plate movement during the early Eocene.