Strontium isotope composition in volcanic rocks from the Northeast Japan arc

Journal of Volcanology and Geothermal Research, 18 (1983)531--548

531

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

STRONTIUM ISOTOPE COMPOSITION IN VOLCANIC ROCKS FROM THE NORTHEAST JAPAN ARC

KENJI NOTSU

Institute of Chemistry, University of Tsukuba, Sakura-mura, Ibaraki 305 (Japan) (Received October 20, 1982)

ABSTRACT

Notsu, K., 1983. Strontium isotope composition in volcanic rocks from the Northeast Japan arc. In: S. Aramaki and I. Kushiro (Editors), Arc Volcanism. J. Volcanol. Geotherm. Res., 18: 531--548. In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. aTSr/86Sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the Northeast Japan arc area. The results reveal that the ratios change systematically in space. Decreasing sTSr/seSr ratios across the arc were confirmed over a wide area of Northeast Japan. In the same direction, increases in both Rb and Sr contents were also found. The regular trends are considered to be a strong constraint for elucidation of subduction-originated magma genesis at the Eurasia plate vs. Pacific plate boundary. In the northern region of the Northeast Japan arc, 87Sr/86Sr ratios in volcanic rocks along the volcanic front were almost constant (0.7038--0.7045) and slightly higher than those from the Izu-Ogasawara arc (0.7032--0.7038). This suggests that "interactions" between the Eurasia plate and the Pacific plate, and those between the Philippine Sea plate and the Pacific plate are slightly different. The southern region of the Northeast Japan arc, where the direction of the volcanic front bends from southward to westward, showed anomalously high 87Sr/8~Sr ratios, reaching to 0.7077. This region coincides with the triple junction of the Eurasia, Pacific and Philippine Sea plates, suggesting "anomalous interaction" at the triple junction.

INTRODUCTION V o l c a n i c a c t i v i t y is i n t e n s e a l o n g i s l a n d arcs a t s u b d u c t i o n z o n e s o f p l a t e m a r g i n s . In e a s t e r n J a p a n , t h e r e are a n u m b e r o f Q u a t e r n a r y v o l c a n o e s , m a k i n g a l o n g , n a r r o w b e l t , f r o m H o k k a i d o via n o r t h e a s t e r n a n d c e n t r a l H o n s h u t o I z u a n d O g a s a w a r a islands, p a r a l l e l t o t h e J a p a n a n d Izu-Ogas a w a r a T r e n c h e s . In t h e n o r t h e r n p a r t o f t h i s v o l c a n i c b e l t , w h i c h is n a m e d the N o r t h e a s t Japan arc, the Pacific plate s u b d u c t s beneath the Eurasia plate, w h i l e in t h e s o u t h e r n p a r t , t h e I z u - O g a s a w a r a arc, t h e P a c i f i c p l a t e goes ben e a t h t h e P h i l i p p i n e Sea p l a t e . T h e N o r t h e a s t J a p a n arc is j o i n t e d to t h e K u r i l e arc t o t h e n o r t h e a s t a n d t h e I z u - O g a s a w a r a arc is j o i n t e d t o t h e Mar i a n a arc t o t h e s o u t h .

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© 1983 Elsevier Science Publishers B.V.

532 In the Northeast Japan arc, distribution of volcanoes is n o t uniform in space. The population of volcanoes is abundant on the eastern side facing the Japan Trench and decreases from east to west. A sharp boundary, which is called "volcanic front" (Sugimura, 1960), is observed at the eastern edge of the volcanic belt. Physical properties of subduction have been investigated most precisely on this arc. A visualized feature of subducting oceanic slab was first reported by Hasegawa et al. (1978), who discovered the doubleplaned structure of the deep seismic zone over a wide area, more than 300 km X 200 km, through the determination of microearthquake hypocenters. Corresponding with the subducting feature of the oceanic slab, various kinds of lateral variations in petrology, chemical and isotopic compositions of arc volcanic rocks have been discussed since Kuno's (1966) work. He pointed out that three types of basalts, namely tholeiite, high-alumina basalt and alkali-olivine basalt, occur zonally from east to west along the island arc. Before Kuno's work, Kawano et al. (1961a) noticed that alkali contents in volcanic rocks from the Northeast Japan arc increased from east (tholeiite series) to west (calc-alkali rock series) based on chemical compositions of 180 rocks. Increase in K~O contents, when normalized to SiO~ contents, away from the volcanic front was c o m m o n l y observed in many island arc systems on earth (Dickinson, 1968) and Ui and Aramaki {1978) summarized the K20 zoning on Japanese island arcs in connection with the Bouguer anomaly. Recent works on volcanic rocks from the Northeast Japan arc have shown that some trace-elements contents, such as Ba, Th, U~ REE and F contents, also increased along with K~O content from east to west (Masuda, 1979; Ishikawa et al., 1980). Chondrite-normalized REE patterns also changed. The inclination of the pattern for lighter REE was fiat in volcanic rocks from the volcanic front side and was gradually steeper away from it (Fujimaki and Kurasawa, 1980; Fujitani and Masuda, 1981). Furthermore, Sakuyama (1979) pointed out that H20 contents in arc magma increased from east to west in the Northeast Japan arc. Isotopic studies were also done in this region. Hedge and Knight (1969) measured Sr and Pb isotope compositions in volcanic rocks from a 40°N traverse cross-section across the Northeast Japan arc, showing that both 87Sr/ 86Sr ratios and radiogenic components of Pb isotopes decreased away from the volcanic front. Nohda and Wasserburg (1981) determined Sr and Nd isotope compositions in volcanic rocks from the same 40°N traverse cross section, confirming the previous Sr data and revealed that they fell near the "mantle array" (DePaolo and Wasserburg, 1979) on the Nd and Sr isotope diagrams. Some models a b o u t the origin of arc magma have been proposed to compromise these lateral variations in volcanic rocks, underground structure ~obtained from geophysical data, such as gravity, earthquake, heat flow, etc., and the underground condition estimated from phase petrological investigations, b u t a unique model has not been reported yet. In order to construct a realistic model, isotopic data, especially 87Sr/86Sr ratios, in volcanic rocks

533 are essential. However, in the case of the Northeast Japan arc, 87Sr/86Sr ratios have been previously determined only for a limited few volcanoes. Except the traverse works of 87Sr/86Sr ratios by Hedge and Knight (1969) and Nohda and Wasserburg (1981), we can enumerate: Oshima-oshima (Hedge and Knight, 1969), Oshima-oshima and Oshima-kojima (Katsui et al., 1978), Osoreyama (Togashi, 1981), Ichinomegata (Kaneoka et al., 1978), Akitakomagatake (Kurasawa, 1979), Towada (Kurasawa, 1972), Asama, Eboshi and M y o k o (Nohda and Wasserburg, 1981) and Iizuna, Kurohime, Myoko and Yakeyama (Ishizaka et al., 1977). Even if all these data are compiled, the spatial distribution of 87Sr/86Sr ratios in volcanic rocks from the Northeast Japan arc remains unclear. The purpose of this study is to present a comprehensive set of 87Sr/86Sr ratios of volcanic rocks from many Quaternary volcanoes situated in a wide area of the Northeast Japan arc. From this approach, we can expect to clarify the regional variations of 8~Sr/86Sr ratios, both in across-arc and along-arc directions. This approach is considered to be essential to understand the "interaction" between the Eurasia plate and the Pacific plate in the northeastern Honshu area, as well as among the Eurasia plate, the Pacific plate and the Philippine Sea plate at the central Honshu area (Kanto-district) where there is a unique triple junction. SAMPLES R o c k samples analyzed in this work were taken from 28 representative volcanoes on the Northeast Japan arc, which were selected with regard to their spatial distribution. Localities of volcanoes investigated are shown in Fig. 1, together with those of other Quaternary volcanoes in the Northeast Japan arc. For an individual volcano, usually two to four representative rock samples were analyzed. These rock samples were selected considering the rock types or chemical compositions of rocks and their erupting stages. R o c k samples from some volcanoes were newly collected by the present author with the aid of other persons, referring to the previous publications a b o u t geology and volcanology of them. Rock samples of other volcanoes were provided by several persons. Comments on each volcano are listed in Table I. ANALYTICAL METHODS Strontium isotope ratios were determined using a VG-Micromass MM-30 double~ollector-type mass spectrometer with on line computer facilities, after the Sr in the rock samples was separated by a conventional cation exchange technique. Standard strontium isotope reagent, NBS 987, was analyzed with every five rock samples to check reproducibilities of the isotope measurements. The measured range of 8~Sr/86Sr ratios of NBS 987 was 0.71028 to 0.71033 during the period of this study. As the statistical error

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Fig. 1. Localities of Quaternary volcanoes in the Northeast Japan arc. Volcanoes investigated are indicated as solid circles. Numerals for volcanoes correspond to those in Table I. o f each isotope analysis was usually less than 0.00002 (20 value of standard deviation in 87Sr/86Sr ratios), which was less than the reproducibility range ( < 0 . 0 0 0 0 5 ) , the u n c e r t a i n t y associated with 87Sr/86Sr ratio analysis o f rock samples was estimated less than 0.00005. Concentrations o f Rb and Sr were measured by (wave length dispersive) X-ray fluorescence s p e c t r o m e t r y on a few grams of pressed p o w d e r disk samples. Matrix effects were corrected using background radiation as an internal standard (Murad, 1973). Uncertainties in Rb and Sr determinations were within + 5% for concentrations o f more than 10 ppm. Fo r 6 samples, Rb and Sr were det er m i ned by energy-dispersive X-ray fluorescence spectro-

535

metry. The latter X R F technique enabled us to determine Rb and Sr in a b o u t 5 mg of powdered rock sample as precise as the former conventional one, although the detection limit was several times higher. RESULTS

Analytical results are presented in Table I. In this study, 87Sr/86Sr ratios for several rocks from the same volcano were found to agree with one another usually within 0.0003, regardless of rock types and/or their erupting stages. Though the difference of 0.0003 in 87Sr/86Sr ratio is meaningfully greater than the uncertainty of each isotope analysis of this work, this level of difference does not seem significant in comparison with the 87Sr/86Sr ratio variations among volcanoes. It is, of course, important to discuss the minute difference in 87Sr/86Sr ratios in several rocks from one volcano, with regard to the evolution of magma composition in the magma chamber of the volcano. It is a very interesting problem whether or not there is any difference in 87Sr/SSSr ratio among basalt, andesite and dacite from a given volcano and/or between associated tholeiite and calc-alkaline series rocks and/or among rocks from successive volcanic stages throughout the life of the volcano. In this work, however, we dared to neglect this problem to make clear the wide-scale variation of the 87Sr/86Sr ratio related to a global tectonic p h e n o m e n o n , such as subduction of the Pacific plate beneath the Eurasia plate. Throughout this paper, thereafter, the variation in 87Sr/86Sr ratios in several volcanic rocks from one volcano is not treated, but it will be discussed elsewhere in a separate paper. In Fig. 2 the spatial distribution of 87Sr/86 Sr ratios in Quaternary volcanic rocks from the Northeast Japan arc is given. In this figure, 87Sr/86Sr ratios are classified into 5 groups: less than 0.7032, 0.7032--0.7038, 0.7038-0.7045, 0.7045--0.7055 and more than 0.7055. This figure also includes previously measured results by other investigators for the volcanoes which were not studied in this work. They are identified by circles with small alphabetical symbols corresponding to their references. 87Sr/SSSr ratios in volcanic TABLE I A n a l y t i c a l results of volcanic r o c k s f r o m t h e N o r t h e a s t Japan arc No.* N a m e o f volcano

S a m p l e No.

Rock type Rb (ppm)

Sr (ppm)

8 7 R b / ~ S r 87Sr/86Sr

1

Oshima-oshima

OO2A 003-2

Andesite Basalt

94 58

637 573

0.430 0.295

0.70311 0.70313

2

Osoreyama

OSO04 OSO07 OSO13

Andesite Dacite Andesite

3.7 18 11

215 218 221

0.046 0.240 0.145

0.70406 0.70419 0.70388

3

Hakkoda

SA64090102

Andesite

25

266

0.274

0.70405

536 TABLE I

(continued)

No.* N a m e o f volcano

Sample No.

R o c k t y p e Rb (ppm)

Sr (ppm)

87Rb/8~Sr 87Sr/86Sr

4

Iwaki

Iw.797121 Iw.797122 Iw.797123 Iw.7910134

Andesite Andesite Andesite Andesite

25 27 28 21

326 299 304 324

0.223 0.263 0.268 0.189

0.70407 0.70406 0.70409 0.70437

5

Towada

TOW01 TOW06

Andesite Andesite

7.4 11

300 289

0.072 0.111

0.70421 0.70415

6

Iwate

IWT13-2 IWT19

Basalt Andesite

2.2 4.8

280 250

0.023 0.056

0.70440 0.70432

7

Moriyoshi

MOR7-1 MOR7-13 MOR7-14

Andesite Dacite Andesite

17 59 25

403 289 416

0.123 0.594 0.175

0.70379 0.70403 0.70395

8

Kampu

KMP01 KMP03

Andesite Andesite

79 78

668 651

0.344 0.349

0.70343 0.70356

9

Kurikoma

KUR03 KUR04-01

Andesite Andesite

31 28

206 217

0.439 0.376

0.70416 0.70425

10

Chokai

CHO01 TU7461502

Andesite Andesite

69 68

386 396

0.520 0.500

0.70352 0.70347

11

Funagata

FUN05 FUN07-01

Andesite Basalt

18 3.1

250 169

0.210 0.053

0.70407 0.70425

12

Gassan

GAS13 GAS14

Andesite Andesite

63 64

413 382

0.444 0.488

0.70352 0.70347

13

Zao

ZAO09 ZAO10

Andesite Basalt

334 364

0.070 0.078

0.70414 0.70384

14

Azuma

AZ01 AZ02

Andesite Andesite

45 63

219 201

0.598 0.912

0.70471 0.70485

15

Nasu

52414 52714

Andesite Basalt

34 7.7

250 265

0.396 0.095

0.70500 0.70489

16

Asakusa

A7 A203

Andesite Andesite

42** 50**

588** 588**

0.208 0.248

0.70391 0.70425

17

Sumon

KC30 KC57

Andesite Andesite

65** 52**

420** 383**

0.451 0.395

0.70437 0.70414

18

Takahara

SA62101315

Dacite

72

209

1.003

0.70588

19

Nikkoshirane

ET78080405

Dacite

60

255

0.685

0.70593

20

Hiuchi

HIU04

Andesite

45

357

0.367

0.70538

8.0 9.7

21

Sukai

SA7808050

Andesite

38

313

0.353

0.70639

22

Hotaka

TY7782612 TY78101408

Andesite Dacite

22 59

290 270

0.221 0.636

0.70563 0.70577

23

Akagi

AK399-7 AK528-1 AK838-1

Andesite Dacite Andesite

9.4 68 9.6

368 308 359

0.074 0.643 0.078

0.70759 0.70704 0.70775

24

Haruna

00661023-6 Dacite O O 6 8 1 1 2 7 - 1 9 Basalt

--*** --***

264** 319"*

---

0.70488 0.70544

537 25

Asama

AS5292 Andesite SA77103006 Andesite

26

Eboshi

SA77112506 Andesite

27

Kusatsushirane

Nsl

Andesite

38 37 8.1 48

294 289

0.376 0.373

0.70417 0.70416

410

0.058

0.70404

278

0.503

0.70396

*This number corresponds to that in Fig. 1. **Energy dispersive XRF data (see the text). ***Not determined. 1. Oshima-oshima: provided by Dr. Y. Nakamura, Univ. Tokyo. OO2A and 0 0 3 - 2 are Nishiyama upper lava and parasitic crater lava of central cone, respectively. 2. Osoreyama: collected by the author, referring to Togashi (1977). OSO04, OSO07 and OSO13 are Kamabuseyama lava, Shojiyama lava and Tsurugiyama lava, respectively. 3. Hakkoda: provided by Prof. S. Aramaki, Univ. Tokyo. 4. Iwaki: provided by Dr. F. Uemura, Geological Survey of Japan. Iw.797121 was collected from somma, Iw.797122 and Iw.797123 were from central cone and Iw.7910134 was from upper stream of Ushironagane-zawa. 5. Towada: collected by the author, referring to Taniguchi (1972). TOW01 and TOW06 are 2nd stage lavas from Nakayama peninsula. 6. Iwate: collected by the author, referring to Onuma (1962). IWT13-02 IWT19 are lavas of Yakushidake cone and Yakehashiri, respectively. 7. Moriyoshi: provided by Dr. M. Sakuyama, Univ. Tokyo. MORT-1 was collected from summit of central cone and MOR7-13 and MOR7-14 were from southern flank. 8. Kampu: provided by Dr. M. Sakuyama. 9. Kurikoma: collected by the author. KUR03 and KUR04-01 are lavas of Kurikomayama and Tsurugisan, respectively. 10. Chokai: CHO01 (Shinzan lava erupted in 1 8 0 l A D ) was provided by Prof. S. Aramaki and TU7461502 was provided by Dr. T. Ui, Kobe Univ. 11. Funagata: collected by the author. FUN05 and FUN07-01 are lavas of Izumigatake and Kurohanayama, respectively. 12. Gassan: collected by the author, referring to Shibahashi (personal communication, 1981 ). AS13 is Gassan central cone lava and GAS14 is Shinsen-ike lava. 13. Zao: collected by the author. ZAO09 and ZAO10 are laves of Fubosan. 14. Azuma: provided by Dr. M. Sakuyama. AZ01 and A Z 0 2 are lavas of Higashiazumasan and Issaikyosan, respectively. 15. Nasu: provided by Prof. S. Aramaki. 52414 is upper Chausudake lava and 52714 is Numahara lava. 16. Asakusa: provided by Prof. K. Chihara, Niigata Univ. 17. Sumon: provided by Prof. K. Chihara. KC30 is lava of Eboshidake and K C 5 7 is Kitaone lava. 18. Takahara: provided by Prof. S. Aramaki. S A 6 2 1 0 1 3 1 5 is pumice block in pumice flow deposit. 19. Nikkoshirane: provided by Prof. S. Aramaki. 20. Hiuchi: provided by Prof. S. Aramaki. 21. Sukai: provided by Prof. S. Aramaki. 22. Hotaka: provided by Mr. T. Yamaguchi, Univ. Tokyo. TY7782612 and TY78101408 correspond to Ap-1 and Tg-2 in Yamaguchi's (1981) paper. 23. Akagi: provided by Prof. I. Moriya, Kanazawa Univ. 24. Haruna: provided by Dr. O. Oshima, Univ. Tokyo. 0 0 6 6 1 0 2 3 - 6 is Futatsudake pumice flow and OO681127-19 is main cone lava. 25. Asama: provided by Prof. S. Aramaki. AS5292 is Shimonobutai lava and SA77103006 is Onioshidashi lava (Aramaki, 1963). 26. Eboshi: provided by Prof. S. Aramaki. SA77112506 is Takamine lava flow. 27. Kusatsushirane: provided by Mr. Y. Hayakawa, Univ. Tokyo. Nsl is Ishizu lava.

538 0

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I Fig. 2. Spatial distribution of 87Sr/86Sr ratios in Quaternary volcanic rocks from the Northeast Japan arc. Circles without alphabetical symbols were analyzed in this work. Others are: a, Katsui et al. (1978); b, Kaneoka et al. (1978); c, Nohda and Wasserburg (1981); d, Kurasawa (1979); and e, Ishizaka et al. (1977).

rocks from some volcanoes were determined both in this work and in the previous ones, being found to agree with each other according to the 5 groups of classification (Oshima-oshima, Iwate and Kampu by Hedge and Knight (1969), Oshima-oshima by Katsui et al. (1978), Osoreyama by Togashi (1981) and Moriyoshi, Kampu, Asama and Eboshi by Nohda and Wasserburg (1981)). Figure 2 demonstrates that the spatial distribution of 87Sr/86Sr ratios in volcanic rocks from the Northeast Japan arc is significantly characteristic and that the ratios change systematically in both across-arc and along-arc directions. Spatial distributions in Rb and Sr concentrations in andesites and basalts are shown in Figs. 3 and 4, from which lateral variations in both elements are clearly observed. Rb and Sr contents in both andesites and basalts increase in across-arc direction with increasing depth of the Wadati-Benioff zone.

539

Andesite

i

o/

i i i

/ O/ /

Rb (ppm)

,(

0 0-10 ~) 10-30 (9 30-50 • 50-70

7 /

/ ~,/

O/ /

~

•

is J

>70

Fig. 3. Spatial d i s t r i b u t i o n o f Rb c o n c e n t r a t i o n s in Quaternary volcanic rocks from the Northeast Japan arc.

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540

VARIATION OF 87Sr/S6SrRATIOS IN ACROSS-ARCDIRECTION Figure 2 indicates a decrease in 87Sr/86Sr ratios of volcanic rocks from the volcanic front side to the back arc one, in other words, with increasing depth of the Wadati-Benioff zone, is observed in a wide area of the Northeast Japan arc. This observation expands the earlier finding by Hedge and Knight (1969), confirming that zones with similar 87Sr/86Sr ratios spread from north to south parallel to the Japan Trench. Figure 5 shows decreasing gradients of s7 Sr/86Sr ratios against the distance from the volcanic front. From this figure, the gradients are found to be roughly equal for all volcanoes on the back arc side, except the case of Iwaki volcano, in which the STSr/86Sr ratio is similar to that from the volcanic front side. It has been pointed out that alkali, especially K:O, contents in volcanic rocks from Iwaki volcano are rather similar to those from the volcanoes in the volcanic front side (Kawano et al. 1961b). 87Sr/S6Sr ratio variation away from the volcanic front, or with increasing depth of the Wadati-Benioff zone, has been observed from other island arc o 138 E 42°N~

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541

systems. In the Scotia arc, tholeiites from the South Sandwich Islands (volcanic front side) had significantly higher STSr/S6Sr ratios than those from Scotia Sea Rise (back-arc side) (Hawkesworth et al., 1977). This tendency was similar to that observed in the Northeast Japan arc. Light REE enrichment in the back-arc side was c o m m o n l y observed in b o t h the Scotia arc and the Northeast Japan arc (Fujitani and Masuda, 1981). But, in the Scotia arc, 14SNd/144Nd ratios were indistinguishable in rocks from both the back-arc side and the volcanic front side, unlike in the Northeast Japan arc (Nohda and Wasserburg, 1981). In the Sunda arc, 87Sr/S6Sr ratios in normal calcalkaline and tholeiitic rocks increased with increasing depth of the WadatiBenioff zone (Whitford, 1975). This tendency is opposite in sense to that observed in the Northeast Japan arc. In the Northeast Japan arc, many kinds of lateral variations in petrology and geochemistry of volcanic rocks have been observed, as was mentioned before. Lateral variation in K20, Ba, Th, U, REE and F contents and chondrite-normalized REE patterns in volcanic rocks and estimated H20 contents in magmas strongly suggest the degree of partial melting of mantle material is small in the back arc side, growing larger toward the volcanic front side, because these elements have a tendency to concentrate in a liquid phase during the partial melting process. The smaller the degree of partial melting, the higher are the concentrations of these elements in partially melted liquid. Sakuyama (1979) explained the relation between decrease of the H20 contents in the magma and increase of the degree of partial melting. Variation of REE patterns was also understood in terms of the degree of partial melting. Lateral variations of Rb and Sr contents, which have already been reported in other volcanic arcs (Hutchison, 1976) and were newly observed in the Northeast Japan arc by this work, are compatible with the above-mentioned model, because the behaviors of Rb and Sr during partial melting processes resemble those of K20, etc. According to Kushiro (1981), the degree of partial melting is closely related to the depth of primary magma generation, based on recent results from experimental petrology. As lateral variation in petrology of volcanic rocks has been explained in terms of the differences in the depth of primary magma generation; the depth increased from the volcanic front side to the back-arc side (Kuno, 1966), "the degree of partial melting" model becomes consistent with "the depth of primary magma" model. Next, we will discuss the strontium isotope data with relation to the subduction model mentioned above. Suppose that STSr/86Sr ratios in the melted regions in mantle are almost identical, the following two processes are speculated to explain the observed variation in 87Sr/8~Sr ratios in volcanic rocks. One process is that 87Sr/86Sr ratios in initially melting material and later melting material are different under an unequilibrated partial melting process. This means that a few phases of the mantle material melt in the back arc side and that more phases, including the first, melt in the volcanic front side under the condition that isotope exchange processes do n o t take place between the melted phase and

542 the residual solid phase. If so, an apparent isochronic relation, analogous to a mantle isochron (Brooks et al., 1976), is expected on a(~TSr/86Sr--87Rb/86Sr) correlation diagram from arc volcanic rocks with different 878r/86Sr ratios. But it is n o t the case for the Northeast Japan arc, based on data in this work. The other process is that some materials with different 87Sr/86Sr ratios are mixed with the originally melting material of either or both sides. As volcanic rocks from the back-arc side have slightly higher 87Sr/8~Sr ratios than normal-type MORB (Sun et al., 1979) and those from the volcanic front side have even higher 87Sr/8~Sr ratios, we must explain the slight difference in 8~Sr/86Sr ratios between in the back-arc side rocks and in MORB as well as lateral variation of 87Sr/86Sr ratios in an across-arc direction. As to the latter problem, it is reasonable to suppose that original material of volcanic rocks from the volcanic front side may be more contaminated with materials with a higher aTSr/86Sr ratio. Candidates of the contaminating materials are: (1) crustal material; (2) subducting slab suffering from sea water alteration, and (3) marine sediments accompanying the subducting slab. The thickness of the crust under the volcanoes concerned is almost constant in an across-arc direction except under the Oshima-oshima volcano (Yoshii, 1972) and the STSr/S6Sr ratios of crustal materials, which are represented by those of plutonic rocks near the volcanoes concerned, do not change significantly and seem rather higher in the back-arc side (Shibata and Ishihara, 1979). These findings suggest that it is not easy to explain the 87Sr/86Sr ratio difference in across-arc direction by contamination of crustal materials. Kobayashi and Hori (1979) proposed that stress-field controlled crustal contamination might change the 87Sr/86Sr ratios in volcanic rocks, taking account of Nakamura's (1977) idea that upward penetratability of original magma was dependent on the regional stress field. He suggested that, where magma ascended easily, crustal contamination might be less and resulting in lower 87Sr/86Sr ratios in volcanic rocks. As, at present Northeast Japan, the stress field is less compressional on the Japan Sea Coast (back-arc side) than on the trench side (Nakamura and Uyeda, 1980), the observed lateral variation of 87Sr/86Sr ratios is consistent with Kobayashi and Hori's (1979) proposal. Although crustal material may not be the best candidate to explain the lateral variation of 87Sr/8~Sr ratios, it may remain a promising candidate to explain the slight increase of 87Sr/86Sr ratios in volcanic rocks from the back arc side in comparison with those in MORB, because the lowest 87Sr/86Sr ratio was obtained from Oshima-oshima volcano which is situated on the thinnest crust in the studied region. DePaolo and Wasserburg (1977) pointed o u t that sea water contributed to island arc magma in some regions through Nd and Sr isotope studies of arc volcanic rocks. This supports the second candidate, subducting slab suffering from sea water alteration, as the contaminating material. However, Nd and Sr isotope studies o f volcanic rocks on the 40 ° N across-arc traverse in Northeast Japan arc showed no evidence of sea water involvement (Nohda and Wasserburg, 1981).

543

The third candidate, oceanic sediment accompanying the subducting slab, is prospective. It seems reasonable that the contribution of the subducting slab c o m p o n e n t becomes smaller away from the volcanic front, because the upper surface of the subducting slab is deeper toward the back-arc side. However, the mechanical process of mixing of the mantle wedge material with the subducting oceanic slab c o m p o n e n t and forming the island arc magma is, at present, n o t well known. Possible pictures are: strontium in the sediment is removed with water emanating from the surface of the subducting slab to the mantle wedge or melt zone observed in the lowest portion of the mantle wedge (Okada, 1979) expands in the surface of the subducting slab. Chemical aspects of mixing, or "interaction" process can be related to geophysical properties of subduction, the degree of coupling of two plates and/or evolutional stage of subduction (Uyeda and Kanamori, 1979). VARIATION OF 87Sr/86Sr RATIOS IN ALONG-ARC DIRECTION

Figure 6 shows the variation of 87Sr/S6Srratios in along-arc direction. This figure also includes the data of volcanoes located to the south of Asama volcano by Kurasawa (1979) and Nohda and Wasserburg (1981). Volcanoes along the volcanic front are divided into two groups by Asama volcano as a boundary position, so far as S Sr/S'Sr ratios are concerned. Volcanoes northward from Asama volcano have STSr/86Sr ratios of higher than 0.7038, while southern volcanoes have those from 0.7032 to 0.7038. Asama volcano is situated at the boundary point of STSr/8~Sr ratio variation and actually stands where the direction of the volcanic front is twisted. In the region northward of Asama volcano, STSr/86Sr ratios are constant with values of 0.7038 to ez Sr oe Sr0.7080

l

'

I

'

I

:

'

I

'

I

'

I

0.7060 m

It

L

0.7040

i

0.7020

•

Z

I

I

I

|__

I

t o

--

ZI--

I

I

400

200

Southward

i

I 0

,

ql~

I 200

N

t

I 400

i

I 6 0 0 Km Northward

Fig. 6. Variation of 87Sr/8~Sr ratios of volcanic front volcanoes in the along-arc direction. Data of Hakone and O-shima are from Kurasawa (1979) and Miyake-jima and Hachijojima from Nohda and Wasserburg (1981).

544 0.7045 between Osoreyama and Zao volcanoes (in Northeastern Honshu area), where the directions of the volcanic front and the Japan Trench are almost parallel. On the other hand, between Azuma and Haruna volcanoes (in central Honshu area), 87Sr/86Sr ratios are significantly higher than those in volcanoes in Northeastern Honshu area, with the maximum value at Akagi volcano. In this area, the volcanic front curves from southward to westward and parallelism between the volcanic front and the Japan Trench is lost. Further going to Asama, Eboshi and Kusatsushirane volcanoes along the volcanic front, 87Sr/86Sr ratios decrease to the level of Northeastern Honshu area. Anomalously high 87Sr/86Sr ratios around Akagi volcano are suggested to have relation to the bending feature of the volcanic front. Similar anomalously high 87Sr/86Sr ratios were reported in other island arc systems on earth. In the Banda arc, 87Sr/86Sr ratio was maximum at Serua island (Whitford and Jezek, 1979), and in the Lesser Antilles arc at St. Lucia island (Smith et al., 1980). These three cases mentioned above have the following c o m m o n features. The volcano with the highest 87Sr/86Sr ratio stands on nearly the central position along a bending arc and have similar maximum values of 87Sr/86Sr ratios (the Northeast Japan arc: 0.7077, the Banda arc: 0.7095 and the Lesser Antilles arc: 0.7092). In the case of the Banda arc, Whitford and Jezek (1979) explained the high 8~Sr/86Sr ratio as the result of mixing a crustal c o m p o n e n t of subducted lithosphere with a mantle derived component. In the Lesser Antilles arc case, Pushkar et al. (1973) interpreted the high 87Sr/86Sr ratio as due to mixing of marine sediment with mantle material. Although it is possible to explain high STSr/86Sr ratios around Akagi volcano in the Northeast Japan arc as a result of these types of contamination, we have no other chemical data, at present, to support this hypothesis. The possibility that magmas around Akagi volcano are generated by remelting of crustal material cannot be excluded, until new evidence a b o u t the genesis of magma in this area has been presented. Akagi and its neighboring volcanoes are situated in a very unique environment in terms of subduction-related volcanism, unlike other volcanoes with anomalously high 87Sr/86Sr ratios on bending arcs, such as the Banda and Lesser Antilles arcs. Under the central Honshu area around Akagi volcano, three plates meet (triple junction). The Pacific plate and the Philippine Sea plate subduct beneath the Eurasia plate, from east to west and from south to north, respectively. Nakamura and Shimazaki (1981) showed the descending features of both the Pacific and Philippine Sea plates under the central Honshu area (Kanto district) through the determination of microearthquake hypocenters, showing that the head of the Philippine Sea plate reached to the close south of Akagi volcano under the ground. If the Philippine Sea plate extends under Akagi volcano, the depth of the plate surface is extrapolated to a b o u t 100 km. Around Akagi volcano, the depth of the Pacific plate surface is between 100 and 140 km. This situation of a triple junction under the Kanto district is unique in the world and may be related to the

545

anomalously high 87Sr/86Sr ratios in volcanic rocks. Further systematic investigations of not only 87Sr/S6Sr ratios but also other isotope ratios and chemical compositions in volcanic rocks from this area are necessary. Considering that high S~Sr/S6Sr ratios in volcanic rocks from the central Honshu area are due to a peculiar geographical or tectonic situation, normal 87Sr/S6Sr ratios in volcanic rocks along the volcanic front of the Northeast Japan arc, where the Pacific plate subducts beneath the Eurasia plate, are 0.7038--0.7045, which are obtained from volcanoes in the Northeastern Honshu area. These values are slightly higher than those from volcanic rocks in the Izu-Ogasawara arc, where the Pacific plate thrusts under the Philippine Sea plate. Physical properties of the two subduction systems are different, as shown schematically in Fig. 7. The dip angle of the Wadati-Benioff zone is sharper and the thickness of the crust over the mantle wedge is thinner in the Izu-Ogasawara arc than in the Northeast Japan arc. These differences may affect 87Sr/86Sr ratios in volcanic rocks from both arcs. 100 km

0.7032-38

(0.7032 0.7032-38 0.7038-45

® 100 .o

200 km

I z u - O g a s a w a r a arc

N o r t h e a s t J a p a n arc

Fig. 7. C o m p a r i s o n b e t w e e n t h e N o r t h e a s t J a p a n arc and the Izu-Ogasawara arc.

CONCLUSIONS

(1) The decreasing values of 87Sr/S6Sr ratios from the volcanic front side to the back-arc side in the Northeast Japan arc are confirmed for all acrossarc traverses except the 40.5°N traverse. (2) STSr/S6Sr ratios in the along-arc direction are constant (0.7038--0.7045) over 350 km in northern region of the Northeast Japan arc. While, in southern region of this arc (central Honshu), 87Sr/86 Sr ratios change with an anomalously high maximum value reaching to 0.7077. (3) The anomalously high STSr/86Sr ratio area coincides with the triple junction of the Eurasia plate, the Philippine Sea plate and the Pacific plate, suggesting "anomalous interaction" at the triple junction. (4) There is a systematic difference in STSr/S6Sr ratios between the Northeast Japan arc and the Izu-Ogasawara arc. 87Sr/86Sr ratios of the former (0.7038--0.7045) are higher than t h o s e of the latter (0.7032--0.7038), suggesting that "interactions" between the Eurasia plate and the Pacific plate,

546 and t h o s e b e t w e e n the Philippine Sea plate and the Pacific plate are slightly different. ACKNOWLEDGEMENTS I wish t o t h a n k t o Prof. N. O n u m a for c o n t i n u o u s discussion and for critical reading o f the m a n u s c r i p t . I a m also grateful to Prof. S. A r a m a k i for enc o u r a g e m e n t and discussion during t h e course o f this w o r k as well as for d o n a t i n g r o c k samples. C o n s t r u c t i v e c o m m e n t s have been p r o v i d e d b y Drs. K. Aoki, N. Fujii, N. Isshiki, I. K a n e o k a , Y. K o b a y a s h i , H. Mabuchi, M. S a k u y a m a and H. Wakita. My t h a n k s go t o these persons. R o c k samples were d o n a t e d by Drs. S. A r a m a k i , M. S a k u y a m a , Y. N a k a m u r a , F. U e m u r a , T. Ui, K. Chihara, T. Y a m a g u c h i , I. Moriya, O. O s h i m a and Y. H a y a k a w a and field assistance was p r o v i d e d by Drs. M. S a k u y a m a (in T o w a d a , Iwate, F u n a g a t a and Zao volcanoes), H. Y u r i m o t o ( O s o r e y a m a ) , Y. N a k a m u r a ( K u r i k o m a ) and F. Masuda (Gassan). I also wish t o t h a n k these persons. Dr. M. S a k u y a m a k i n d l y gave me i n f o r m a t i o n a b o u t s o m e r o c k samples. X R F facilities were p r o v i d e d by G e o p h y s i c a l I n s t i t u t e , University o f T o k y o and Rigaku Industrial C o r p o r a t i o n . This w o r k was partially s u p p o r t e d b y a grant f r o m the Ministry o f E d u c a t i o n , Science and Culture, J a p a n (No. 574212). REFERENCES Aramaki, S., 1963. Geology of Asama volcano. J. Fac. Sci. Univ. Tokyo, Sect. II, 14: 229--443. Brooks, C., Hart, R.S., Hofmann, A. and James, D.E., 1976. Rb-Sr mantle isochrons from oceanic regions. Earth Planet. Sci. Lett., 32: 51--61. DePaolo, D.J. and Wasserburg, G.J., 1977. The sources of island arcs as indicated by Nd and Sr isotopic studies. Geophys. Res. Lett., 4: 465--468. DePaolo, D.J. and Wasserburg, G.J., 1979. Petrogenetic mixing models and Nd-Sr isotopic pattern. Geochim. Cosmochim. Acta, 43: 615--627. Dickinson, W.R., 1968. Circum-Pacific andesite types. J. Geophys. Res., 73: 2261--2269. Fujimaki, H. and Kurasawa, H., 1980. Lateral variation of REE pattern of basaltic magma across the Japan arc. J. Jpn. Assoc. Miner. Petrogr. Econ. Geol., 75: 313--322. Fujitani, T. and Masuda, A., 1981. Light REE inclination and distance from volcanic front; a case of volcanic rocks in Northeastern Japan. Geochem. J., 15: 269--281. Hasegawa, A., Umino, N. and Takagi, A., 1978. Double-planed structure of the deep seismic zone in the northeastern Japan arc. Tectonophysics., 47: 43--58. Hawkesworth, C.J., O'nions, R.K., Pankhurst, R.J., Hamilton, P.J. and Evensen, N.M., 1977. A geochemical study of island-arc and back-arc tholeiites from the Scotia Sea. Earth Planet. Sci. Lett., 36: 253--262. Hedge, C.E. and Knight, R.J., 1969. Lead and strontium isotopes in volcanic rocks from northeastern Honshu, Japan. Geochem. J., 3: 15--24. Hutchison, C.S., 1976. Indonesian active volcanic arc: K, Sr and Rb variation with depth to the Benioff zone. Geology, 4: 497--408. Ishikawa, K., Kanisawa, S. and Aoki, K., 1980. Content and behavior of fluorine in Japanese Quaternary volcanic rocks and petrogenetic application. J. Volcanol. Geotherm. Res., 8: 161--175.

547 Ishizaka, K., Yanagi, T. and Hayatsu, K., 1977. A strontium isotopic study of the volcanic rocks of Myoko volcano group, central Japan. Contrib. Mineral. Petrol., 63: 295-307. Kaneoka, I., Matsuda, J., Zashu, S., Takahashi, E. and Aoki, K., 1978. Ar and Sr isotopes of mantle-derived rocks from the Japanese islands. Bull. Volcanol., 41 : 424--433. Katsui, Y., Oba, Y., Ando, S., Nishimura, S., Masuda, Y., Kurasawa, H. and Fujimaki, H., 1978. Petrochemistry of the Quaternary volcanic rocks of Hokkaido, North Japan. J. Fac. Sci. Hokkaido Univ. Ser. IV, 18: 449--484. Kawano, Y., Yagi, K. and Aoki, K., 1961a. Petrography and petrochemistry of the volcanic rocks of Quaternary volcanoes of northeastern Japan. Sci. Rep. Tohoku Univ. Ser. III, 7 : 1--46. Kawano, Y., Aoki, K. and Kadowaki, K., 1961b. Petrology of Iwaki volcano. J. Jpn. Assoc. Miner. Petrogr. Econ. Geol., 4 6 : 1 0 1 - - 1 1 0 (in Japanese with English abstract). Kobayashi, Y. and Hori, K., 1979. A relationship between a tectonic stress and s~Sr/S6Sr ratios of calc-alkaline rocks in the Sunda and Banda arcs. Bull. Volcanol. Soc. Jpn., 24: 83--84 (in Japanese with English abstract). Kuno, H., 1966. Lateral variation of basalt magma type across continental margins and island arcs. Bull. Volcanol., 24: 195--222. Kurasawa, H., 1972. Isotopic composition of strontium in volcanic rocks from Towada volcano, northeast Japan. Bull. Volcanol. Soc. Jpn., 1 7 : 1 5 8 - - 1 5 9 (abstract in Japanese). Kurasawa, H., 1979. Isotopic composition of strontium in volcanic rocks from Fuji, Hakone and Izu area, central Japan. Bull. Volcanol. Soc. Jpn., 2 4 : 1 3 5 - - 1 5 2 (in Japanese with English abstract). Kushiro, I., 1981. Origin of magmas in island arcs: a discussion based on experimental petrology. J. Geol. Soc. Jpn., 8 7 : 7 6 9 - - 7 8 0 (in Japanese with English abstract). Masuda, Y., 1979. Lateral variation of trace element contents in Quaternary volcanic rocks across Northeast Japan. Bull. Univ. Osaka Pref. Ser. A, 28: 105--125. Murad, E., 1973. Determination of trace elements in unfused rock and mineral samples by X-ray fluorescence. Anal. Chim. Acta, 67 : 37--53. Nakamura, K., 1977. Volcanoes as possible indicators of tectonic stress orientation -principle and proposal. J. Volcanol. Geotherm. Res., 2: 1--16. Nakamura, K. and Uyeda, S., 1980. Stress gradient in arc--back arc regions and plate subduction. J. Geophys. Res., 85: 6419--6428. Nakamura, K. and Shimazaki, K., 1981. Sagami and Suruga troughs and plate subduction. Kagaku, 5 1 : 4 9 0 - - 4 9 8 (in Japanese). Nohda, S. and Wasserburg, G.J., 1981. Nd and Sr isotopic study of volcanic rocks from Japan. Earth Planet. Sci. Lett., 52: 264--276. Okada, T., 1979. New evidence of the discontinuous structure of the descending lithosphere as revealed by ScSp phase. J. Phys. Earth, 2 7 : 5 3 - - 6 4 (suppl.). Onuma, K., 1962. Petrography and petrochemistry of the rocks from Iwate volcano, northeastern Japan. J. Jpn. Assoc. Miner. Petrogr. Econ. Geol., 47 : 192--204. Pushkar, P., Steuber, A.M., Tomblin, J.F. and Julian, G.M., 1973. Strontium isotopic ratios in volcanic rocks from St. Vincent and St. Lucia, Lesser Antilles. J. Geophys. Res., 78: 1279--1287. Sakuyama, M., 1979. Lateral variation of H=O contents in Quaternary magmas of northeastern Japan. Earth Planet. Sci. Lett., 43 : 103--111. Shibata, K. and Ishihara, S., 1979. Initial STSr/SSSr ratios of plutonic rocks from Japan. Contrib. Mineral. Petrol., 70: 381--390. Smith, A.L., Roobol, M.J. and Gunn, B.M., 1980. The Lesser Antilles -- a discussion of island arc magmatism. Bull. Volcanol., 43: 287--302. Sugimura, A., 1960. Zonal arrangements of some geophysical and petrological feature in Japan and its environs. J. Fac. Sci. Univ. Tokyo, Sect. II, 12: 133--153.

548 Sun, S.-S., Nesbitt, R.W. and Sharaskin, A.Y., 1979. Geochemical characteristics of midocean ridge basalts. Earth Planet. Sci. Lett., 44: 119--138. Taniguchi, H., 1972. Petrological study on Towada volcano. J. Jpn. Assoc. Miner. Petrogr. Econ. Geol., 67 : 128--138 (in Japanese with English abstract). Togashi, S., 1977. Petrology of Osore-yama volcano, Japan. J. Jpn. Assoc. Miner. Petrogr. Econ. Geol., 7 2 : 4 5 - - 6 0 (in Japanese with English abstract). Togashi, S,, 1981. Petrology and geochemistry of andesite and dacite from Osoreyama volcano, northern Honshu, Japan. Abstr. 1981 IAVCEI Symposium, Arc Volcanism. Tokyo, pp. 381--382. Ui, T. and Aramaki, S., 1978. Relationship between chemical composition of Japanese island-arc volcanic rocks and gravimetric data. Tectonophysics, 45: 249--259. Uyeda, S. and Kanamori, H., 1979. Back-arc opening and the mode of subduction. J. Geophys. Res., 84: 1049--1061. Whitford, D.J., 1975. Strontium isotopic studies of the volcanic rocks of the Sunda arc, Indonesia, and their petrogenetic implications. Geochim. Cosmochim. Acta, 39: 1287--1302. Whitford, D.J. and Jezek, P.A., 1979. Origin of late-Cenozoic lavas from Banda arc, Indonesia: Trace element and Sr isotope evidence. Contrib. Mineral. Petrol., 68: 141-150. Yamaguchi, T., 1981. Geology of Hotaka volcano. J. Geol. Soc. Jpn., 8 7 : 8 2 3 - - 8 3 2 (in Japanese with English abstract). Yoshii, T., 1972. Terrestrial heat flow and features of the upper mantle beneath the Pacific and the Sea of Japan. J. Phys. Earth, 20: 271--285.