Lateral variation of major and trace elements in the ...

J. Min. Petr. Econ. Geol.85, 364-389, 1990

Lateral variation of major and trace elements in the Pliocene volcanic rocks of the Northeast Japan arc

Kenji Shuto* and Ryuichi Yashima**

* Department of Geology and Mineralogy, Faculty of Science,

Niigata University, Niigata 950-21, Japan** Faculty of Education, Fukushima University, Fukushima 960-12, Japan

Abundances of major elements and seven trace elements, Rb, Sr, Nb, Y, Zr, Ni and Cr were determined by X-ray fluorescence analysis for 111 samples from twelve Pliocene volcanic centers in the NE Japan arc, The petrographical and analytical results on these samples and other Pliocene volcanic rocks indicate that systematic zonal variation of petrological features, i.e., mafic

phenocryst assemblages and rock series, as well as gradual increase in K2O, Rb and Sr contents are recognized in the Pliocene volcanic rocks from the Pacific Ocean side to the Japan Sea side across the NE Japan arc.

Similarity of across-arc variations with regard to both the petrological and the geochemical features among the volcanic rocks from the late Miocene through Pliocene to Quaternary time suggests that a mechanism of magma generation due to plate subduction has been contineous in the NE Japan arc at least from 8 Ma to the present.

Introduction

It has been well-known that the Quater

nary volcanic rocks increase gradually in alkali

contents, especially in K2O, other large ion

lithophile elements (LIL elements), and high

field strength elements (HFS elements) from the

volcanic front toward the back-arc side across

the NE Japan arc (Kuno, 1960; Kawano et al.,

1961; Masuda, 1979; Fujimaki and Kurasawa,

1980; Fujitani and Masuda, 1981; Yoshida and

Aoki, 1984; Sakuyama and Nesbitt, 1986;

Nakagawa et al., 1988). More recently, sev

eral workers suggested the existence of the

lateral variation of LIL elements including K,

Rb and Sr of the volcanic rocks during the late

Miocene time, based on the geochemical investi

gations of the Tertiary volcanic rocks from both the central Honshu and northern Honshu

to southwest Hokkaido (Shuto et al ., 1988; Tamura and Shuto, 1989; Okamura, 1987).

In the NE Japan arc, the volcanic rocks

formed by the Pliocene volcanism from 4 to 2

Ma show a widespread distribution which coin

cides roughly with that of the Quaternary vol

canic rocks (Fig. 1). We examined the abun

dances of major and trace elements for 111

rock samples from twelve Pliocene volcanic

centers in the NE Japan arc. Based on these

results and the published analyses, an across

arc compositional variation for the Pliocene

volcanics will be examined in comparison with

that for the Quaternary volcanics in the NE

Japan arc.

Quaternary volcanic zone in the NE Japan

arc

The Quaternary volcanic zone of the NE

Japan arc has been divided into four subzones, Aoso-Osore, Sekiryo, Moriyoshi and Chokai

zones across the arc from the Pacific Ocean side

(volcanic front side) to the Japan Sea side

(Manuscript received, May 9, 1990; accepted for publication, June 22, 1990)

Pliocene volcanic rocks of the Northeast Japan arc 365

(back-arc side) based on differences in mafic

phenocryst assemblages and incompatible element abundances of the volcanic rocks from

each volcanic zone (Nakagawa et al., 1986;

Fig. 1). The former two subzones form the

outer volcanic zone (Nasu zone) and the latter

two subzones the inner volcanic zone (Chokai

zone).

Aoso-Osore zone close to the Japan trench

is characterized by occurrence of rocks belong

ing to the hypersthenic rock series of Kuno

(1950) (H series, nearly equivalent to the calcalkaline series) rather than to his pigeonitic

rock series (P series, nearly equivalent to the

tholeiitic series). In Sekiryo zone, on the other

hand, most volcanoes comprise rocks of both

Fig. 1. Locations of the Pliocene volcanics and the representative Quaternary volcanoes in the NE Japan arc.Solid triangles indicate the localities of the Pliocene volcanics and open triangles show those

of the Quaternary volcanoes. Thin dashed lines show the zonal arrangement of the Quaternary volcanoes in the NE Japan arc (Nakagawa et al., 1986): A, Aoso-Osore zone; S, Sekiryo zone; M, Moriyoshi zone; C, Chokai zone. Thick dashed lines indicate depth to WadatiBenioff zone (Utsu, 1974).

366 Kenji Shuto and Ryuichi Yashima

the tholeiitic and the calc-alkaline series.

Hornblende phenocrysts are present in the an

desites and dacites of Aoso-Osore zone, but

phenocrystic hydrous minerals are absent in the volcanic rocks of Sekiryo zone. Most an

desites and dacites in Moriyoshi and Chokai

zones belong to the calc-alkaline series and

frequently contain phenocrysts of hornblende together with pyroxene. Biotite phenocrysts

are also found in some dacites of Chokai zone.

Chemically, the andesites and dacites of

Aoso-Osore zone are characterized by lower K2O contents than those of Sekiryo zone when

compared at similar SiO2 contents. The for

mer rocks refer roughly to Gill's (1981) low-K andesites (including basalts and dacites), and

the latter to his low to medium-K andesites . Most of the volcanic rocks of Moriyoshi and Chokai zones have K2O contents corresponding

to those of Gill's medium-K and high-K an

desites, respectively.

Geological setting

Several geological and petrographical

studies have been carried out for the Pliocene volcanic rocks in the NE Japan arc (e.g. Hanzawa et al., 1985; Yashima et al ., 1987; Yashima et al., 1989a, b; Sato and Yoneyama

Research Group, 1975; Suzuki et al., 1985; Hakaseyama Research Group, 1990). However, the zonal variation of rock chemistry across the NE Japan arc has not been paid

much attention for these volcanic rocks . Recently, Shuto (1989) suggested the existence

of the lateral variation of K2O contents of the volcanics in Pliocene time , based on a preliminary regional study of the Pliocene volcanics in

the NE Japan arc. More recently , Fukudome et al. (1990) found an alkali basalt yielding K

- Ar age of 3.32 Ma from Kyuroku-shima Island

in the northeast margin of Japan Sea and

revealed the existence of the lateral variation

back within Japan Sea from the chemical study

of the basalt.

The locations of the Pliocene volcanic

rocks investigated in this study are shown in

Fig. 1, together with the zonal arrangements of

the Quaternary volcanoes and the depth to the

Wadati-Benioff zone proposed by Nakagawa et

al. (1986) and Utsu (1974), respectively.

These volcanic rocks are divided into two

groups, the eastern Pliocene volcanic zone (the EPV zone, nearly corresponding to the Quater

nary Aoso-Osore and Sekiryo zones) and the

western Pliocene volcanic zone (the WPV zone, nearly corresponding to the Quaternary Mori

yoshi zone). The volcanics in the EPV zone comprise the Ajarayama volcanics, 20km west

of the Quaternary Towada volcano, Aomori

Prefecture; the Kodosan andesites , 20km southeast of Towada volcano; the Inaniwadake basalts, Takakurayama andesites and Ara

kidayama andesites to the west of the Quater

nary Nanashigure volcano, Iwate Prefecture;

the Aonokimori andesites of Hanamaki City, Iwate Prefecture; the Kunimiyama andesites, 30km southeast of the Quaternary Yakeishi volcano, Iwate Prefecture; the Nanatsumori

dacites to the east of the Quaternary Funagata

volcano, Miyagi Prefecture; the Sasamori

yama andesites and Ohnagurayama volcanics to the east and southeast, respectively, of the

Quaternary Adatara volcano, Fukushima Prefecture and the Hakaseyama basalts , 40km southwest of Lake Inawashiro , Fukushima Prefeture. The remaining eleven volcanic

groups in the EPV zone, except Ajarayama and Hakaseyama, lie on a narrow zone along the

boundary between Aoso-Osore and Sekiryo

zones, 300km inland from the Japan trench, where the depth to the Wadati-Benioff zone is

130km (Fig. 1). The Ajarayama volcanics and Hakaseyama basalts are situated in the western areas of Sekiryo zone , 350km inland from the Japan trench, with 150km depth of the

Wadati-Benioff zone. The Masukawadake


andesites, the only one volcanic group studied in the WPV zone occur in Tsugaru Peninsula which lies within the Quaternary Moriyoshi zone, about 400km from the Japan trench, with 170km depth of the Wadati-Benioff zone.

K-Ar ages of volcanic rocks

The following K-Ar ages have been reported for the studied Pliocene volcanic rocks; Takakurayama andesites: 3.2 Ma (Agency of Natural Resources and Energy, 1985), Arakidayama andesites: 2.9 Ma (Agency of Natural Resources and Energy, 1985), Aonokimori andesites: 2.1 Ma (Yashima, 1990), Kunimiyama andesites: 3.6 Ma (Kimura, 1986), Nanatsumori dacites: 1.9 and 2.3 Ma (Yashima, 1990; Ozawa et al., 1987), Sasamoriyama andesites: 2.3 Ma (Yashima, 1990), Hakaseyama basalts: 2.6 and 2.8 Ma (Kobayashi and Inomata, 1986), Ajarayama volcanics: 1.9 Ma (Yashima, 1990). Although the radiometric ages for the Kodosan andesites, Inaniwadake basalts, Ohnagurayama volcanics and Masukawadake andesites have not yet been determined, the stratigraphical studies indicate that these volcanic rocks are possibly formed by the Pliocene volcanism during 4 to 2 Ma ago (Ohguchi et al., 1986; Iwai et al., 1986 and this study).

Brief description of volcanic rocks

Kodosan andesitesThe Kodosan andesites are composed

mainly of augite-hypersthene andesite, augitehornblende-hypersthene dacite and augite

hypersthene-hornblende dacite, all of H series, with intersertal or felsitic groundmass, composed largely of plagioclase with subordinate amounts of silica minerals, augite, hypersthene and magnetite. Quartz phenocrysts are present only in dacite specimens. The Kodosan andesites occur as lava flows, associated with Pyroclastic flows.

Inaniwadake basalts

The main rock types of the Inaniwadake

basalts are olivine-hypersthene-augite basalt,

augite-olivine basalt, hypersthene-augite

olivine basalt and hypersthene-augite basalt,

with minor augite-hypersthene andesite, all of

P series, with intergranular or intersertal

groundmass, composed mainly of plagioclase,

augite and pigeonite, with a minor amount of

magnetite. The Inaniwadake basalts form

lava flows, associated with pyroclastic flows.

Takakurayama andesites

The Takakurayama andesites comprise

large amounts of augite-hypersthene-andesite

and small amounts of olivine bearing augite

- hypersthene andesite, augite-hypersthene

dacite and hypersthene-hornblende dacite, most

of H series. Quartz phenocrysts are present

only in dacite. The groundmass of the

Takakurayama andesites shows intersertal,

hyaloophitic or hyalopilitic texture, composed

of plagioclase, augite, hypersthene, magnetite,

apatite, silica minerals and pale brown glass.

These rocks mostly form lava flows associated

with pyroclastic flows.

Arakidayama andesites

The Arakidayama andesites comprise vari

ous types of andesites including augite-hyper

sthene andesite, olivine bearing hornblende-

augite-hypersthene andesite, hornblende-

augite-hypersthene andesite and occasionally

hypersthene-hornblende dacite, all of H series.

The groundmass is intersertal or hyalopilitic in

texture, composed of plagioclase, augite, hyper

sthene, silica minerals, glass and occasionally

magnetite and apatite. The Arakidayama an

desites occur as lava flows with subordinate

amounts of pyroclastic flows.

Aonokimori andesites

The Aonokimori andesites comprise hyper

sthene-augite-andesite, olivine-augite-hyper

sthene andesite, augite-hypersthene dacite and

occasionally augite-hypersthene-hornblende


Table 1. Phenocrystic minerals and rock series of the volcanic rocks

Ol: Olivine, Au: Augite, Hy: Hypersthene, Ho: Hornblende , Bi: Biotite, Pl: Plagioclase, Qz: Quartz. P and H are Kuno's pigeonitic rock series and hypersthenic rock series, respectively.

dacite (Yashima et al., 1989a) . The groundmass of andesites is intergranular , intersertal or partly felsitic in texture, composed of plagio

clase, augite, pigeonite or hypersthene , magnetite and minor amounts of dark brown glass . The groundmass of the dacites is felsitic or

hyaloophitic in texture, composed of plagio

clase, silica minerals, pale brown glass and

occasionally augite and hypersthene. Mostly

the dacites belong to H series, whereas the

andesites are of both P and H series. The

former rocks form lava domes while the latter

rocks occur as numerous lava flows and as

sociated pyroclastic units.


Fig. 2. Lateral variations of mafic phenocryst assemblages of the Pliocene volcanics investigated in this study.Open circles represent volcanics always containing phenocrysts of pyroxene and sometimes olivine and/or hornblende but no biotite phenocrysts; solid circles represent volcanics always containing phenocrysts of pyroxene and sometimes olivine but neither hornblende nor biotite phenocrysts; open square represents volcanic rocks containing phenocrysts of pyroxene, hornblende and/or biotite but no olivine phenocrysts. Crosses represent other Pliocene volcanic centers. Dashed lines are the same as those of Fig. 1.

Kunimiyama andesites

The Kunimiyama andesites are composed mainly of augite-hypersthene andesite with

minor amounts of olivine bearing augite-hyper

sthene andesite , all of P series, with intergranular groundmass composed of plagioclase, clinoppyroxene (pigeonite and/or augite) , silica

minerals, magnetite and brown glass . These

andesites form the alternation of at least 70

layers of lava flows and pyroclastic flows,

which reach to 350m in maximum thickness

(Hanzawa et al., 1985).

Nanatsumori dacites

According to Abe et al. (1976) and Kitamu

ra et al. (1983) the Nanatsumori dacites are

composed mainly of several types of dacites


associated with a minor amount of andesite,

most of H series. Dacitic rocks include

augite-hypersthene dacite, hypersthene-augite

dacite and rarely hornblende bearing hypersth

ene-augite dacite, all of which have no quartz

phenocrysts. The groundmass is hyalopilitic

or felty in texture, composed of plagioclase,

silica minerals, hypersthene, augite, magnetite

and glass. The andesites contain only plagio

clase phenocrysts in the intergranular ground

mass consisting of plagioclase, clinopyroxene,

magnetite and glass. The dacites mostly

belong to H series whereas the andesites to P

series. The Nanatsumori dacites form lava

domes extruded through the Nanakita Forma

tion and the Miyatoko Tuff of the late Miocene

to Pliocene time.

Sasamoriyama andesites

The Sasamoriyama andesites are mostly

augite-hypersthene andesite with small

amounts of olivine bearing augite-hypersthene

basalt. Magnetite phenocrysts are rarely

found in some andesites. The groundmass is

intergranular or intersertal in texture , and composed of plagioclase, augite, pigeonite , magnetite, silica minerals and occasionally

apatite. The andesites and basalts correspond

to P series. The Sasamoriyama andesites

mostly occur as lava flows with frequent inter

calations of pyroclastic flows .Ohnagurayama volcanics

According to Yashima et al . (1987), the Ohnagurayama volcanics are mainly composed

of olivine-augite-hypersthene basalt , hypersthene-augite basalt, olivine bearing augite-hyper

sthene andesite and augite-hypersthene an

desite, with intergranular groundmass com

posed of plagioclase, pigeonite, magnetite , silica minerals and glass . They belong to P series. These volcanics occur as lava flows

with subordinate amounts of pyroclastic flows .Ajarayama volcanics

The Pliocene Ikarigaseki Cauldron , was

newly found in the Hirosaki district, Aomori

Prefecture (Yashima et al., 1989b). The Ajara

yama volcanics comprise basalts and andesites,

which form five cone-shaped volcanic bodies

arranged along the inner marginal zone of the

cauldron characterized by a polygonal shape

about 8•~15km in size. Each body is com

posed of lava flows and associated pyroclastic

flows. The main rock types of Ajarayama

volcanics are augite-olivine basalt, hypersth

ene-augite-olivine basalt, augite-hypersthene

andesite and augite-hypersthene dacite, all of P

series. The groundmass of the rocks shows

intersertal or hyalopilitic texture and is com

posed of plagioclase, augite, pigeonite, silica

minerals, dark brown glass and minor amounts

of magnetite.

Hakaseyama basalts

Hakaseyama Research Group (1990)

divided the Pliocene volcanic fomation located

at the Hakaseyama district into the lower and

upper units. The lower unit consists of

tuffaceous conglomerates and the upper unit of

volcanic products (the Hakaseyama basalts)

which occur as numerous lava flows intercalat

ed with pyroclastic flows. The Hakaseyama

basalts comprise various types of basalts such

as olivine-augite-hypersthene basalt, hypersth

ene-augite-olivine basalt, augite-hypersthene-

olivine basalt and augite-olivine basalt with

subordinate amounts of andesites whose essen

tial mafic phenocryst minerals are augite,

hypersthene and olivine. The groundmass is

intergranular in texture , and composed essen

tially of plagioclase , augite, pigeonite and a

minor amount of glass . They belong to P

series.

Masukawadake andesites

The Masukawadake andesites comprise

hornblende-biotite-hypersthene-augite an

desite and hornblende-hypersthene-augite an

desite, in which quartz also occurs as phnocryst

minerals. The groundmass is hyalopilitic or


microfelsitic in texture, composed of plagio

clase, silica minerals, glass, hypersthene and

magnetite, indicating that the Masukawadake

andesites belong to H series. They form a

large lava dome and small dikes, intruding into

the Nagane and Kodomari Formations of the

middle to late Miocene.

Phenocrystic mineral assemblages and the

variation of mafic phenocryst assemblages of

the analyzed volcanic rocks in this study are

summarized in Table 1 and Fig. 2, respectively.

Geochemistry

Analytical methods and results

Major element compositions except for

FeO and trace element compositions were

determined by X-ray fluorescence method.

Detailed procedures and accuracy of analysis

by this method were given by Tamura et al.

(1999). In this work, JB-1 (standard basalt sample of Geological Survey of Japan) was used

to check the accuracy and precision of the

method., The detection lower limits for seven

trace elements and the results of twenty re

plicate analyses of these elements for JB-1 are shown in Table 2, together with the data recom

mended by Ando et al. (1987). Analysis of FeO

Table 2. Results of twenty replicate analyses of JB-1

All results are in ppm.I, Results of this study. Values in parentheses are

mean values.II, Values recommended by Ando et al. (1987).L. I. D., Lower limit of detection, calculated using Norrish and Chappell's (1977) equation 37.

was made by permanganate titration method.

Major and trace element analyses of 111 sam

ples are given in Table 3.

Major element chemistry

In order to examine rock series of the

Pliocene volcanic rocks investigated in this

study, the data are plotted in a AFM diagram

(Fig. 3) together with the fractionation trends of the tholeiitic and calc-alkaline series of the

Quaternary Nasu volcanic zone.The Inaniwadake basalts, Kunimiyama

andesites, Sasamoriyama andesites, Ohnagura

yama volcanics, Ajarayama volcanics and Hakaseyama basalts plot within the field of the

tholeiitic series of Nasu zone and each volcanic

group follows the respective fractionation trends of island arc tholeiitic series (Fig. 3). Two samples of augite-hypersthene andesite of

the Kodosan andesites plot in the field of the

tholeiitic series, one sample of augite-hypersth

ene-hornblende dacite within the field of the calc-alkaline series and the another one sample

of augite-hornblende-hypersthene dacite plots

just on the boundary between the two rock series of Nasu zone. The Arakidayama an

desites and Aonokimori andesites are also plotted both in the fields of the tholeiitic and calc

alkaline series. These evidences indicate that

the andesitic rocks from the Kodosan, Ara

kidayama and Aonokimori districts comprise the rocks of both the tholeiitic and calc-alka

line series. Most of the Takakurayama an

desites and Nanatsumori dacites lie within the

calc-alkaline field and all of the Masukawada

ke andesites below the field of the calc-alkaline

series.Variation diagrams of CaO, Na2O+K2O,

Na2O and K2O plotted against SiO2 are shown

in Fig. 4. Each volcanic group shows the

different ranges in major element compositions.

Among these volcanic groups, the andesitic

rocks including acidic andesite and dacite, from

*: Total Fe as FeO.

Table 3. Whole-rock chemical compositions of the volcanic rocks

*

: Total Fe as FeO.


*: Total Fe as FeO.

Table 3. continued

*: Total Fe as FeO.

Pliocenne volcanic rocks of the Northeast Japan arc 373

*: Total Fe as FeO.

Table 3. continued


Fig. 3. MgO-FeO*(FeO+0.9•~Fe2O3)-Na2O+K2O diagram.

T, differentiation trend of the tholeiitic series of Nasu zone, NE Japan arc; C, calc-alkaline series of Nasu zone (Aoki and Fujimaki,

1982).



Fig. 4. Major elements vs . SiO2 variation diagrams.Symbols are the same as those of Fig . 3. Solid lines A and Bin Na2O+K2O vs . SiO2 diagram sh

ow the general boundaries between the fields of the low-alkali tholeiite series, high-alkali tholeiite series

and alkali rock series (Kuno , 1968). Solid lines C and D in K2O vs. SiO2 diagram are the general boundaries between the fields of low -, medium- and high-K andesites in an orogenic belt (Gill, 1981) and their extention , and broken line E is that between the fields of the Q

uaternary vocanic rocks in Aoso-Osore and Sekiryo zones.


the Kodosan, Takakurayama, Ajarayama and

Aonokimori districts in the EPV zone, have

K2O contents as low as those of the Quaternary

andesitic rocks in Aoso-Osore zone (Fig. 4). These Pliocene andesitic rocks belonging to

tholeiitic or calc-alkaline series, also resemble

in mafic phenocryst assemblages to the Quaternary Aoso-Osore andesites because both the

andesitic rocks frequently contain phenocrysts

of hornblende together with pyroxene. These

facts support the similarity of petrographical

and chemical features between the Pliocene andesitic rocks in above four volcanic centers

of the EPV zone and the Quaternary andesitic

rocks in Aoso-Osore zone. The other Pliocene andesitic rocks from the Ajarayama,

Kunimiyama, Sasamoriyama and Ohnagura

yama districts in the EPV zone are mostly of tholeiitic series without hydrous phenocrystic minerals and have K2O contents similar to

those of the Quaternary andesites in Sekiryo

zone when compared at a given SiO2 content. These results also indicate the similarity of

both mafic phenocryst assemblages and K2O

contents of these Pliocene andesites to those of

the Quaternary andesites from Sekiryo zone.All of the basaltic rocks from the Ha

kaseyama, Inaniwadake, Ajarayama, Sasamoriyama and Ohnagurayama districts belong to

the tholeiitic series and their K2O contents are similar to those of the Quaternary basaltic

rocks in Sekiryo or Aoso-Osore zone (Fig. 4).

It is concluded from these results that the

EPV zone is characterized by occurrence of rocks whose petrological and chemical charac

teristics are consistent with those of the Quater

nary volcanic rocks in Aoso-Osore or Sekiryo

zone, but further subdivision of the EPV zone into two subzones such as the Quaternary

Aoso-Osore and Sekiryo zones is not possible.

The andesites in the Masukawadake district lying within the WPV zone are of the calc

alkaline series, in which hornblende and/or

biotite phenocrysts are present together with

phenocrystic pyroxenes, and have K2O contents

similar to those of the Quaternary andesitic

rocks distributed in Moriyoshi zone (Fig. 4).

Figure 4 also shows that the Na2O contents

of the Ajarayama volcanics and Hakaseyama

basalts in the western region of the EPV zone

and the Masukawadake andesites in the WPV

zone tend to be slightly higher than those of

most volcanics in the eastern EPV zone at a

given SiO2 content.Although most Pliocene volcanic rocks in

the EPV zone have lower alkali contents corre

sponding to those of rocks of Kuno's (1968) low

alkali-tholeiite series, the Ajarayama volcanics

and Hakaseyama basalts are slightly higher in

Na2O+K2O than other group located at the

eastern region in the EPV zone (Fig. 4), whereas

the Masukawadake andesites in the WPV zone

distinctly lie in the field of Kuno's (1968) high

alkali-tholeiite series.

The CaO contents of the volcanics in the

EPV zone decrease with increasing SiO2 con

tents and the similarity of CaO variation pat

terns of these volcanics to those of the Quater

nary volcanics in Aoso-Osore and Sekiryo

zones is evident (Fig. 4).

Trace element chemistry

The relationship of incompatible trace ele

ments vs. SiO2 contents is shown in Fig. 5, in

which the variation patterns of Rb and Zr are

essentially similar to that of K2O in Fig. 4. In

the individual volcanic group of the EPV zone,

Rb and Zr increase gradually with increasing

SiO2 and fall within the distribution field of the

Quaternary volcanics from Aoso-Osore and

Sekiryo zones. Among these volcanic groups,

the Hakaseyama basalts are characterized by

slightly higher Rb contents than rocks of other

volcanic groups. In the Masukawadake an

desites, Rb and Zr also exhibit the same ten

dency as K2O values. These incompatible ele


Fig. 5. Incompatible trace elements vs. SiO2 variation diagrams .Symbols and two fields are the same as those of Fig. 3 and Fig. 4, respectively.

ments show a gradual increase with SiO2 and

lie within the field of the Quaternary volcanics

in Moriyoshi and Chokai zones.

As illustrated in Fig. 5, eleven volcanic

groups in the EPV zone have Sr contents falling

within the Quaternary volcanic field of Aoso

Osore and Sekiryo zones and , respective volcanic group have fairly constant Sr contents

regardless of SiO2 contents throughout each

group. Among these volcanic groups, the


Hakaseyama basalts have slightly higher Sr

contents than the remaining volcanic groups,

similarly to the relationship between K2O and

Rb against SiO2. The Sr contents of the

Masukawadake andesites in the WPV zone are

higher than those of andesitic rocks in the EPV

zone, and correspond to those of the Quater

nary andesites from Moriyoshi and Chokai

zones.

Systematic differences of Nb and Y con

tents do not exist not only among the volcanic

groups in the EPV zone but also between the Masukawadake andesites in the WPV zone and

the volcanics in the EPV zone (Fig. 5).

As the Quaternary volcanic rocks in Aoso

- Osore and Sekiryo zones, in general, have

undergone extensive fractional crystallization,

they are characterized by low abundances of Ni

(less than 100 ppm) and Cr (less than 250 ppm). The analyzed Pliocene volcanics in the EPV zone also have low Ni and Cr contents decreas

ing from 71 to 2 ppm and from 240 to 2 ppm,

respectively, with decreasing MgO content

from 7.55 to 1.35%, similarly to the variation

patterns of Ni and Cr against MgO for the Quaternary volcanics from Aoso-Osore and

Fig. 6. Cr and Ni vs. MgO variation diagrams.

Symbols are the same as those of Fig. 3.


Sekiryo zones (Fig. 6).

Incompatible element characteristics of

basaltic rocks

The relatively primitive basaltic rocks

whose FeO*/MgO and SiO2 are less than 2 and

53%, respectively, are associated with the rocks

from five volcanic groups including the Aja

Fig. 7. Abundance pattern of incompatible elements normalized against N-type MORB of Pearce (1982).Data sources: Pliocene basalts, Fukudome et al. (1990) and this study; Quaternary basalts, Hayashi et al. (1984); Ishikawa et al. (1984); Sakayori et al. (1984); Togashi et al. (1986) and Shimotori et al. (1983).

rayama volcanics, Inaniwadake basalts, Sasamoriyama andesites, Ohnagurayama volcanics and Hakaseyama basalts in the EPV zone (Table 3).

Mid-ocean ridge basalt (MORB) normalized patterns of these basalts are shown in Fig. 7 to examine the incompatible element natures of them, together with patterns of the Pliocene alkali basalt of Kyuroku-shima Island in northern Japan Sea and the several basaltic rocks from the Quaternary volcanoes in the NE Japan arc for comparison. One of the most conspicuous geochemical characteristics of the island arc basalts is the positive anomalies of LIL elements such as Ba, Rb and K, and the negative anomalies of HFS elements such as Nb and Zr, as shown on the primordial mantle or Ntype MORB normalized patterns for incompatible elements (Sun and Nesbitt, 1977; Pearce, 1982).

Figure 7 shows that both the Pliocene basalts in the EPV zone and the Quaternary basalts in Aoso-Osore and Sekiryo zones have similar incompatible element patterns, indicating the positive K and Rb anomalies and the negative Nb and Zr anomalies. These chemical features suggest an essential similarity in primary processes such as chemical characteristics of source material and the degrees of

partial melting for both the Pliocene and Quaternary basaltic rocks in the volcanic front side of the NE Japan arc.

Lateral variation of incompatible elements in the Pliocene volcanics from the NE Japan arc

From the petrographical and analytical results in this study and the analyses of other Pliocene volcanic rocks an across-arc compositional variation for these volcanic rocks is examined in comparison with that for the Quaternary volcanic rocks in the NE Japan arc.

As pointed out previously, among the


Pliocene volcanics in the EPV zone which geo

graphically coincides with the combined region of the Quaternary Aoso-Osore and Sekiryo

zones, the Kodosan andesites, Takakurayama

andesites, Arakidayama andesites, Aonokimori andesites and Nanatsumori dacites resemble to

the Quaternary andesites and dacites in Aoso-

Osore zone with respect to not only K2O con

tents but also mafic phenocrystic mineral

assemblages and rock series, whereas the

petrological and chemical characteristics of the Ajarayama volcanics, Kunimiyama andesites,

Ohnagurayama volcanics and Hakaseyama

basalts are consistent with those of the Quaternary volcanic rocks from Sekiryo zone.

Furthermore, it is noted that the former groups

are not accompanied by basaltic rocks, whereas

the latter are composed mostly of the rock suites ranging from basalt to andesite with

subordinate amounts of dacite. These two

types of rock assemblage are also consistent with those of the Quaternary volcanoes in the

Aoso-Osore and Sekiryo zones, respectively.

The Inaniwadake basalts are characterized by

extremely low K2O contents ranging from 0.06 to 0.18% with increasing SiO2 content from 50.4

to 52.5% and some of them have K2O content

comparable to that of the basaltic andesite

from the Quaternary Osore volcano (Togashi, 1977). The Masukawadake andesites distribut

ed in the WPV zone have the petrological and

chemical features similar to those of the

Quaternary andesitic rocks from Moriyoshi and Chokai zones and have higher contents of K2O,

Rb, Sr and Zr than the volcanics in the EPV

zone as shown in Figs. 4 and 5.

The Pliocene volcanic rocks petrologically investigated by some earlier workers are also

located within the Quaternary Chokai zone and its southwestward extension along the back arc

region of the NE Japan arc (Fig. 1). They

comprise the basalt from Kyuroku-shima

Island in the northeast margin of Japan Sea, the

Yoneyama volcanics to the south of Kashiwaza

ki City, and the Umikawa volcanics to the east

of Itoigawa City, Niigata Prefecture. The

Kyuroku-shima Island basalt which was recent

ly dated by K-Ar method to be about 3.3 Ma

belongs to the alkali basalt series of island arc

type, based on their incompatible element

chemistry (Fukudome et al., 1990). Fukudone

et al. (1990) found a similarity of mineralogy

and chemistry of the alkali basalt and the

associated andesite of Kyuroku-shima Island to

those of rock suites from alkali basalt to calc

alkaline andesite of the Quaternary Oshima

- Oshima volcano, 100km to the north.

Both the Yoneyama and Umikawa vol

canics form numerous lava flows and associated

pyroclastic rocks, and are composed mainly of tholeiitic basalts and calc-alkaline andesites

with hornblende phenocrysts (Sato and

Yoneyama Research Group, 1975; Suzuki et

al., 1985). Four andesites specimens of the

Yoneyama volcanics yielded K-Ar ages of 2.5

to 2.8 Ma, whereas the Umikawa volcanics

were geologically estimated to be in the Ni

shiyama Formation of the Pliocene age though

the radiometric age detemination has not yet

been carried out (Tsunakawa et al., 1983;

Suzuki et al., 1985).

The published analyses and the new ones of

these volcanic rocks are compiled on variation

diagrams of K2O, Rb, and Sr against SiO2 (Fig.

8). Figure 8 shows that the Pliocene volcanic

rocks increase gradually in K2O contents from

the Pacific side toward Japan Sea side across

the NE Japan arc and that the variation pattern

in K2O contents is similar between the Pliocene

and the Quaternary volcanic rocks. Rb and Sr

contents are also higher in the rocks from the

Japan Sea side than those from the Pacific side,

indicating the lateral variations similar to the

case of K2O. Furthermore, all of the Pliocene

and Quaternary basalts shown in Fig. 7 have the

incompatible element patterns typical of island


Fig. 8. Rb, Sr and K2O vs. SiO2 variation diagrams for the Pliocene volcanic rocks in the NE Japan arc.

1, Kyuroku-shima Island basalt (Fukudome et al., 1990); 2, Yoneyama volcanics (Sato and Yoneyama Research Group, 1975); 3, Umikawa volcanics (Suzuki et al., 1985); 4, Masukawadake andesites (this study); 5, Kodosan andesites, Inaniwadake basalts, Takakurayama andesites, Arakidayarna andesites, Aonokimori andesites, Kunimiyama andesites, Sasamoriyama andesites, Ohnagurayama volcanics, Nanatsumori dacites, Ajarayama volcanics and Hakaseyama basalts (this study). Solid lines C and D and broken line E are the same as those of Fig. 4, respectively and two fields are the same as those of Figs. 4 and 5.


arc-type basalts but the absolute abundances of

these elements, especially K2O, Rb and Sr, are higher in the basalts from the Japan Sea side

than those from the Pacific side through

Pliocene to Quaternary time.To sum up, although the Pliocene volcanic

zone on the Pacific Ocean side (the EPV zone)

cannot be divided into two subzones like the

Quaternary Aoso-Osore and Sekiryo zones, systematic across-arc variations of chemical

compositions and petrological features, essen

tially similar to those of the Quaternary volcanic rocks, are observed in the Pliocene vol

canic rocks of the NE Japan arc.

Implication of across-arc variation

Recent petrological and geochemical

studies show that the NE Japan arc of the

middle Miocene (16-12 Ma) are characterized

by occurrence of rocks of non island arc-type volcanic rocks such as E-type MORB like

basalt enriched in Ti and other HFS elements,

basaltic rocks having intermediate feature

between the island arc tholeiite and the back arc basin basalt in bulk rock chemisrty and

icelandite-like andesites and dacites, in associa

tion with volcanics of the island arc tholeiite

series and talc-alkaline series (Isshiki, 1974; Shuto and Yashima, 1985; Shuto et al., 1985;

Shuto, 1988, 1989; Shiramizu et al., 1983; Tsu

chiya, 1988a, 1988b). It is also noted that these volcanic rocks have no systematic across-arc

differences in K2O, Na2O+K2O and light REE

(Ebihara et al., 1984; Shuto and Chihara, 1987; Shuto et al., 1988; Tsuchiya, 1988a).

More recent geochemical investigations by Okamura (1987), Shuto et al. (1988) and Tamura

and Shuto (1989), however, clearly demonstrate

the presence of systematic across-arc varia

tions in chemical compositions and petrological

features, similar to those in the Quaternary

volcanic rocks, among the late Miocene (8-6

Ma) volcanic rocks throughout the NE Japan

arc.

Most Japanese consider that the zonal

variations of LIL elements such as K, Rb and Sr

in the Quaternary volcanic rocks may be rea

sonably explained by the mechanism of magma

generation due to subduction of oceanic litho

sphere (the Pacific Plate). Presumably the lat

eral variation of basaltic rocks are produced

either by different degrees of partial melting or

by the lateral heterogeneity of the wedge man

tle beneath the NE Japan arc or by a combina

tion of these two mechanisms (e.g., Sakuyama

and Nesbitt, 1986; Nakagawa et al., 1988;

Tatsumi et al., 1989).

From these evidences and the results of the

present study we are led to the conclusion that

the mechanism for magma generation due to

plate subduction has been contineous in the NE

Japan arc at least during the period from the

late Miocene to Quaternary.

Acknowledgements: The authors wish to

express their thanks to Emeritus Professor K.

Yagi of Hokkaido University for critical read

ing of the manuscript and his encouragements.

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Appendix

Locality and rock name of the analyzed sam

ples.Numbers correspond to those of in Table 3.

1: au-ho-hy-qz-pl dacite (H), 1.1km north

of Kainamoriyama.

2: au-hy-ho-qz-pl dacite (H), 1.5km north

of Kainamoriyama.

3: au-hy-pl andesite (H), 1.5km south of

Kodosan.

4: au-hy-pl andesite (H), 0.75km southwest

of Kodosan.

5: hy-au-ol-pl basalt (P) , a road, 4.1km southeast of Inaniwadake .

6: au-ol-pl basalt (P), a road , 2.9km southeast of Inaniwadake.

7: au-ol-pl basalt (P), a road , 1.4km southeast of Inaniwadake.

8: ol-hy-au-pl basalt (P) , a road, 4.7km east-southeast of Inaniwadake .

9: ol-by-au-pi basalt (P) , a road, 4.7km southeast of Inaniwadake .

10: hy-au-ol-pl basalt (P), upper stream of Ashinazawa, 4.2km northeast of Inaniwadake.

11: au-ol-pl-basalt (P), a road, 1.5km southsouthwest of Inaniwadake.

12: ol-au-pl basalt (P), a road, 1.3km southsouthwest of Inaniwadake.

13: hy-au-pl basalt (P), a road, 1.8km southsouthwest of Inaniwadake.

14: au-hy-pl andesite (H), upper stream of Hokaizawa.

15: au-hy-pl andesite (H), upper stream of Hokaizawa.

16: au-hy-qz-pl dacite (H), 3km east of Takakurayama.

17: au-hy-qz-pl dacite (H), 2.6km east of Takakurayama.

18: ho-au-hy-pl andesite (H), 0.5km south of Fukazawayama.

19: ho-au-hy-pl andesite (H), 1.2km east of Fukazawayama.

20: au-hy-pl andesite (H), 0.8km southeast of Fukazawayama.

21: ho-au-by-qz-pl andesite (H), 0.8km northeast of Ryugamoriyama.

22: ol-ho-au-hy-pl andesite (H), southern flank of Ryugamoriyama.

23: ho-au-hy-pl andesite (H), 0.5km south of Fukazawayama.

24: au-hy-qz-pl dacite (H), 1km south-southwest of Aonokimori.

25: au-hy-pl andesite (H), 1.2km south-southwest of Aonokimori.

26: au-hy-qz-pl dacite (H), a road, 0.4km north-northeast of 765m-peak, north of Aonokimori.

27: au-hy-pl andesite (P), a road, 1.5km west-northwest of 765m-peak.

28: ol bearing au-hy-pl andesite (H), a road, 1.7km west-northwest of 765m-peak.

29: ol bearing au-hy-pl andesite (P), a road, 1km northwest of 765m-peak.

30: ol bearing au-hy-pl andesite (P), a road,


0.97km northwest of 765m-peak.

31: ol bearing au-hy-pl andesite (P), a road,

0.87km northwest of 765m-peak.

32: au-hy-pl andesite (H), a road, 0.65km

northwest of 765m-peak.



34: ol-au-hy-pl andesite (P), a road, 0.93km

west-northwest of 765m-peak.

35: ol-au-hy-pl andesite (P), a road, 1.4km

west-northwest of 765m-peak.

36: by-au-pl andesite (P), a road, 0.45km

northeast of 765m-peak.

37: au-hy-ho-qz-pl dacite (H), a road, 1.43

km west-northwest of 765m-peak.




southwest of 765m-peak.

40: au-by-pl-andesite (P), left bank of Sengo

kuishi pond, 10km west of Kanegasaki.

41: au-hy-pl andesite (P), a road, 1km north

east of Izawa, Kanegasaki.

42: au-hy-pl andesite (P), Kunimiyama line,

2.9km northeast of Kunimiyama.


2.45km northeast of Kunimiyama.


1.3km east of Kunimiyama.

45: au-hy-pl basaltic andesite (P),

Kunimiyama line, 0.75km west of

Kunimiyama.


0.25km west of Kunimiyama.


0.1km north of Kunimiyama.





50: au bearing hy-pl andesite (P),

Kunimiyama line, 1.3km east of

Kunimiyama.

51: au-hy-pl dacite (H), summit of Tochiku

rayama.

52: au-hy-pl dacite (H), Sarusuberiiwa, south

ern flank of Nadekurayama.

53: au-hy-pl dacite (H), Tengunotobiiwa,

northern flank of Hachikurayama.

54: au-hy-pl dacite (H), southwestern flank of Kamakurayama.

55: au-hy-pl dacite (H), western flank of

Kamakurayama.

56: au-hy-pl dacite (H), a road, 1km north

west of Sasakurayama.

57: au-hy-pl dacite (H), Kunimizaki, 0.2km east of Sasakurayama.

58: au-hy-pl dacite (H), a cliff, 0.3km south

east of Sasakurayama.

59: ol bearing au-hy-pl basalt (P), a road, 3km west of Tyusakuyama, 2.5km to the

north-northwest of Sasamoriyama.

60: au-hy-pl andesite (P), a quarry, 3.15km

west of Tyusakuyama.

61: au-hy-pl andesite (P), a quarry, 3.15km west of Tyusakuyama.

62: hy-au-pl andesite (P), a quarry, 3km

west-northwest of Tyusakuyama.

63: au-hy-pl andesite (P), a quarry, 2.4km west-northwest of Tyusakuyama.

64: hy-au-pl andesite (P), a quarry, 2.4km




66: au-hy-pl andesite (P), a road, 1.8km north-northeast of Tyusakuyama.

67: au-hy-pl andesite (P), a road, 1.6km northwest of Tyusakuyama.

68: ol-au-hy-pl basalt (P), a quarry, 0.3km

east of Ohnagurayama.

69: ol bearing au-hy-pl andesite (P), a road, 0.25km northeast of Ohnagurayama.

70: ol bearing au-hy-pl andesite (P), a road, 0.5km southwest of Aaahikurayama, 0.65

km to the northwest of Ohnagurayama,

71: ol-au-hy-pl basalt (P), a road, 0.2km


west-northwest of Asahikurayama.

72: hy-au-pl basalt (P), a quarry, 0.9km east

of Ohnagurayama.

73: au-hy-pl andesite (P), a road, 0.4km

southeast of Ohnagurayama.


southwest of Asahikurayama.

75: au-hy-pl andesite (P), 1.4km northeast of

Asahikurayama.


Asahikurayama.


Asahikurayama.

78: au-hy-pl basalt (P), eastern foot of 554m

peak, 2km to the south of Oguni.79: au-hy-pl dacite (P), summit of

Yasuteyama.


northwest of Asogatake.

81: hy-au-ol-pl basalt (P), a road, 1.25km

southwest of Oguni.

82: au-ol-pl basalt (P), a road, 1.1km south

west of Oguni.


northeast of Mitsumori.

84: au-ol-pl basalt (P), 0.25km south of

Yasuteyama.

85: aphyric basalt (P), a quarry, 0.6km south

east of Asogatake.

86: au-hy-pl andesite (P), a road, 0 .3km northeast of Mitsumori.

87: au-ol-pl basalt (P), a road, 1.3km north

east of Yasuteyama.

88: hy-au-ol-pl basalt (P), a road , 2.4km southwest of Hakaseyama.

89: hy-au-ol-pl basalt (P) , a road, 2.35km southwest of Hakaseyama .

90: au-ol-pl basalt (P), a road , 2.35km southwest of Hakaseyama.

91: hy-au-ol-p1 basalt (P), a road , 2.3km southwest of Hakaseyama .

92: au-hy-ol-pl basalt (P), a road , 2km southeast of Hakaseyama.

93: au-ol-pl basalt (P), a road, 1.75km southwest of Hakaseyama.

94: au-ol-pl basalt (P), a road, 1.8km southwest of Hakaseyama.


southwest of Hakaseyama.

96: au-ol-pl andesite (P), a road, 1.85km

southwest of Hakaseyama.

97: by-au-ol-pl basalt (P), a road, 1.9km

southwest of Hakaseyama.98: hy-au-ol-pl basalt (P), a cliff, 1.85km

west of Hakaseyama.

99: au-ol-pl basalt (P), a cliff, 1.85km west of

Hakaseyama.100: ol-au-hy-pl basalt (P), a cliff, 1.9km west

of Hakaseyama.

101: au-by-ol-pl basalt (P), a cliff, 1.95km

west of Hakaseyama.

102: by-au-pl andesite (P), a cliff, 2km west of

Hakaseyama.103: au-ol-pl basalt (P), a cliff, 2.1km west of

Hakaseyama.

104: au-hy-ol-pl basalt (P), a road, 1.75km

west of Hakaseyama.

105: au-hy-ol-pl andesite (P), a road, 1.8km west of Hakaseyama.


west of Hakaseyama.

107: au-ol-pl basalt (P), a road, 2.4km west

northwest of Hakaseyama.108: ol-au-hy-pl andesite (P), a road, 2.4km

west-northwest of Hakaseyama.

109: ho-bi-hy-au-qz-pl andesite (H), western

flank of Masukawadake.

110: ho-bi-hy-au-qz-pl andesite (H), western


111: ho-hy-au-qz-pl andesite (H), western


ol, olivine; hy, hypersthene; au, augite; ho,

hornblende; bi, biotite; pl, plagioclase; qz,

quartz. P and H are Kuno's pigeonitic rock series and hypersthenic rock series, respectively


東北日本弧の鮮新世火山岩の主要元素と微量元素の水平変化

周藤賢治・八島隆一

東北日本弧の太平洋側から脊梁帯にかけての広い地域に南北に配列する鮮新世(4～2Ma)火山岩111試

料について,主要元素と微量元素(Rb, Sr, Nb, Y, Zr, Ni, Cr)の分析を螢光X線法で行った。本研究結

果と既発表の他の鮮新世火山岩のデータは,太平洋側から日本海側に分布する火出岩(玄武岩～デイサイ

ト)にむかって, K2O, Rb, Srが漸次増加することを示す。これらの火山岩は,苦鉄質斑晶鉱物組合せや

岩石系列の点においても,東北日本弧の第四紀火山岩の場合とほぼ同様な水平変化を示す。このような化

学組成と岩石学的性質の水平変化は, 8～6 Maの火山岩にも認められているので,東北日本弧においては,

太平洋プレートの沈み込みに関連した火山活動が少なくても8 Ma前から現在まで継続して起っているも

のとみられる。

Lateral variation of major and trace elements in the ...

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