[S-MP28] 変形岩・変成岩とテクトニクス

[SMP28-01] Ignimbrite flare-up in Late Cretaceous to Paleocene Japan driven by hot mantle inflow

*山岡 健¹、諸星 暁之²、佐藤 大介¹、三國 和音^1,3 (1.国立研究開発法人産業技術総合研究所地質調査総合センター、2.東京大学大学院理学系研究科地球惑星科学専攻、3.東京科学大学理学院地球惑星科学系)

キーワード：白亜紀、テクトニクス、イグニンブライトフレアアップ、火成活動、イザナギプレート

The mechanisms by which subduction parameters, mantle dynamics, and magmatic systems drive ignimbrite flare-ups remain incompletely understood. By reanalyzing spatial and temporal variations in igneous activity and Sr–Nd isotopic compositions from the Cretaceous to Paleogene periods, based on newly compiled data from the Japan arc and the Korean Peninsula, we identify two isotopically distinct mantle sources. These sources are delineated by thresholds of SrI = 0.706 and εNd(t) = −4, with the more enriched mantle source entering the mantle wedge between 100 and 60 Ma. The enrichment of isotopic signatures within magmas in the Japan arc reached its zenith at 85–60 Ma, coinciding with a significant ignimbrite flare-up. This isotopic variability can be attributed to passive asthenospheric upwelling, triggered by the rollback of the flat-slab of the Marginal sea plate, which had subducted beneath the North China Craton. While the slab dip and convergence rate of the subsequent Izanagi plate subduction likely played a secondary role in facilitating magmatic activity in southwest Japan, our dataset highlights that increased thermal input from the mantle to the lower crust of the Japan arc was the dominant factor in producing the region’s extensive ignimbrite deposits.