Japan Geoscience Union Meeting 2014

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Oral

Symbol S (Solid Earth Sciences) » S-VC Volcanology

[S-VC53_28PM2] volcanic activities and tectonics

Mon. Apr 28, 2014 4:15 PM - 4:45 PM 413 (4F)

Convener:*Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Takuya NISHIMURA(Disaster Prevention Research Institute, Kyoto University), Ryuta FURUKAWA(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Kiyoshi Toshida(Geosphere Science Sector, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Chair:Yasuhisa Tajima(NIPPON KOEI CO.,LTD.)

4:30 PM - 4:45 PM

[SVC53-P04_PG] Magma genesis of Miocene basalts from Ootsu district, Yamaguchi Prefecture, SW Japan arc

3-min talk in an oral session

Yoji HIGASHIYAMA1, *Toshiaki HASENAKA1, Tomoyuki SHIBATA2, Masako YOSHIKAWA2, Takashi NAGAO3 (1.Grad School Sci. & Tech., Kumamoto Univ., 2.Beppu Geothermal Res. Lab., Kyoto Univ., 3.Grad School Sci. & Eng., Yamaguchi Univ.)

Keywords:alkaline rock, tholeiite, boron, rare earth element, mantle

The geological, petrological and geochemical studies of Miocene Ootsu basalts, distributed in Tsunoshima and Yuyashima islands along the coast of the Japan Sea, revealed the temporal and vertical changes in mantle melting processes. Based on their volcanic stratigraphy and petrological data, Ootsu basalts were grouped into; (1) clinopyroxene-olivine basalt (COB), (2) olivine basalt, magnetite-rich type 1 (MRB1), (3) olivine basalt, magnetite-rich type 2 (MRB2), and (4) olivine basalt, magnetite-poor type (MPB). MRB1 and MRB2 are rich in FeO* and TiO2 contents, and MPB is rich in SiO2 and Al2O3 contents. MRB1, MRB2 and COB are alkalic and MPB is tholeiitic.Phase diagram and mass balance calculations indicate that these four groups were derived from different primary magmas, and had experienced polybaric crystallization. The compositions of primary magmas for these four groups suggest that MRB1 and MPB were generated at the deepest and shallowest depths, by the lowest and the highest degrees of melting, respectively. Multi trace element plots (normalized by the primitive mantle values) of Ootsu basalts show the strong enrichment of LILE (Rb, Ba, and K), and distinct negative anomaly of Nb and Sm. The compositions of coexisting olivine and spinel (OSMA) suggest that MPB's mantle source is the most fertile among four groups. The different ratios of LREE/HREE among four groups suggest different mantle source and different degree of partial melting. B/Nb ratio of four groups is getting higher with decreasing segregation depth. These systematic differences in B/Nb ratio indicate that the upper mantle beneath Ootsu district is characterized by an increased degree of metasomatism at shallow level. We concluded that the diversities of chemical composition in Ootsu basalts attribute largely to different segregation depth and heterogeneous mantle source.