New Detrital-zircon Ages from the Hitachi Area in Northeast Japan and Their Geological Significance

Abstract

We measured the LA-ICP-MS U-Pb age distribution of detrital zircons in three psammitic schist samples of Hitachi and Nishidohira medium P / T metamorphic rocks from the southern part of Abukuma Belt, Northeast Japan. It has been proposed that these medium P / T metamorphic rocks mark the eastern extension of the Triassic collisional suture between the North China and South China blocks. Therefore, we aim to obtain the age of sedimentation, stratigraphy, and provenance of their protolith from the measurements, and evaluate the above proposition. The psammitic schist sample （DIO-9） of Hitachi Metamorphic Rocks, originating from quartzose sandstone at the lowest part of the Daioin Formation, contains detrital zircons of the youngest age clustered around 410 Ma and the youngest zircon at 395±20 Ma （ 206 Pb / 238 U age; 2）. Considering that the upper part of the Daioin Formation contains Visean （Lower Carboniferous） corals and that the formation intercalates abundant felsic tuff layers, the lowest part of the Daioin Formation is likely to be correlated with Devonian Nakasato or Lower Carboniferous Hikoroichi Formation of South Kitakami Belt, Northeast Japan. Nishidohira Metamorphic Rocks lie beneath ultramafic rocks along the base of Hitachi Metamorphic Rocks, and consist of mafic, siliceous, calcareous, pelitic, and psammitic schists or gneisses. Because the siliceous schist of Nishidohira Metamorphic Rocks is meta-pelagic chert, the metamorphic rocks presumably originated from an accretionary complex. The ages of detrital zircons in two psammitic schist samples （ND-12 and-13） of Nishidohira Metamorphic Rocks mostly fall between ca. 300 Ma and 200 Ma, with the youngest two zircons at 154±6 Ma （ND-12） and 175 ±3 Ma （ND-13） （ 206 Pb / 238 U age, 2）. The protolith age of the psammitic schists must be ca. 154±6 Ma （Kimmeridgian of Late Jurassic） and ca. 175±3 Ma （Aalenian of Middle Jurassic） or younger, suggesting that Nishidohira Metamorphic Rocks originated from a Jurassic accretionary complex. In the Hitachi area three tectonostratigraphic units superpose, i.e., in ascending order, （1） the Jurassic accretionary complex of Nishidohira Metamorphic Rocks, （2） ultramafic rocks, and （3） Hitachi Metamorphic Rocks that are at least partly correlated with the ＊ 富山大学大学院理工学研究部 ＊＊ 京都大学大学院理学研究科 ＋ 現所属：東京大学大学院総合文化研究科