[SIT23-P05] Discovery of plagiogranitic rocks and their effusive equivalents in the Nakanogawa Group, southern Hidaka Belt, Hokkaido, Japan
Keywords:Plagiogranite, Hiroo Complex, Nakanogawa Group, Hidaka Belt, Izanagi Plate
The granitic rock is tonalite and composed mainly of quartz, plagioclase, and amphiboles. A small number of opaque minerals is also present. The whole-rock major elemental chemistry is rich in silica (SiO2 = 71.0–73.7 wt%), and the alumina saturation index (Al2O3/CaO + Na2O + K2O in molar ratio) is 0.9–1.1. The tonalite is characterized by an extremely low content of K2O (0.2–0.3 wt%). This geochemical feature, together with lithological features, make the tonalite quite similar to plagiogranites found in ophiolites. In fact, so-called plagiogranites in ophiolites and ocean floor show a wide range of composition (e.g., Koepke et al., 2007); however, plagiogranites from many representative ophiolites commonly show exceptionally low content of K2O in accordance with the traditional definition of plagiogranite (e.g., Coleman and Donato, 1979). The tonalities in this study are characterized by relatively flat chondrite-normalized rare-earth-element patterns (La/Yn[N]A.V. = 2.3; A.V. denotes averaged value). This feature is also concordant with plagiogranites in the Elder Creek ophiolite, California (La/Yn[N] A.V. = 2.2; Shervais, 2008). On the other hand, greenstones intruded by tonalities show 71–72 wt% of SiO2, and 0.1–0.4 wt% of K2O. These compositions closely resemble those of tonalities, and suggest that both effusive and plutonic phases formed by contemporaneously common igneous activity. Overall geochemical features of the tonalite-dacite are distinct from ~46-Ma granites in the Hidaka Magmatic Zone in the Hidaka Belt (e.g., La/Yn[N] A.V. = 4.6), and from presumable lithologies from the Tokoro, and Nemuro belts; i.e. Daimaruyama Greenstones in the Nakanogawa Group (Hidaka Belt; e.g., La/Yn[N] A.V. = 6.4), greenstones in the Toyokoro area (Tokoro Belt; e.g., La/Yn[N] A.V. = 2.7), and granitic gravels from the Nemuro Belt (e.g., La/Yn[N] A.V. = 5.0). Thus, the studied samples exhibit a unique composition compared to that of all the felsic igneous rocks thus far reported from the Hidaka, Tokoro, and Nemuro belts.
The major- and trace-element characteristics are consistent with partial melting of amphibolite-facies oceanic crust at relatively low pressures (5–10 kbar) outside the garnet stability field. Melting of oceanic crust at these pressures can only occur under the influence of an active spreading center. Recent reconstruction of global plate motion suggests that the Izanagi plate had subducted beneath the Eurasia plate by 60 Ma, and the Izanagi–Pacific ridge was subducted from approximately 60–50 Ma (Müller et al., 2016). In addition, intra-oceanic subduction on the Izanagi Plate has recently been proposed (Yamasaki and Nanayama, 2018). Thus, subduction of the Izanagi–Pacific ridge in either an intra-oceanic or Eurasian margin subduction zone, or melting of oceanic crust at Izanagi–Pacific ridge, can be assumed as the origin of the plagiogranite-dacite, at present. Determination of formation age remains an important issue to constrain the locality of its formation and requires further study.