Nagoya University
NagoyaUniversity

Radiometricages

andNd-Sr isotopic compositions

from the Ulleungdo yolcanicrocks

offelsicplutonicisland,South Korea

Han Kima, Keisuke Nagaob, Tsuyoshi TanakaC,HirochikaSuminob, Takamasa
HayashiC,Toshio Nakamurad, Kazuhiro SuzukiCand Mitsum Okunoe

Kyu"Department

Education,Ewha PVbmans Uhivensity,
SeouL Kbrea
ofSeience
Earthquake Chemistt:y,Graduate School ofScience,
Uhiversity
7bkyo,
of'7bkyo,bLaboratotyfor.lapanClitiversity,
Nbgqya , .Jbpan
qf'Earthand EhvironmentalSciences,?Vltgop?aandimterialResearch Center,Ndgaya Clhivensity,lapan

CDepartment

dDatinge

flacultyofScience.
Jilefkuoka
Uhiversity,
Fletkuoka,
Jtipanis aThe Ulleungdoin size)withvalcanic

activities areasmallsubmarine

to subaerial

dormant strato

volcano(12km 10 km×Nari caldera(2 km in diameter)at the summit.

The Ulleungdo

divided,onthe basisof lithology,
occurrence

andradiometricMa, basaltic
Stage 1 (1.37-O,97
agglomerate,basalticlava,picritic
basaltand trachyte),Stage 2 (O.83-O.77
Ma, trachyte,
trachybasalt and trachyte agglomerate), Stage 3 (O.24-O.73
Ma, trachyte,trachyandesite,trachybasalt, phonolite,trachyte agglomerate), Stage 4 (9300-6300
YBP, pyroclatics,tuff block,pumice and ash) and Stage 5 (O,OlMa, leucitebearingtrachyandesite).
Numerous fragments of felsicplutonicrocks (monzenites)
werefound in therecent (6300-9300
YBP) tephra fbrmed by the Stage 4 eruption of the Ulleungdo
volcano,an oceanicisland,which consists mainly of Quaternary
alkali volcanics,
South Korea,
This unusual occurrence of felsicplutonicrocks in the tephra fbrmationgives ustheimportant inforrnationabout felsic (monzonitic)
magmatism
andbasement
underlying
the alkali volcanic province of a back arc basin.
Herein we document theage,

into 5stages

in the order of eruptionldeposition:

fragmentsof Ulleungdo rnonzonites,
present their age and propose apetrogeneticmodel for the evolution of the extremely young plutonicrocks. We alsomagmasindiscuss the origin and source characteristics of the felsic(monzonitic)
terms of Nd and Sr isotopesand whole rock geochemistry and the relationshipbetween volcanic and plutonicrocks in a back arc basinlocated along the active
occurrence

ofcontinental

marginof the southeastern

cornerof the Eurasianplate.

Geochemical characteristics of the felsicplutonicrocks, which are typically silica
as monzonites,
are similar
toundersaturated
alkali felsic
rocks, which can be classified-25-

NII-Electronic Library ServiceNagoya University
NagoyaUniversity

those ofandalkali

volcanics

in the Ulleungdo in terrns ofconcentrations

ofmajor,trace

RI]E elements,

The Rb-Sr mineralisochronage fbr the monzonites
is O.12 Ma, K-Ar biotite
agesfrom the same monzonitesamples
gave relatively concordant ages of O,19 Ma. The
initialNd-Sr isotopicratios of the monzonites are also comparabledwiththose of the87Srf86Sr
during Stage 3 Ulleungdo votcanism,
The high initialvaluesof the monzonites
imply that seawater
andcrustal
contaminatedpre-exisited
trachytic rocks
may
have been melted
or assimilated
during differenciationofthealkali basaltic
magma.
The radiometricages,
and Nd-Sr isotopicsigriaturesofgeochemical characteristics
the Ulleungdo monzonites
strongly suggest that the felsic(monzonhic)
magma
wasalkali volcanics

eruptedStage 3 alkali basalticmagmaviafractionating
felsictrachytic magma,
fbrmed in the back arc basin and related to the opening of the East
Sea and the Japan Sea.
cogenetically

evolvedfrom-26-

NII-Electronic Library Service