Late Cretaceous to early Paleogene forearc magmatism and subduction initiation in the Paleo-Kuril arc-trench system, eastern Hokkaido, Japan. (December 2018)

• Record Type: Journal Article
• Authors: Ikeda, Yasuo; Goto, Motojiro
• Is Part Of: Journal of geodynamics. Volume 122(2018)
• Page Start: 41
• Page End: 53
• Publication Date: 2018-12
• Subjects: Nemuro group -- Paleo-Kuril arc -- Forearc basin -- Alkaline magma -- Fluid -- Phengite; Geodynamics -- Periodicals; Earth movements -- Periodicals; Rock deformation -- Periodicals; Earth -- Internal structure -- Periodicals; 551.1
• DOI: 10.1016/j.jog.2018.11.002
• Languages: English
• ISSNs: 0264-3707

Abstract

Abstract: New major and trace elemental data presented here imply a temporal and spatial change in the magmatic evolution and mode of subduction initiation in the Paleo-Kuril arc-trench system, eastern Hokkaido, Japan. Late Cretaceous to early Paleogene igneous rocks from the Nemuro Group (Nokkamappu, Hamanaka, and Kiritappu Formations) in the forearc basin of the Paleo-Kuril arc consist of the tholeiitic, alkaline and calc-alkaline rock series, based on petrographic and major-element (K2 O, total alkalies [K2 O + Na2 O], and SiO2 contents) data. Immobile-element features on the Zr/TiO2 vs. Nb/Y diagram indicate that a majority of the alkaline rocks are classified as non-alkaline rock. The low TiO2 (< 1%), high V/Ti, and low Nb/Yb ratios of the igneous rocks show that the original mantle compositions of alkaline and non-alkaline rocks were highly refractory in nature. The magmas were produced with a 10–20% partial melting of the depleted mantle, which is common in arc-related non-alkaline magma. The high Th/Yb, Ba/La, Rb/La, and Ba/Th ratios and low La/Sm ratios of the alkaline and tholeiitic rocks from the Nokkamappu and Hamanaka Formations strongly reflect the imprint of substantial fluid components from the subducting slab. The enriched K and Rb in the alkaline magma source are indicative of fluid derived from mica breakdown. Cs fractionation in the slab-derived fluids is indicated by variations in Y/Cs and Nb/Cs, and the fluids are controlled by fluid derived from the breakdown of mica such as phengite. Tholeiitic and calc-alkaline rocks from the Kiritappu Formation show less aqueous fluids. Late Cretaceous magmatism, with decompression melting and formation of the forearc basin, resulted from rollback during subducting initiation, which was associated with a rift system created by the upwelling of depleted mantle following the onset of the sinking of the slab into the mantle. Subsequently, the rollback of the subducting slab slowed down and stabilized, and consequently, normal tholeiitic and calc-alkaline magmas were produced in the early Paleogene.