comparative study of quaternary arc volcanic belts ......the physical properties of subducting...

1.Introduction

Southern Chile and Northeast Japan are typical Qua-

ternary volcanic arcs situated at the opposite side in the

circum-Pacific region. They are characterized by com-

mon arc features, such as the presence of a trench, the

Wadati-Benioff zone, an outer non-volcanic arc, an

inner volcanic arc, and an inter-arc basin. Several dis-

tinctive differences are also present between the two

arcs; the Southern Chilian volcanic arc lacks a back-arc

marginal sea basin and the occurrence of frontal low-

alkali tholeiite. The most remarkable contrast is the

age of the subducting lithosphere; very young in South-

ern Chile and rather old in Northeast Japan. It is impor-

tant for the study of magma generation in subduction

zones to compare the differences as well as similarities

between two contrasting volcanic arcs. The main pur-

pose of this paper is to clarify the differences between

Southern Chile and Northeast Japan and then consider

the origin of such dissimilarities. We also intend to dis-

cuss about the generation of magmas at convergent

plate boundaries based on this comparative study.

─　　　─135 （111）

1）日本大学文理学部地球システム科学科：〒156－8550 世田谷区桜上水 3－25－40

2）茨城大学理学部地球生命環境科学科：〒310－8512 水戸市文京 2－1－1

3）東京大学大学院理学研究科：〒113－0033 文京区本郷 7－3－1

4）チリ大学地質学地球物理学教室：チリ共和国サンチャゴ市カシジャ13518

Proceedings of the Institute of Natural Sciences, Nihon UniversityNo.37 (2002) pp.135 -156

1）Department of Geosystem Sciences, College of Humanitiesand Sciences, Nihon University: 3-25-40, Sakurajousui,Setagaya-ku Tokyo 156-8550 Japan

2）Department of Environmental Sciences, Faculty of Science,Ibaraki University: 2-1-1 Bunkyo, Mito 310-8512 Japan

3）Graduate School of Science, University of Tokyo: 7- 3- 1Hongo, Bunkyo-ku Tokyo 113-0033 Japan

4）Department of Geology and Geophysics, University of Chile:Casilla 13518, Santiago, Chile

Comparative Study of Quaternary Arc Volcanic Belts:Southern Chile vs. Northeast Japan

Masaki TAKAHASHI 1), Michio TAGIRI 2), Kenji NOTSU 3), Leopoldo LOPEZ-ESCOBAR 4)

and Hugo MORENO-ROA4)

(Received September 30, 2001)

AbstractThe comparative study of arc volcanism in Southern Chile and Northeast Japan reveals that the crustal

effect, mantle process and crustal stress field are essential for the genesis of subduction zone magmatism.The crustal effect appears to be reflected in the along-arc variation of upper limit of K2O content in frontalvolcanic rocks. The mantle process seems to be related to the rock-series of frontal basalts and across-arcvariation of alkaline content or rock-series of basalts. While, the condition of crustal stress field may beimportant for the occurrence of large calderas with voluminous felsic pyroclastic flows and abundance ofandesite. The physical properties of subducting oceanic lithosphere is contrasting between the two arcs;young, warm and buoyant in Southern Chile and old, cold and dense in Northeast Japan. On the basis ofcomparative study of arc volcanic belts with contrasting characters, it may be concluded that the adiabaticupwelling of hotter mantle materials caused by the induced counter flow, which is controlled by the physi-cal properties of descending slab, is a plausible process to produce arc basaltic magmas.

Keywords: island arc, magma, volcanic belt, subduction zone, Quaternary, Northeast Japan,Southern Chile

Masaki TAKAHASHI, Michio TAGIRI, Kenji NOTSU, Leopoldo LOPEZ-ESCOBAR and Hugo MORENO-ROA

─　　　─136（112）

2.Age of subducting lithosphere

The most conspicuous difference of the two arcs is

the age of the subducting oceanic lithosphere (Fig. 1).

In Southern Chile, the subducting plate is very young

(0～36Ma) (Fig. 1A). The geologic age of the oceanic

lithosphere north of the Valdivia-Mocha fracture zone

(VMFZ) is Oligocene (23-36Ma). In the south of the

VMFZ, the age of the subducting plate becomes

younger southward (from 19 to 2Ma), and the mid-

oceanic ridge (the Chile Rise) is subducting under the

South American continent at a latitude of about 46 S゚.

The absolute convergent rate of this young and warm

oceanic lithosphere is about 6.4cm/year and the rela-

tive rate is 9.2 cm/year (Minster and Jordan, 1980).

Contrarily, the age of the subducting plate is very old in

Northeast Japan, which is Jurassic to early Cretaceous

(122 to 145Ma) (Fig. 1B). The absolute convergent rate

of this old and cold lithosphere is 10.4cm/year and the

relative rate is 10.6cm/year (Minster and Jordan, 1980).

The depth of the trench is shallower in Southern

Chile where the younger lithosphere is subducting; it is

-4, 500 to -5, 000m in the north of the VMFZ, but no top-

ographical trench is observed to the south of it, where a

younger plate is underthrusting (Fig. 5A). To the con-

trary, the depth of the trench is deeper than 7, 000m in

Northeast Japan, where the old oceanic lithosphere is

subducting (Fig. 5B).

3.The Wadati-Benioff zone

The Wadati-Benioff zone or deep seismic zone reaches

a depth of about 200km in Southern Chile (Hanus and

Vanek, 1978) and about 500 to 600 km in Northeast

Japan (Yoshii, 1979) (Fig. 2). The dip angle of the

Wadati-Benioff zone is gentle in Southern Chile; 20 i゚n

the northern section (from 33 t゚o 36 S゚) and 15 i゚n the

southern section (from 36 S゚ to 45 S゚). Contrarily, the dip

angle is relatively steep in Northeast Japan; 30 i゚n north-

Fig.1 Map showing the age and convergent rate of subducting oceanic lithosphere in Southern Chile and NortheastJapan (Moore, 1982). A:Southern Chile, B:Northeast Japan. solid circle: Quaternary volcano; line with solid trian-gles: trench; stippled rectangle: mid-oceanic ridge (Chile Rise); line with numeric number: magnetic lineation;arrow with numeric number: absolute convergent rate (cm/year) and direction. The absolute age of magnetic lin-eation is as follows; 2-5E corresponds to 2-9Ma, 7-13 to 26-36Ma and M8-M23 to 122-145Ma.

Comparative Study of Quaternary Arc Volcanic Belts: Southern Chile vs. Northeast Japan

─　　　─137 （113）

ern section (around 40 N゚) and 40 i゚n southern section

(around 33 N゚). The differences in depth and dip angle

of the Wadati-Benioff zone are probably due to the dis-

similarities in the physical properties of the subducting

lithosphere: young, warm and buoyant oceanic plate in

Southern Chile; and old, cold and dense descending

slab in Northeast Japan. The depth from the volcanic

front to the Wadati-Benioff zone is around 100km in the

northern section of Southern Chile and Northeast

Japan, but less than 50km in southern section of South-

ern Chile where the younger oceanic lithosphere is sub-

ducting.

4.Crustal thickness

The thickness of the crust is approximately reflected

in the topographical altitudes of main mountain ranges

(Carr, 1984). The variation of altitude of the back-bone

ranges, on which the frontal volcanic edifices are con-

structed, are shown in Fig. 3.

In Southern Chile, the altitude exceeds 4,000m above

sea level in the area north of 36 S゚, and it gradually

decreases southward from 3,000 to 2,000m between lati-

tudes 36 a゚nd 45 S゚. The crust in the north of 36°S is

thicker than that in the south; the crustal thickness of

the former probably exceeds 40km and that of the latter

is less than 40km (Lowrie and Hey, 1981).

While the altitude is generally between 1,000 and

2,000m in Northeast Japan, it exceeds 2,000m only at

the arc-arc junction regions. In the area south of 35 N゚,

the altitude decreases to less than 1,000m, and the land

submerges under the sea to form a chain of volcanic

islands. The thickness of the crust in Northeast Japan

is nearly 30 km (Yoshii and Asano, 1972), but it

decreases in the area south of 35 N゚ and is less than

20km at 32 N゚.

The detailed crustal sections at 38 N゚ in Southern

Chile and at 39 t゚o 40 N゚ in Northeast Japan are shown

in Fig. 4. Fig. 4A is a density model based on the data of

Fig.2 Cross-sections showing the Wadati-Benioff zone in Southern Chile (Hanus and Vanek, 1978) and Northeast Japan(Yoshii, 1979). A: southern section of Northeast Japan (around 33°N); B: northern section of Northeast Japan(around 40°N); C: northern section of Southern Chile (from 33° to 36°S); D: southern section of Southern Chile(from 36° to 45°S). open reversed triangle: the position of trench; closed triangle: the location of volcanic front.


─　　　─138（114）

Fig.3 Along-arc variation of topographical altitude in Southern Chile and Northeast Japan. A: Southern Chile; B: North-east Japan; solid circle: altitude of a peak in the mountain range; solid triangle: the location of Quaternary volcano.

Fig.4 Across-arc crustal section in Southern Chile and Northeast Japan. A: density model based on the free-air gravityanomaly for across-arc section at 38°S in Souterh Chile (Couch et al., 1981); a unit of numeric number is g/cm3.B: P seismic wave velocity model obtained by explosion seismological method in Northeast Japan for across-arcsection at 39° to 40°N (Yoshii and Asano, 1972); a unit of numeric number is cm/sec. arrow: the position oftrench; solid triangle: the location of volcanic front.


─　　　─139 （115）

free-air gravity anomaly (Couch et al., 1981), and Fig.4B

is a P seismic wave velocity model obtained by the

explosion seismological method (Yoshii and Asano,

1972). The crustal thickness under the volcanic front in

these sections is nearly 40 km in Southern Chile and

30km in Northeast Japan.

5.Dimension of the arc volcanic belts

The volcanic arc belt in Southern Chile extends for 1,

400km with a maximum width of 80km at a distance of

260km from the trench; the distance from the trench

decreases southwards and less than 200km at 46 S゚ where

the Chile Rise is subducting. It continues southward to

the Austral Andes volcanic belt with a non-volcanic gap

of 370km and northward to the Central Andes volcanic

belt also with a non-volcanic gap of 550km (Fig. 5A).

The volcanic belt in Northeast Japan extends for

more than 1,500km, the width of which is 150km from

37 t゚o 43 N゚, 60 km in the south of 35 N゚, and 210 to

230km at the arc-arc junction areas. The volcanic front

is situated at a distance of 260km to the north of 35 N゚

and 160km to the south of 35 N゚ from the trench. It con-

tinues southward to the Izu-Bonin volcanic arc and

northward to the Kurile volcanic arc without any non-

volcanic gaps (Fig. 5B).

The width of the volcanic belt in Southern Chile is

Fig.5 Dimension of the arc volcanic belts in Southern Chile and Northeast Japan (the same scale) A: Southern Chile;B: Northeast Japan; contours: trench; solid area: Quaternary volcanic edifice (showing only polygenetic volca-noes and excluding the distribution of voluminous pyroclastic flows).


─　　　─140（116）

narrow and is divided into two zones in the northern

area of 42 S゚: the frontal western zone and eastern zone

at the back-arc side (Moreno-Roa, 1976; Lopez-Escobar,

1984). However, it becomes a single chain consisting of

only the western frontal volcanic zone in the south of 42

S゚. The volcanoes are concentrated in the western

frontal volcanic zone.

To the contrary, the width of the volcanic belt is broad

in Northeast Japan; it cannot be distinctly divided into two

zones but is composed of the frontal volcanic chain and a

wide area with sporadically scattered volcanic edifices at

the back-arc side. The volcanoes are densely distributed

in the frontal volcanic chain also in Northeast Japan.

6.Type of polygenetic volcanoes

Moriya (1983) classified polygenetic volcanoes into

three main types: stratovolcanoes with or without

horseshoe-shaped calderas (Type-A1); stratovolcanoes

with calderas and central cones or lava domes (Type-

A2); and polygenetic lava domes with or without small

calderas (Type-A3). The distribution of these three

types of volcanoes in Southern Chile and Northeast

Japan is shown in Figs. 6A and 6B, respectively. The

type-A2 volcanoes appear to be predominant at the cen-

tral segment in Southern Chile and at the arc-arc junc-

tion areas in Northeast Japan.

Fig.6 Type of polygenetic volcanoes. A: Southern Chile; B: Northeast Japan. open circle: type A1 (stratovolcano withor without horseshoe-shaped caldera); solid circle: Type A2 (stratovolcano with caldera and central cones ordomes); half-solid circle: Type A3 (polygenetic lava domes with or without small caldera).


─　　　─141 （117）

7.Distribution of large calderas with voluminousfelsic pyroclastic flows

Large calderas with voluminous felsic pyroclastic

flows in Southern Chile are distributed only to the north

of 37 S゚, where the altitude and crustal thickness exceed

4,000m and 40km, respectively (Fig. 7A). They are the

caldera-like depression at the Maipo volcano (Stern et

al., 1984) and the Calabozos caldera (Hildreth et al.,

1984). In the area south of 37 S゚, the large calderas with

voluminous pyroclastic flows are completely lacking.

In Northeast Japan, large calderas with voluminous

felsic pyroclastic flows are present in the area where

the thickness of the crust is nearly 30 km (Fig. 7B).

From north to south Tokachi, Hakkoda, Tamagawa,

Onikobe, and Shirakawa erupted during the early Qua-

ternary, and Kutcharo, Akan, Shikotsu, Toya and

Towada during the late Quaternary.

It appears that the thick crust exceeding 30 km in

thickness is necessary for the generation of voluminous

felsic pyroclastic flows and related large calderas. How-

ever, it is not a sufficient requirement because crustal

thickness to the south of 37 S゚ in Southern Chile is

nearly the same as that to the north of 35 N゚ in North-

east Japan. Other factors, such as differences in the tec-

tonic stress field, may be needed to explain the produc-

Fig.7 Distribution of large calderas with voluminous felsic pyroclastic flows. A: Southern Chile; B: Northeast Japan.large star: Quaternary; middle-sized star: early Quaternary; small star: late Quaternary.


─　　　─142（118）

tion of voluminous felsic pyroclastic flows and forma-

tion of large calderas. Takahashi (1995) proposed that

low crustal strain rate is responsible for the large-scale

felsic vocanic activity with large calderas.

8.Dominant type of volcanic rocks

Andesite is predominant to the north of 37 S゚ in South-

ern Chile, where the thickness of the crust exceeds

40km. Contrarily, the dominant rock type to the south

of 37 S゚ is basalt (Fig. 8A). In Northeast Japan, andesite

is the major rock type where the crust is thick (ca

30km in thickness), but basalt is dominant to the south

of 35 N゚ where the crustal thickness is less than 30km.

There are no significant differences in age and physical

nature of the subducting lithosphere between the areas

in which basalt is dominant (south of 35 N゚) and

andesite is dominant (north of 35 N゚).

Although basalt is the major rock type to the south of

37 S゚ in Southern Chile and andesite predominates to

the north of 35 N゚ in Northeast Japan, the crustal thick-

ness of both regions is similar. Thus, it may be con-

cluded that the dominant rock type does not depend not

only upon the age and physical properties of the sub-

ducting oceanic plate but also upon the crustal thick-

ness. The state of tectonic stress field may also play an

important role for determining the major rock types.

Fig.8 Dominant type of volcanic rocks. A: Southern Chile; B: Northeast Japan. open circle: dominantly andesite; dou-ble circle: mainly basalt with subordinate andesite; half-solid circle: bimodal (basalt and dacite-rhyolite); solidcircle: dominantly basalt.


─　　　─143 （119）

For example, both regions are compressional, but

reverse faults are predominant to the north of 35°N in

Northeast Japan and strike-slip faults predominant to

the south of 37°S in Southern Chile.

9.Rock-series of basalts

In Southern Chile, high-alumina basalt is common

along the volcanic front, which is occasionally accompa-

nied by alkaline basalt with relatively low alkaline con-

tent. The Hudson volcano situated in the southern-

most portion of the frontal volcanic chain, where the

mid-oceanic ridge (the Chile Rise) is subducting,

mainly consists of alkaline basalt (Fig. 9A). To the con-

trary, the frontal volcanoes of Northeast Japan are char-

acterized by the occurrence of low alkaline tholeiite,

excluding the arc-arc junction area where high-alumina

basalt appears on the volcanic front (Fig. 9B). In the

back-arc side of the volcanic front in Northeast Japan,

high-alumina basalt is present, and alkaline basalt

occurs in the farthest region from the trench. The

Na2O content of frontal high-alumina basalt in Southern

Chile is higher than that in Northeast Japan (Taka-

hashi, 1989).

It is manifest that rock-series of basalt is closely related

to the age and physical properties of the subducting

oceanic lithosphere and not to the thickness of the

Fig.9 Rock-series of basalts. A: Southern Chile; B: Northeast Japan. solid circle: low alkali tholeiite; open circle: high-alumina basalt; circle with cross: alkaline basalt.


─　　　─144（120）

crust; the age of the subducting plate is Miocene to

present in Southern Chile and Jurassic to early Creta-

ceous in Northeast Japan.

10. Mafic phenocryst assemblage of andesite,dacite and rhyolite

Sakuyama (1977; 1979) classified the Quaternary vol-

canoes of Northeast Japan into three types on the basis

of mafic phenocryst assemblage of intermediate to fel-

sic volcanic rocks: volcanic rocks with biotite and horn-

blende; rocks with hornblende and no biotite; and rocks

without hydrous mafic minerals.

The volcanoes composed of rocks without horn-

blende and biotite are predominantly distributed in the

frontal volcanic chain (Fig. 10B). Those with horn-

blende and no biotite occur mainly at the back-arc side

of the volcanic front, and those with hornblende and

biotite mostly appear in the farthest region from the

trench (Fig. 10B).

On the other hand, andesite and dacite-rhyolite with

biotite and hornblende are restricted to the north of 37 S゚

in Southern Chile, where the crust is thick. Those with

hornblende are distributed in the eastern volcanic belt

(the Tronador volcano) and in the southern-most por-

tion of the western frontal volcanic chain (the Mentolat

and Cay volcanoes). Andesite without hornblende and

Fig.10 Mafic phenocryst assemblage of andesite, dacite and rhyolite. A: Southern Chile; B: Northeast Japan. large cir-cle: andesite; large square: dacite and rhyolite; small circle: andesite, dacite and rhyolite; open: with biotite withor without hornblende; half-solid: with hornblende; solid: without hydrous mafic phenocryst.


─　　　─145 （121）

biotite, and dacite-rhyolite with fayalite and no hydrous

mafic phenocrysts are predominant to the south of 37 S゚

of the frontal volcanic belt (Fig. 10A).

The variation of mafic phenocryst assemblage is

ascribed to the difference of H2O content in magma

(Sakuyama, 1977; 1979); magma without hydrous mafic

phenocryst is relatively dry and the H2O content of

magma increases as the phenocryst assemblage of vol-

canic rocks changes from hornblende only to hornblende

plus biotite. The H2O content of magma increases from

the volcanic front to the back-arc side in Northeast Japan.

In Southern Chile, H2O is most abundant to the north of

37 S゚, moderate in the eastern volcanic zone and south-

ern-most portion of the frontal volcanic belt, and the least

to the south of 37 S゚ of the frontal volcanic chain.

The mafic phenocryst assemblage, namely the H2O

content of magma, is not necessarily related to the

crustal thickness as is shown in Northeast Japan, but it

seems to be closely related to the age and physical

properties of the subducting lithosphere in Southern

Chile. The older plate in the north of VMFZ is descend-

ing under the region north of 37 S゚, and the younger lith-

osphere in the south of VMFZ is underthrusting to the

south of 37 S゚. Furthermore, the active mid-oceanic

ridge (the Chile Rise) is subducting in the southern-

most area of the frontal volcanic chain. It may be con-

cluded that the difference of H2O content of magma is

ultimately originated in the magma generation process

in the mantle wedge under the arc, although the effect

of crustal materials is not completely excluded.

11. Across-arc variation of alkaline content

It is a well known fact that the alkaline content, espe-

cially K2O, increases across the arc from the volcanic

front to the back-arc side. In order to examine the across

arc variation of alkaline content, two areas with similar

crustal thickness (about 30km) are selected: one is the

across-arc section between 40 3゚0’ and 41 3゚0’S in South-

ern Chile (Fig. 11A); and the other the region between

39 3゚0’ and 40 N゚ in Northeast Japan (Fig. 11B). The

K2O content increases from the volcanic front towards

the back-arc side in both areas, but the content at the

volcanic front is lower in Northeast Japan than in South-

ern Chile (Figs.12A and B). The Na2O content

increases from the volcanic front toward the back-arc

side in Northeast Japan, but no significant increase is

observed in Southern Chile (Fig. 13A and B).

The total alkalis increase toward the back-arc side

from the volcanic front in both volcanic arcs. All the

data of volcanic rocks in Southern Chile are plotted in

Fig.11 Map showing the locality of volcanoes examined the across-arc variation of alkaline content. A: Southern Chile(40°30’-41°30’S); B: Northeast Japan (39°30’-40°20’N).


─　　　─146（122）

the field of high-alumina basalt series on a Na2O+K2O

vs. SiO2 diagram (Fig. 14B), whereas the data of frontal

volcanoes (Nanashigure, Iwate, Akita-Komagatake,

Hachimantai, Kayo and Akita-Yakeyama) in Northeast

Japan are plotted in the low aikali tholeiitte region, and

those of the back-arc side (Moriyoshi, Kanpu, Megata)

are plotted in the high-alumina basalt series and partly

in the alkaline basalt series regions (Fig. 14A).

12. Along-arc variation for alkaline content offrontal volcanoes

In order to examine the along-arc variation of alkali

contents, the frontal volcanoes of both arcs are divided

into six along-arc segments, A-I to A-VI in Southern

Chile and B-I to B-VI in Northeast Japan (Fig. 15).

The K2O content is highest in the A-I segment (33 t゚o

36 3゚0’S) where the crust is thick. The lowest limit of

K2O content decreases in A-II (36 3゚0’ to 39 S゚) and is the

minimum in A-III (39 t゚o 41 3゚0’S). It slightly increases

in A-IV (41 3゚0’ to 43 S゚) and A-V (43 S゚ to 45 S゚) , and the

K2O content is relatively high in A-VI (45 t゚o 46 S゚)

where the mid-oceanic ridge is subducting (Fig. 16).

Most volcanic rocks are medium-K series by Gill

(1981), but many in A-I are high-K series and a lot of

Fig.12 Across-arc variation of K2O content. A: Southern Chile (solid square: Calbuco; solid circle: Osorno; solid star:Cordillera Nevada; half-solid circle: Cayutue-Pichilaguna-La Vigueria; half solid square: Antillanca; half-solid tri-angle: Puyehue; open circle: Tronador; open square: Mirador); B: Northeast Japan (solid star: Nanashigure;solid circle: Iwate; solid square: Akita-Komagatake; solid triangle: Hachimantai; solid reversed triangle: Akita-Yakeyama; double circle: Kayo; half-solid circle: Moriyoshi; open circle: Kanpu; open star: Megata); upper line:upper limit of the Moriyoshi zone (Nakagawa et al., 1987); lower line: lower limit of the Moriyoshi zone. Thelist of data source is avairable. Request to the author (M.Takahashi).


─　　　─147 （123）

them in A-III are low-K series.

In Northeast Japan, the upper limit of K2O content is

the lowest in B-I (33 t゚o 35 3゚0’N) where the crust is

thin. It increases in B-II (35 3゚0’ to 37 N゚) and is maxi-

mum in B-III (37 t゚o 39 N゚), and then decreases in B-IV

(39 t゚o 41 N゚), B-V (41 t゚o 43 N゚) and B-VI (43 t゚o 45 N゚).

The lower limit of the K2O content is constant through-

out all the segments except for the arc-arc junction

area. Most volcanic rocks in B-I are low-K series and

those in other segments are both low-K and medium-K

series, but the arc-arc junction areas are characterized

by medium-K series.

The Na2O content is slightly high in A-I; it decreases

in A-II and is the minimum in A-III and A-IV. It increases

again in A-V and A-VI (Fig. 17). On the other hand, in

Northeast Japan, the Na2O content is the lowest in B-I

but nearly constant in other segments excluding the

arc-arc junction areas characterized by rather high

Na2O contents. It is clear that the Na2O content is gen-

erally higher in Southern Chile than in Northeast Japan.

The total alkali content is highest in A-I, most vol-

canic rocks of which are both high-alumina basalt and

alkaline basalt (Fig. 18). The lowest limit decreases in

A-II and is the minimum in A-III and A-IV, belonging to

Fig.13 Across-arc variation of Na2O cotent. A: Southern Chile; B: Northeast Japan; symbols are the same as in Fig.12;the line shows the lower limit of Moriyoshi zone.


─　　　─148（124）

the high-alumina basalt series. The total alkali content

increases in A-V and is nearly the same level as A-I in

A-VI.

The total alkali content is the lowest in B-I and nearly

constant from B-II to B-VI. Most volcanic rocks are low

alkali tholeiite except for the arc-arc junction area

where high-alumina basalt is predominant.

The along-arc variation of the lower limit of K2O,

Na2O and total alkali content seem to be related to the

crustal thickness and/or the age of subducting oceanic

lithosphere in Southern Chile. They are highest in A-I

where the crust is thickest and the older plate is under-

thrusting. The upper limit of K2O and lower limit of

Na2O and total alkali content seem to be related to the

crustal thickness in Northeast Japan. They are the low-

est in B-I where the crust is thinner than in other seg-

ments. The lowest limit of K2O, however, is nearly con-

stant and appears to be unrelated to the thickness of the

crust.

13. Along-arc variation of Sr and O isotopes infrontal volcanoes

In Southern Chile, the 87Sr/86Sr ratio of volcanic

rocks is the highest (0.7047 to 0.7062) in the northern

Fig.14 Across-arc variation of total alkali content. A: Southern Chile; B: Northeast Japan; symbols are the same asFig.12; the upper line is the boundary between alkali olivine basalt series (AOB) and high-alumina basalt series(HAB), and the lower line is that between high-alumina basalt series (HAB) and low alkali tholeiite series(LAT).


─　　　─149 （125）

part of A-I and decreases in its southern portion

(Fig. 19). It is the lowest in A-II (0.7040>) and increases

in A-III. The 87Sr/86Sr ratio is slightly higher in A-IV, A-

V, A-VI (0.7041<); biotite rhyolite in the Chaiten volcano

in A-IV shows a high ratio (0.7058). The highest ratio in

the northern portion of A-I may be related to the pres-

ence of thick continental crust exceeding 40km.

In Northeast Japan, the 87Sr/86Sr ratio is the lowest

(0.7040>) in B-I and B-VI. It is the highest in B-II

(0.7078>) and decreases in B-III, B-IV and B-V (Fig. 19).

The lowest ratio in B-I may possibly be related to the

thin crust less than 30km in thickness, but it is not the

case in B-VI where the crust is not so thin. The extraor-

dinary high ratio may be related to the collision of sub-

ducted Pacific plate and Philippine Sea plate beneath

the B-II segment (Notsu, 1983).

No systematic variation ofδ18O in volcanic rocks is

observed in either Southern Chile or Northeast Japan

(Fig. 20), but the variation of 87Sr/86Sr ratio seems to be

weakly correlated to that of theδ18O value except for

the A-I segment in Southern Chile. In A-I,δ18O of vol-

canic rocks is not so high in spite of their high 87Sr/86Sr

ratio; the crustal thickness does not appear to be

related toδ18O value.

Fig.15 Map showing the segmentation of the frontal volcanoes by which the along-arc variation of alkaline content isexamined. A: Southern Chile; B: Northeast Japan.


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Fig.16 Along-arc variation of K2O content in the frontal volcanoes. open circle in A: volcanoes in western volcanic zone;open circle in B: volcanoes at the arc-arc junction area; upper line: boundary between high-K and medium-K seriesby Gill (1981); lower line: boundary between medium-K and low-K series. The list of data source is available.Request to the author (M.Takahashi).


─　　　─151 （127）

Fig.17 Along-arc variation of Na2O content in the frontal volcanoes. Symbols are the same as in Fig.16.


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Fig.18 Along-arc variation of total alkali content in the frontal volcanoes. Upper line: boundary between alkali olivine basaltseries (AOB) and high-alumina basalt series (HAB); lower line: boundary between high-alumina basalt series (HAB)and low alkali tholeiite series (LAT). Other symbols are the same as in Fig.16.


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14. Concluding remarks and discussions

It is concluded from the comparative study of the arc

volcanism in Southern Chile and Northeast Japan that

two factors, the crustal effect and mantle process, must

be taken into account when the magma genesis is inves-

tigated. In this case, the mantle process includes vari-

ous dynamic physico-chemical processes caused by the

subduction of lithosphere with different ages and physi-

cal properties.

The crustal effect appears to be reflected in the along-

arc variation of upper limit of K2O content in frontal vol-

canic rocks. The existence of thick continental crust is

favorable for the crustal remelting and/or assimilation,

which may bring about high K2O content of magmas.

On the other hand, the mantle process seems to be

related to (1) the rock-series of frontal basalts and (2)

across-arc variation of alkaline content and rock-series

of basalts.

In addition to above two factors, the condition of

Fig.19 Along-arc variation of 87Sr/86Sr ratio in the frontal volcanoes. A: Southern Chile; data from Hildreth et al. (1981);Hickey et al. (1982); Deruelle et al. (1983); Klerkx et al. (1977); Godoy et al. (1981); Stern et al. (1984); Lopez-Escobar (1984); Notsu & Lopez-Escobar unpublished data. B: Northeast Japan; data from Katsui et. al. (1978);Notsu (1983); Kurasawa (1984).


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crustal stress field may be important for (1) the occur-

rence of large calderas with voluminous felsic pyroclas-

tic flows and (2) abundance of andesite.

The cause of variation of Sr and O isotope ratios is

rather complex. It may be related to both the crustal

effect and mantle process, though the difference

between two arcs is not so remarkable as the alkaline

content.

Recent experimental studies revealed the physical

conditions of magma generation in the mantle wedge.

According to Tatsumi et al. (1983), low alkali tholeiite

magma is produced under relatively low pressure (1.1

GPa) with higher degree of partial melting, while alka-

line basalt magma is formed under higher pressure (2.3

GPa) with lower degree of partial melting. On the other

hand, the melting pressure and degree of partial melt-

ing of high-alumina basalt magma show intermediate

values ( the pressure is 1.7GPa )between those of low

alkali tholeiite and alkaline basalt.

The decompression melting is thought to bring about

the difference of degree of partial melting, because the

melting temperature of these magmas are nearly the

Fig.20 Along-arc variation of δ18O value in the frontal volcanoes. A: Southern Chile. Data from Deruelle et. al. (1983);Gerlach et. al. (1983); Stern et. al. (1984). B: Northeast Japan. Data from Matsuhisa (1979).


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same (about 1320℃). The adiabatic ascent of mantle

material with high temperature causes the decompres-

sion melting; the degree of partial melting increases as

the pressure dcreases.

The depth of magma generation beneath the volcanic

front is shallower and degree of partial melting is

higher in Northeast Japan than in Southern Chile,

because basalt erupted in the volcanic front is low alkali

tholeiite in Northeast Japan and high-alumina basalt in

Southern Chile. It may be explained if the adiabatic

upwelling flow with similar temperature reaches to the

shallower level in the mantle wedge beneath the vol-

canic front in Northeast Japan but stagnates in rather

deeper portion under the volcanic front in Southern

Chile.

The physical properties of subducting oceanic litho-

sphere is contrasting between the two arcs; it is old,

cold and dense in Northeast Japan and young, warm

and buoyant in Southern Chile. The cold and dense

lithosphere descends into the deeper level in the upper

mantle with high subduction rate, which may promote

the upwelling counter flow with high temperature

reaching to the shallower level in the wedge mantle

beneath the volcanic front (e.g. Furukawa, 1996). Con-

trarily, the subduction of warm and buoyant plate with

low descending rate is restricted to the shallower level

in the mantle, hence in this case it may be difficult for

the upwelling counter flow to ascend to higher level

beneath the volcanic front.

The across-arc variation of magma series of basalts is

also explained by this model, because the upwelling

counter flow is inclined parallel to the subducting plate

and the depth of magma generation becomes deeper

from the volcanic front towards the back-arc side.

On the basis of comparative study of arc volcanic

belts with contrasting characters, it may be concluded

that the adiabatic upwelling of hotter mantle materials

caused by the secondary induced counter flow, which is

controlled by the physical properties of descending

slab, is a plausible process to generate arc basaltic mag-

mas.

Acknowledgement

The start of this study was the Overseas Scientific

Research (No. 59043009) titled “Geochemical Investiga-

tion of Southern Andes Volcanic Belt” carried out in

1982 to 1985, which was a cooperative project by

Ibaraki University with University of Chile. We wish

express our thanks to the late Prof. Naoki ONUMA

(Ibaraki University) who gave us a chance to participate

the project. We are also grateful to Prof. Kazuo

AMANO (Ibaraki University) and Dr. Andrew James

MARTIN (Japan Nuclear Cycle Development Institute)

for critical reading of the manuscript.

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comparative study of quaternary arc volcanic belts ......the physical properties of subducting...

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