42. CONGLOMERATES OF VOLCANIC ROCKS OF DEEP SEA DRILLING PROJECT SITE 439

Kantaro Fujioka, Ocean Research Institute, University of Tokyo, Nakano, Tokyo, Japan

ABSTRACTAcidic to intermediate volcanic rocks were obtained as boulders,

pebbles, and clasts with intercalated matrix sediments near the JapanTrench. A 47.5-meter conglomerate bed unconformably overliesacoustic basement consisting of Upper Cretaceous siltstone and isoverlain in turn by massive coarse-sandstone and siltstone beds withmany fossil mollusks. The volcanic cobbles and boulders in the con-glomerate show pronounced porphyritic texture. Their phenocrystsare plagioclase, hornblende, and biotite; the groundmass consists ofplagioclase, K-feldspar, quartz, iron oxide, and altered interstitialglass.

The Plagioclase content of these volcanic rocks is very high,whereas iron oxide minerals are rare. The chemical composition ofthese volcanic rocks was analyzed to determine the rock series. Mat-rix sediments were also analyzed chemically, and their chemical com-position was found to be similar to that of volcanic rocks, except fora lower CaO content. SiO2 content of the volcanic rocks ranges from60.23 to 73.90, corresponding to that of andesite to rhyolite.

All the samples show extremely high A12O3 content, which reflectsthe high amounts of modal plagioclase.

These volcanic rocks belong to both the calc-alkalic and tholeiiticrock series, and the differentiation trend is controlled by fractionalcrystallization, mainly of plagioclase, K-feldspar, and hornblende.The assemblage of calc-alkalic and tholeiitic rock series is frequentlyobserved in island arcs and active continental margins. These vol-canic rocks are derived from the Oyashio ancient landmass, which isa slightly matured island arc (see site chapters, this volume, Pt. 1).

INTRODUCTION

At DSDP Site 439, acidic volcanic rocks were re-covered in Core 32, below a unit consisting of massivesandstone and siltstone beds. These acidic volcanicrocks are the first recovered during DSDP drilling. Thevolcanic rocks occur as boulders, pebbles, and clasts ina 47.5-meter conglomerate bed of Lithologic Unit 6 atSites 438 and 439. No plutonic rocks, such as graniteand granodiorite, were found. The largest boulder ofvolcanic rock is about 70 cm. Clasts of sedimentaryrocks also occur throughout the conglomerate bed.These clasts are always angular to subangular. Small-scale grading of clasts is locally observed in Unit 6.Judging by the angular sedimentary clasts, the large sizeof boulders, and the absence of definite marine fossils inthe matrix, the source region for the conglomerate can-not be very far from Site 439.

The multichannel airgun records suggest that the Up-per Cretaceous acoustic basement rises gradually east-ward from this site. A significant hiatus exists betweenthe Upper Creatceous siltstone of Unit 7 and the con-glomerate bed of Unit 6.

The shipboard scientists have proposed the existenceof an exposed landmass called the Oyashio ancient land-mass (von Huene, Nasu, et al., 1978). In order to deter-mine the volcanic rock series of these conglomerates and

the features of volcanism, I performed major-elementchemical analyses by X-ray fluorescence (XRF), usingthe fusion method.

In this article, I discuss the volcanic rock series ofthese conglomerates and their provenance.

PETROGRAPHY OF VOLCANIC ROCKSConglomerates of Lithologic Unit 6 contain clasts of

volcanic rocks which exhibit a pronounced porphyritictexture; their phenocrysts consist of plagioclase, horn-blende, and biotite (rarely), and their groundmass con-sists of plagioclase, K-feldspar, quartz, iron oxide, andaltered interstitial glass. The amount of mafic mineralsis always less than 15 per cent. The color index ofvolcanic rocks is very low. The amount of modalplagioclase is extremely high and is always greater thanthat of alkali feldspar. The texture of the volcanic rocksis almost intersertal and (or) intergranular, and rarelysub-ophitic. Small amounts of interstitial glass alwaysoccur. Therefore these volcanic rocks were probablyderived mostly from flows of acidic to intermediatecomposition, and partly from hypabyssal rocks. Noclasts of plutonic rocks were observed in the con-glomerate bed.

Plagioclase occurs both as phenocrysts and as agroundmass phase. Phenocryst plagioclase, nearly euhe-dral and up to 3 mm, sometimes shows pronounced

1075

K. FUJIOKA

compositional zoning: a broad, highly calcic core sur-rounded by narrow, intensely zoned rims. Groundmassplagioclase occurs as small laths. Compositions ofplagioclases are shown in Figure 1.

Plagioclase phenocrysts of andesitic and dacitic cob-bles are very calcium-rich, sometimes reaching aboutAn80 in the grain core, whereas the plagioclase of more-acidic rocks is less calcic. The K2O content ofplagioclase increases as Na2O content increases, and theK2O content of phenocryst plagioclase is very low.Some plagioclase phenocrysts extremely rich in CaOhave small glass inclusions, now altered to brown clayminerals. This type of plagioclase phenocryst is geneti-cally significant and is sometimes observed in basalticrocks. Judging from the very high calcium content ofthe broad core and the resorbed phenocrysts, this typeof plagioclase is possibly a xenocryst derived from more-mafic magma.

An Ab

V/ V V i t Ѥȥ n, *An Ab

Ab

• • •An 50 Ab

Figure 1. Or-Ab-An plot of plagioclases of the conglom-erate bed.

Hornblende occurs as a phenocryst in andesite anddacite, but is absent in rhyolite. Figure 2 is photomicro-graph of a typical hornblende dacite. Hornblende pheno-crysts always change to aggregates of clay minerals at theirperipheries. It is clear from Figure 2 that hornblendecrystals are partly replaced along their rims, while thecore of the crystals remains unaltered. These cores showpale-brownish-green, weak pleochroism. In some rocksonly hornblende pseudomorphs are observed.

Chemical compositions of unaltered hornblende arelisted in Table 1. The MgO and A12O3 contents ofphenocryst plagioclase are higher than those of typicalgranitic rocks of the Kitakami Mountains, whereasK2O, and total Fe contents are lower than those ofgranitic rocks (Kanisawa, 1972). In rims of hornblendethe total Fe2O3 or Fe2O3/MgO ratio becomes higherthan in the core.

Iron-oxide minerals are minor constituents and theyare very fine grained; therefore the intensity ofmagnetization is very weak. The groundmass of thesevolcanic rocks has been altered to various degrees. In-terstitial glass has mostly altered or devitrified to small

Figure 2. Photomicrograph of hornblende dacite. Horn-blende phenocryst is partly replaced by small aggre-gates of clay minerals.

1076

CONGLOMERATES OF VOLCANIC ROCKS

TABLE 1Chemical Compositions of Hornblende Phenocrystsa

SiO2

Tiθ2A12O3

FeO*MnO

MgOCaONa2θK2O

46.151.478.187.620.11

16.9711.17

2.810.18

45.061.688.219.240.14

17.0210.69

3.030.15

34-2, 120-122 cm

46.061.47

11.317.620.07

15.5611.24

2.450.16

45.531.368.409.160.14

16.9611.47

2.740.16

46.971.438.72

10.050.18

16.4010.91

2.940.14

44.471.63

10.0510.15

0.18

16.1111.06

2.900.15

46.431.08

10.187.830.04

17.0810.58

2.740.06

34-2, 148-150 cm

44.001.559.91

10.330.11

15.7210.283.380.04

42.231.329.888.130.16

14.839.713.130.12

42.321.58

10.107.430.07

15.3510.48

2.950.14

* Total iron as FeO.&EPMA analyses.

aggregates of brown clay minerals. In some rocks,spherulitic chlorite and chalcedony are observed.

CHEMICAL COMPOSITION OF THEVOLCANIC ROCKS

General Statement and Method of Analysis

In order to classify the volcanic rocks of the con-glomerate bed and determine their rock series, conven-tional XRF analyses were carried out for SiO2, TiO2,A12O3, total iron as Fe2O3, MnO, MgO, CaO, Na2O,K2O and P2O5. The analytical method is as follows:First, the volcanic rocks are crushed with a ball mill,using a tungsten carbide cylinder for about 10 minutes,and then 1.000-gram powders are measured precisely;lithium borate powders of five times this weight are add-ed as a flux. These powders are then fused in a furnacein a Pt95-Au5 crucible or dish at a temperature of about1100°C for several tens of minutes. When all the crystalsmelt, the crucible is taken off the furnace. In this way,glassy pellets are prepared. Weight per cents of majorelements are then measured by XRF calibration curvesfor all the elements, based on glass pellets of knownchemical composition. The standards JB-1 and JG-1 arealso used when unknown glass is measured. The precisemethod and experimental conditions were reported byOhmori (1976), Ohmori and Ohmori (1976), Goto (1976),and Sugisaki et al. (1977).

Twenty-five volcanic rocks and three samples ofmatrix sediment were analyzed and are listed in Table 2.The SiO2 content of these volcanic rocks ranges from60.23 to 73.90 weight per cent in water-free samples,corresponding to that of andesite to rhyolite. The mostfrequent rock type is dacite. All the samples show veryhigh A12O3 content, which reflects the large amount ofmodal plagioclase. The chemical composition of thematrix sediment is similar to that of the volcanic rocks,except that CaO content is low compared with that ofthe volcanic rocks. The low CaO content of the matrixsediments reflects the lower amount of plagioclase.Because the conglomerate clasts and matrix sedimentare similar it is likely that the provenance of this con-glomerate bed is not far from this site.

SiO2-(Na2O + K2O) Relation

Classification of volcanic rocks according to theirchemical compositions is very important, and manyauthors have proposed various diagrams and methods.In order to divide the volcanic rocks into alkalic rockseries and non-alkalic rock series, the SiO2-(Na2O +K2O) diagram of Kuno (1960, 1966, 1968) is very useful.Kuno (1966) has classified many volcanic rocks of thecircum-Pacific region by using this diagram.

Macdonald and Katsura (1964) proposed a similardiagram for the Hawaiian basaltic rocks, but in thepresent case it seems to be better to use Kuno's diagrambecause the volcanic rocks have intermediate to acidiccompositions. Figure 3 shows the SiO2-(Na2O + K2O)relation of the volcanic rocks of the conglomerate bed.In this diagram, the areas A, B, and C represent alkalibasalt, high-alumina basalt, and tholeiite respectively.All the volcanic rocks of the conglomerate bed fall infields B and C, suggesting that all the volcanic rocksbelong to non-alkalic rock series. These volcanic rockshave suffered alteration to various degrees, and it ispossible that SiO2 and alkalis have been removed duringthe alteration process. This problem seems to be signifi-cant and difficult to solve for the older volcanic rocks.The mobility of SiO2, alkalis, and other elements hasbeen dealt with by many petrologists and geochemists(e.g., Hart, 1973), but this problem has not yet beenclearly solved. Alteration products of these volcanicrocks do not exceed 15 volume per cent. Petrographicobservations suggest that the most abundant alterationproducts are the small aggregates of clay minerals. Inthe case of volcanic rocks, as alteration proceeds there isa possibility that the amount of alkalis, especially K2O,increases (Hart, 1973; Melson and Thompson, 1973).

In the SiO2-(Na2O + K2O) diagram, a critical factorused to separate alkalic from non-alkalic rocks is thetotal alkali content.

(FeO*/MgO)-SiO2 Relation

Recently, Miyashiro (1974, 1975) tried to classify vol-canic rocks of the non-alkalic series into two groups,calc-alkalic and tholeiitic, by compiling analyses of

1077

K. FUJIOKA

TABLE 2XRF Bulk Analyses of Volcanic-Rock Clasts and Matrix of Conglomerate (wt.

Component

SiO2

TiO2

A12O3

Fe2θ3*MnO

MgOCaO

Na2OK 2 O

P2O5

Total Alkali

FeO

MgO/FeO*

1

32-2,61-62

62.290.43

24.341.790.00

0.494.714.251.600.11

5.85

1.613.29

233-1,18-19

73.900.18

16.671.570.04

0.191.814.101.440.09

5.54

1.417.42

333-1,94-95

60.230.43

24.923.130.00

0.945.193.701.440.03

5.14

2.823.00

433-1,

122-124(a)

62.660.40

23.913.220.00

0.914.403.301.170.03

4.47

2.903.19

Sample

533-1,

122-124(b)

62.900.40

23.683.160.00

0.924.393.351.180.02

4.53

2.843.09

Number and Interval (in cm)

633-1,

137-139

64.000.42

24.842.220.00

0.793.632.921.170.02

4.09

2.002.53

734-1,34-35

72.510.72

18.233.450.00

0.860.251.652.300.03

3.95

3.113.62

834-1,51-52

69.350.77

20.093.690.03

1.070.481.652.850.01

4.50

3.323.10

934-1,

140-143

62.380.43

24.452.050.00

0.254.774.401.090.19

5.49

1.857.40

1034-2,0-4

60.680.41

24.981.820.00

0.245.574.951.110.24

6.06

1.646.83

11

34-2,30-32

61.610.43

25.631.430.00

0.214.584.741.050.31

5.79

1.296.14

12

34-2,96-98

71.120.54

15.973.520.05

1.481.802.972.470.07

5.44

3.172.14

13

34-2,99-101

65.630.37

23.471.630.03

0.443.623.691.040.09

4.73

1.473.34

*Total iron as Fe2C>3 or FeO.

many volcanic rocks from various tectonic settings. Heproposed an (FeO*/MgO)-SiO2 diagram as a critical in-dicator. (FeO* is total iron as FeO.)

His method is thought to be most useful for the in-termediate to basic rocks with FeOVMgO ratios of 2 to5. However, we have no critical factor to distinguishbetween calc-alkalic rocks and tholeiitic rocks withmore acidic chemical composition. Therefore, in orderto classify the volcanic rocks in the present study, the(FeO*/MgO)-SiO2 diagram proposed by Miyashiro wasused. These data are plotted in Figure 4. The equationseparating the calc-alkalic and tholeiitic fields was ob-tained by Miyashiro (1974) using the relation SiO2 =[6.4 × (FeOVMgO)] + 42.8.

In Figure 4, the data are somewhat scattered, butthere is a concentration in a narrow region. The ana-lyzed matrix sediments fall within the calc-alkalic area.FeO* and MgO and the FeOVMgO ratio may not be aschangeable during the alteration process compared withother components, and the matrix sediments may be de-rived from calc-alkalic source rocks. The ratio of thecalc-alkalic to the tholeiitic points is high, showing thatthe dominant rock series forming the volcanic rocks ofthe conglomerate bed is the calc-alkalic one.

Variation Diagram

Figure 5 is an oxide variation diagram. The horizon-tal axis is SiO2, because the Fe2O3/FeO ratio of the vol-canic rocks is unknown, and normative minerals there-fore could not be calculated. In the calc-alkalic rocks,SiO2 content is a good indicator of the degree of differ-entiation. In this figure, A12O3 and CaO decrease asSiO2 increases, suggesting that differentiation may havebeen controlled by fractional crystallization of plagio-clase. However, Na2O and K2O do not show a distinctrelation to SiO2. FeO* and MgO do not change much asSiO2 increases. The FeOVMgO ratio changes signifi-cantly in the (FeOVMgO)-SiO2 diagram (Figure 4).This suggests that differentiation of these volcanic rockswas controlled not only by fractional crystallization ofplagioclase, but also by that of hornblende and K-feld-

spar, and that the FeO*/MgO ratio depends on the localamount of mafic minerals.

FeO*-(Na2O + K2O) - MgO (AFM) Diagram

In AFM diagrams, differentiation trends of the calc-alkalic rock series and the tholeiitic series are different.The plots for acidic volcanic rocks of this study areshown in Figure 6. The data plot near the total-alkalicorner, indicating highly differentiated rocks. A goodlinear relation is exhibited in this diagram, suggestingthat all of the analyzed volcanic rocks, although theyoccur as clasts in a conglomerate, have a close geneticrelationship with one another and that differentiationwas controlled by fractional crystallization of plagio-clase. The trend shown by these volcanic rocks is similarto that of typical calc-alkalic rocks.

TECTONIC SETTING OF THE VOLCANIC ROCKS

Miyashiro (1974, 1975) compiled analyses of non-alka-lic rocks and proposed a method of classification of thecalc-alkalic and tholeiitic series. He also discussed therelationship between volcanic rock series and tectonics.

The acidic volcanic rocks of the Site 439 conglomer-ate belong to both the calc-alkalic and tholeiitic series.In modern island arcs along active continental margins,assemblages of tholeiitic and calc-alkalic volcanic rocksare always observed. In highly matured island arcs, andassemblage of tholeiitic, calc-alkalic, and small amountsof alkalic volcanic rocks is frequently observed.

Judging from the assemblage of rock series and thefrequency of calc-alkalic and tholeiitic rock series of thevolcanic rocks of the conglomerate bed in LithologicUnit 6 of Site 439, the source region of the conglomer-ates is thought to be an island arc (Oyashio ancient land-mass; Site 438-439 Report, this volume).

MAGNETIC PROPERTIES OF THEVOLCANIC ROCKS

In order to determine whether these volcanic rockshave suffered extensively from low-temperature oxida-

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CONGLOMERATES OF VOLCANIC ROCKS

TABLE 2 -Continued

Sample Number and Interval (in cm)

14 15 16 17 18 19 20 21 22 23 24 25 26 27 2834-2, 34-2, 34-3, 35-1, 35-1, 35-1, 35-1, 35-1, 36-1, 36-1, 36-1, 36-1, 36-1, 36-2, 36-2,

120-122 148-150 50-52 52-53 69-71 78-79 105-106 148-149 23-27 35-36 62-64 120-122 123-125 28-30 72-74

64.030.39

20.293.620.04

1.314.744.390.990.20

5.38

3.262.49

64.220.39

20.593.690.04

1.414.853.770.970.06

4.74

3.322.35

63.610.40

24.542.350.00

0.663.953.291.170.03

4.46

2.123.21

62.380.39

23.422.870.00

0.824.834.191.010.10

5.20

2.583.15

62.300.4 i

23.103.040.00

0.944.854.211.040.11

5.25

2.742.91

68.170.30

18.772.820.04

0.853.554.231.240.04

5.47

2.542.99

66.200.53

20.354.090.05

1.352.372.792.240.02

5.03

3.682.73

65.090.35

19.094.370.04

1.804.363.870.930.09

4.80

3.932.18

63.350.39

23.372.290.03

0.344.943.741.520.03

5.26

2.066.06

65.490.33

26.801.880.03

0.702.251.730.800.01

2.5 3

1.692.41

66.100.36

21.763.020.04

1.473.223.000.970.05

3.97

2.721.85

71.190.28

19.131.430.16

0.392.133.971.280.03

5.25

1.293.31

69.360.54

17.824.410.05

1.311.272.602.580.07

5.18

3.973.03

70.120.66

17.444.750.04

1.240.992.162.530.07

4.69

4.283.45

61.030.39

24.212.880.00

0.785.294.181.100.12

5.28

2.593.32

SiO2 (wt.'

Figure 3. SiO2-(Na2O + K2O) diagram.

tion a study of their magnetism was carried out. Fromthe results, the environment in which the volcanic rockswere erupted can be estimated.

The total magnetic intensity of a whole-rock samplewas measured with a spinner magnetometer. The inten-sity is 6.58 × 10 ~6 emu/g. This value is slightly lowerthan that of other acidic volcanic rocks. As these vol-canic rocks have very small modal amounts of iron-oxide minerals, this low value seems reasonable. A ther-momagnetic measurement was also carried out with amagnetic balance in order to estimate the Curie temper-ature and to draw a thermomagnetic curve of the vol-canic rock (Figure 7). The Curie point of this rock is545 °C, and this temperature suggests that the magneticminerals of the volcanic rock are nearly stoichiometricmagnetite, deviating slightly from the stoichiometricmagnetite-titanomagnetite line. The thermomagneticcurve resembles a reversible type, but suggests that this

Fe*/MgO

Figure 4. (FeO*/MgO)-SiO2 diagram. FeO* representstotal iron as FeO.

rock suffered neither from low- nor high-temperatureoxidation. Nevertheless, there is some possibility thatthis rock was erupted subaerially. Further investigationof the magnetism of these volcanic rocks is necessary.

AGE OF THE VOLCANIC ROCKS40Ar/39Ar dating of these volcanic rocks was carried

out by Yanagisawa et al. (this volume). According tothem, the absolute isochron age ranges from 21.2 to22.8 m.y. and averages 21.4 ±1.0 m.y. (that is, upper-most Oligocene to lowermost Miocene, correspondingto magnetic anomaly 6B).

Volcanism of the same age is recorded on the OgaPeninsula (Konda, 1974; Nishimura and Ishida, 1972),in the western part of the Tohoku arc, and in the easternpart of the Tohoku arc of Honshu (Takadate andesite).

1079

K. FUJIOKA

25

15

2 1

0

3

SM 2

1

0

•

70

%60 65

* .

•60

I

70

#

60

,»«»»» <> > ‰ m S60 65

SiO,

FeO"

Na2O

Figure 6. FeO*-(Na2O + K2O)-MgO (AFM) diagram.FeO* represents total iron as FeO.

600

Figure 5. Oxide variation diagram. All values in weight Figure 7. Thermomagnetic curve of volcanic rock fromper cent. 439-33-1, 122-124 cm.

1080

CONGLOMERATES OF VOLCANIC ROCKS

DISCUSSION

Major-element chemical analyses of the acidicvolcanic rocks of the conglomerate bed show that all thevolcanic rocks belong to the non-alkalic rock series, ac-cording to Kuno's silica-alkali diagram. The volcanicrocks plot in both the calc-alkalic and the tholeiiticfields of the (FeO*/MgO)-SiO2 diagram of Miyashiro.The volcanic rocks have a genetic similarity to oneanother in various diagrams, and therefore the tectonicsetting of these volcanic rocks is inferred to have beenan island arc.

The K2O content of volcanic rocks is a useful in-dicator of the depth of the Benioff zone and distancefrom the trench axis (Dickinson, 1968; Dickinson andHatherton, 1967; Nielson and Stoiber, 1973). Recently,Miyashiro showed variation between K2O and SiO2 ofvolcanic rocks of various island arcs and active con-tinental margins with advancing development of conti-nental-type crust (that is, with the degree of maturity ofthe arcs and continental margins). He compared manyvolcanic rocks from various island arcs, plotting themon the K2O-SiO2 diagram.

In the K2O-SiO2 diagram, the volcanic rocks fromimmature island arcs plot around the low-K2O regions,whereas those from highly mature island arcs plot in thehigh-K2O areas. To estimate the maturity of the Oyash-io ancient landmass, which presumably supplied con-glomerates and elastics to Site 439, the data are plot-ted on this diagram (Figure 8A), with Miyashiro's figurefor comparison (Figure 8B). In this figure, the data forthe present study plot in a narrow low-K2O area. Thisrelation is very similar to that for volcanic rocks obtainedbetween the northeast Japan outer arc and inner arc ofMiyashiro's figure. This suggests that in size and matu-rity the Oyashio ancient landmass was the same as theabove-mentioned arcs.

ACKNOWLEDGMENTS

The author sincerely thanks Drs. N. Nasu, K. Kobayashi,and T. Ishii, Ocean Research Institute, University of Tokyo,who read this manuscript critically and gave fruitful discus-sions in the course of preparing this paper. The author also ex-presses his thanks to Drs. R. von Huene and M. Arthur and allthe shipboard scientists for their continuous encouragement.

REFERENCES

Dickinson, W. R., 1968. Circum-Pacific andesite types. /. Geo-phys. Res., 73, 2261-2269.

Dickinson, W. R., and Hatherton, T., 1967. Andesitic volcan-ism and seismicity around the Pacific. Science, 157, 801-803

Goto, H., 1976. Automatic analysis of major elements in sili-cate rocks by X-ray fluorescence spectrometry. Bull. Geol.Surv. Jap., 27, 595-611. [In Japanese with English ab-stract]

Kanisawa, S., 1972. Coexisting biotites and hornblendes fromsome granitic rocks in southern Kitakami Mountains, Ja-pan. /. Jap. Assoc. Mineral. Petrol. Econ. Geol., 67, 332-344.

Konda, T., 1974. Bimodal volcanism in the Northeast Japanarc. /. Geol. Soc. Jap., 80, 81-89.

50 60 70

SiO2 (wt. %)

4 -

3 .

2 -

1 -

40

B

tholeiitic

— — calc-alkalic

Kermadecs-?' •\o^ S>

A /

. i/ur"\\es-0

"―'― NE Japan-0

1 i ,

50 60 70

SiO2 (wt. %)

80

Figure 8. A. K2O-SiO2 plot of the volcanic rocks. B.Same diagram compiled by Miyashiro (1974).

Kuno, H., 1960. High-alumina basalt. J. Petrol., 1, 121-145., 1966. Lateral variation of basalt magma type across

continental margins and island arcs. Bull. Volcanol., 29,195-222.

., 1968. Origin of andesite and its bearing on the islandarc structure. Bull. Volcanol., 32, 141-176.

Hart, R., 1973. A model for chemical exchange in basalt-sea-water system of Oceanic Layer II. Can. J. Earth Sci., 10,799-816.

Macdonald, G. A., and Katsura, T., 1964. Chemical composi-tion of Hawaiian lavas. /. Petrol., 5, 82-133.

Melson, W. G., and Thompson, G., 1973. Glassy abyssal ba-salts, Atlantic sea floor near St. Paul's rocks: petrographyand composition of secondary clay minerals. Geol. Soc.Am. Bull., 84, 703-716.

Miyashiro, A., 1974. Volcanic rock series in island arcs and ac-tive continental margins. Am. J. Sci., 274, 321-355.

, 1975. Volcanic rock series and tectonic setting. EarthPlanet. Sci. Lett. Ann. Rev., 16, 251-269.

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K. FUJIOKA

Nielson, D. R., and Stoiber, R. E., 1973. Relationship of po- Ohmori, T., and Ohmori, E., 1976. X-ray fluorescence anal-tassium content in andesite lavas and depth of the seismic ysis of major elements in rocks and minerals. Part 1. Bull.zone. /. Geophys. Res., 78, 6887-6892. Geol. Surv. Jap., 27, 195-211.

Sugisaki, R., et al., 1977. An automatic X-ray fluorescenceNishimura, S., and Ishida, S., 1972. Fission-track ages of tuffs method for the analysis of silicate rocks. J. Geol. Soc. Jap.,

of the Neogene-Tertiary in Oga Peninsula, Akita Prefecture 83, 725-733.Japan. J. Jap. Assoc. Mineral. Petrol. Econ. Geol., 67, 166- von Huene, R., and Nasu, N., et al., 1978. Japan Trench tran-168. [In Japanese with English abstract] sected. Geotimes, 23, 16-21.

1082