Petrological and geochemical features of ultramafic cumulates from the Nikoro Group of the Tokoro Belt, Hokkaido, Japan: deeper facies of seamount on the Izanagi Plate

The Nikoro Group of the Tokoro belt, which extends in a N-S direction along the central axis of Hokkaido, northern Japan, has been regarded as a subduction complex accreted to the Paleo-Kuril arc-trench system during the late Cretaceous to Neogene. The Nikoro Group consists of Mesozoic rocks of seamount-origin, namely basaltic-trachytic pillow lavas, hyaloclastites, dikes and clastic sediments with minor amounts of chert and limestone. In addition, it has been reported that ultramafic cumulates occurred closely with olivine (Ol) gabbro, dolerite, and basalt as co-magmatic products. Therefore, ultramafic cumulates can provide valuable information about deeper portions of the magmatic and crustal architecture of accreted seamount on the Izanagi Plate in the Paleo-Pacific Ocean.

The samples were collected from two localities; Kuton-nikoro river in Kitami City (Loc-1) and Penke-kunbetsu river in Rikubetsu Town (Loc-2). Samples from both of the localities consisted mainly of spinel (Spl)-Ol cumulates. While the interstices of cumulus phases were occupied by anhedral plagioclase (Pl) and clinopyroxene (Cpx) in general, Spl-Ol-Pl cumulates with euhedral Pl were also obtained. Therefore, the crystallization order of cumulus phases is Spl + Ol → Pl → Cpx. Cr# [= Cr/(Cr+Al)] and TiO₂contents of Spl from Loc-1 were 0.29-0.42 and 1.55-4.36 wt%, respectively, and those from Loc-2 were 0.41-0.57 and 0.42-1.52 wt%, respectively. The compositions of Loc-1 were plotted in the field of within-plate basalt (WPB) and those of Loc-2 were plotted in the overlapping area of WPB and mid-ocean ridge basalt (MORB). Ol Mg# [= Mg/(Mg+Fe)] were 0.81-0.84 (Loc-1) and 0.83-0.88 (Loc-2). Estimated NiO₈₉/MnO₈₉of the Ol was ~1.5 and corresponds to a lithosphere-asthenosphere boundary depth of ~0 km. Pressure estimates from Cpx barometer yielded 0.25-0.3 GPa (Loc-1) and ~0.55 GPa (Loc-2). Calculated equilibrium melt compositions from trace element geochemistry of Cpx showed typical ocean island basalt composition for Loc-1 and E-MORB composition for Loc-2. The crystallization order and geochemistry of constituent minerals of the samples suggest that ultramafic cumulates at Loc-2 were formed at plume-influenced mid-ocean ridges, and that subsequent ocean island magmatism on the Izanagi Plate formed ultramafic cumulates at Loc-1.