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)

3:30 PM - 3:45 PM

[SMP32-13] Early Paleozoic subduction-zone metamorphism in Japan: A geochronological reappraisal of Kitomyo Schists

*Shota Matsunaga 1, Tatsuki Tsujimori1, Atsushi MIYASHITA2, Shogo Aoki3, Kazumasa Aoki3 (1.Tohoku University, 2.Seikei University, 3. Okayama University of Science)

Keywords:metamorphic zircon, zircon geochronology

Early Paleozoic subduction-related high-pressure metamorphic rocks have been reported from the Kitomyo Schists of the Kurosegawa Belt (Maruyama and Ueda 1975 [doi: 10.2465/ganko1941.70.47]) and the Fuko Pass Metacumulates (FPMC) of the Oeyama Belt (Tsujimori and Liou 2004 [doi: 10.1111/j.1525-1314.2004.00515.x]). In order better to understand the earliest subduction zone record of the ‘proto-Japan’, we revisited the Kitomyo Schists petrologically and gecohronologically. The metasedimentary sample is a well-deformed, quartzo-feldspathic mica schist, with the mineral assemblage of garnet + clinozoisite + phengite (Si p.f.u. = 3.3–3.5) + quartz + albite. Euhedral grains show distinct prograde chemical zoning in spessartine decrease toward the rims. The lack of biotite, oligoclase and paragonite indicate that the schist underwent a HP intermediate type metamorphism. The metabasaltic rock is a well-deformed, amphibolite. Although the amphibolite was overprinted severely at a greenschist-facies condition, relict barroisitic amphibole [B]Na (Na in the B-site) = ~0.69 and rutile, indicating HP metamorphism. LA-ICPQMS and HR-SIMS zircon geochronology yielded 445.5 ± 3 Ma for the timing of HP intermediate type metamorphism of the Kitomyo Schists. Metamorphic zircon in metasedimentary sample is characterized by lower U (117–132 µg/g) and lower Th (34–143 µg/g) than detrital magmatic zircons. Our reapproval suggests that the Kitomyo Schists formed in an Early Paleozoic subduction zone with geothermal gradient in the order of ∼15°C/km. Such a relatively high geothermal gradient in the subduction zone has produced the HP intermediate type metamorphism. The earliest subduction record in the ‘proto-Japan’ was not ‘cool’ enough to produce blueschist.