Detrital zircon U–Pb dating and Raman spectral analysis of carbonaceous material along the boundary between the Sanbagawa metamorphic complex and Chichibu accretionary complex, central Kii Peninsula, southwest Japan

Abstract

We conducted detrital zircon U–Pb dating and Raman spectral analysis of carbonaceous material (CM) around the Takihara thrust, which is thought to be the boundary between the Sanbagawa metamorphic complex and Chichibu accretionary complex in the central Kii Peninsula, southwest Japan. The Mikabu greenstones, which are normally used to define the boundary, are missing from this area. U–Pb dating of psammitic rocks yields a youngest age of ∼118 Ma on the northern side of the Takihara thrust and ∼158 Ma on the southern side. The youngest age and age distribution obtained for the sample from the northern side is consistent with data for the Sanbagawa metamorphic complex in other regions, and the age data obtained for the sample from the southern side are consistent with the Chichibu accretionary complex, demonstrating that the Takihara thrust is the geological boundary between these complexes. We examined two parameters of the Raman spectra from CM: the temperature estimated from the full width at half maximum of the D1 band (T_D1) and the ratio of the intensities of the D4 and D1 bands (D4/D1_int). We obtained T_D1 values of ∼275–290 °C on the northern side of the Takihara thrust and ∼275–280 °C on the southern side, with no clear difference across the thrust. In contrast, there is a clear difference in D4/D1_int values across the thrust: ∼0.30–0.33 on the northern side and ∼0.47–0.50 on the southern side. This suggests that the D4/D1_int value can not only be used to distinguish between the Sanbagawa and Chichibu complexes in the central Kii Peninsula, but it is also useful for detecting differences in metamorphic grade in low-grade metamorphosed rocks that cannot be detected by the geothermometer that uses the full width at half maximum of the D1 band.