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He-Ar and Nd-Sr isotopic compositions of late Pleistocene felsic plutonic back arcbasin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesisof young plutonic rocks in a back arc basin

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Title
He-Ar and Nd-Sr isotopic compositions of late Pleistocene felsic plutonic back arcbasin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesisof young plutonic rocks in a back arc basin
Other Titles
을릉도의 화강암질 심성암에 He, Ar, Ne, Sr 동위원소 분석연구로 맨틀헬륨의 확인과 excess Ar을 연구
Authors
Keisuke Nagao
Tsuyoshi Tanaka
Kim, Kyu Han
Lee, Jong Ik
Toshio Nakamura
Takamasa Hayashi
Hirochika Sumino
Subject
Geochemistry & Geophysics
Keywords
Alkali basaltic magma; Excess argon; Felsic plutonic rocks; Mantle helium; Nd– Sr isotopes
Issue Date
2008
Publisher
ELSEVIER
Citation
Keisuke Nagao, et al. 2008. "He-Ar and Nd-Sr isotopic compositions of late Pleistocene felsic plutonic back arcbasin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesisof young plutonic rocks in a back arc basin". Chemical Geology, 253(3-4): 180-195.
Abstract
We report analyses of noble gases and Nd?Sr isotopes in mineral separates and whole rocks of late Pleistocene (b0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb?Sr mineral isochron age for the monzonites is 0.12±0.01 Ma. K?Ar biotite ages from the same samples gave relatively concordant ages of 0.19±0.01and 0.22±0.01 Ma. 40Ar/39Ar yields a similar age of 0.29±0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K2O+Na2O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd?Sr isotopic ratios of the monzonites (87Sr/86Sr=0.70454?0.71264, 143Nd/144Nd=0.512528?0.512577) are comparable with those of the alkali volcanics (87Sr/86Sr=0.70466?0.70892, 143Nd/144Nd=0.512521?0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24?0.47 Ma). The high initial 87Sr/86Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma. A mantle helium component (3He/4He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd?Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium an
URI
http://repository.kopri.re.kr/handle/201206/5614
DOI
http://dx.doi.org/10.1016/j.chemgeo.2008.05.009
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