Proceedings of the Institute of Natural Sciences, Nihon University No.37 (2002) pp.135 -156

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

Masaki TAKAHASHI 1), Michio TAGIRI 2), Kenji NOTSU 3), Leopoldo LOPEZ-ESCOBAR 4) and Hugo MORENO-ROA4) (Received September 30, 2001)

Abstract The 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 frontal volcanic rocks. The mantle process seems to be related to the rock-series of frontal and across-arc variation of alkaline content or rock-series of basalts. While, the condition of crustal stress field may be important for the occurrence of large calderas with voluminous felsic pyroclastic flows and abundance of andesite. 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 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 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, , subduction zone, Quaternary, Northeast Japan, Southern Chile

alkali tholeiite. The most remarkable contrast is the 1.Introduction age of the subducting lithosphere; very young in South- Southern Chile and Northeast Japan are typical Qua- ern Chile and rather old in Northeast Japan. It is impor- ternary volcanic arcs situated at the opposite side in the tant for the study of magma generation in subduction circum-Pacific region. They are characterized by com- zones to compare the differences as well as similarities mon arc features, such as the presence of a trench, the between two contrasting volcanic arcs. The main pur- Wadati-Benioff zone, an outer non-volcanic arc, an pose of this paper is to clarify the differences between inner volcanic arc, and an inter-arc basin. Several dis- Southern Chile and Northeast Japan and then consider tinctive differences are also present between the two the origin of such dissimilarities. We also intend to dis- arcs; the Southern Chilian volcanic arc lacks a back-arc cuss about the generation of magmas at convergent marginal sea basin and the occurrence of frontal low- plate boundaries based on this comparative study.

1）日本大学文理学部地球システム科学科： 1）Department of Geosystem Sciences, College of Humanities 〒156－8550 世田谷区桜上水 3－25－40 and Sciences, Nihon University: 3-25-40, Sakurajousui, Setagaya-ku Tokyo 156-8550 Japan 2）茨城大学理学部地球生命環境科学科： 2）Department of Environmental Sciences, Faculty of Science, 〒310－8512 水戸市文京 2－1－1 Ibaraki University: 2-1-1 Bunkyo, Mito 310-8512 Japan 3）東京大学大学院理学研究科： 3）Graduate School of Science, University of Tokyo: 7- 3- 1 〒113－0033 文京区本郷 7－3－1 Hongo, Bunkyo-ku Tokyo 113-0033 Japan 4）チリ大学地質学地球物理学教室： 4）Department of Geology and Geophysics, University of Chile: チリ共和国サンチャゴ市カシジャ13518 Casilla 13518, Santiago, Chile

─ ─135 （111） Masaki TAKAHASHI, Michio TAGIRI, Kenji NOTSU, Leopoldo LOPEZ-ESCOBAR and Hugo MORENO-ROA

Fig.1 Map showing the age and convergent rate of subducting oceanic lithosphere in Southern Chile and Northeast Japan (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.

relative rate is 10.6cm/year (Minster and Jordan, 1980). 2.Age of subducting lithosphere The depth of the trench is shallower in Southern The most conspicuous difference of the two arcs is Chile where the younger lithosphere is subducting; it is the age of the subducting oceanic lithosphere (Fig. 1). -4, 500 to -5, 000m in the north of the VMFZ, but no top- In Southern Chile, the subducting plate is very young ographical trench is observed to the south of it, where a (0～36Ma) (Fig. 1A). The geologic age of the oceanic younger plate is underthrusting (Fig. 5A). To the con- lithosphere north of the Valdivia-Mocha fracture zone trary, the depth of the trench is deeper than 7, 000m in (VMFZ) is Oligocene (23-36Ma). In the south of the Northeast Japan, where the old oceanic lithosphere is VMFZ, the age of the subducting plate becomes subducting (Fig. 5B). younger southward (from 19 to 2Ma), and the mid- oceanic ridge (the Chile Rise) is subducting under the 3.The Wadati-Benioff zone South American continent at a latitude of about 46゜S. The Wadati-Benioff zone or deep seismic zone reaches The absolute convergent rate of this young and warm a depth of about 200km in Southern Chile (Hanus and oceanic lithosphere is about 6.4cm/year and the rela- Vanek, 1978) and about 500 to 600km in Northeast tive rate is 9.2cm/year (Minster and Jordan, 1980). Japan (Yoshii, 1979) (Fig. 2). The dip angle of the Contrarily, the age of the subducting plate is very old in Wadati-Benioff zone is gentle in Southern Chile; 20゜in Northeast Japan, which is Jurassic to early Cretaceous the northern section (from 33゜to 36゜S) and 15゜in the (122 to 145Ma) (Fig. 1B). The absolute convergent rate southern section (from 36゜Sto45゜S). Contrarily, the dip of this old and cold lithosphere is 10.4cm/year and the angle is relatively steep in Northeast Japan; 30゜in north-

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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.

ern section (around 40゜N) and 40゜in southern section In Southern Chile, the altitude exceeds 4,000m above (around 33゜N). The differences in depth and dip angle sea level in the area north of 36゜S, and it gradually of the Wadati-Benioff zone are probably due to the dis- decreases southward from 3,000 to 2,000m between lati- similarities in the physical properties of the subducting tudes 36゜and 45゜S. The crust in the north of 36°Sis lithosphere: young, warm and buoyant oceanic plate in thicker than that in the south; the crustal thickness of Southern Chile; and old, cold and dense descending the former probably exceeds 40km and that of the latter slab in Northeast Japan. The depth from the volcanic is less than 40km (Lowrie and Hey, 1981). front to the Wadati-Benioff zone is around 100km in the While the altitude is generally between 1,000 and northern section of Southern Chile and Northeast 2,000m in Northeast Japan, it exceeds 2,000m only at Japan, but less than 50km in southern section of South- the arc-arc junction regions. In the area south of 35゜N, ern Chile where the younger oceanic lithosphere is sub- the altitude decreases to less than 1,000m, and the land ducting. submerges under the sea to form a chain of volcanic islands. The thickness of the crust in Northeast Japan 4.Crustal thickness is nearly 30km (Yoshii and Asano, 1972), but it The thickness of the crust is approximately reflected decreases in the area south of 35゜N and is less than in the topographical altitudes of main mountain ranges 20km at 32゜N. (Carr, 1984). The variation of altitude of the back-bone The detailed crustal sections at 38゜N in Southern ranges, on which the frontal volcanic edifices are con- Chile and at 39゜to 40゜N in Northeast Japan are shown structed, are shown in Fig. 3. in Fig. 4. Fig. 4A is a density model based on the data of

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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 gravity anomaly 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-arc section at 39° to 40°N (Yoshii and Asano, 1972); a unit of numeric number is cm/sec. arrow: the position of trench; solid triangle: the location of volcanic front.

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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). free-air gravity anomaly (Couch et al., 1981), and Fig.4B the Austral volcanic belt with a non-volcanic gap is a P seismic wave velocity model obtained by the of 370km and northward to the Central Andes volcanic explosion seismological method (Yoshii and Asano, belt also with a non-volcanic gap of 550km (Fig. 5A). 1972). The crustal thickness under the volcanic front in The volcanic belt in Northeast Japan extends for these sections is nearly 40km in Southern Chile and more than 1,500km, the width of which is 150km from 30km in Northeast Japan. 37゜to 43゜N, 60km in the south of 35゜N, and 210 to 230km at the arc-arc junction areas. The volcanic front 5.Dimension of the arc volcanic belts is situated at a distance of 260km to the north of 35゜N The volcanic arc belt in Southern Chile extends for 1, and 160km to the south of 35゜N from the trench. It con- 400km with a maximum width of 80km at a distance of tinues southward to the Izu-Bonin volcanic arc and 260km from the trench; the distance from the trench northward to the Kurile volcanic arc without any non- decreases southwards and less than 200km at 46゜S where volcanic gaps (Fig. 5B). the Chile Rise is subducting. It continues southward to The width of the volcanic belt in Southern Chile is

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

narrow and is divided into two zones in the northern area of 42゜S: the frontal western zone and eastern zone 6.Type of polygenetic volcanoes at the back-arc side (Moreno-Roa, 1976; Lopez-Escobar, Moriya (1983) classified polygenetic volcanoes into 1984). However, it becomes a single chain consisting of three main types: stratovolcanoes with or without only the western frontal volcanic zone in the south of 42 horseshoe-shaped calderas (Type-A1); stratovolcanoes ゜S. The volcanoes are concentrated in the western with calderas and central cones or lava domes (Type- frontal volcanic zone. A2); and polygenetic lava domes with or without small To the contrary, the width of the volcanic belt is broad calderas (Type-A3). The distribution of these three in Northeast Japan; it cannot be distinctly divided into two types of volcanoes in Southern Chile and Northeast zones but is composed of the frontal volcanic chain and a Japan is shown in Figs. 6A and 6B, respectively. The wide area with sporadically scattered volcanic edifices at type-A2 volcanoes appear to be predominant at the cen- the back-arc side. The volcanoes are densely distributed tral segment in Southern Chile and at the arc-arc junc- in the frontal volcanic chain also in Northeast Japan. tion areas in Northeast Japan.

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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.

the thickness of the crust is nearly 30km (Fig. 7B). 7.Distribution of large calderas with voluminous From north to south Tokachi, Hakkoda, Tamagawa, felsic pyroclastic flows Onikobe, and Shirakawa erupted during the early Qua- Large calderas with voluminous felsic pyroclastic ternary, and Kutcharo, Akan, Shikotsu, Toya and flows in Southern Chile are distributed only to the north Towada during the late Quaternary. of 37゜S, where the altitude and crustal thickness exceed It appears that the thick crust exceeding 30km in 4,000m and 40km, respectively (Fig. 7A). They are the thickness is necessary for the generation of voluminous caldera-like depression at the volcano (Stern et felsic pyroclastic flows and related large calderas. How- al., 1984) and the caldera (Hildreth et al., ever, it is not a sufficient requirement because crustal 1984). In the area south of 37゜S, the large calderas with thickness to the south of 37゜S in Southern Chile is voluminous pyroclastic flows are completely lacking. nearly the same as that to the north of 35゜N in North- In Northeast Japan, large calderas with voluminous east Japan. Other factors, such as differences in the tec- felsic pyroclastic flows are present in the area where tonic stress field, may be needed to explain the produc-

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

tion of voluminous felsic pyroclastic flows and forma- There are no significant differences in age and physical tion of large calderas. Takahashi (1995) proposed that nature of the subducting lithosphere between the areas low crustal strain rate is responsible for the large-scale in which basalt is dominant (south of 35゜N) and felsic vocanic activity with large calderas. andesite is dominant (north of 35゜N). Although basalt is the major rock type to the south of 8.Dominant type of volcanic rocks 37゜S in Southern Chile and andesite predominates to Andesite is predominant to the north of 37゜S in South- the north of 35゜N in Northeast Japan, the crustal thick- ern Chile, where the thickness of the crust exceeds ness of both regions is similar. Thus, it may be con- 40km. Contrarily, the dominant rock type to the south cluded that the dominant rock type does not depend not of 37゜S is basalt (Fig. 8A). In Northeast Japan, andesite only upon the age and physical properties of the sub- is the major rock type where the crust is thick (ca ducting oceanic plate but also upon the crustal thick- 30km in thickness), but basalt is dominant to the south ness. The state of tectonic stress field may also play an of 35゜N where the crustal thickness is less than 30km. important role for determining the major rock types.

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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.

For example, both regions are compressional, but trary, the frontal volcanoes of Northeast Japan are char- reverse faults are predominant to the north of 35°N in acterized by the occurrence of low alkaline tholeiite, Northeast Japan and strike-slip faults predominant to excluding the arc-arc junction area where high-alumina the south of 37°S in Southern Chile. basalt appears on the volcanic front (Fig. 9B). In the back-arc side of the volcanic front in Northeast Japan, 9.Rock-series of basalts high-alumina basalt is present, and alkaline basalt In Southern Chile, high-alumina basalt is common occurs in the farthest region from the trench. The along the volcanic front, which is occasionally accompa- Na2O content of frontal high-alumina basalt in Southern nied by alkaline basalt with relatively low alkaline con- Chile is higher than that in Northeast Japan (Taka- tent. The Hudson volcano situated in the southern- hashi, 1989). most portion of the frontal volcanic chain, where the It is manifest that rock-series of basalt is closely related mid-oceanic ridge (the Chile Rise) is subducting, to the age and physical properties of the subducting mainly consists of alkaline basalt (Fig. 9A). To the con- oceanic lithosphere and not to the thickness of the

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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 with or without hornblende; half-solid: with hornblende; solid: without hydrous mafic phenocryst.

crust; the age of the subducting plate is Miocene to blende and biotite are predominantly distributed in the present in Southern Chile and Jurassic to early Creta- frontal volcanic chain (Fig. 10B). Those with horn- ceous in Northeast Japan. blende and no biotite occur mainly at the back-arc side of the volcanic front, and those with hornblende and 10. Mafic phenocryst assemblage of andesite, biotite mostly appear in the farthest region from the dacite and rhyolite trench (Fig. 10B). Sakuyama (1977; 1979) classified the Quaternary vol- On the other hand, andesite and dacite-rhyolite with canoes of Northeast Japan into three types on the basis biotite and hornblende are restricted to the north of 37゜S of mafic phenocryst assemblage of intermediate to fel- in Southern Chile, where the crust is thick. Those with sic volcanic rocks: volcanic rocks with biotite and horn- hornblende are distributed in the eastern volcanic belt blende; rocks with hornblende and no biotite; and rocks (the Tronador volcano) and in the southern-most por- without hydrous mafic minerals. tion of the western frontal volcanic chain (the The volcanoes composed of rocks without horn- and Cay volcanoes). Andesite without hornblende and

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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). biotite, and dacite-rhyolite with fayalite and no hydrous most area of the frontal volcanic chain. It may be con- mafic phenocrysts are predominant to the south of 37゜S cluded that the difference of H2O content of magma is of the frontal volcanic belt (Fig. 10A). ultimately originated in the magma generation process The variation of mafic phenocryst assemblage is in the mantle wedge under the arc, although the effect ascribed to the difference of H2O content in magma of crustal materials is not completely excluded. (Sakuyama, 1977; 1979); magma without hydrous mafic 11. Across-arc variation of alkaline content phenocryst is relatively dry and the H2O content of magma increases as the phenocryst assemblage of vol- It is a well known fact that the alkaline content, espe- canic rocks changes from hornblende only to hornblende cially K2O, increases across the arc from the volcanic plus biotite. The H2O content of magma increases from front to the back-arc side. In order to examine the across the volcanic front to the back-arc side in Northeast Japan. arc variation of alkaline content, two areas with similar

In Southern Chile, H2O is most abundant to the north of crustal thickness (about 30km) are selected: one is the 37゜S, moderate in the eastern volcanic zone and south- across-arc section between 40゜30’ and 41゜30’S in South- ern-most portion of the frontal volcanic belt, and the least ern Chile (Fig. 11A); and the other the region between to the south of 37゜S of the frontal volcanic chain. 39゜30’ and 40゜N in Northeast Japan (Fig. 11B). The

The mafic phenocryst assemblage, namely the H2O K2O content increases from the volcanic front towards content of magma, is not necessarily related to the the back-arc side in both areas, but the content at the crustal thickness as is shown in Northeast Japan, but it volcanic front is lower in Northeast Japan than in South- seems to be closely related to the age and physical ern Chile (Figs.12A and B). The Na2O content properties of the subducting lithosphere in Southern increases from the volcanic front toward the back-arc Chile. The older plate in the north of VMFZ is descend- side in Northeast Japan, but no significant increase is ing under the region north of 37゜S, and the younger lith- observed in Southern Chile (Fig. 13A and B). osphere in the south of VMFZ is underthrusting to the The total alkalis increase toward the back-arc side south of 37゜S. Furthermore, the active mid-oceanic from the volcanic front in both volcanic arcs. All the ridge (the Chile Rise) is subducting in the southern- data of volcanic rocks in Southern Chile are plotted in

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Fig.12 Across-arc variation of K2O content. A: Southern Chile (solid square: ; solid circle: ; 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. The list of data source is avairable. Request to the author (M.Takahashi).

the field of high-alumina basalt series on a Na2O+K2O contents, the frontal volcanoes of both arcs are divided

vs. SiO2 diagram (Fig. 14B), whereas the data of frontal into six along-arc segments, A-ItoA-VI in Southern volcanoes (Nanashigure, Iwate, Akita-Komagatake, Chile and B-ItoB-VI in Northeast Japan (Fig. 15).

Hachimantai, Kayo and Akita-Yakeyama) in Northeast The K2O content is highest in the A-I segment (33゜to Japan are plotted in the low aikali tholeiitte region, and 36 ゜30’S) where the crust is thick. The lowest limit of

those of the back-arc side (Moriyoshi, Kanpu, Megata) K2O content decreases in A-II (36゜30’ to 39゜S) and is the are plotted in the high-alumina basalt series and partly minimum in A-III (39゜to 41゜30’S). It slightly increases in the alkaline basalt series regions (Fig. 14A). in A-IV (41゜30’ to 43゜S) and A-V (43゜Sto45゜S) , and the

K2O content is relatively high in A-VI (45゜to 46゜S) 12. Along-arc variation for alkaline content of where the mid-oceanic ridge is subducting (Fig. 16). frontal volcanoes Most volcanic rocks are medium-K series by Gill In order to examine the along-arc variation of alkali (1981), but many in A-I are high-K series and a lot of

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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.

them in A-III are low-K series. The Na2O content is slightly high in A-I; it decreases

In Northeast Japan, the upper limit of K2O content is in A-II and is the minimum in A-III and A-IV. It increases the lowest in B-I (33゜to 35゜30’N) where the crust is again in A-V and A-VI (Fig. 17). On the other hand, in thin. It increases in B-II (35゜30’ to 37゜N) and is maxi- Northeast Japan, the Na2O content is the lowest in B-I mum in B-III (37゜to 39゜N), and then decreases in B-IV but nearly constant in other segments excluding the (39゜to 41゜N), B-V (41゜to 43゜N) and B-VI (43゜to 45゜N). arc-arc junction areas characterized by rather high

The lower limit of the K2O content is constant through- Na2O contents. It is clear that the Na2O content is gen- out all the segments except for the arc-arc junction erally higher in Southern Chile than in Northeast Japan. area. Most volcanic rocks in B-I are low-K series and The total alkali content is highest in A-I, most vol- those in other segments are both low-K and medium-K canic rocks of which are both high-alumina basalt and series, but the arc-arc junction areas are characterized alkaline basalt (Fig. 18). The lowest limit decreases in by medium-K series. A-II and is the minimum in A-III and A-IV, belonging to

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Fig.14 Across-arc variation of total alkali content. A: Southern Chile; B: Northeast Japan; symbols are the same as Fig.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).

the high-alumina basalt series. The total alkali content thrusting. The upper limit of K2O and lower limit of

increases in A-V and is nearly the same level as A-Iin Na2O and total alkali content seem to be related to the A-VI. crustal thickness in Northeast Japan. They are the low- The total alkali content is the lowest in B-I and nearly est in B-I where the crust is thinner than in other seg-

constant from B-II to B-VI. Most volcanic rocks are low ments. The lowest limit of K2O, however, is nearly con- alkali tholeiite except for the arc-arc junction area stant and appears to be unrelated to the thickness of the where high-alumina basalt is predominant. crust.

The along-arc variation of the lower limit of K2O, 13. Along-arc variation of Sr and O isotopes in Na2O and total alkali content seem to be related to the crustal thickness and/or the age of subducting oceanic frontal volcanoes lithosphere in Southern Chile. They are highest in A-I In Southern Chile, the 87Sr/86Sr ratio of volcanic where the crust is thickest and the older plate is under- rocks is the highest (0.7047 to 0.7062) in the northern

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

part of A-I and decreases in its southern portion case in B-VI where the crust is not so thin. The extraor- (Fig. 19). It is the lowest in A-II (0.7040>) and increases dinary high ratio may be related to the collision of sub- in A-III. The 87Sr/86Sr ratio is slightly higher in A-IV, A- ducted Pacific plate and Philippine Sea plate beneath V, A-VI (0.7041<); biotite rhyolite in the Chaiten volcano the B-II segment (Notsu, 1983). in A-IV shows a high ratio (0.7058). The highest ratio in No systematic variation of δ18O in volcanic rocks is the northern portion of A-I may be related to the pres- observed in either Southern Chile or Northeast Japan ence of thick continental crust exceeding 40km. (Fig. 20), but the variation of 87Sr/86Sr ratio seems to be In Northeast Japan, the 87Sr/86Sr ratio is the lowest weakly correlated to that of the δ18O value except for (0.7040>)inB-I and B-VI. It is the highest in B-II the A-I segment in Southern Chile. In A-I, δ18O of vol- (0.7078>) and decreases in B-III , B -IV and B-V (Fig. 19). canic rocks is not so high in spite of their high 87Sr/86Sr The lowest ratio in B-I may possibly be related to the ratio; the crustal thickness does not appear to be thin crust less than 30km in thickness, but it is not the related toδ18O value.

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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 series by 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).

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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 basalt series (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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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).

The crustal effect appears to be reflected in the along- 14. Concluding remarks and discussions arc variation of upper limit of K2O content in frontal vol- It is concluded from the comparative study of the arc canic rocks. The existence of thick continental crust is volcanism in Southern Chile and Northeast Japan that favorable for the crustal remelting and/or assimilation, two factors, the crustal effect and mantle process, must which may bring about high K2O content of magmas. be taken into account when the magma genesis is inves- On the other hand, the mantle process seems to be tigated. In this case, the mantle process includes vari- related to (1) the rock-series of frontal basalts and (2) ous dynamic physico-chemical processes caused by the across-arc variation of alkaline content and rock-series subduction of lithosphere with different ages and physi- of basalts. cal properties. In addition to above two factors, the condition of

─ ─153 （129） Masaki TAKAHASHI, Michio TAGIRI, Kenji NOTSU, Leopoldo LOPEZ-ESCOBAR and Hugo MORENO-ROA

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).

crustal stress field may be important for (1) the occur- magma is produced under relatively low pressure (1.1 rence of large calderas with voluminous felsic pyroclas- GPa) with higher degree of partial melting, while alka- tic flows and (2) abundance of andesite. line basalt magma is formed under higher pressure (2.3 The cause of variation of Sr and O isotope ratios is GPa) with lower degree of partial melting. On the other rather complex. It may be related to both the crustal hand, the melting pressure and degree of partial melt- effect and mantle process, though the difference ing of high-alumina basalt magma show intermediate between two arcs is not so remarkable as the alkaline values ( the pressure is 1.7GPa )between those of low content. alkali tholeiite and alkaline basalt. Recent experimental studies revealed the physical The decompression melting is thought to bring about conditions of magma generation in the mantle wedge. the difference of degree of partial melting, because the According to Tatsumi et al. (1983), low alkali tholeiite melting temperature of these magmas are nearly the

（130） ─ ─154 Comparative Study of Quaternary Arc Volcanic Belts: Southern Chile vs. Northeast Japan same (about 1320℃). The adiabatic ascent of mantle the upwelling counter flow to ascend to higher level material with high temperature causes the decompres- beneath the volcanic front. sion melting; the degree of partial melting increases as The across-arc variation of magma series of basalts is the pressure dcreases. also explained by this model, because the upwelling The depth of magma generation beneath the volcanic counter flow is inclined parallel to the subducting plate front is shallower and degree of partial melting is and the depth of magma generation becomes deeper higher in Northeast Japan than in Southern Chile, from the volcanic front towards the back-arc side. because basalt erupted in the volcanic front is low alkali On the basis of comparative study of arc volcanic tholeiite in Northeast Japan and high-alumina basalt in belts with contrasting characters, it may be concluded Southern Chile. It may be explained if the adiabatic that the adiabatic upwelling of hotter mantle materials upwelling flow with similar temperature reaches to the caused by the secondary induced counter flow, which is shallower level in the mantle wedge beneath the vol- controlled by the physical properties of descending canic front in Northeast Japan but stagnates in rather slab, is a plausible process to generate arc basaltic mag- deeper portion under the volcanic front in Southern mas. Chile. The physical properties of subducting oceanic litho- Acknowledgement sphere is contrasting between the two arcs; it is old, The start of this study was the Overseas Scientific cold and dense in Northeast Japan and young, warm Research (No. 59043009) titled “Geochemical Investiga- and buoyant in Southern Chile. The cold and dense tion of Southern Andes Volcanic Belt” carried out in lithosphere descends into the deeper level in the upper 1982 to 1985, which was a cooperative project by mantle with high subduction rate, which may promote Ibaraki University with University of Chile. We wish the upwelling counter flow with high temperature express our thanks to the late Prof. Naoki ONUMA reaching to the shallower level in the wedge mantle (Ibaraki University) who gave us a chance to participate beneath the volcanic front (e.g. Furukawa, 1996). Con- the project. We are also grateful to Prof. Kazuo trarily, the subduction of warm and buoyant plate with AMANO (Ibaraki University) and Dr. Andrew James low descending rate is restricted to the shallower level MARTIN (Japan Nuclear Cycle Development Institute) in the mantle, hence in this case it may be difficult for for critical reading of the manuscript.

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