Evidence of magma mixing: Petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan

Journal of Volcanology and Geothermal Research, 5 (1979) 179--208

179

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

EVIDENCE OF MAGMA MIXING: PETROLOGICAL STUDY OF SHIROUMA-OIKE CALC-ALKALINE ANDESITE VOLCANO, JAPAN

MASANORI SAKUYAMA

Geological Institute, Faculty of Science, University of Tokyo, Tokyo (Japan) (Received April 4, 1978; revised and accepted August 29, 1978)

ABSTRACT Sakuyama, M., 1979. Evidence of magma mixing: petrological study of Shirouma-Oike calc-alkaline andesite volcano, Japan. J. Volcanol. Geotherm. Res., 5: 179--208. Shirouma-Oike volcano, a Quaternary composite volcano in central Japan, consists mostly of calc-alkaline andesitic lavas and pyroclastic rocks. Products of the earlier stage of the volcano (older group) are augite-hypersthene andesite. Hornblende crystallized during the later stage of this older group, whereas biotite and quartz crystallized in the younger group. Assemblages of phenocrysts in disequilibrium, such as magnesian olivine(Fos0)/quartz, iron-rich hypersthene(Enss)/iron-poor augite(Wo43.s , En.~.s , Fsl,.0), and two different types of zoning on the rim of clinopyroxene are found in a number of rocks. Detailed microprobe analyses of coexisting minerals reveal that phenocrysts belong to two distinctly different groups; one group includes magnesian olivine + augite which crystallized from a relatively high-temperature (above 1000~C) basaltic magma; the second group, which crystallized from relatively low temperature (about 800°C) dacitic to andesitic magma, includes hypersthene + hornblende + biotite + quartz + plagioclase + titanomagnetite ± ilmenite (in the younger group) and hypersthene + augite + plagioclase + titanomagnetite -+ hornblende (in the older group). The temperature difference between the two magmas is clarified by Mg/Fe partition between clinopyroxene and olivine, and Fe-Ti oxides geothermometer. The compositional zoning of minerals, such as normal zoning of olivine and magnesian clinopyroxene, and reverse zoning of orthopyroxene, indicate that the basaltic and dacitic-andesitic magmas were probably mixed in a magma reservoir immediately before eruption. It is suggested that the basaltic magma was supplied intermittently from a deeper part to the shallower magma reservoir, in which dacitic-andesitic magma had been fractionating.

INTRODUCTION T h e origin o f the calc-alkaline r o c k series has b e c o m e one o f t h e principal p r o b l e m s in p e t r o l o g y d u r i n g the last t w o decades. This p r o b l e m has been s t u d i e d intensively b y m e a n s o f e x p e r i m e n t a l p e t r o l o g y (e.g., O s b o r n , 1 9 5 9 ; G r e e n a n d R i n g w o o d , 1 9 6 8 ; Y o d e r , 1 9 6 9 ; Mysen a n d B o e t t c h e r , 1 9 7 5 a , b; Kushiro, 1 9 7 4 ) , and several m o d e l s o f t h e origin o f calc-alkaline andesite have been p r o p o s e d . T h e r e are, however, t o o f e w basic p e t r o g r a p h i c and geological

180

data on calc-alkaline andesites to provide constraints on the various models. In particular, features of phenocrystic minerals, such as compositional zoning, degree of equilibration, and change of phenocryst assemblage during eruptive episodes of calc-alkaline magmas, need to be studied in more detail. The purpose of this paper is to describe rocks of Shirouma-Oike volcano, central Japan (Fig. 1), which consist dominantly of calc-alkaline andesite, and to place special emphasis on the equilibrium or disequilibrium relations

~|

• 4

/....-"

0 I

100 I

I

.: A

A

'L

,

200 km *

I

Fig. 1. Index m a p of Shirouma-Oike volcano. Solid triangles are Quaternary volcanoes in central and northeast Japan.

181 among phenocrystic minerals, their compositional zoning and the relationship between the petrographic data and the order of eruption. GENERAL GEOLOGY Lavas and pyroclastic rocks of the Shirouma-Oike volcano cover an area of about 40 km 2 (Fig.2). The basement rocks consist mainly of Paleozoic and Mesozoic sandstone and shale, and partly of Miocene sediments (Ishii, 1937; Saito et al., 1972). The Paleozoic sediments are intruded by granite and granodiorite, probably of Mesozoic age, and Mesozoic sediments are intruded b y quartz porphyry. A large serpentinite b o d y along a north-south-trending fault has been omitted from the geological map for the sake of simplicity. The volcano is composed of alternating lavas and pyroclastic rocks, the volumetric ratio of which is nearly 1:1. Most of the rocks are calc-alkaline andesite. Dacite is rare, and basalt and rhyolite are absent. The volcanic rocks are largely divided into older and younger groups. Augite-hypersthene andesite is dominant in the older group, b u t hornblende phenocrysts appear in rocks of the later stage of the older group (Hiedayama lava series and Norikuradake-shita lava). Biotite and quartz phenocrysts are found in rocks of the younger group. R o u n d e d basic inclusions, which are darker and more porous than the matrix and have an average size of a b o u t 15 cm in diameter, are c o m m o n in rocks of the younger group. Their field occurrence and microscopic textures are similar to those of "cognate inclusion" described by MacGregor (1938) at Montserrat. PETROGRAPHY AND CHEMICAL COMPOSITION OF VOLCANIC ROCKS Representative modal compositions of rocks of the Shirouma-Oike are shown in Table 1. Phenocrysts of orthopyroxene, plagioclase and titanomagnetite are present in all the rocks. Clinopyroxene phenocrysts are observed in all the rocks except for one dacitic flow (Kzs-3) of the younger group. Olivine is present sporadically in rocks of the older group and is c o m m o n in rocks of the younger group. O r t h o p y r o x e n e phenocrysts are consistently more abundant than clinopyroxene phenocrysts within the older group. Ilmenite phenocrysts appear in the later stage of the younger group. The sizes of phenocrysts, particularly those of plagioclase and hornblende, are larger in the younger group (maximum length of about 1 cm) than in the older group (maximum length is at most 5 mm). The groundmass of rocks of the older group exhibits a hyalopilitic texture, whereas that of the younger group has a higher glass component. Groundmass minerals consist of plagioclase, orthopyroxene, clinopyroxene, titanomagnetite, ilmenite and pale-brown to clear glass. The holocrystalline groundmass of some lavas of the older group contains rare biotite. Because of the presence of o r t h o p y r o x e n e in the groundmass of these andesites, they are equated with the hypersthenic rock series of Kuno (1950).

182

The chemical compositions of some volcanic rocks, shown in Table 2, show typical calc-alkaline trends, as illustrated, for example, by AFM diagrams. K20 contents are relatively higher than those of other Japanese Quaternary volcanic rocks.

( \ .....

~i~ ~ ~.-- _

i:,0:

..... / //

o

LEGEND

f, +

, )2:

-9+ + +

/

//

/

+

/

/

,/ / / ,"k

1

. . . . . .

A .

.

~,

A

,'\

--

~"~ ,i

A

,'~,

A

A

/\

/

,°,, "

A

/

A

",

~,

/

/ /

\

I

20OOrn

;~

)2:

°°

Fig. 2. Geological m a p of Shirouma-Oike volcano: 1 = b a s e m e n t rocks, 2 ffi N u k e h i r a z a w a lava series, 3 = H i y o d o r i m i n e lava series, 4 = Tsugadaira lava series, 5 = I c h i n a n b a y a m a lava series, 6 = H i e d a y a m a lava series, 7 = Norikuradake-shita lava, 8 ffi Norikuradake lava, 9 = K a z a f u k i d a k e s o m m a lava series, 10 = K a z a f u k i d a k e central c o n e lava, 11 = K a z a f u k i d a k e pyroclastic f l o w deposit, 12 = talus and fan deposits. A = Lake Shirouma-Oike, B = Mt. S h i r o u m a - N o r i k u r a d a k e ( 2 4 3 6 m), C = Mt. H i y o d o r i m i n e ( 1 9 0 6 m), D = Mt. H i e d a y a m a ( 1 4 4 3 m), E ffi Mt. K a z a f u k i d a k e ( 1 8 8 0 m).

183

TABLE 1 R e p r e s e n t a t i v e m o d a l c o m p o s i t i o n s (vol.%) Geologic u n i t

Sample No.

Ol

Cpx

Opx

Hb

Bt

Qz

P1

Ore

Gm

Kzc-1

0.4

5.7

1.0

4.8

0.8

0.1

26.2

1.5

59.5

Kzs-3 Kzs-2 Nor-1

-0.3 tr

-4.3 4.6

1.4 1.3 2.6

9.0 5.6 5.8

2.7 0.2 1.3

1.2 1.2 0.1

29.9 26.2 31.3

1.1 1.3 1.0

54.8 59.6 51.7

Hie-6

3.4

3.8

4.1

0.8

--

--

24.8

1.0

62.2

Tsg-3

--

3.2

6.4

--

--

--

35.3

1.4

59.5

Hyo-3

0.3

3.4

7.1

--

--

--

33.8

2.0

53.5

Younger group Kazafukidake c e n t r a l c o n e lava Kazafukidake somma lava series N o r i k u r a d a k e lava

Older group H i e d a y a m a lava series T s u g a d a i r a lava series H i y o d o r i m i n e lava series

TABLE

2

Whole-rock chemical compositions (wt. %) of volcanic rocks 1 SiO~ TiO~ A1203 Fe203 FeO MnO MgO CaO Na~O K:O P205 H20

Total

2

3

4

60.2 0.84 17.9 2.50 3.34 0.12 1.99 6.36 3.20 2.28 0.14 1.87

56.3 0.70 16.9 2.82 4.87 0.16 3.12 8.33 2.63 2.06 0.14 1.18

65.0 0.57 15.7 2.19 2.99 0.11 1.72 4.96 3.17 2.79 0.12 0.73

60.1 0.77 15.9 2.58 4.48 0.16 3.11 6.83 2.68 2.22 0.13 0.96

100.54

99.21

100.05

99.92

1 = Tsg-3, Tsugadaira lava series, augite-hypersthene andesite; 2 = Kzs-2, Kazafukidake s o m m a lava series, olivine-augite-hypersthene-biotite-hornblende-quartz-andesite; 3 ---Kzs-3, Kazafukidake s o m m a lava series, hypersthene-biotite-hornblende dacite; 4 = Kzc-1, Kazafukidake central cone lava, olivine-augite-hypersthene-biotite-hornblende-quartz andesite. A n a l y s t , M. S a k u y a m a .

184 MINERALOGY

The constituent minerals were analyzed with the JEOL electron probe microanalyze~ Model JXA-5. Analytical methods utilized are those described by Nakamura and Kushiro (1970), and correction procedures follow the m e t h o d of Bence and Albee (1968).

Olivine. Olivine phenocrysts are generally euhedral, and, in most rocks, reaction rims are absent or very thin (at most 10 ~m). Normal zoning is always observed in olivine. Mg content is nearly constant in the large part of a crystal, and decreases gradually at the rim. The most magnesian olivine phenocryst is FoBs. s in the Norikuradake-shita lava. The most Fe-rich olivine, Fo41.s, is found in the Hiyodorimine lava series of the older group, b u t this is a very minute anhedral grain. Most olivine phenocrysts have compositions in the range Fos0--Fo60. Representative chemical compositions of olivines are shown in Table 3. Many magnesian olivine phenocrysts contain inclusions of picotite. Clinopyroxene. Clinopyroxene is found both as euhedral phenocrysts and as a groundmass constituent. Two types of zoning, normal and reverse, are observed in phenocrysts. The Ca and Mg contents at the rim of normally zoned clinopyroxene are lower than those at the core, whereas the rims of reversely zoned crystals are more enriched in Mg than the core, and are often depleted in Ca. Both reverse and normal zonings are always abrupt at the rim and the larger part of the crystals has a uniform chemical composition. Reversely zoned clinopyroxene phenocrysts are observed only in the eruptives of the older group. In rocks of this group, both types of zoning are often observed together. In these samples, normally zoned phenocrysts are TABLE 3 R e p r e s e n t a t i v e c h e m i c a l c o m p o s i t i o n s (wt. %) of olivine Nrs-2 p.c.

Kzs-2 p.c. 39.0 0.03 0.01 19.5 0.32 41.2 0.12

Kzc-1 p.c

SiO 2 TiO2 A1203 FeO MnO MgO CaO

39.1 0.01 0.03 13.9 0.21 46.3 0.12

40.0 0.03 0.02 20.6 0.33 39.0 0.14

Total

99.6

100.1

100.1

Fo (mole)

85.6

79.0

77.2

A b b r e v i a t i o n : p.c. = core of p h e n o c r y s t .

40.4 35.5 24.2

Wo En Fs

43.3 40.9 15.8

98.8

45.8 7.65 1.58 9.43 0.21 13.7 20.1 0.38 0.0

42.1 40.1 17.8

99.8

50.8 2.17 0.65 11.1 0.37 14.0 20.4 0.28 0.C

43.7 38.2 18.2

98.5

51.6 1.38 0.26 11.1 0.44 13.3 21.0 0.30 0.01

p.c.

Nrs-2

40.8 42.7 16.5

98.7

49.0 4.41 0.98 10.0 0.27 14.5 19.3 0.25 0.02

p.r.

42.3 43.5 14.2

100.4

52.1 2.23 0.46 8.90 0.36 15.4 20.8 0.25 0.0

p.c.

Nor-1

41.8 40.0 18.2

100.7

52.2 1.58 0.58 11.3 0.55 13.9 20.2 0.31 0.06

g.c.

48.3 38.3 13.4

99.9

46.5 8.52 1.34 7.96 0.12 12.8 22.4 0.21 0.01

p.c.

Kzs-2

35.6 47.6 16.9

100.0

52.8 1.64 0.29 10.5 0.15 16.7 17.4 0.20 0.03

g.c.

48.5 37.6 13.9

100.1

47.8 8.20 1.54 8.12 0.14 12.4 22.2 0.31 0.0

p.c.

Kzc-1

44.0 42.7 13.3

99.5

51.1 3.54 0.56 8.17 0.17 14.7 21.1 0.18 0.01

g.c.

A b b r e v i a t i o n s : p.c. -- core o f p h e n o c r y s t , p.r. = r i m o f p h e n o e r y s t , g.c. = c o r e o f g r o u n d m a s s . A b b r e v i a t i o n s for s a m p l e n a m e s are s h o w n in c a p t i o n s o f Figs. 3 a n d 7. p.c. 1 a n d p.c. 2 r e p r e s e n t reversely a n d n o r m a l l y z o n e d p h e n o c r y s t s respectively. *Fe as F e O in Tables 4, 6, 8 a n d 9.

100.2

51.0 1.06 0.31 15.0 0.50 12.4 19.6 0.31 0.0

Total

SiO2 A1203 TiO 2 FeO* MnO MRO CaO Na~O K20

p.c.

p.c. 1

p.c. 2

Hyo-3

Nuk-8

R e p r e s e n t a t i v e c h e m i c a l c o m p o s i t i o n s (wt. %) o f c l i n o p y r o x e n e

TABLE 4

¢9a

186 m

/\

/X

×

m

X

o

1

x

Kzs-I

×

x

°°°c8 o x

x

×

x

×

×

x

Kzp-1 V

v

v

Ca

V

V

~Fe

187 more enriched in Mg than reversely zoned ones (Tables 4, 5, and Fig. 3). Fig. 3 also shows the compositions of the cores of groundmass clinopyroxenes, which fall between normally zoned and reversely zoned phenocrystic clinopyroxenes. A further difference in chemical composition between the two types of clinopyroxene is well shown by the Si/A1 ratios (Fig. 4). Normally zoned clinopyroxene exhibits greater Tschermak's substitution than the reversely zoned types. Clinopyroxene inclusions in plagioclase phenocrysts in the older group are Fe-rich and have the same composition as reversely zoned clinopyroxene phenocrysts (Table 5 and Fig. 3), whereas all clinopyroxene phenocrysts containing small grains of olivine are normally zoned.

Orthopyroxene. Hypersthene (Ens0--EnT0) is present both as phenocrysts and as groundmass minerals in all the rocks observed. Phenocrysts are mostly Alq O = 6 ) 0.4

o • ] Nrs- 2 0.3

• ] Nuk-8 Ca-Tsch.

\\

\,

,\

0,2

o ~\

\

D

'\

o

,,

\\

\,

~ \

0.1

1.7

1.8

1.,g Si(O=6)

2.0

Fig. 4. A1-Si relations o f clinopyroxene phenocrysts in the older group. Open circles and squares represent the core of normally zoned phenocryst, and solid circles and squares represent the core o f reversely zoned phenocryst in the Nukehirazawa lava series (Nuk-8) and Norikuradake-shita lava (Nrs-2). Fig. 3. Chemical compositions o f clinopyroxene plotted in terms o f Mg, Ca and Fe (atomic %). Open circles represent the core of phenocrysts; solid dots, which are tielined with open circles, represent the rim o f phenocryst. Crosses are the core o f groundmass clinopyroxene. Solid squares are inclusions in plagioclase phenocryst; solid triangles are clinopyroxene phenocrysts with olivine inclusions. Nuk- = Nukehirazawa lava series, Tsg- = Tsugadaira lava series, Hie- = Hiedayama lava series, Nrs- = Norikuradake-shita lava, Nor- = Norikuradake lava, Kzs- = Kazafukidake s o m m a lava series, Kzc- = Kazafukidake central cone lava, Kzp- = Kazafukidake pyroclastic flow deposit.

39.8

33.9 39.4

41.1 40.7

19.6

24.2 21.0

16.3 18.2

42.8

47.7

41.9

41,3

15.3

11.0

41.5

43.7 41.8

40.2

40.9 40.7

En

41.2

A b b r e v i a t i o n s f o r s a m p l e n a m e s are t h e s a m e as in F i g . 3. Phenocryst 1 = normally zoned phenocryst; phenocryst 2 = reversely zoned phenocryst.

Groundmass core

40.6

41.9 39.6

P h e n o c r y s t 2: c o r e rim

I n c l u s i o n in p l a g i n c l a s e

42.6 41.2

P h e n o c r y s t 1: c o r e rim

Fs

41.4

38.5

18.3

15.4 17.5

Fs

17.4

18.7

19.1 16.3

42.3 42.5

39.5

47.5 43,2

Wo

En

19.9

42.8

38.9 41.9

Wo

Fs

41.3

20.5

42.0 41.8

43.3 41.5

Wo

Wo

En

38.8

38.7

22.4 17.2

14.8 17.9

Fs

Wo

18.9

40.8

37.3 41.0

41.2 41.1

En

Kzs-1

39.7

23.7

40.3 41.7

44.0 41.0

Wo

Nor-2

41.4

Groundmass core

35.0

25.0 18.8

15.4 18.1

Fs

Hie-2

Nrs-3

41.3

I n c l u s i o n in p l a g i o c l a s e

34.1 39.0

41.4 42.0

En

Tsgo6

Hie-6

41.0 42.2

P h e n o c r y s t 2: c o r e rim

43.2 39.9

40,4 40.3

15.8 17.0

43.9 42.6

P h e n o c r y s t 1: c o r e rim

Wo

Fs

Wo

En

Tsg-4

Nuk-8

P a r t i a l a n a l y s i s of c l i n o p y r o x e n e

TABLE 5

42.6

42.5 41.2

En

34.7 35.6

41.3 41.1

En

17.9

10.0 15.6

Fs

23.0 21.9

15.4 17.4

Fs

41.1

36.5 43.7

48.8 38.7

42.4

En

Wo

Kzp-1

16.5

14.8 17.6

Fs

b.A 0V O0

189

euhedral, and have reversely zoned rims, i.e., the Fe content decreases, and the Mg and Ca contents increase towards the rim (Tables 6, 7). As shown in the EPMA scanning pattern of the rim of orthopyroxene phenocryst (Fig. 5), the compositional change is in all cases abrupt. Such reversely zoned rims on orthopyroxene phenocrysts have been reported in some calc-alkaline volcanic rocks by other authors (Oshima, 1975; Anderson, 1976). The cores of the groundmass orthopyroxene are generally more enriched in Ca and Mg than the cores of the phenocrysts. The compositional relationships between phenocryst and groundmass orthopyroxenes are shown in Tables 6, 7, and Fig. 6. The chemical compositions of the cores of phenocrysts of orthopyroxene in different lavas and pyroclastics are taken from representative geologic units and arranged in the order of eruption from the older to the younger. The following are the significant features: (1) Wo content of orthopyroxene phenocrysts gradually decreases with the order of eruption, from 3.7 Wo mole% in the Nukehirazawa lava series to 1.5 mole% in Kazafukidake pyroclastic flow deposit. (2) Hornblende phenocrysts appear when the Wo content in orthopyroxene falls below about 3.0 mole% (Hie-l~Kzp-1 in Fig. 7). (3) The Mg/Fe ratio of phenocryst orthopyroxene did not change throughout the volcanic history, even though the Wo content gradually decreased. Orthopyroxene occurs in glomeroporphyritic aggregates, composed of orthopyroxene, or with coexisting plagioclase, titanomagnetite and ilmenite. As indicated in Fig. 7, the chemical composition of the cores of orthopyroxenes in glomeroporphyritic aggregates is the same as that of the isolated phenocrysts. Orthopyroxene also forms reaction rims on olivine phenocrysts. It has the same composition as, or is more enriched in Mg than the groundmass orthopyroxene (Table 6 and Fig. 6). Opx phenocryst core <.-

Mg

Fe.. Ca

0/. Fig. 5. E P M A scanning pattern o f a reversely zoned orthopyroxene phenocryst in the N o r i k u r a d a k e - s h i t a lava o f the older group. 0 ~ m corresponds to the rim o f crystal. Enrichment in both Ca a n d Mg, and depletion in F e are shown near the rim.

3.5 57.8 38.7

100.0

51.5 1.03 0.28 24.3 0.73 20.4 1.72 0.04 0.0

3.4 61.5 35.2

99.1

52.1 1.07 0.28 21.9 0.63 21.5 1.63 0.03 0.0

p.c.

p.c.

3.4 63.1 33.5

99.3

52.3 1.16 0.33 21.0 0.69 22.1 1.68 0.0 0.01

p.r.

2.4 58.4 39.2

99.1

51.5 0.78 0.23 24.3 0.88 20.3 1.18 0.04 0.01

p.c.

Nrs-2

2.3 54.4 43.4

100.1

52.0 0.69 0.12 26.2 1.58 18.4 1.07 0.06 0.0

p.c.

Kzs-2

4.0 70.5 25.6

99.5

52.4 2.72 0.30 16.3 0.53 25.2 1.97 0.02 0.02

g.c.

1.6 54.2 44.3

99.9

51.5 0.64 0.09 26.9 1.58 18.4 0.73 0.02 0.0

p.c.

Kzs-3

1.5 54.2 44.4

99.9

52.5 0.35 0.16 26.3 1.60 18.0 0.68 0.02 0.01

p.c.

Kzc-1

A b b r e v i a t i o n : r.r. = r e a c t i o n r i m o f olivine p h e n o c r y s t . O t h e r a b b r e v i a t i o n s are t h e s a m e as in T a b l e 4.

Wo En Fs

Total

SiO 2 Al203 TiO2 FeO* MnO MgO CaO Na20 K20

Tsg-3

Hyo-3

R e p r e s e n t a t i v e c h e m i c a l c o m p o s i t i o n s (wt. %) o f o r t h o p y r o x e n e

TABLE 6

4.1 61.0 34.9

99.9

53.0 1.08 0.21 21.7 0.67 21.3 1.98 0.02 0.0

g. c.

2.8 65.9 31.3

100.1

53.1 2.32 0.28 19.4 0.72 22.9 1.35 0.04 0.0

r.r

3.1

Groundmass core

Inclusion in plagioclase

2.3 2.9

Phenocryst: core rim

67.2

53.7 70.4 29.7

44.0 26.7 3.3

1.8 1.9

Wo

2.9

4.2

2.8 3.5

Wo

Fs

31.1

48.9 45.8

Nor-1

En

64.9

48.8 51.1

Nrs-3

4.0

Groundmass core

Inclusion in plagioclase

3.3 3.1

Phenocryst: core rim

Wo

Fs

Wo

En

Tsg-6

Nuk-8

Partial analysis of orthopyroxene

TABLE 7

69.1

54.3 57.3

En

56.2

66.2

59.7 65.3

En

27.7

43.9 40.8

Fs

40.9

29.6

39.5 31.2

Fs

3.9

1.8 1.9

Wo

Nor-2

2.8 2.9

Wo

Hie-2

69.8

55.7 56.8

En

49.1 49.6

En

26.3

42.6 41.3

Fs

48.1 47.5

Fs

1.8

3.4

1.8 2.9

Wo

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58.7

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53.6

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28.6

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193

in augite. There is a continuous gradation fr()m thin to thick mantles of augite around hypersthene, in which the relative proportion of augite to hypersthene depends on the degree of reaction. Hornblende. All lavas and pyroclastic rocks erupted after the Hiedayama lava series contain hornblende phenocrysts. This mineral is almost always opacitized to aggregates of titanomagnetite, ilmenite, clinopyroxene, orthopyroxene and plagioclase from the rim of crystals. Hornblende phenocrysts in Hiedayama lava series are generally shorter than 0.5 mm along their C-axes, whereas in rocks of the younger group it is larger; a length of 1 cm along the C-axis is common. The chemical composition of analysed hornblende phenocrysts is shown in Table 8. Although there is a wide range in chemical composition from magnesio-hornblende to pargasite in the classification of Leake (1968), the cores of the larger phenocrysts in the younger group is typically magnesiohornblende.

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Fig. 7. Chemical compositions of the core of orthopyroxene phenocryst. Abbreviations of sample names are the same as in Fig. 3, except that Hyo-2 represents Hiyodorimine lava series. Solid squares are orthopyroxenes in glomeroporphyritic aggregates. Mg/(Mg+ Fe) is in atomic ratio.

194

Some hornblende has reverse zoning; the rim exhibits a higher Mg/Fe ratio than the core and is more enriched in A1, Ca, Na and K, and depleted in Si. Such reverse zoning is also found in hornblende in volcanic rocks of Futatsudake, Haruna volcano, central Japan (Oshima, 1975).

Biotite. All rocks of the younger group contain biotite phenocrysts, almost all of which are opacitized like hornblende. The maximum size is about 3 mm in diameter. Representative chemical compositions of biotite are given in Table 9. Biotite has a uniform chemical composition in contrast to the coexisting hornblende in rocks of the younger group.

Quartz. Quartz is present as phenocrysts in all rocks of the younger group. It is typically corroded but sometimes shows the pseudomorphic habit of highquartz. Reaction rims of augite are rare around corroded quartz. Plagioclase. Plagioclase is the commonest phenocryst and groundmass mineral. Many plagioclase phenocrysts have resorbed and honey-combed structures TABLE8 Representative chemical compositions (wt. %) of h o r n b l e n d e 1

2

3

4

SiO 2 A1203 TiO 2 FeO* MnO MgO CaO Na20 K20

40.1 12.3 2.71 11.7 0.15 15.0 11.3 2.41 0.69

48.4 6.94 1.29 16.2 0.56 13.0 10.7 1.23 0.54

46.4 7.33 1.26 16.2 0.57 14.3 11.2 1.42 0.51

44.2 11.3 2.37 13.4 0.24 13.1 11.3 2.12 0.70

Total

96.3

98.8

99.2

98.7

Si A1 Ti Fe" Mn Mg Ca Na K Total

6.020 2.185 0.306 1.466 0.019 3.351 1.817 0.703 0.312

7.072 1.195 0.141 1.979 0.069 2.830 1.678 0.350 0.101

6.812 1.268 0.139 1.987 0.071 3.315 1.755 0.404 0.095

6.462 1.944 0.261 1.640 0.029 2.842 1.763 0.601 0.130

15.999

15.415

15.666

15.671

1 = Hie-1 (Hiedayama lava series), 2 = Kzs-3 (Kazafukidake s o m m a lava series), 3 = Kzs-6 (Kazafukidake s o m m a lava series), 4 = Kzc-1 (Kazafukidake central cone lava). Numbers of a t o m s are for 23.0 oxygens.

195 TABLE 9 Representative chemical compositions (wt. %) of biotite 1

2

3

SiO 2 AI:O~ TiO 2 FeO* MnO MgO CaO Na20 K20

36.9 14.1 4.61 17.4 0.14 13.5 0.01 0.62 8.98

36.1 14.8 4.54 19.4 0.27 12.9 0.03 0.58 8.66

36.0 13.7 4.67 19.3 0.19 12.0 0.02 0.68 9.14

Total

96.2

97.4

95.7

Si A1 Ti

5.539 2.483 0.520 2.180 0.017 3.010 0.002 0.180 1.717

5.404 2.612 0.511 2.430 0.035 2.876 0.004 0.168 1.651

5.504 2.466 0.537 2.466 0.024 2.734 0.003 0.201 1.783

15.648

15.691

15.718

Fe" Mn Mg Ca Na K Total

1 = Kzs-3, 2 = Kzs-6, 3 = Kzc-1. Numbers of atoms are for 22.0 oxygens. ( K u n o , 1 9 5 0 ) , and c o n t a i n d u s t y inclusions, b u t clear e u h e d r a l plagioclase p h e n o c r y s t s are also c o m m o n . T h e m a x i m u m size o f plagioclase p h e n o c r y s t is smaller in t h e o l d e r g r o u p ( < 4 m m ) t h a n in t h e y o u n g e r g r o u p ( < 7 m m ) . T h e following observations have been m a d e with the m i c r o p r o b e . (1) T h e cores o f t h e p h e n o c r y s t s are n o t always m o r e An-rich. (2) T h e r e is n o s y s t e m a t i c d i f f e r e n c e in chemical c o m p o s i t i o n b e t w e e n t h e crystals having d u s t y zones and clear crystals. (3) T h o u g h t h e c o m p o s i t i o n a l z o n i n g occurs in grains with a wide range o f An c o n t e n t s and in c o m p l e x p a t t e r n s , average An c o n t e n t s o f p h e n o c r y s t s gradually decrease with t h e a d v a n c e d stage o f e r u p t i o n (Fig. 8). (4) T h e o u t e r p a r t o f t h e d u s t y z o n e is always m o r e An-rich t h a n t h e i n n e r part. Observations (2) and (3) indicate t h a t all plagioclase p h e n o c r y s t s , including t h o s e with d u s t y zones and h o n e y - c o m b e d s t r u c t u r e , are c o n s i d e r e d t o be p h e n o c r y s t s and n o t x e n o c r y s t s i n c o r p o r a t e d f r o m granitic rocks.

Titanomagnetite. All t h e rocks c o n t a i n p h e n o c r y s t i c and g r o u n d m a s s titanom a g n e t i t e . P h e n o c r y s t s are m o s t l y e u h e d r a l and smaller t h a n 1 m m across.

196 An mote% 90 80 7C 6C 5C 4C

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Fig. 8. Simplified zoning patterns of plagioclase phenocrysts. Left ends of each crystals are the core of phenocrysts. Dotted lines represent the dusty zones. Abbreviations: Hyo-3 = Hiyodorimine lava series; Tsg-3 = Tsugadaira lava series; Kzs-2, 3 = Kazafukidake somma lava series.

The chemical compositions of titanomagnetites were determined in seven rock samples (Table 10) and ulvSspinel mole% in titanomagnetite calculated by the method of Carmichael (1967), is plotted in Fig. 9. The compositions of all titanomagnetites occurring as inclusions in other phenocryst phases, irrespective of their host mineral species, are the same as those of discrete titanomagnetite phenocrysts. Groundmass titanomagnetite is almost always more enriched in the ulvSspinel molecule than the cores of phenocrysts, as pointed by Oshima (1971, 1975). Ilmenite. Ilmenite occurs as phenocrysts in rocks erupted after the Kazafukidake somma lava series. Ilmenite found as inclusions in orthopyroxene, hornblende, biotite and plagioclase, has the same composition as discrete phenocrysts. In the older group, ilmenite appears as rare microphenocrysts and in the groundmass. Representative chemical compositions of phenocrystic ilmenite are

197

shown in Table 10, and are plotted in Fig. 9 in terms of R203 mole% which is calculated by the method of Carmichael (1967). The compositional variation of ilmenite is narrower than that of titanomagnetite. Most ilmenite phenocrysts are homogeneous, and compositional zoning is rare. I N T E R P R E T A T I O N OF PETROGRAPHIC AND MINERALOGICAL DATA

The volcanic rocks of the Shirouma-Oike volcano, especially those of the younger group, show evidence of disequilibriun/mineral assemblages; for example, the coexistence of phenocrysts of quartz and magnesian olivine without reaction rims, coexistence of reverse zoning of orthopyroxene phenocrysts and normal zoning of olivine and clinopyroxene phenocrysts, and strong deviation from the equilibrium value of the Mg-Fe partitioning between olivine and orthopyroxene. These problems are discussed in more detail in the following sections.

TABLE 10 Representative chemical compositions of phenocrystic Fe-Ti oxides Tsg-3

Nor-1

Kzs-3

Kzc-1

Mt

Mt

Mt

SiO 2 TiO~ AI203 V~O 3 Cr203 FeO* MnO MgO CaO

0.02 15.9 2.28 0.80 0.05 75.8 0.56 0.83 0.01

0.08 13.1 1.28 0.86 0.02 78.8 0.47 1.17 0.0

0.0 8.29 2.28 0.70 0.06 82.0 0.57 0.81 0.02

0.0 45.5 0.18 0.56 0.0 49.7 1.03 1.86 0.01

Total

96.3

95.6

94.7

FeO** Fe203**

44.4 34.9

41.2 41.6

Total**

99.8

Usp (mole%) R203 (mole%)

44.7

Ilm

Mt

Kzp-1 Ilm

Mt

0.0 10.9 2.25 0.80 0.05 80.4 0.44 1.11 0.01

0.08 47.0 0.11 0.45 0.0 48.2 0.71 2.02 0.04

0.0 9.06 2.04 0.66 0.07 80.7 0.42 1.25 0.01

0.02 45.5 0.22 0.41 0.0 49.7 0.74 1.98 0.02

98.8

96.0

98.6

94.2

98.5

37.2 49.7

36.6 14.7

39.6 45.3

38.0 11.3

37.2 48.3

36.6 14.5

99.8

99.7

100.3

100.6

99.8

99.1

100.0

37.1

23.4

30.5 14.6

Jim

25.7 11.3

14.4

Abbreviations for sample names are shown in Fig. 3. Mt and Ilm represent titanomagnetite and ilmenite respectively. **: Recalculated values for FeO, Fe203 and Total according to Carmichael (1967).

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I

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Kzp-1

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199

Mg-Fe partition between orthopyroxene and olivine Matsui and Nishizawa (1974) compiled the experimental data on the Mg-Fe partition between orthopyroxene and olivine at pressures from 1 atm to 30 kbar, and at temperatures from 800 to 1300°C. Their data, shown in Fig. 10, indicate that olivine is slightly more Fe-rich than the coexisting orthopyroxene in wide ranges of temperatures and pressures. The chemical compositions of olivine and orthopyroxene phenocryst of volcanic rocks of ShiroumaOike volcano are plotted in a similar figure (Fig. 11), in which the range of Mg/(Mg+ Fe) ratio of the core of orthopyroxene is given by a vertical bar and the compositional change of olivine is given by an arrow. Most pairs deviate from the equilibrium curve and indicate that most of the coexisting olivine and orthopyroxene phenocrysts are not in equilibrium.

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200

0% Mg

0.1

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Fig. 11. Mg-Fe partition between olivine and orthopyroxene phenocrysts in rocks of the Shirouma-Oike volcano. Vertical bars represent the range of compositional variation of the core of orthopyroxene phenocryst; arrows represent the compositional change of coexisting olivine phenocryst. Mg/(Mg+Fe) is indicated in atomic ratios. Disequilibrium relations are also suggested from the evidence that all olivine is normally zoned, whereas almost all orthopyroxene is reversely zoned. The compositional zonings of both the minerals indicate t h a t t h e y were approaching the equilibrium composition. In Fig. 11, there are three samples in which olivine and o r t h o p y r o x e n e are close to equilibrium compositions. In these samples, however, olivine crystals are very fine grained and reverse zoning of orthopyroxene phenocryst is not clear.

Mg-Fe partition between clinopyroxene and olivine The Mg-Fe partition between clinopyroxene and olivine can also be used as an indicator of equilibration. The ranges of Mg/(Mg+ Fe) ratio of the cores of olivines and normally zoned clinopyroxene phenocrysts are given in Fig. 12, and it is evident t h a t the Mg/Fe ratios of the cores of olivine and clinopyroxene vary similarly from sample to sample; the samples with Mg-rich olivine contain Mg-rich clinopyroxene, whereas those with Fe-rich olivine contain Fe-rich clinopyroxene.

201

Obata et al. (1974) pointed out that Mg-Fe partition between olivine and clinopyroxene can be used as an empirical geothermometer. Judging from their conclusions, the results in Fig. 12 indicate that olivine and normally zoned clinopyroxene were equilibrated at temperatures higher than 1000°C. It is most likely that these two minerals crystallized in a cotectic relation from magma with a temperature higher than 1000°C. The clinopyroxene phenocrysts which have reversely zoned rims would not be in equilibrium with olivine phenocryst because such pairs would plot in the upper side of the equilibrium curve of Fig. 12.

Fe-Ti oxide equilibration temperatures Phenocrysts of ilmenite and titanomagnetite are present in rocks of the younger group except for the Norikuradake lava and lower part of the Kazafukidake somma lava series. The average ulvSspinel mole% of titanomagnetite and R203 mole% of ilmenite were calculated, and the equilibration temperature and oxygen fugacity were obtained from these values using the method of Buddington and Lindsley (1964); they are about 800°C and 10 -t4 atm f o , respectively (Table 11). The oxidation state is nearly equal to that of the ~qi-NiO buffer at about 800°C. Mg Mg,,Fe Cpx

J 0.6

0.7

0.8 /

a9

1.0

o

ob

01s

o:7

I

0.50liv.

M9

Mg.Fe Fig. 12. Mg-Fe partition between olivine and normally zoned clinopyroxene phenocrysts. Ranges in Mg/(Mg+ Fe) (atomic ratio) of the core of both phases in respective rock samples are indicated.

202 TABLE 11 Fe-Ti oxides phase equilibration temperature and oxygen fugacity Sample No.

Usp (mole%)

R203 (mole%)

T(°C)

-log fo2

Kzs-3 Kzc-1 Kzp-1

21.0 (7) 32.0 (12) 26.1 (11)

14.2 (12) 11.1 (9) 14.2 (13)

740 810 780

14.7 13.6 13.8

Number in parentheses denotes the number of averaged grains.

It is evident that crystallization temperature of olivine and normally zoned clinopyroxene, and that of ilmenite and titanomagnetite are different. It is likely that these two pairs have crystallized from different magmas.

Equilibrium phenocryst assemblage in dacite Kzs-3 The above-mentioned equilibration must be examined among other phenocrystic minerals. Because no reliable method to examine such equilibration exists, reasonable deductions must be made from the detailed petrographic and mineralogical investigations. Modal abundances of olivine and clinopyroxene vary considerably more in rocks of the younger group than do those of other phenocrystic minerals, and in some cases olivine is absent. For example, the dacite sample, Kzs-3, does not contain both olivine and clinopyroxene. Phenocrysts in this dacite are orthopyroxene, hornblende, biotite, quartz, plagioclase, ilmenite and titanomagnetite, which are always observed in rocks of the younger group. Orthopyroxene phenocrysts show no reverse zoning and groundmass orthopyroxene is only slightly more magnesian than the phenocrysts (Fig. 6). Plagioclase phenocrysts are clear and rarely contain dusty inclusions. Apparently dacite Kzs-3 has a mineral assemblage that is closest to equilibrium of all rocks of the younger group. This is also supported by the following observations. (1) Chemical compositions of titanomagnetite and ilmenite inclusions are uniform irrespective of the host minerals. (2) In other samples of the younger group, such as Kzs-1 (Table 7), chemical compositions of orthopyroxene inclusions in plagioclase phenocrysts are the same as those of the discrete orthopyroxene phenocrysts. This suggests that orthopyroxene has co-crystallized with plagioclase. It can be concluded, therefore, that o~hopyroxene, hornblende, biotite, quartz, plagioclase, titanomagnetite and ilmenite were probably in equilibrium in the dacite Kzs-3 at a temperature of about 740°C (Table 11).

Magma mixing inferred from the disequilibrium phenocryst assemblage As mentioned above, volcanic rocks of the younger group contain olivine,

203 clinopyroxene, orthopyroxene, hornblende, biotite, quartz, plagioclase and titanomagnetite (+ ilmenite) as phenocrysts. Among these phenocrysts, olivine and clinopyroxene always have normal zoning and seem to be in equilibrium with each other in terms of their Mg-Fe partition. On the other hand, orthopyroxene has a reverse zoning and is not in equilibrium with olivine. Orthopyroxene, hornblende, biotite, quartz, plagioclase and Fe-Ti oxides are considered to have been in equilibrium with one another, as observed in the dacite (Kzs-3) described above. Therefore, there are two groups of phenocrysts in the rocks of the younger group. One includes olivine and clinopyroxene equilibrated at high temperatures, and the other includes phenocrysts other than olivine and clinopyroxene that equilibrated at low temperatures. The evidence for a temperature difference, which was discussed in previous sections, is summarized as follows. (1) The equilibration temperature of olivine and clinopyroxene phenocrysts is high, probably higher than 1000°C as inferred from the Mg-Fe partition. (2) The equilibration temperature of ilmenite and titanomagnetite phenocrysts is about 750--850°C. The temperature is consistent with the reasoning that ilmenite and titanomagnetite phenocrysts were crystallized in equilibrium with biotite, because recent experimental works on andesitic melts indicate that the upper stability limits of biotite in such magma under crustal pressure (<10 kbar) is at most 900°C (Piwinskii and Wyllie, 1968; Robertson and Wyllie, 1971) if the effect of fluorine is neglected. The compositional zoning of the phenocrysts described above strongly suggests that two magmas have been mixed immediately before eruption; normal zoning of olivine and clinopyroxene indicates falling temperature and accompanying chemical fractionation of the high-temperature magma, whereas reverse zoning of orthopyroxene indicates increasing temperature and a change of chemical composition of the low-temperature magma accompanied with magma mixing. The mixing just before eruption is also supported by the absence of reaction rims around olivine and quartz phenocrysts. Reversed zoning of orthopyroxene phenocrysts and the existence of magnesian groundmass orthopyroxene are also common features of rocks of the older group (see page 189). This can be interpreted as evidence that magma mixing also has occurred in pyroxene andesites and hornblende pyroxene andesites of the older group. The above interpretation is confirmed by the coexistence of two types of clinopyroxene phenocrysts: one which is enriched in Mg and has normal zoning would have crystallized from basaltic magma, whereas the other, which is enriched in Fe and has reverse zoning on its rim, would probably have crystallized from andesitic magma in a cotectic relation with orthopyroxene. This is consistent with the previously mentioned petrographic observations: (1) clinopyroxene containing olivine grains is always normally zoned, (2) orthopyroxene inclusions in plagioclase grains have the same compositions as discrete phenocryst orthopyroxenes, (3) clinopyroxene inclusions in plagioclase phenocrysts have the same compositions as reversely zoned clinopyroxene phenocrysts.

204

Mixing of t w o magmas is also indicated by the presence of heterogeneous textures on various scales, which are sometimes observed in rocks of the younger group. Quenched products of basaltic melt, which exhibit a mesh structure composed of clinopyroxene, plagioclase and interstitial glass with bubbles, exist around olivine phenocrysts (Fig. 13). Pargasitic hornblende

Fig. 13. Heterogeneous groundmass in a lava of Kazafukidake s o m m a lava series. Basaltic

groundmass is present around olivine phenocryst.

sometimes occurs in such parts in place of clinopyroxene. Banded textures, composed of mafic and silicic streaks on a microscopic scale (0.5--1.0 mm wide), are sometimes observed. The mafic bands contain olivine and clinopyroxene phenocrysts, whereas plagioclase phenocrysts incorporated in these bands show resorption texture, such as honey-combed structures and have dusty zones around crystals, and hornblende phenocrysts in these bands are also strongly opacitized. On the other hand, the silicic bands contain hornblende, biotite, orthopyroxene, plagioclase, quartz, titanomagnetite and ilmenite as phenocrysts in a groundmass of clear glass. All of the phenocrysts in these bands are euhedral and free from resorption textures. The banded structures are also observed on a macroscopic scale (2--10 cm wide). The bandings are composed of white and dark bands with a gradual change from one to the other.

205 IMPLICATIONS OF MAGMA MIXING

Crystallization of the two previously mentioned groups of phenocrysts must have taken place in different places. The high-temperature fraction in mixed magma is varied and is sometimes absent, whereas the low-temperature fraction is present in all the volcanic rocks. It is most reasonable to assume, therefore, that the low-temperature magma was stagnant in a shallower magma reservoir under crustal pressures and the high-temperature magma has been supplied intermittently to this reservoir, probably from deeper part (Anderson, 1976). Mixing of magmas is an important volcanological tool for inspecting conditions within a magma reservoir just before eruption (Anderson, 1976). The crystallization sequence of minerals in the shallower magma reservoir can be obtained by excluding the effect of high-temperature magma (Fig. 14). The crystallization sequence of phenocrysts is the same as that observed in several experiments on calc-alkaline andesitic melt (Piwinskii and Wyllie, 1968; Robertson and Wyllie, 1971; Eggler, 1972; Eggler and Burnham, 1973), and Wo=

An= T

1.5 1.5

42

W

42

1.6

50 1.8

50

Kazafukidake cenral cone |ava

~

Kazafukidake

~

somma lava series

O >'

Norikuradake lava

"

2.3

3.5

6oi

3.5

7o!

(:Z lava series D

o Tsugadaira o~ lava series ~,

O 'l

Hiyodorimine lava series Nukehirazawa lava series

C

0

._c

Norikuradakeshita lava Hiedayama

2.7

C

Kazafukidake pyrodastic flow

=

C

"~

"~

0

c-

r-

~

o

~

~

,

,

Fig. 14. Crystallization sequence of minerals in the shallower magma reservoir under Shirouma-Oike volcano. ~-phase is titanomagnetite and s-phase is ilmenite. Numbers along bars of orthopyroxene and plagioclase are Wo rnole% in orthopyroxene and An mole% in plagioclase respectively.

206 can be explained simply by crystallization due to falling temperature. It is also supported by the gradual decrease of Wo content of orthopyroxene phenocrysts (Fig. 7). Because orthopyroxene coexisted with clinopyroxene to the stage of the Hiedayama lava series and Norikuradake-shita lava (Fig. 14) and its Mg-Fe ratio is nearly fixed through all the volcanic rocks (Fig. 7), the gradual decrease in Wo content of orthopyroxene indicates a continuous temperature decrease at least through the stage of the older group. Another aspect of two different assemblages of phenocrysts is that the nature of the introduced magma can be estimated to some extent and the mechanism of magma mixing can be tentatively discussed. The magma supplied from depth is considered to have been basaltic, because of its phenocrysts (olivine and clinopyroxene), estimated temperature (higher than 1000°C) and basaltic quenched products around olivine phenocrysts. Though the chemical composition of this basalt can not be determined, the phenocryst assemblage (olivine and clinopyroxene without orthopyroxene) is very characteristic of alkali olivine basalts (Kuno, 1960). Although the high-temperature basaltic magma may have been intermittently supplied to the shallower magma reservoir, the magma in the reservoir seems to have fractionated continuously with decreasing temperature. It is suggested from this observation that the introduced magma did not significantly influence the magma in the reservoir at least thermally. The following two possibilities are consistent with this observation: (1) the volume of the introduced magma was much smaller than that of the magma in the reservoir, and (2) the introduced magma did not effectively mix with the magma in the reservoir, and mixing occurs only at narrow zones along the passage of the incoming magma. Magnesian olivine and quartz phenocrysts co-existing without noticeable reaction have also been found in lavas of many other calc-alkaline andesite volcanoes in Japan. The author examined assemblages of phenocrysts from 140 Quaternary volcanoes in the East Japan volcanic zone from the published descriptions. Thirty volcanoes contain lavas with both magnesian olivine and quartz with or without hornblende. Quartz "phenocrysts" in some could be xenocrysts from basement granitic rocks; however, other lavas could have been formed by mixing of basaltic and dacitic magmas before eruption. The importance of magma mixing for the compositional diversity of volcanic rocks from continental margins and other environment has also been emphasized by several authors (Eichelberger, 1975; Anderson, 1976; Sparks et al., 1977; Johnston and Schmincke, 1977). If mixing of magmas is indeed an important petrogenetic process, the bulk chemical composition of rocks cannot be used directly to evaluate trends of fractional crystallization or various melting processes during magma generation (O'Hara, 1977). More detailed petrographic and mineralogical studies are needed in order to interpret the whole-rock chemistry of volcanic rocks, especially those of the calcalkaline suite of andesites and dacites.

207 ACKNOWLEDGEMENTS

The author wishes to express his hearty appreciation to Professors I. Kushiro and S. Aramaki of University of Tokyo for discussions, encouragement and critical reading of the manuscript. He is also indebted to Dr. D. Bostok for valuable suggestions in editing the manuscript.

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