[M-IS17] New frontiers in geology

[MIS17-P06] Zircon geochronology of indialite-staurolite-bearing hornfelses in the Unazuki area, Toyama Prefecture

*Mami Takehara¹, Kenji Horie^1,2, Yoshikuni Hiroi³, Tatsuro Adachi⁴, Tomokazu Hokada^1,2 (1.National Institute of Polar Research, 2.The Graduate University for Advanced Studies (SOKENDAI), 3.Chiba University, 4.Kyushu University)

Keywords:U-Pb zircon geochronology, Unazuki area

The Unazuki area, situated at the northeastern margin of the Hida metamorphic complex, is affected by medium P/T (525-700℃, 0.5-0.6 GPa) metamorphism characterized by a clockwise P-T path, which is representatively observed in kyanite-staurolite-bearing schists (Hiroi, 1983). The occurrence of peraluminous pelitic schists and carboniferous limestone (Hiroi, 1978) and the coexistence of bimodal volcanic rocks suggest that protoliths of the Unazuki metasediments are likely derived from the continental shelf sediments accumulated on a rifted continental margin (Isozaki, 1997 and references therein). Therefore, it is important to determine the Permian-Triassic thermal history in the Unazuki area for discussing the crustal developments between the Japanese Islands and the eastern margin of the Eurasian continent, including the Korean Peninsula, before the opening of the Japan Sea. Previous studies provided constraints on the duration of metamorphism. Horie et al. (2018) constrained the duration of the medium P/T metamorphism in the Unazuki area based on zircon U-Pb dating of quartz-feldspathic schist (258 ± 2.3 Ma) and granite samples (253 ± 1.9 Ma). Hiroi (2022) reported the existence of indialite in staurolite-bearing hornfelses from Nakatani in the Unazuki area, based on detailed microscopic observation. Indialite, a high-temperature polymorph of cordierite, is generally observed in the coalfields on fire in India and volcanic rocks. On the other hand, indialite transitions to cordierite in hornfelses close to plutons (e.g., Kitamura & Yamada, 1987). Therefore, indialites in the staurolite-bearing hornfelses from Nakatani suggest that the hornfelses were rapidly cooled after contact metamorphism. Zircon U-Pb ages were obtained from the hornfels using a sensitive high-resolution ion microprobe (SHRIMP-IIe) at the National Institute of Polar Research, and the age is consistent with those reported by Horie et al. (2018). In this presentation, we report the U-Pb zircon ages with zircon trace element geochemistry and petrology of the indialite-staurolite-bearing hornfels samples and discuss the Permian-Triassic thermal history in the Unazuki area.

References: Hiroi (1983) Contributions to Mineralogy and Petrology, 82, 334-350; Hiroi (1978) The Journal of the Geological Society of Japan, 84, 521-530; Isozaki (1997) Island Arc, 6, 2-24; Horie et al. (2018) Chemical Geology, 484, 148-167; Hiroi (2022) The 129th Annual meeting of the Geological Society of Japan abstract (T1-O-25); Kitamura & Yamada (1987) Contributions to Mineralogy and Petrology, 97, 1-6.