We report analyses of noble gases and Nd–Sr isotopes in
mineral separates and whole rocks of late Pleistocene (< 0.2 Ma)
monzonites fro
m 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 fro
m the sa
me sa
mples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma.
40Ar/
39Ar yields a si
milar age of 0.29 ± 0.09 Ma. Geoche
mical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt % in K
2O + Na
2O), are si
milar to those of 30 alkali volcanics fro
m Ulleungdo in ter
ms of concentrations of
major, trace and REE ele
ments. The initial Nd–Sr isotopic ratios of the
monzonites (
87Sr/
86Sr = 0.70454–0.71264,
143Nd/
144Nd = 0.512528–0.512577) are co
mparable with those of the alkali volcanics (
87Sr/
86Sr = 0.70466–0.70892,
143Nd/
144Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanis
m (0.24–0.47 Ma). The high initial
87Sr/
86Sr values of the
monzonites i
mply that seawater and crustally conta
minated pre-existing trachytes
may have been
melted or assi
milated during differentiation of the alkali basaltic
mag
ma.
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 and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.