HR: 1340h
AN: V13B-1489    [Abstracts]
TI: Petrology of Early Miocene volcanic rocks from Okushiri Island, Japan: geochemical characteristics of lithospheric mantle beneath the back-arc side of the NE Japan arc.
AU: * Sato, M
EM: f04j007g@mail.cc.niigata-u.ac.jp
AF: Depertment of geology, Faculty of Science, Niigata University, 2-8050 Igarashi, Niigata, 950-2181 Japan
AU: Shuto, K
EM: shuto@geo.sc.niigat-u.ac.jp
AF: Depertment of geology, Faculty of Science, Niigata University, 2-8050 Igarashi, Niigata, 950-2181 Japan
AU: Ishimoto, H
EM: f01l221c@mail.cc.niigata-u.ac.jp
AF: Depertment of geology, Faculty of Science, Niigata University, 2-8050 Igarashi, Niigata, 950-2181 Japan
AU: Yagi, M
EM: yagi@rc.japex.co.jp
AF: JAPEX Research Center, 1-2-1 Mihama, Chiba, 261-0025 Japan
AU: Takazawa, E
EM: takazawa@geo.sc.niigata-.ac.jp
AF: Depertment of geology, Faculty of Science, Niigata University, 2-8050 Igarashi, Niigata, 950-2181 Japan
AB: The Sr and Nd isotopic studies on Tertiary to Quaternary basaltic rocks of the NE Japan arc have shown that isotopic characteristics of basaltic rocks found on the back-arc side of the NE Japan arc changed drastically from an undepleted isotopic signature (initial 87Sr/86Sr (SrI)=0.7040-0.7060 and initial 143Nd/144Nd (NdI)=0.51260-0.51284) to a depleted isotopic signature (SrI=0.7030-0.7040 and NdI=0.70284-0.51308) at around 15 Ma (Shuto et., 2004). This feature may have resulted from changes around 15 Ma in the isotopic compositions of the magma source beneath the back-arc side in the NE Japan arc due to the thinning of the undepleted subcontinental lithospheric mantle by upwelling of depleted asthenospheric mantle material during the opening of Japan Sea. Based on major and trace element data as well as SrI and NdI values for Early Miocene basaltic rocks from the back-arc side of the NE Japan arc, we examined geochemical characterization of the magma source (lithospheric mantle) for these basaltic rocks. Early Miocene (23-18 Ma) basalts and associated more felsic volcanic rocks form seven volcanic fields (Okushiri Is., Matsumae Pen., Fukaura, Oga Pen., Honjo, Atsumi and Sado Is.) delineating a 500 km-long array in the back-arc side of the NE Japan arc. In terms of major, trace element and Nd isotopic compositions, two groups of Early Miocene basalts can be distinguished. Group 1 is composed of tholeiitic basalts and alkali basalts, and is characterized by high TiO2 contents (1.5-2.5 %) and high (La/Yb)n ratios ($>$5.5), and high Zr/Y ratios ($>$6). These samples show the chondrite-normalized LREE-enriched patterns and have NdI values ranging from 0.51259 to 0.51282. Group 2 is composed of tholeiitic basalts, and is different from Group 1 by lower TiO2 contents ($<$1.5 %), lower (La/Yb)n ratios ($<$5) and lower Zr/Y ratios ($<$5.5). These samples show modelately LREE-enriched patterns and have NdI values ranging from 0.51250 to 0.51278. In contrast, Middle Miocene (after 15 Ma) basaltic rocks from the Niigata and Honjo areas in the back-arc side of the NE Japan arc have NdI values of 0.51286 to 0.51310, which are higher than those of Early Miocene basalts. These Middle Miocene samples are lower in TiO2 contents and (La/Yb)n and Zr/Y ratios than Early Miocene Group 1 samples, but partly overlap with Early Miocene Group 2 samples. The geochemical differences between Group 1 and 2 basalt magmas possibly originated from subcontinental lithospheric mantle may be due to differences in degrees of partial melting or heterogeneity of the mantle.
DE: 8439 Physics and chemistry of magma bodies
DE: 9320 Asia
DE: 9604 Cenozoic
DE: 8105 Continental margins and sedimentary basins
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting