Possible partial melting and production of felsic melt in a Jurassic oceanic plateau of the Izanagi Plate: Insights from 159 Ma plagiogranites from northern Japan

A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous.

在日本北部日高带的中野川群中发现一处小型英云闪长岩-英安岩岩体。该中野川群为形成于欧亚大陆东北缘古日本海沟的古近纪俯冲杂岩。该英云闪长岩以极低的K₂O含量（0.2~0.3 wt.%）和平坦的球粒陨石标准化稀土元素配分模式（La/Yb[N]≈2.3）为特征，与蛇绿岩中的斜长花岗岩相似。其主量与微量元素特征与石榴石稳定域外洋壳的低压部分熔融产物一致。对该英云闪长岩开展锆石U-Pb定年，获得加权平均²⁰⁶Pb/²³⁸U年龄为159.1±1.6 Ma。晚侏罗世洋高原型玄武岩与少量斜长花岗岩类岩石，在古日本弧-沟系统沿线的俯冲杂岩中呈零星分布。沿古千岛弧-沟系统分布的白垩纪俯冲杂岩中的绿岩，曾被报道获得相似的锆石U-Pb年龄（151.6±1.8 Ma）。该英云闪长岩-英安岩岩块可能源自古日本与古千岛弧-沟系统交汇点处的俯冲杂岩。因此，绿岩与该英云闪长岩体大概率属于大型洋高原的组成部分，该洋高原形成于晚侏罗世-早白垩世的伊佐奈岐-太平洋-法拉隆洋脊三联点处。