HR: 1340h
AN: V13B-1479    [Abstracts]
TI: Middle Miocene to Quaternary Primary Basalt Magmas of North Hokkaido, Japan: Segregation Depths and Degrees of Partial Melting of Mantle Peridotite
AU: * Ishimoto, H
EM: F01L221C@mail.cc.niigata-u.ac.jp
AF: Depertment of Geology, Faculty of Sciense, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Shuto, K
EM: shuto@geo.sc.niigata-u.ac.jp
AF: Depertment of Geology, Faculty of Sciense, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Takazawa, E
EM: takazawa@geo.sc.niigata-u.ac.jp
AF: Depertment of Geology, Faculty of Sciense, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Goto, Y
EM: ygoto@mmm,muroran-it.ac.jp
AF: Department of Civil Engineering and Architecture, Muroran Institite of Technology, Mizumoto-cho 27-1, Muroran, 050-8585 Japan
AB: On the basis of Sr and Nd isotopic studies on middle Miocene to Quaternary basaltic rocks from north Hokkaido (NH basaltic rocks) and the back-arc side of the NE Japan arc (BA basaltic rocks), the generation of both NH and BA basaltic rocks is considered to be closely related to mantle evolution which involves upwelling of asthenosphere into the continental lithosphere beneath north Hokkaido and the back-arc side of the NE Japan arc, at the same time as spreading of the Kurile back-arc basin and Japan Sea back-arc basin. Most of the NH basaltic magmas younger than 12 Ma and the BA basaltic magmas younger than 15 Ma were derived from the upwelled asthenospheric mantle which is slightly different from MORB source in terms of major and trace element characteristics such as TiO2/K2O and Zr/Y as well as Sr and Nd isotopes. Using the olivine maximum fractionation method, primary magma compositions were calculated for the NH primitive basalts (younger than 12 Ma) from eight locations in north Hokkaido, Japan. The modelled primary basalt magma compositions suggest segregation depths of about 23-56 km, with no across-arc variation in terms of segregation depth. Based on Fo versus NiO relations for measured and calculated equilibrium olivine compositions, the relation between Cr/(Cr+Al) ratios in chromian spinel inclusions and Fo content in host olivine phenocrysts, and the N-MORB normalized pattern of HFS elements, and the chondrite normalized patterns of REE of calculated primary magmas, most of the 10-12 Ma, 7-9 Ma, and $<$ 3 Ma primary magmas were probably derived from a common mantle peridotite source, which has depleted Sr, Nd isotope ratio, but by different degrees of partial melting, with the degree of partial melting decreasing with age. Generation of these primary magmas has been associated with twice asthenospheric mantle upwellings accompanied by spreading of the Japan Sea and Kurile back-arc basins. The older basalt magmas have been produced from high temperature mantle material beneath the northwest Hokkaido during upwelling at 10-12 Ma, by greater degrees of partial melting. The younger basalt magmas were generated from similar material under the northeast Hokkaido during upwelling at 7-9 Ma, at lower temperatures and by smaller degrees of partial melting. The older Ohmu magma may be derived from the subcontinental lithospheric mantle, which was thinned by the upwelling of the asthenospheric mantle, which is characterized by undepleted Sr and Nd isotopic compositions, and lower Zr/Y ratios than the upwelled asthenospheric mantle material.
DE: 9320 Asia
DE: 9604 Cenozoic
DE: 3625 Descriptive mineralogy
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting