Jour. Geol. Soc. JaPan, Vol. 105, No. 9, p. 625-642, September 1999
Coeval volcanism due to interaction of back-arc
basin basalt (BABB) magma with the island-arc
crust in the late Miocene Engaru volcanic field,
northeastern Hokkaido, Japan:The evidence of Sr
and Nd isotopic ratios combined with major- and
trace-element compositions
Abstract
Satoshi Yamashita*, The.Iate Miocpne Engaru volcanic field in northeasterR
Hokkaido contains basalts, Temeoka basalt (TM) and
Kenl'IShUtO**, chiyoda-Kakurezawa basalt (CK), and rhyolite, Wakamatsu
YaszeyuklKakihara*** '
,i ,i it te,[sWKK),),.::Ighh.s,u.bpotr.ddinda.t,einagm7o.ugnMz,ofandesite•Sakaeno
.h.YdO
and Hiroo KagaMi*** Both the TM and CK basalts have geochemical charaeteristics
similar to those of baek-arc basin basalt (BABB). Based on
ReceivedJuly23,lgg,s. differenees in major and trace element abundan.ces and the
Accepted April 2o, lggg. initial values of Sr and Nd isotopic ratios (Srl and Ndl), the CIK
' Fukken Gijutu consultants co.Ltd. basalt, SK andesite and WK rhyoLite can be divideq into two
Nishiki-cho IL7-2s, Aoba-ku, sendai gso- tYPeS (I and II), respectively. The SK type I andesite and WK
oo12, Japan typeIrhyolite are of calc-alkaline series, whereas the SK type
"' Department of Geology, Faculty of sci- II andesite (including icelandite-Iike andesiteÅr and WK type II
ence, Niigata University, Ikarashi 2-machi rhyolite are of tholeiitic series.
8050, Niigata 95o-21sl, Japan The WK type I rhyolite has remarkably high Srl and low Ndl
*"' Graduate School of Science and Tech- ValUes comPared with other volcanic rocks from the study area.
nology, Niigata university, Ikarashi 2- The similarity in Srl of the rhyolite to S--type granitoids and
machi 8050, Niigata 9sO-2181, Japan PelitiC-PSaMMitic rocks of the Hidaka belt suggests the crllstal
origin of the rhyolite. The combined inajor- and traeeelement and Sr- and Nd- ,isotopic data indicate that the main
v generation process of the SK typeI andesite was the mixing of
basaltic magma (the CK type I basalt) with some felsic magrnas.
The felsic magmas are the WK type I rhyolite magma and
rhyolitic magmas having higher Sr aRd Nd contents, and higher
Srl and lower Ndl values than the WK type I rhyolite. On the
other hand, the SK type II andesite and WK type II rhyolite may
•- have been derived from the CK type II basaltic magma by
fraetional crystallization aceompanied by a minor degree of
assimilation of crustal materials,
The genetic relationship of these coeval basalts, andesite and
rhyolite can be attributed to spreading of the Kurile basin,
BABB magma which is a partial melt in a hot asthenosphere
uprising below the island arc during the basin spreading, could
have heated the erust to generate calc-alkaline rhyolitic
magma. Andesitic rocks were deriyed both by mixing of BABB
magma with the crust-derived rhyolitic magma and by
fractional ctystallization of BABB magma,
Key words : Engaru district, Alorth Hokkaido, Late Miocene, back-arc
basin basalt, crust, magma mixing, fractionat cTystallization, Sr and
Nd isotopes, sPreading of the KuriZe basin
@The Geologiqal Society of Japan 1999 625
626 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
'Åqiib"l,S,,,,,, Elilllili8.",aslle.rn.a.ry,,.v.ol.c,ano
14oeE bss"
7 -xeS"
45eN '• r
Northern Hokkaido se;,SOC
Iilii: 22iZ 6-9Mavolcanics
-4soN RIggiii g-14Mavolcanics
o
Japan Sea
See Okhotsk Sea
&se"oXÅr
Japa"
Basy sNe
Monbetsu Stud Area Fi .2) e
,`e
e
NO"
g
GifWdp•!
N''
ot Engaru baShiri l"' Åí sas '5
)"ceez)e :drP '
-44eN
or
0
50km
:::i::.i •(gÅrscis' ' (C7
-:•::::'
..•.'-" "o
2(ll;i
142eE 144eE
Fig.1. Index map showing the study area and
distribution of Cenozoic
volcanic rocks in northern
Hokkaido, Japan. Boundary of western (WZ) and
eastern (EZ) zones is
slightly modified from
Watanabe and Yamaguchi
(1988).
Introduction rpiddle tO late Miocene• . Recent K-Ar age determipations show that volcamc rocks were erupted during
y
Tertiafy volcanic rocks in north Hokkaido are two periods of volcanic
activity (Yahata and Nishido,
widely distributed•in a back arc region behind the 1995). The first episode (11-13Ma) involves eruption
junction of the Northeast Japan and Kurile arcs (Fig. of andesitic magma, whereas the second episode (7-9
1), K-Ar age data for these volcanic rocks indicate Ma) involves eruption of voluminous basalt and rhyothat an intense volcanic activity has taken place since lite magmas accompanied by small amounts of andes-'
about 14 Ma (e.g. Shibata et al. 1981 ; Watanabe et al., ite magma.
1991;Nakagawa et al. 1993;Goto et al. 1995; In this paper we examine the origin of coeval basalt,
Sugawara et al., 1995;Yahata and Nishido, 1995; andesite '
and rhyolite produced during the second
Aoki et aL, 1999). Recent works have revealed re- volcanic episodq, and show that' this activity was
gional petrological and geochemical differences in causedbytheinteractionofBABBmagmawithisland
middle to late Miocene (6-14Ma) volcanic rocks be- arc crust, resulting from upwelling of asthenospheric
tween the eastern and western regions (Kokubu et al., mantle during spreading of the Kurile basin.
Igg4;watanabe and Yamaguchi, 1988;GOtO et al" stratigraphy
1995;Okamura etal. 1995). Rocks m the eastern
region (northeastern Hokkaido) are characterized by Tertiary volcanic rocks are exposed over a 15km
non-island arc type volcanics, including icelandite- (N-S)Å~12km (E-W) area in the Engaru district, north-
likeandesite,basaltandandesiteenrichedinTi02,and western Hokkaido (Fig.2). Lithologically volcanic
voluminous calc-alkaline dacite and rhyolite. Rocks rocks can be divided into six units ; in the ascending
in the western region (northwestern Hokkaido) are order, the Inaushi andesite (IN andesite), Gakenosawa
characterized by large amounts of calc-alkaline andes- tuff (GK tuff), TM basalt, WK rhyolite, SK andesite
ites.
and CK basalt (Fig. 3). The SK andesite interfingers
Based mainly on petrologic and geochemical data, with the WK rhyolite. The IN andesite and GK tuff
the fo]Iowing models have been proposed as the tec- were possibly erupted in the middle Miocene (11-13
tonic background for the generation of these volcanic Ma), while the TM basalt, WK rhyolite, SK andesite
rocks ; a volcanic activity was caused by back-arc and CK basalt were emplaced during the late Miocene
spreading related to the formation of the Kurile basin (7-9 Ma).
(Goto et al., 1995;Ikeda, l998), due to an increase in 1. Inaushi andesite
the dip-angle of the subducting Pacific plate and The IN andesite is restricted to the south of the
resulting injection of hot asthenosphere (Watanabe, district (Fig. 2), and is composed of massive lava flows
1995), or due to during collision of the Eurasian, and pyroclastic rocks (tuff breccia and volcanic brec-
Okhotsk and Pacific plates (Okamura et al., 1995). cia) with a total thickness of about 500m. They are
The Engaru district is located in northeastern underlain and overlain by sedimentary rocks. The
Hokkaido, about 15km inland from the Okhotsk Sea underlying sedimentary rocks consisit mainly of
coast (Fig. 1), where an intense terrestrial volcanism sandstone with intercalations of conglomerate and
associated with regional uplift took place during the mudstone (50 to 70 m thick), and unconformably over-
Jour. Geol. Soc. cJopan, 105 (9) Coeval volcanism in the Engaru volcanic field 627
Åë
Inaushi kve Iittl..t:tt'Li'lill'llli:iliSi';SINiN;::ll':Slilil:llliX":Sflllf)li::l'ii;E.li,ll'il'ill'S:N:';l:l:iil.ISi., yo Riyer
L,L tttttttt ;KakurezawaL 's:s:s;s:s't't .:.;..
Legend
tttttttt Chiyoda-Kakurezawa ll'i•:•,i•i':• Gakenosawa tuff
tttttttt basalt(CK) '''`iii:' (GK)
- sakaeno andesite (SK) g EaabnodvS80inneaushi andesite)
liliiliiiliiiii/d'2a:kR,gP///2,Ii'is,"Cgl.:h'goltl"kttWw"KiEEi:ii]zz{•2a:•eerio:c.:.ISSIZ.(LN.i,,.,
l/i,I/ll/li,i/i•i/i,l/Si://l,7Y,,a.k,a,,m•,a,t.s,u,shyoiite(WK)IilSI[IEggP.a,s,e.m,e.nfi.R.o.c,k,,..)
'
YYYYYYY:• Tomeoka basalt(TM) .' inferred
fault
'
'
Fig.2. Geologic map of
the Engaru district. See
Fig, 1 for location.
lie the late Cretaceous Hidaka Formation. 2. Gakenosawa tuff
To the west of this area is extensive exposures of The GK tuff is found as isolated exposures in the
andesite (the Kohonomai Formation), which corre- southandnorthofthedistrict(Fig. 2). Itiscomppsed
sponds both stratigraphically and lithologically to the of beds of weakly altered, gray to white glassy tuff
IN andesite. Two specimens from the Kohonomai containing angular glassy clasts of about lcm in diFormation have K-Ar ages of 11.4Ma and 12.8Ma ameter, The thickness of the tuff beds is uncertain
(Yahata and Nishido, 1995). because of their poor exposure. Geological mapping
628 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
Age
Lithofacies
Stratigraphy
cuyoda-Kakurezawabasalt 7.3Å}O.4Ma
(CK
e
8
g
gg
: g
s
Sakaene andesite (SK) 7.2 Å}O.4Ma
(epiclastic lapilli tuff and tuffi
[FE
(rbyolitic lava 7.0Ma)
LLLLLLLLLLLLLLLL
LLLLLLLLLLLLLLLL
LLLLLLLLLLLLLLLL
tltltttttlt-ltl
NSSSSsNNNNNSNSSS
lllitttlttt/ltt
IM:l;l;l;l:l:l
LLLLLLLL
NSNNSSSS
vvvvvv
vvvvvv
vvvvvv
vvvvvv
vvvvvv
vvvvvv
vvvvvv
vvvvvv
vvvvvv
Tomeoka
basalt (IIM)
8.7Å}O.7Ma
5
co
8
z
([iakenosawa
8
g
s
e
E
tuff(GK)
1:ill:l:iil:
:::::::::l::
]iilgiliili
Sa dst c
Inaushiandesite(IN) '
1 1 .4Å}O.Na
12,8Å}O.7Ma
Clastic rock
gereotaus-
The age of the WK rhyolite is probably about 7Ma,
judging from a K-Ar age of the overlying SK andesite.
lttttttt/ttlttt
5. Sakaenoandesite
NSSSSSSNSNSSNSSS
tttttltltttittt
SNSSSSSNSNSSSSSS
-tttttlt-XttttSNSNNSNSSNSSNSNS
The SK andesite occurs as lava fiows and pyoclastic
t:l::::::::;;::;l:IEiliiilggi
ttttttltttllitl :l
SNSSNSSNSNSSSSSN
rocks in the south of the district (Fig. 2). The maxitttttlltttttttt
SSNSNNNNSSSSSSNS
ttttttittltttll
SNNNSSSNSSS
(cl ticrck) 't3t`eJS.e s:s's'rt'sJs:N:s's;N
o
Layers of felsic epiclastics unconformably overlie
the TM basalt at Miharashi in the east of the district.
mum thickness of lava flows reaches about 200m.
The contact between the SK andesite and the layers of
felsic epiclastics of the WK rhyolite is observed north
of a dam on the Yubetsu River at Sakaeno. The SK
andesite may interfinger with the upper part of the
WK rhyolite (felsic epiclastics).
Yahata and Nishido (1995) obtained a K-Ar age of
7.2Ma for a sample from the Setaniushiyma andesite
lava, which is a stratigraphically and lithologically
correlative to the SK andesite.
6. Chiyoda-Kakurezawabasalt
Basement Rock Small-scale lava plateaus, consisting of basaltic lava
fiows and associated volcanic breccias, are distributed
Fig• 3. Stratigraphic sequences in the study area. K-Ar in tOPOgraphically isolated, two high areas;south-
age data;Yahata and Nishido (1995). west of Chiyoda to Kakurezawa, and northeast of the
study area. The CK basalt unconformably overlies
indicates an unconformable relationship between the the WK rhyolite (Fig. 3). These basaltic rocks afe
GK tuff and the underlying IN andesite, and the prob- lithologically and petrographically similar to each
able eruptive age of the tuff between 11-12Ma and9 other. The total thickness of the CK basalt is about
3,. Tomeokabasalt A basalt specimen' collected in a quarry at
The TM basalt is distributed in the east of the KakurezawayieldedaK-Arageof7.3Ma(Yahataand
district, as massive lava fiows associated with auto- Nishido, 1995).
brecciated facies, and well-stratified hyaloclastite.
the latter consists mainly of fine-grained basaltic PetrOgraphy and mineral chemistry
fragments (2-3 mm in diameter) embedded in a matrix The modal compositions of representative volcanic
of fine-grained volcanic glass. The unit is more than rocks from the five units, IN andesite, TM basalt, WK
200m thick, and shows no direct contact with the rhyolite, SK andesite and CK basalt are shown in
underlying the GK tuff. A K-Ar age of Yahata and Table 1.
Nishido (1995) for the TM basalt is 8.7Ma. 1. Inaushi andesite
4. Wakamatsu rhyolite The lN andesitet's porphyritic with between 16 and
The WK rhyolite, which is extensively distributed 34vo19o phenoerysts of plagioclase (9.6-29.00/o),
in this district (Fig. 2), is composed of three different clinopyroxene,(1.1-4.5%o), orthopyroxene (O.8-5.20/o)
rock facies including layers of clastic rocks, felsic and Fe-Ti oxide (trace). Plagioclase phenocry$ts are
epiclastic lapilli tuff and tuff, and rhyolitic masses. I-3mm long, and some grains have di.sty zones,
Clastic rocks are found at the base of the rhyolite Fragmented and corroded plagioclase gralns are alsQ
flows, and consist of about 30 meters of alternating found. Clinopyroxene and orthopyroxene
beds of well-rounded conglomerates and weakly phenocrysts commonly form glom, erporphyritic agcross-beddedsandstone,s,. Felsicepiclastic.rocksare gregates with plagioclase. The groundmass is
the most abu.ndant constituent of the WK.;rhyolite. hyalopilitic, and contains plagioclas,e microlites.
W. eakly stratified layers Qf lapilli tuff contain sub- 2. Tomeoka basalt ., ,
angulartosubroundedclastsoffibrouspumice,obsid- Phenocrysts in the TM basalt are hard tQ be
ian and rhyolite in a white-colored matrtx of fine- identified from coarse graihs in. the grQundmass.
grained, unconsolidated volcanic glass and crystals of Howeyer, the TM basalt contains 2-6 vol% of euhedralG
biotite and quartz. Rhyolitic masses of mappably and subhedral phenocrysts of olivine (2-5%) and plavarious sizes, possibly consisting of lava fiows and gioclase (less than 10/o). Olivine phenocrysts (O.5-1
lqVa domes, p,66ur at several locations, being mmlong)aregenerallyfreshbutarealteredinpartto
sandWitched by layers of the felsic epiclastics. The clay minerals. Phenocryst plagioclase (O.5-2mm
total thickness of the WK rhyolite ranges from 200 to long) is unzoned. The groundmass is intergranular,
300m -';• • and consists of plagioclase, clinopyroxene, olivine,
Jour. Geol. Soc. JaPan, 105 (9) Coeval volcanism in the Engaru volcanic field 629
Table 1. Modal compositions'
of volcanic rocks from the study area.
SampleNo. IN03 IN06 INIO INII IN13 IN191 TMOI TM02 TM08 TM12
Phenocryst 16.2 32.4 24.8 22.1 20.7 33.6: 2.7 2.3 2.9 5.5
Ol - - - - - -l 2.7 2.3 2.9 4.8
1
Cpx 1.4 2.6 1.2
Ll 4.5 3.91, - ..- - Opx 5.2 O.8 1.1 2.8 1.7 1.8: - i•. - -
Bi - - - - - -l
- •,, -. - .
'
Fe-Tioxide
- '' -18.2
r r14.5
r -,27.9I
- - .- --r o.7
Pl 9.6 29.0 22.5
i
Groundmass 83,8 67.6 75.2
77.9 79.3 66.41 97.3 97.7 97.1 94:5
SampleNo. WKOI WK061 SK03 SK06 SKCr7 SKIO SK13 SK17 SK181 CK04 CK06
(type) (typeII) (type IÅrl (type I) (type I) (type I) (type II) (type II) (typeII) (type II)l (type II) (type II)
Phenocryst 3.2 9.51 15.4 30.7 24.4 18.l 8.1 23.3 17.41 4.2 8.9
Ol - -i - - - - -- -• -i 2.9 6.8
Cpx - --i O.4 2.8 2.9 1.3 2.4 2.8 2.6i - -
Opx - -1 4.5 3.9 2.6 O.5 r O.1 O.71 - -
1I
Pl 3.2 5.41 10.5 24.0 18.8 16.2 5.5 19.6 13.81 1.3 2.1
Qz 968
- 3.ol
- -69.3
-11
.75.6
- -81.9-:91.9
- -76,7 82,61 95.8 91.1
Groundmass
90.5: 84.6
11
-• 1.ll
- -O.1
- - O.2
-1 -O.8
- O.3i -. r
Fe-TiBi
oxide
- ri --r -O.1
SampleNo.
CK12 CK13 CK15 CK17 CK23 CK30 CK31 CK38.CK48 CK56 CK60
(type) (type II) (type II) (type II) (type II) (type II) (type IIÅr (type II) (type II) (type I) (type I) (type I)
Phenocryst 1.0 22,8 11.6 23.4 24.0 5.8 11.0 7.1 5.5 6.8 4.0
Ol O.6 5.8 8.3 7.1 5.7 - 2.4 2.1 2.9 2.8 2.0
Cpx -- r - - - O.1 O.9 O.1 - r -
Bi - - -. -..r -- -- ---- -.-r---- - -Fe-Tioxide
Pl O.4 17.0 3.3 16.3 18.3 5.7 7.7 4.9 2.6 4.0 2.0
QZ - - -,- - - - -- - - -
Groundmass 99.0 77.2 88.4 •76.6 76.0 94.2 89.0 92.9 94.5 93.2 96.0
Ol=olivine, Cpx=clinopyroxene, Opx=orthopyroxene, Bi=biotite, Pl=plagioclase, Qz=quartz, tr-rare (modal qoÅqO.1)
IN=Inaushi andesite, TM=Tomeoka basalt,WK=Wakamatsu rhyolite, SK=Sakaeno andesite,
CK=Chiyoda-Kakurezawa basalt
iron oxides and glassy mesostasis. (Åq 1-30/oÅr, orthopyroxene (Åq 1-50/o) and Fe-Ti oxides
Cores of 10 to 20 grains of olivine phenocrysts for (less than 1%), These minerals are found as not only
four samples show a relatively narrow compositional discrete phenocrysts but also glomeroporphyritic agrange (FoO/o ==74"v85). Most olivine phenocrysts dis- gregates of 2-3mm in diameter.
play slightlynormal zoning. ' The cores of orthopyroxene phenocrysts for six
3. Wakamatsu rhyolite samples (SK02, SK03, SK07, SK13, SKI8 and SKIO)
Based on the phenocryst mineral assemblage, haveanarrowMg-valuerange(Fig.4)showingaun-
rhyolitic rocks (constituting lava flows and lava imodal distribution, whereas those for two samples
domes) can be divided into two types. One type has (SK06 and SK17) have a wide range showing a
5-10volfoO phenocrystsofplagioclase(2-5%),corroded bimodalMg-valuedistributton. Mostorthopyroxene
quartz (2-30%), biotite (rvl%o) and a minor Fe-Ti oxide, phenocrysts have uniforrn core compositions and are
whereas the other type contains only plagioclase zoned normally in Mg in the margins (Fig. 5-A), but
(about 3%) as a phenocryst phase. The groundmass some phenocrysts are zoned reversely (Fig. 5-B).
of the formershowsaheterQ. geneous fe.ature in which Core compositions of clinopyroxene phenocrysts
a colorless glass is mingled with a brown glass con- Iall within ranges of Mg-value between 60 and 68 for
taining plagioclase microlites, whereas the ground- SK02, SK18, SKIO and SK17, ttnd between 68 and 74
mass of the latter is hyalopilitic and less heterogene- for SK13, Clinopyroxene cores in SK06 also show a
ous color. bimodal distribution (Fig. 4).
4. Sakaeno andesite Although cores of plagioclase phenocrystsii'n seven
The SK andesite is porphyritic, containing 8-31vol andesite samples (except $K06) have a relatively
% phenocrysts of plagioclase (6-24%), clinopyroxene narrow compositional range in An% between 40 and
630 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
SK02 (FeO"IMgO= 1.30)
(type I)
cpx
SK13 (FeO*/MgO=3.04) 80
(typell)
=O
cpx ?
-es70
:oo60
1--"'"' -'-'1------n
50
opx
SK03 (FeO*IMgO=1.40)
(type I)
opx
core
OPX "327ptmn
(StyKpi,8 ii(
o
)FeO*iMgO=4•i9)
tSi7o
[lii
T-"--"1- --1---"`--n
SK06 (FeO"fMgO=2.32)
cpx
:.i6o
opx
50
core
(type I)
cpx
SKIO (FeO*IMgO=4.45)
(typeii)tEL/ cpx gefi.5,',,R.e.P.r.e,Se",`,a,ti.'V.e,,S,t8,P?,C.a."ni2ff,Pr,O.fi'eS..a,Cg,9.2S,:
-----Eril,-------,
i--d--.i-dii..----n ShOWing Variation in Mg values.
opx
OPX phenocrysts are found in samples with FeO*/MgO
ratios rnore than 1,6. They are found as stubby prismatic grain's less than 1 mm Iong, being often in cori-
SK17 (Feo*IMgo=s.oo) taCt With Plagioclase phenocrysts and sometimes en.
(typeii) cpx :•108e,S.2;,.P,l•Zg,gZCga.S,e.',,,9Z`.h,O,P,\•g,9X,e."e,,RB2".99r.Y,St.S
R ==2.12 and 52.25 Si02%), occur) as isolated, prismatic
i-'-'""i'--"H-"--'-i euhedral grains. Fe-Ti oxides (less than O.5mm
OPX across) are titanomagnetite. The groundmass is intergranular or intersertal in texture, and consists of
plagioclase, clinopyroxene, Fe-Ti oxides, and glass.
80 Mg-values
70 60 50 Mg-values
80 70 60
s-: 50
The phenocryst assemblage and occurrence of
phenocryst minerals suggest that the crystallization
Fig•4• Frequency of Mg values (Mg/(Mg+Fe)) of Se.qUenCe iS frOM OIivine, through plagioclase and
cores of clino- and ortho-pyroxene phenocrysts from CIMOpyroxene, to orthopyroxene.
the SK andesite. FeO'/MgO ratios are those of whole The cores for 4-25 olivine phenocryst grains in ten
rocks. basalt samples show compositional ranges of Fo =80
t-87 for CK 15, CK 06, CK 12, CK 56 and CK 48, of Fo == 74
56, those of SK06 show a wide compositional range "v84 for CK60, and of Fo =64nv79 for CK04, CK13, CK
from An ==40 to An =88. Although normal zoning is 38 and CK31. 01ivine phenocryst cores tend to grad-
common among phenocryst plagioclases, reverse ually decrease in Fo with FeO*/MgO ratios of the
zonirig is included in the Iatter plagioclas,es. rocks (from less than 1.3 to more than 1.5). They are
5. Chiyoda-Kakurezawa basalt • usually normally zoned.
The CK basalt has 4 to 24 vol% phenocrysts, mostly Clinopyroxene phenocrysts in four specimens exless than IO%. ,Based on phenocryst assemblages, the amined have the core with the Mg-values of 68N82 for
basalt can be•,,divided into two types;one contains CK13, CK38, CK30 and CK31. 0rthopyroxene
olivine (Åq '1 -8%) and plagioclase (Åq 1- 19%) phenocrysts in CK30 range in Mg-value from 68 to 76,
phenocrystsj andLt.he other contains olivine (2-3%), indicating partial equilibrium with the coexisting
clinopyro•xene (Åq•.-19o) and plagioclase (2 - 8%) clinopyroxene with the core of 74 to 76 as Mg-value.
phenocrysts, Fe7Ti. oxide phenocrysts are rare in Both clino- and ortho-pyroxene are usually normally
:both [types. -f One exceptional sample (CK30) contains, . zoned.
clinopyroxene, orthopyroxene and plagioclase
phenocrysts but no olivine.
Geochemistry and Sr- and Nd- isotopic compositions
of the late Miocene (7-9 Ma) volcanic rocks
Olivine phenocrysts aqe up to 2mm in length, and
partly altered to clay minerals. Plagioclase 1. Majorandtraceelementcompositions
phenocrysts are1tQ3mm in length. Clinopyroxene Major and trace element analyses of ninety-eight
cJoux Geol. Soc. cJaPan, 105 (9) Coeval volcanism in the Engaru volcanic field 631
988 ee Uth za- #G as "th -n eg F vae
ee oo ; t., ". pt ge O. ".
ti $- ve Nv O- xt th NON8t
.-co VÅr co MO st` O en pt N ,-- ts oo
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632 S. Yamashita,KShuto, Y. Kakihara and H. Kagami 1999-9
6
A CK typel Å~ SK typel " WK typel
'A CK type l X SK type l e WK typel
O CK type ll + SK type ll Q WK type ll
U CK type ll + SK type ll Q WK type ll
e TM
e TM
80
A4
lvEEitD
70
A
VN
eptb
O.
M2
o
•." 60
50
o
(B)
g•
5s
66
NN
Z4
40
MN
FeO"IMgO
Fig.7. FeO'/MgO vs. Si02 diagram for the 7-9Ma
volcanic rocks from the study area. Line shows the v
boundary between the fields of calc-alkaline series (CA)
and tholeiitic series (TH) (after Miyashiro, 1974•).
2
'
40 50 60 70 80 alkali tholeiite and high-alkali tholeiite
fields (Fig• 6' -
Si02(91o) B).
The CK basalt, SK andesite and WK rhyolite can be
Fig.6. Si02 vs. K20 and .Na20+K20 VariatiOn dividedintotwogroupsinFeO"/MgO-Si02 diagraM
diagrams for the 7-9Ma volcamc rocks from the study
area• The fields of low-, med.- and high-K andesites in (Fig• 7);tYPe I volcanic group (CK type I basalt, SK
Si02-K20 diagram are from Gill (1981) and the fields of tyPe I andesite and WK type I rhyolite) falls in the
LT (low-alkali tholeiite), HT (high-alkali tholeiite) and calc-alkaline (CA) field of Miyashiro (1974), where Si02
AB (alkai basalt) in Si02-(Na20+K20) diagram from content increases abruptly from basalt through an-
Kuno(1968)• desite to rhyolite despite of moderate increase in
FeO*/MgO ratios, whereas type II volcanic group (CK
samples from the late Miocene volcanic rocks have type II basalt, SK type II andesite and WK type II
been determined by X-ray fluorescence spectrometry rhyolite) falls in the tholeiitic (TH) field, where FeO*/
at Niigata University, using analytical methods MgO ratios increase from basalt through andesite to
described by Tamura et al, (1989). Analyses of forty- rhyolite. Since andesites with extremely high FeO'/
four selected samples are listed in Table 2. In the MgO ratios (about 10) among the SK type II andesite
following discussion, major element compositions are are similar to "icelandites" of Carmichael (1964), they
treated on an anhydrous basis, and all analyses have are termed here as icelandite-like andesite. As shown
been plotted in Figs. 6, 7,8and 9. in Figs. 8 and 9, two types of volcanic group have
The Si02 contents of basalt and andesite vary con- different major and trace element abundances.
tinuously from 49% to 660/o, whereas there is a com- TypeIof the CK basalt is relatively enriched in Si02,
positional gap of about 8906o Si02 between the SK an- MgO, Ni and Cr compared with type II. The basalts
desite and WK rhyolite (Fig. 6). Most rocks from the forming lava flows in the Kakurezawa area and the
TM basalt, CK basalt and SK andesite lie within the northeastern part of the study area correspond to type
medium-K andesite field and on its extension of Gill I, whereas voluminous lava flows in the Chiyoda area
(1981) (Fig. 6-A), and also in the high-alkali tholeiite correspond to type II. TypeIof the SK andesite has
field of Kuno (1968) (Fig. 6-B), The WK rhyolite lower Si02, MnO, P2 0s, Na2 0, Ti02, Sr, Zr, Y, Nb and V
shows a wide variation in K20 and Na20+K20 con- abundances than type II. Each andesite type constitents, compared to basaltic and andesitic rocks, and is tutes lava flows which are separately distributed in
plotted in the medium-K and high-K andesite fields the studyarea. The WKrhyoliteshows awide range
and their extensions (Fig. 6-A), and also in the low- for high field strength elements (HFS elements), such
Jour. Geol, Soc, cJaPan, 105 Åq9) Coeval volcanism in the Engaru volcanic field 633
A CK typel X SK typel " WK typel
M CK type II + SK type II Q WK type II
e TM
2
Ti02t "N-.
tN,t
MgO
iN
fssN
s
s
sA
i1s
-Fx
1s
""' lliSIIiliiii--t. ,}s, Å~SsÅ~Å~.
1
A
sN
NN
NN
XN Ns
NN
SN
NS
ss ;liz4;" N .- .
sN S
N.9Q
S"t b'--. e
o
tN' s,
CaO
Al2O3ptlleq
x
16
'. -N
.---NNS
N'sNO NN
NNNNI Nsx
12
NN`9..bl.'
8
12
FeO* ,t x
N
Ns
s- .. gE.itrN
8
Å~-NS
Ns SNN
4
NN
ss
NN9Å~
Ns QNN
sN
NI
o
O.3
MnO
tt
O.2
O.1
t-t +N
•Nsx
il 4Ei+
l
s
N.. N.-+ gX
-b.---- ÅriS(!
"'Ns. QXs
o
P2 0s ,'- "S .
.. - - -- - - --;"' x
O.2
O.1
o
NQ 1
40 50 Si02(91o)
60 70 80Si02(91e)
40 50 60 70 80
Fig.8. Si02 vs.major element variation diagrams for the 7-9Ma volcanic rocks from the study area. Solid line
shows possible mixing lines between the CK type I basalt and WK type I rhyolite. Field enc]osed by broken line
shows the general trend of type II volcanic group.
as Zr, Nb and Y. Type I of the WK rhyolite, contain- trends in Figs. 8 and 9, Major element data of type I
ing plagioclase, quartz and biotite phenocrysts, is volcanic group plot around a single line with increaslower in abundance of HFS elements compared with ing Si02 (Fig. 8). In contrast, the analyses of major
type II containnig only plagioclase phenocrysts. In elements of the SK type II andesite do not plot on the
general, typeIhas higher abundance of LIL elements, trends traced by typeIvolcanic group. ,,
such as Rb, Ba and K20. Each type of the WK rhyo- The differences between types I and II are also
lite also constitutes different lava flows or lava domes. recognized in Si02-trace element variation diagrams
These two volcanic groups show different variation (Fig. 9). On Si02-Sr, Rb, Ba, Zr, Nb, Y and V diagrams,
634 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
(ppm)
16
Nb ...--- -6 b- N.
12
,,ti"
..-/'+
tt QQ5
N
Å~%lrt......-i--"'
A CK typel X SK typel " WK typel
8
O CK type II + SK type II Q WK type II
1,
N t/A.
Å~
4
e TM
(ppm)
-tt
o
60
Y ...ÅqÅr---Å~
//Q o5År
400
.- /'e".t..----ij
300
40,•
Zr ..• "Q oN N.,
....4 Qo)
200
20
o
600
400
1OO
o
Cr
A
tdiNs
400
i e,
A
300
NA-• -- •-}s?)`1-' F- "" - "' ---'
200
200
'"
etr ,
1OO
)eÅq rR++ Q e'
- -" - "" "-++- -- -- --. .Q. - .e. .t
o
o
Ni
150
150
,E,
t Os
1OO
1,A
1OO
50
o
400
3OO
200
50
o
1OOO
/N
v /[iSfx
800
lltl
ltIl
i tt
s s
NX
600
400
SNNNNNNs sibÅqiSitSii "' '" "- - "
1OO
o
sN .- ....
200
s
'
o
40 50Si02(9o)
60 70 80
40 50 60 70 80
Si02(91o)
Fig. 9. Si02 vs. trace element variation diagrams for 7-9Ma volcanic rocks from the study area. Solid line and
field enclosed by broken line are the same as in Fig. 8.
cJour. Geol. Soc. c.TaPan, 105 (9) Coeval volcanism in the Engaru volcanic field 635
Table 3. Sr and Nd isotopic data for the late Miocene volcanic rocks from the study area.
N2 P.:•tOf .Sulllllgeler RtyOpC,k evRb/"sr orsr/s6sr srl (pSpMm) (plj:,) '`'sml'"Nd '`3Ndli"Nd Ndl (AMg.e)
CK CK13 Ba(II) O.024 o.7o3372Å}17o.7o336gÅ}17 5 18 O.1679 o.s12g5oÅ}15o.512933Å}15 7•3
basalts CK15 Ba(II) O,151 o.7o32sgÅ}14o.7o3273Å}14 5 19 O.1591 o.s12g6gÅ}290.512953Å}29 7-3
CK60 An(I) O.265 o.7o32gsÅ}14o.7o327oÅ}14 4 17 O.1423 o.s12g4sÅ}270.512934Å}27 7•3
CK63 An(I) O.243 o.7o337sÅ}13 o.7o33soÅ}13 4 14 O.1727 o.s12g62Å}7oo.512945Å}70 73
TM TM02 Ba O.135 o.7o33ssÅ}13 o.7o333sÅ}13 4 19 O.1273 o.s12gslÅ}17o.s12975Å}17 8•7
basalts. TM08 Ba O.135 o.7o337oÅ}12o.7o33s3Å}13 5 20 O-1512 o.s12gssÅ}310.512967Å}17 8•7
i,,TM12 Ba O.143 o.7o33ggÅ}13 o.7o33slÅ}13 5 20 O•1512 o.s12gs7Å}41o.s12g39Å}41 8•7
SK SK02 An(I) O.372 o.7o4142Å}12o.7o41o4Å}12 3 14 O•1296 o.s12s63Å}42o.s12s5oÅ}42 7-2
andesites SK04 An(I) O.401 o.7o43o6Å}11o.7o426sÅ}12 2•2 7•8 O•1705 O.512s23Å}14o.512815Å}14 7-2
SK08 An(I) O.454 o.7o3s6sÅ}13 o.7o3s22Å}12 3-1 10 O•1874 o.s12g21Å}13o.s12g12Å}B 7•2
SK09 An(I) O.527 o.7o41ssÅ}14o.7o413sÅ}12 3-6 13•5 O•1612 o.s12sloÅ}12o.s12so4Å}13 72
SKIO An(II) O.466 o.7o3746Å}16o.7o36ggÅ}12 3 12 O•1512 o.s12g47Å}3oo.s12g4oÅ}3o 7•2
SKII An(II) Q404 o.7o37gsÅ}14o.7o37s7Å}14 2-7 8 O-2041 o.s12ss4Å}14o.s12s74Å}14 7•2
y SK12 An(II) O.416 o.7o3746Å}14o.7o37o3Å}14 2•8 8•8 O•1924 o.512g16Å}120.512go7Å}12 7•2
SK13 An(II) O.468 o,7o3s23Å}14o.7o3476Å}14 7 30 O•1411 o.s12gs6Å}22o.s12g42Å}22. 7t2
SK15 An(II) O.393 o,7o37sgÅ}lso.7o371gÅ}ls 6.2 20.3 O•1847 o.s12ss4Å}11o,s12s75Å}11 7•2
SK17 An(II) O.469 o,7o3722Å}14o.7o367sÅ}14 7 26 O.1628 o.s12g32Å}3oo.sl'2g16Å}3o 7-2
SK18 An(II) O.401 o.7o3724Å}o7 o.7o36s3Å}o7 6.4 21•1 O.1834 o.s12sg4Å}13o.512ss5Å}13 7•2
SK19 An(II) O.517 o,7o3777Å}14o.7o372sÅ}14 5.9 19.7 O.1811 o.s12sgsÅ}13o.s12ssgÅ}13 7•2
WK WKOI Rhy(II) 1.530 o.7o3727Å}14o.7o3s71Å}14 77 26.8 O.1733 O.512glOÅ}14Q512902Å}14 7.0
rhyolites WK02 Rhy(I) 10.443 o.7o63ssÅ}12o.7os347Å}12 3•3 10.6 O•1882 O,512781Å}140.512772Å}14 7•O
WK03 Rhy(II) 2.993 o.7o3s73Å}13 o.7o3s74Å}13 9 37 O.1471 o.512g50Å}400.512936Å}40 7iO
WK04 Rhy(II) 2.321 o.7o3734Å}14o.7o3so3Å}13 6.9 24.5 O.1703 o.512922Å}130.512914Å}13 7•O
" WK05 Rhy(II) 1.312 o,7o3sg4Å}13 o.7o3764Å}13 8.3 29.5 O.1701 o.512sg4Å}100.512886Å}10 7.0
WK06 Rhy(I) 4.494 o,7osnlÅ}2o o.7o4723Å}2o 3 15 O.1209 o.s12solÅ}25o.5127s9Å}25 7•O
WK07 Rhy(I) 3.678 o.7os466Å}21 o.7oslooÅ}21 3 10.2 O.1778 o.512789Å}130.512781Å}13 7•O
WK08 Rhy(I) 9.023 o.7o62ggÅ}12o.7o54o2Å}12 3 9.4 O.1930 O,512792Å}130.512783Å}13 7•O
WK09 Rhy(II) 1.333' o.7o3sosÅ}13 o.7o3676Å}13 6.9 23.9 O.1745 o.512s99Å}100.512891Å}10 7nyO
WKIO Rhy(I) 3.725 o.7oso2gÅ}14o.7o46sgÅ}14 3.3 11.6 O.1720 O.5127gsÅ}130.512790Å}13 7•O
Ba;basalt, An;andesite, Rhy;rhyolite,I;type I, II;type II, Srl;initial 87Sr/86Sr ratio, Ndl;initial i`3Sr/i"Sr ratio.
K-Ar ages are from Yahata and Nishido (1995År. Rb and Sr contents are shown in Table 2.
the data of type I volcanic group plot on or near the University. 87Sr/86Sr and i43Nd/'`4Nd ratios were
straightlines. ChromiumandNirichsamplesofthe normalized to 86Sr/88Sr==O.1194 and i`6Nd/'"Nd==
SK typeIandesite plot on a tie line between the CK O.7219, respectively. Blank tests with the present
type I basalt and WK type I rhyolite (Fig. 9År, On the analytical procedure yielded errors of 1.0ng for Sr,
other hand, Zr, Nb and Y contents of type II volcanic O.06 ng for Sm and O.6ng for Nd.
group gradually increase with the increase in Si02. Sr isotopic ratios for NBS987 were measured ten
This is quite different frorn the variation trends of times during this study, the average being O.710251Å}
type I volcanic group in these diagrams (Fig. 9). In O,OOOO03 (2o, N=51). Rb and Sr concentrations were
Si02-Cr and Ni diagrams, the contents of Ni and Cr of determined by X-ray fiuorescence. The mean '`3Nd/
the CK type II basalt and SK type II andesite occupy i"Nd ratio of JB-1a was O.512782Å}O.OOOO07 (2a, N=9).
to the below from the tie lines connecting the CK type During the course of this study, we also measured
I basalt with WK type I rhyolite, and decrease with i`3Nd/i"Nd ratios of two the La Jolla and JNdi-1
increasing Si02. The WK type II rhyolite has higher standards, the latter of which is a new standard proNi and Cr contents as same as those of the SK type II vided from the Geological Survey of Japan. The La
andesite. Jolla standard gaveamean i43Nd/i44Nd ratio of
2. Sr and Nd isotopic cornpositions O.511851Å}O.OOOO06 (2o, N=17) and JNdi-1 standard a
A. Analytical procedures mean i`3Nd/i"Nd ratio of O.512106Å}O,OOOO03 (2o, N==
Extraction of Rb, Sr, Sm and Nd from rock powder 44). Sm and Nd contents were determined with the
has been described by Kagami et al, (1987). The mass mass spectrometer, using a i49Sm-i50Nd mixed spike.
spectrometric analysis follows the procedures of B. Results
Kagami et al. (1987, 1989) and Miyazaki and Shuto The analytical results and Srl and Ndl values are
(1998). The mass spectrometric analysis was con- presented in Table 3.
ducted using MAT262 mass spectrometer at Niigata Three specimens from the TM basalt have Srl be-
636 S. Yamashita,KShuto, Y. Kakihara and H. Kagami 1999-9
A CKtypeI
A CK typeI
V(PPM)e TM
OCK type ll Ba (ppm) MCK ype ll
500
le /rv"
/t'g.l"';"' ,,i,,!,S,iF" ]\I9,RB
"IEi
Ag es
,, A
/ 1i DBABB
IAT ii g XX .
400
// //iC [S1 E]..///////
3OO
200
svwce Xei/
/,//zi i. /6/ ..-gLB,-,i,Il.Åq,,-si)
200
1OO ,cl.ifi;/sÅqs-.//csE--x!---
o
o2468 10 12 14 16 18 Oo so loo 150 200 250
Ti(ppm)110oo Zr (ppm)
Fig. 10. V vs. Ti/1000 diagram for basaltic rocks from
Fig. 11. Ba vs. Zr diagram for basaltic rocks from the
6Mh.[it.SBti.?,mY.ld:,f8.g.63n/i',S,lddg.e,f,Ob:a.i,a",:Tg,/(.i'fiada."kdA,aB.iC,I/,bh,2,E:/ll'g`::cj s,S",?.y,agga.,,,Fis.'d,sKio6.igia29,aE,c,.Pas,ag.`si,gg,ckha.rs
are after She'
rvais (1982). Tarney (1991).
tween O.70334 and O.70338, and four samples from the higher than those of the typeIrhyolie, but similar to
CK basalt also have a narrow Srl range between those of the SK type II andesite.
o.7o327 and O.70337. Both types I (CK60 and CK63) Generation of late Miocene andesite and rhyolite
and type II (CK13 and CK15) of the CK basalt are
similar'in Srl to each other (Table 3). In contrast, 1. Geochemical characteristics of the Tomeoka and
samples from the SK andesite show a wide variation Chiyoda-Kakurezawa basalts
in Srl from O.70337 to O.70427, and the difference be- Asshown in Figs.6and 7, the CK basalt is composed
tween types I and II of the SK andesite. The former of relatively primitive tholeiitic basalt (with FeO*/
has both higher Srl (O.70410-O.70427) and lower Srl MgO ratios less than 1.2), evolved basalt, and basaltic
(O.70352) than the latter;Srl values of type II are andesite, all of which are characterized by intermedi-
between O.70348-O.70376 which are slightly higher ate K20 and Na20+K20 contents. The CK type I
than those for the TM and CK basalts. Eleven speci- basalt is characterized by higher abundance of Si02,
mens from the WK rhyolite show an wider variation MgO, Ni and Cr, cornpared to type II basalt. The TM
in Srl than the SK andesite, and the difference be- basaltis also tholeiitic, butshowsless variability inits
tween typesIand II of the rhyolite. Type Irhyolite chemical composition (51.17-53.15% Si02 and 5.67has Srl values ranging from O.70466 to O.70540, the 7.03906 MgO) (Table 1). The TM basalt is similar in
highest values among the late Miocene volcanic rocks major- and trace- element contents to the CK type II
in the Engaru district, while the type II rhyolite has basalt rather than type I basalt (Figs.8 and 9).
the Srl values between O.70350 and O.70376, similar to The Ti vs. V diagram (Fig. 10) shows that the CK
those for the SK type II andesite. and TM basalts have higher Ti contents than those of
Ndl values of the TM and CK basalts are between island arc tholeiite at similar V contents. These
O.51293-O.51298. These values are higher than those basalts are similar to MORB and BABB in terms of
of the SK andesite and WK rhyolite. In contrast with Ti-V relation (Fig. 10). The Zr vs. Ba diagram (Fig.
Srl values, most Ndl values.of the SK type I andesite 11) also shows that most rocks of the CK and TM
arelowerthanthoseoftheSKtypellandesite. Type basalts are plotted in the field of the BABB. The
I of the WK rhyolite has Ndl values between O.51277 N-MORB-normalized incompatible element spiderdiaand O.51279, the lowest values among the measured gram for the CK and TM basalts, some island arc
rocks. The type II rhyolite has Ndl values which are tholeiitic basalts from the NE Japan arc and a sample
cJour, Geol. Soc. ,JaPan, 105 (9) Coeval volcanism in the Engaru volcanic field 637
of BABB from East Scotia Sea is shown in Fig. 12,
This figure shows that HFS element abundances of 100
the CK and TM basalts are higher than those of MORB
and island arc tholeiitic basalts, but similar to those of
BABB.
Thus, several trace element signature of the 7-9Ma
10
basalts in the study area is different from that of
island arc basalts and MORB, but resembles the chem- iF iX
istry of BABB. Ikeda (1998) has argued, on the basis of ee
major and trace element contents and elemental -
" from northeast- 8 1
ratios, that the 7-9Ma basaltic rocks
ern Hokkaido (including the present study area and ts
its environs) have geochemical characteristics of cgl
BABB, The present data support his idea, V
2. 0rigin of calc-alkaline WK type I rc
rhyolite
n4
O.1 and
SK type I andesite- partial melting of the crust o
and mixing of basaltic and rhyolitic magmas :iE
The WK rhyolite is characterized by 1) a wide 2
variational range in incompatible element contents N
(such as K20, Na20, Rb, Ba, Zr, Nb and Y) compared .9 10
(B) -'-C)- TM02
t SAPO03
t ASRO06
----"
'- --'-- --- --- - East Scotia Sea
;lr,it,h5:2T,I)f,I.g,nd,,P.5,gfi2a,gts,s"S..g.K.a,".le,xige87,`il:zh6,• S
contents and Srl- and Ndl-values betWeen type l and ll U)
rhyolites (Figs.6and9; Table 3), and 3) higher Srl and 1
lower Ndl values of type I rhyolite compared the TM
and CK basalts (Table 3). The latter indicates that
the WK type I rhyolite cannot be formed by simple
fractional crystallization of these basaltic magmas.
Crustal rocks of the Hidaka belt show a large range O-1
in Srl from around 07o2s to hi her than o7o6o Sr K Rb Ba Nb P Zr Ti Y
(Shibata and Ishihara, 1979a, b;Ikeda, 1984, 1991;
Owada and Osanai, 1989 ; Owada et al. 1991 ; Shimura,
1992 ; Maeda and Kagami, 1994 ; Honma and Fig•12• Abundance pattern of incompatible elgments
for the CK basalt (A) and TM basalt (B) normalized to
Fujimaki, 1997). The Srl values of type I rhyolite are
. . ". N-type MORB of Pearce (1982). Solid square and solid
similar to those of S-type granitoids and PelitiC- circle are low-K island arc tholeiites from Asari-dake
psammitic rocks among them. The rhyolite is and Sapporo-dake in the western region of Sapporo,
peraluminous (Al/(Ca+Na+K)=År1.0), and rich in K, respectively ÅqNakagawa, 1992). East Scotia Sea basalt
Rb, and Ba (Figs. 6 and 9). These features suggest (D23•5) is from Saunders and Tarney (1979). Basalts
either the rhyolitic magma may have assimilated frOM the study area are selected less evolved basalts.
granitoids and sedimentary rocks, or the source mate-
rial of the magma may have contained these crustal
materials. Neither xenolithic fragment nor rhyolite are similar to those of I-type granitoids,
xenocryst derived from crustal materials was ob- gabbros and MORB-type basalts of the Hidaka belt
served in the WK type I rhyolite, suggesting that (Owada and Osanai, l989;Shimura, 1992;Maeda and
crustal assimilation had not an important effect on Kagami,1994;HonmaandFujimaki, 1997). However,
the generation of the rhyolite, Therefore, it is possi- the type II rhyolite Srl values are not distinguishable
ble that type I rhyolite was derived by partial melting from those of the SK type II andesite, and difference in
of the crust. Watanabe et al. (1995) also reported the Srl values between type II rhyolite and the basaltic
Srl values mostly lower than O,7040 for the middle to rocks from this district is very small. Thus, it is not
late Miocene rhyolites from northeastern Hokkaido, clear from these chemical characteristics and Sr
and argtued that these rhyolites resulted from partial isotopic features whether the origin of type II rhyolite
melting of the oceanic crust based on similarity in the is crustal melting or not.
Srl values between these rhyolites and mafic rocks of The large differences in Srl and Ndl between the SK
the Hidaka belt. typelandesite and basaltic rocks (TM and CK basalts)
On the other hand, type II rhyolite is also peralumi- clearly indicate that type I andesite could not be
nous, but poor in K, Rb, and Ba compared with type I derived by simple fractional crystallization of the pri-
rhyolite (Figs. 6 and 9). The Srl va]ues of type II mary basaltic magma. The SK typeIandesite (such
638 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
e TM m CK type ll
A CK type l + SK type ll
Å~ SK typeI Q WK type ll
O.7060
(A)
" WK typeI
as sample SK 06) has phenocrysts showing disequilibrium rela'tion to the coexisting liquids;that is, 1)
bimOdal core compositions for orthopyroxene and
clinopyroxene phenocrysts, and 2) reverse zoning of
orthopyroxene and plagioclase phenocrysts. These
phenomena suggest that the SK type I andesite
magma resulted from mixing of magmas with differ-
"t
O.7050
ent compositions.
Basaltic rocks in this district are characterized by
remarkably lower Srl and higher Ndl values com-
N
pared with the WK type I rhyolite, and the SK type I
ca
b
andesite has Srl and Ndl values which are roughly
intermediate between those of the basalts and WK
type I rhyolite. This suggests that the generation of
the SK type I andesite can be ascribed to the mixing of
O.7040
the CK type I basaltic magma with the WK type I
rhyolitic magma.
A mixing line (a) in Si02 vs. Srl diagram (Fig. 13-A)
O.7030
O.51 30
connects the end-member pair of the CK typeIbasalt
(Si02=50%, Sr=300ppm, Srl=O.7033) and the WK
(B) 1
type I rhyolite (Si02=800%, Sr=55ppm, Srl=O.7051).
A mixing line (d) in Si02 vs. Ndl diagram (Fig. 13-B)
x same pair of the basalt (Si02==50%, Nd=
connects the
16 ppm, Ndl == O.51296) and the rhyolite (Si02= 80%, Nd
O.5 l 29
=11ppm, Ndl==O.51278). Although one SK type I an-
e
z
desite sample plots on two mixing lines (a) and (d) in
Fig. 13, other three SK type I andesite samples plot
away from these mixing lines.
The following possibilities for the wide deviation
from the mixing lines are considered ; 1) fractional
crystallization subsequent to mixing, 2) alteration or
crustal assimilation after mixing, and 3) heterogeneity
in Sr and Nd contents, and Srl and Ndl values of the
O.5 1 28
fe
O.5 1 27
45 55 65 75 85 rhyoiitic end-member.
Si02(9e) The fractional crystallization of the mixed magma
with about 75% Si02 (points M and N in Fig. 13) may
result in the formation of rocks with Srl and Ndl
Fig.13. Si02 vs. Srl and Ndl diagrams for late values similar to those of the above three andeistes
ini'
iM
oXOLCaaigdgatreOdCksSjmfrp9eMmtihxeinSgtUcduYrveasr,ea6etiOeiei: because87sr/86srandi43Nd/i44Ndratiosdonotchange
eOsCesnhe
the CK type I basaltic magma and rhyolitic magmas thrOUgh fractional crystallization. However, it is imwith different Srl and Ndl values, and Nd and Sr possible to obtain andesitic compositions with Si02
contents, which were drawn using equation of contentofaround600/obyfractionalcrystallizationof
Lqngmulr et .al. (.1978). End-me{nber compositiOnS Of such mixed magma enriched in Si02. These andesite
mixmg lmes m Fig. 13-A ; basaltic magma : Si02 == 50%,
samples
do not contain
Srl=O,7033, Sr =300ppm, rhyolitic magma
: Si02=80%,
xenolithic fragments and
Srl=O.7051, Sr=55ppm for mixing line (a), sio,==so%, XenOCrYStS, and thier phenocryst minerals and
Srl=O.7054, Sr=300ppm for mixing line (b), and Si02= groundmass are generally fresh. These suggest that
800/o, Srl=O.7060, Sr=250ppm for mixing line (c). the firstandsecond featuresdid not giveasignificant
End-mgmber composltions of mixing lines in Fig• 13-B ; effect for the wide deviation of andesite plots from the
basaltic magma:Si02==50%, Ndl=O.51296, Nd=16
mixing lines in Fig. 13.
ppm, rhyolitic magma : Si02=80%, Ndl=O.51278, Nd=
11ppm for mixing line (d), Sio2=so/o, Ndl==o.s127o, Nd If the felsic end-member of the mixing line (b) in Fig.
==25ppm for mixing line (e) and Si02=80s06, Ndl= 13-A is represented by rhyolite with Si02 content of
O.51265, Nd=30ppm for mixing line (f). Field of type II 80% and Srl value of O.7054 (highest value among the
volcanic group is enclosed by broken line• Points M wK typel rhyolite samples), its Sr content is calculatand N on mixing lines (a) and (d) show the mixed
ed to be 300ppm. This value is higher than the avermagma with 75% Si02.
age Sr content (55ppm) of the WK typeIrhyolite.
Using Srl value of O.7060 and Si02 content of 80%, Sr
content of the felsic end-member on the mixing line (c)
Joux Geol. Soc. eJaPan, 105 (9) . Coeval volcanism in the Engaru volcanic field 639
in Fig. 13-A is estimated to be 250 ppm, This Sr con-
tent is still higher than the average value of the wK M CK type ll + SK type ll
type I rhyolite. The data of three andesite samples O WK type ll
plotting away from amixing line (d) lie near other two 10
mixing lines (Fig. 13-B). Nd contents and Ndl values
O
ZrlY
of the felsic end-members containing Si02 content of 8
80% are required to be 25ppm and O.51270 for mixing
line (e), and to be 30 ppm and O.51265 for mixing line (f), 6
O
+
(gSSge. dis
respectively.
O
OO
Consequently, more than one felsic end-member is 4
required to explain the Srl and Ndl values of the SK
type I andesite in the mixing model, We can choose
2
the following rhyolitic magmas as candidates of the
felsic end-member ; 1) the WK type I rhyolite magma,
o
and 2) rhyolite magmas with high Sr and Nd contents,
and high Srl and low Ndl values, compared to the WK
Zr/Nb
type I rhyolite.
O
O
The rhixing model is almost consistent with fact 30
,+
oO O
m #Å}
that the data of most rocks of typeIvolcanic group
are plotted on or near mixing lines in the variation
diagrams (Figs. 8 and 9), The data of some SK type I
+
20
andesite samples, plotting away from the mixing lines
in Figs, 8 and 9, suggest a minor degree of fractional
#
a
crystallization subsequent to the mixing event.
However, the data of some SK type l andesite samples 10
plotting to the beiow from the mixing line in Y vs.
Si02 daigram (Fig. 9) can not be exp]ained by fractional crystallization. The reason for this data is ambig-
uous.
Y/Nb
D
6
m
+
.,-;I!
Thus, combined major- and trace- element and Srand Nd- isotopic data indicate that the mixing of
basaltic magma (forming the CK type l basalt) with 4
felsic magmas was a major process to produce the SK
type I andesite. The felsic magmas are the WK type
o
+H:
oQ
++ OO
+
O
I rhyolite magma and rhyolitic magmas having
higher Sr and Nd contents, and higher Srl and lower 2
40 50 60 70 80
idi,",?.i,",e.Sg,h2n,g,h,e,,IM,,KsielkP2,ii.hl,Oi;'ie,.I:ii,M.ai,,. Si02(9io)
type ll andesite- fractional cryStallization of .
Fig. 14. Si02 vs. Zr/Y, Zr/Nb and Y/Nb diagrams for
b.asaltic magma
MaJor and trace element concentrations of tyPe II from the study area.
volcanic group plot in the area which varies with
. , type ll volcanic group in late Miocene volcanic rocks
increasing Si02, and most of the data of the SK type II
andesite occupy the area apart from straight (mixing) rhyolite.
Iines linking with analyses of the CK type II basalt The all geochemical and Sr- and Nd- isotopic feaand WK type II rhyolite (Figs. 8 and 9År. This indi- tures of type II volcanic group, except MnO and P20s,
cates that the SK type II andesite was not derived by suggest that the SK type II andesite and WK type II
mixing between these basaltic and rhyolitic magmas. rhyolite were derived mainly by fractional crystallizaThe concentrations of Nb, Zr and Y increase with tion of basaltic magma forming the CK type II basalt.
increasing Si02 in type II volcanic group, and are However, the causes of the enrichment in MnO in the
opposite variations of type I volcanic group (Figs. 8 SK type II andesite than in the CK type II basalt, and
and 9). Moreover, Zr/Y, Zr/Nb and Y/Nb ratios of of the similar enrichment in P20s between these basalt
the former volcanic group either gradually increase or and andesite are not clear.
decrease with increasing Si02 (Fig. 14). The restrict- Although petrographical evidences suggesting
ed values of Srl and'Ndl of type II volcanic group (Fig. crustal assimilation are not observed in the SK type II
13) suggest the genetic relationship among the CK andesite, the andesite has slightly higher Srl- and
type II basalt, SK type II andesite and WK type II lower Ndl- values than the CK type II basalt (Fig. 13).
640 S. Yamashita, K. Shuto, Y. Kakihara and H. Kagami 1999-9
Thus, it is probable to consider that during fractional of northeastern Hokkaido during spreading of the
crystallization of basaltic magma (the formation stage Kurile basin.
of the SK type II andesite), it has been somewhat The above mentioned middle to late Miocene volaffected by assimilation of crustal materials enriched canic rock association in northeastern Hokkaido is
in radiogenic Sr and depleted in radiogenic Nd. different from a typical island arc association consistReiationshipb.eR,w2s,",sa.t,2M,v,ofi:"evoica"'sm laCngd,Zd`a.a;R":g",d,g.gs`s"sdu:cghiieC&:t2hZkiQ.gueag,eEMg.5".,9'.y2oZi2ae,/',,il
The geology in the middle to late Miocene north- Nakagawa etal., 1995;Yoshida and Aoki, 1984),
eastern Hokkaido is characterized by the association Thus, we conclude that the middle to late Miocene
of non-island arc volcanic rocks, such as icelandite- volcanism in northeastern Hokkaido is not related to
like andesite and Ti02- rich basalt and andesite, to-,? subduction of the Pacific plate.
gether with island arc calc-alkaline felsic rocks (Goto Recently, based on K-Ar age data for Tertiary vol-
et al.1995;Okamuraet al.-1995). In addition,chemi- canic rocks from South Sakhalin, Takeuchi (1997)
cal characteristics of these volcanic rocks show no suggested that the 9-23Ma volcanic activity in Sakhasystematic regional variation such as across-arc vari- lin and Hokkaido resulted from southward migration
ation in K20 (Kokubu et al., 1994 ; Goto et al., 1995 ; of the Kurile during back-arc spreading of the Kurile
Okamura et al., 1995). basin and ceased at about 9Ma. Ikeda (1998) argued
Goto et al. (1995) interpreted the reason for no spa- that a N-S trending graben in northeastern Hokkaido
tial variations in chemistry of these volcanic rocks (Okamura et al., 1995;Watanabe, 1995;Yahata and
and the appearance of non:island arc volcanism after Nishino, 1995;Yahata, 1997) corresponds to the south
14Ma,notassubductionofthePacificplatebutasthe terminate of the rift formed by the Kurile basin
uprise of asthenospheric mantle related to spreading spreading. The results of this study may support the
of the Kurile arc. Ikeda (1998) provided the same inter- opinion of Ikeda (1998).
?,'.ellha/rg",,fi2'.g,2",fi'i}tgR",.O,.2t.?egl.R,M,Z,b,IS.aalfigB.2?,B,' Acknowiedgements
Kurile basin was considered by Kimura and Tamaki The authors wish to express their gratitude to Mr.
(1986) and Maeda (1990) to have been occured before 15 T. Miyazaki and Dr. T. Hamamoto and Dr. M. Yuhara
-16Ma ; that is, oldet than the main Neogene volca- of Niigata Univ. for their help with Sr and Nd isotope
nism in northern Hokkaido (Fig. 1), Okamura et al. analysis. We are also indebted to Dr. P.A. Morris of
(1995) and Watanabe (1995) considered that the volca- the Geological Survey of Western Australia for his
nism was not related to spreading of the Kurile basin. revision of the English and improving the early ver-
The present study has shown that the WK type I sion of manuscript. This study was supported in
rhyolite magma could have been derived by crustal part by Grant-in Aid (No. 09640541) from the Japanese
melting, and that mixing between such crust-derived Ministry of Education, Science and Culture.
rhyolitic magma and basalSic magMa (O.riginating in References
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