Japan Geoscience Union Meeting 2019

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP32] Deformed rocks, Metamorphic rocks and Tectonics

Wed. May 29, 2019 3:30 PM - 5:00 PM A08 (TOKYO BAY MAKUHARI HALL)

convener:Yumiko Harigane(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)), Yoshihiro Nakamura(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Chairperson:Shunsuke Endo(島根大学 総合理工学部 地球科学科), Ryosuke Oyanagi(国立研究開発法人 海洋研究開発機構 地球内部物質循環研究分野)

3:45 PM - 4:00 PM

[SMP32-14] Dating a 'princess': U–Pb age determination of 'nunakawaite' (strontio-orthojoaquinite)

*Tatsuki Tsujimori1, Tomomi Hara1, Yuzuki Shinji1, Tomohiro Ishizaka1, Hiroshi Miyajima2, Jun-Ichi Kimura3, Shogo Aoki4, Kazumasa Aoki4 (1.Tohoku University, 2.Fossa Magna Museum, 3.JAMSTEC, 4.Okayama University of Science)

Keywords:nunakawaite' (strontio-orthojoaquinite), U–Pb dating, metasomatism, serpentinite mélange, Itoigawa–Omi area

Nunakawaite' (strontiojoaquinite) is an orthorhombic variety of strontiojoaquinite [Sr2Ba2(Na,Fe)2Ti2[Si4O12]2O2(O,OH)2·H2O]; it is a rare joaquinite group mineral that is only found in a riebeckite-bearing albitite in the serpentinite-matrix mélange of the Itoigawa–Omi area. The mineral was originally named after 'Princess Nunakawa' (nunakawa hime) in the Japanese Shinto mythology 'Kojiki'.

'Nunakawaite' is characterized by remarkably high Ba, Zr, Nb, Zn, LREEs, MREEs, and enriched in U (35.8–721 µg·g-1), Pb (2.2–31 µg·g-1), and Th (7.42–2365 µg·g-1). LA-ICPMS analyses show highly variable U/Pb (238U/206Pb = 9.245–68.98) and Pb (207Pb/206Pb = 0.0758–0.756) isotope ratios, and the scattered trend define an isochron line with a lower intercept at 89.19 ± 1.07 Ma. The 'nunakawaite' U–Pb age confirms that the 'nunakawaite'-hosted riebeckite-bearing albitite formed at late Cretaceous. This implies that the serpentinite-matrix mélange unit with early Paleozoic jadeitites and late Paleozoic blueschist, eclogite and amphibolite was reactivated by a significantly younger tectonic event.

In-situ Sr-Pb isotope analyses show two different isotope trends between Sr-rich accessory minerals in riebeckite-bearing albitite ('nunakawaite' and ohmilite) and those in jadeitite (itoigawaite, stronalsite, vesvianite, Sr-rich epidote). The Sr-Pb isotopes also support the idea that the riebeckite-bearing albitite formed by a fluid-induced metasomatic event different from the jadeitite-forming metasomatism at early Paleozoic. The formation of riebeckite-bearing albitite at ~90 Ma is coeval with late Cretaceous granitic intrusion of the Omi area (youngest zircon U–Pb: 90.8 ± 1.1 Ma: Nagamori et al. 2018). The granitic intrusion might have acted an important role in the formation of 'nunakawaite'. In other words, reactivation of metasomatic mineralization in the Paleozoic serpentinite mélange is recorded in the Cretaceous riebeckite-bearing albitite.