Japan Geoscience Union Meeting 2021

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-VC Volcanology

[S-VC30] Volcanic and igneous activities, and these long-term forecasting

Thu. Jun 3, 2021 5:15 PM - 6:30 PM Ch.13

convener:Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Teruki Oikawa(GSJ, National Institute of Advanced Industrial Science and Technology), Daisuke MIURA(Department of Physical Science, Graduate School of Science, Osaka Prefecture University), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology)

5:15 PM - 6:30 PM

[SVC30-P03] The origin of bimodal magmatism in the Okinawa Trough, Japan

*Arran Peter Murch1, Kenichiro Tani1, Takashi Sano1, Shigekazu Yoneda1 (1.National Museum of Nature and Science, Tsukuba, Ibaraki, Japan)

Keywords:Okinawa Trough, Bimodal volcanism, Magma Formation

The Okinawa Trough (OT) is an incipient continental back-arc basin that extends ~1200 km south from Kyushu to Taiwan, and can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT). Back-arc volcanism in the OT is bimodal (basaltic to rhyolitic) and restricted to volcanic centres located in en-echelon grabens the MOT and SOT.

Here we present petrological descriptions, along with major, trace element and Sr–Nd isotopic data for a bimodal sample suite collected during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. Samples were recovered as lavas and pumice dredged from various seafloor knolls and ridges located in the Io and Iheya grabens and from Izena Hole in the MOT, and from a single volcanic ridge in the Yaeyama graben and a single isolated knoll in the SOT.

The mafic end member ranges from ~50 to 56 wt% SiO2. Samples from the SOT show a fractionation trend with an initial enrichment of FeO indicating a tholeiitic trend, while MOT samples are characterised by a calc-alkaline trend. Major and trace element data also suggests at least two groups of basalt in the MOT defined by the relative depletion in incompatible elements (K, Na, Si, Zr, Nb, Rb, Ba, Th, Ce, etc.), and enrichment in Al2O3, V, Cr, Sr of one group vs the other. However, the entire mafic end member sample suite records a strong subduction signature with MORB normalised spider plot patterns showing enrichment of incompatible elements, especially LIL, and Pb, along with the depletion of Nb and Ta. Chondrite normalised REE trends also show enrichment in LREE.

The silica end member ranges from ~71 to 77 wt% SiO2. However, major and trace element data shows the silica end member is highly diverse with at least four identifiable groups based on differences in the degree of enrichment in incompatible elements (K, Rb, Ba, Nb, Zr, Y, Th). Chondrite normalised REE diagrams indicate the same diversity with variable (La/Sm)N ratios. While MORD normalised spider diagrams show the same subduction signature present in the mafic end member. Each group contains at least one dense lava sample suggesting the chemical diversity is a primary feature of magmatism in the Okinawa Trough rather than a result of the floating in of pumiceous material from various locations.

Using petrological descriptions and the chemistry of samples along with MELTS modelling we plan to evaluate the conditions and processes of magma formation and evolution. In doing so we hope to provide a model to explain the magma chemistry in the MOT and SOT.