magma mixing process of calc-alkalic andesites from

Magma mixing process of calc-alkalic andesites from Funagata volcano

KEUI WADA*

Department of Geology and Mineralogy, Faculty of Science,

Hokkaido University, Sapporo 060, Japan

Calc-alkalic andesites from the Funagata volcano are characterized by disequilibrium phase assemblages, reverse zoning of plagioclase and pyroxenes, wide compositional range of plagio-clase, and coexistence of basaltic and rhyolitic glass inclusions. These mineralogical features can be ascribed to magma mixing process involved in the formation of the calcalkalic andesites. Compositional profiles of plagioclase and pyroxenes suggest that the phenocrysts have crystallized from three distinct magmas such as mafic and silicic end-member magmas and mixed magma. Mixing ratio of mafic end-member magma to silicic end-member magma within individual andesite was calculated by available mineralogical data. Whole-rock chemical composition of andesites exhibits linear correlation with both the mixing ratio and SiO2 content. Two mixing lines in MgO-K2O diagram are constricted at the MgO-rich tholeiitic basalts which represent the mafic end-member magma. Chemical composition of the silicic end-member magmas was deduced by the mixing ratio. Calculated two silicic end-members are felsic andesite (SiO2=64%, K2O=1.1%) and dacite (SiO2=66%, K2O=1.8%). Available Sr-isotopic data suggest that the dacite magma is not produced by fractional crystallization of the tholeiitic magmas.

Introduction

A number of authors have discussed the

origin of voluminous andesites of the calc-al-

kalic series which characteristically occur in

island arcs and continental margins. More

recently, on the basis of petrographic and chem

ical evidence an increasing effort has been

extended toward magma mixing that plays an

important role in the genesis of calc-alkalic

volcanic rocks (Anderson, 1976; Eichelberger,

1975, 1978; Sakuyama, 1979, 1981 ; Gerlach

and Grove, 1982 and others). The present

study is concerned with mixed andesites from

the Funagata volcano group situated on the

volcanic front of northeast Japan.

Quaternary volcanoes in northeast Japan are densely distributed along the volcanic front,

while they gradually decrease in number

toward the back-arc side. The volcanic rocks

along the volcanic front comprise the low-

alkali tholeiitic series and the calc-alkalic

series. Distinct contrasts in the whole-rock

chemical compositions between the two rock

series have been described, i.e. the tholeiitic

series show a higher degree of iron-enrichment,

lower level of incompatible elements such as K,

Th and U and lower SiO2 mode than the calc-

alkalic series (Kuno, 1950; Kawano et al.,

1961; Katsui, 1961; Aoki, 1978; Masuda and

Aoki, 1979 and others). It has been considered

that the whole-rock chemical variations of the

calc-alkalic series represent either (a) a liquid

line of descent by fractional crystallization

from primary tholeiitic or calc-alkalic mag-

mas, or (b) assimilation line of tholeiitic magma

(Manuscript received, June 14, 1985;accepted for publication, August 23, 1985)* Present Address: Department of Earth Science, Asahikawa College, Hokkaido University of Education,

#Asahikawa 070, Japan

468 Keiji Wada

and granitic rocks. On the other hand, disequilibrium petrographic features such as

coexistence of phenocrystic olivine and quartz, and bimodal composition and reverse zoning of

phenocrystic plagioclase and pyroxenes in the talc-alkalic andesites can be best explained by magma mixing (Sakuyama, 1979, 1981; Wada, 1981). Sakuyama (1981) has documented from detailed phase petrological study that the chem-

ical variations of the talcalkalic andesites from Myoko and Kurohime volcanoes repre-sent mixing lines developed by the mixing of high-alumina basalt magma and dacite mag-

mas from different stages of fractional crystallization. He has also suggested that most of

the calc-alkalic andesites in northeast Japan are products of "internal magma mixing. However, some calc-alkalic dacites along the volcanic front which represent possible silicic

end-member magmas before mixing may not be formed by fractional crystallization of tholeiitic magma as inferred from their erup-

tive volume ratios, chemical relations and Srisotopic data.

The purpose of this paper is to clarify the

magma mixing process involved in the forma

tion of talc-alkalic andesites from the

Funagata volcano group and to deduce the

characteristics of the end-member magmas.

Geological setting

Funagata volcano group is situated about

35 km northwest of Sendai city, and it is on the

central portion of the volcanic front of north-

east Japan arc. The volcano group overlies

the Plio-Pleistocene Pre-Funagata volcanic

rocks, and consists of three composite vol-

canoes : Mt. Funagata, Ushiro-shirahige and

Kita-izumigatake (Wada, 1981). These vol-

canoes have been highly dissected and neither

craters nor fumaroles are found. Among three

volcanoes Mt. Funagata seems to be the young-

est, judging from the degree of erosion. Mt.

Funagata is composed mainly of talc-alkalic

andesite and intercalates tholeiitic basalt and

andesite. The first product of the Kita-

izumigatake is tholeiitic basalt and the rocks of

latter stages are calc-alkalic andesite with

subordinate tholeiitic andesite. The rocks of

the Ushiro-shirahige are dominantly calc-al-

kalic andesite with subordinate tholeiitic an-

desite.

Analytical method

Chemical analyses of minerals were perfor

med with electronprobe microanalyzers both of

Hitachi Model 5A of the Department of Earth

Science, Kanazawa University and JEOL

Model 50A of the Faculty of Engineering,

Hokkaido University, using data reduction and

matrix correction procedures of Bence and

Albee (1968) and Albee and Ray (1970). Ana

lyses of glass inclusions in phenocryst were

made under a specimen current of 0.01-0.015

microampere and an electron beam of 5ƒÊm in

diameter. Major element chemical analyses of

rocks were also carried out with electronprobe

microanalyzer on fused glass by the method of

Fukuyama and Sakuyama (1976). Analytical

precision and reproducibility of this method are

given in Wada (1981). Representative chemi

cal analyses are listed in Table 1.

Sr-isotopic ratios of 9 samples from calc

alkalic andesites and of the same number from

tholeiitic basalts and andesites were deter-

mined by Dr. H. Kurasawa, Geological Survey

of Japan.

Petrography

Outline of petrography of the tholeiitic and

calc-alkalic rocks was already described by

Wada (1981), only significant features of the

analysed samples are given here. Since the

tholeiitic rock series show no petrographic

evidence of magma mixing, the petrography of

basalts is presented as a possible mafic end-

Magma mixing process, Funagata volcano 469

Table 1. Selected whole-rock major chemical composition and Sr-isotopic ratio of calc-alkalic andesites (Nos. 1-10) and tholeiitic basalts

member.

1) Calc-alkalic andesites

Calc-alkalic andesite lavas contain vari

able amounts of discrete phenocrysts, glomero

porphyritic clumps and sometimes small basaltic inclusions in hand specimen. The

andesitic rocks have phenocryst assemblages of

plagioclase, orthopyroxene, augite, titano-magnetite with or without olivine, quartz and

hornblende (Fig. 1 and Table 2). Plagioclase

phenocrysts are most dominant phase (20-30

vol.%) and show various zoning pattern : nor

mal and reverse, oscillatory and other complex

zoning. The Ca/(Ca+Na) ratio of the broad

core of plagioclase phenocrysts attains a maxi-

mum range from 0.90 to 0.42 (sample No. JA-

37), but differs from specimen to specimen (Fig.

9). Plagioclase phenocrysts frequently contain

glass inclusions in certain zones and/or dust inclusion zones of fine-grained opaque minerals

and alkali feldspar. Olivine phenocrysts are

present in several andesites, and their modal

Fig. 1. Mode and phenocryst assemblage of the talc-alkalic andesites. Numbers are the same as sample Nos. of Tables 1, 2 and 3.

470 Keiji Wada

Table 2. Modal analyses of calc-alkalic andesites

content correlates positively with the whole

rock MgO content. Olivine phenocrysts show

commonly euhedral to subhedral and normal

zoning without reaction rim. Olivine pheno

crysts in the lavas of YH-29 and SJ-35 have

maximum Fo contents of Fo86-80, but coexist

with quartz phenocrysts in these lavas.

Chromian spinels are frequently included in

olivine phenocrysts even in low Fo contents

(Fo75-70, in FG-I-13). Orthopyroxene phenocrysts are commonly euhedral and usually both

normally zoned and reversely zoned. The

sharp boundary between the iron-rich inner

core and the magnesian outer core is sometimes

recognized by difference in interference color or

Beck's line. Microphenocrysts of orthopyrox

ene commonly display euhedral form and are

generally high in Mg/(Mg+Fe) ratio. Augite

phenocrysts commonly have a compositionally homogeneous core, whereas some of them have

outer core of reverse zoning. Augite micro

phenocrysts commonly display secter zoning and are considerably higher in Mg/(Mg+Fe)

ratio, Al and Ti contents than the core of

phenocrysts. Some Fe-rich orthopyroxene and augite phenocrysts contain subspherical

glass inclusions of dacitic to rhyolitic composition with a bubble.

2) -a Tholeiitic basalts from Mt. Funagata

The basaltic rocks (FG-II-1•`3) are por

phyritic and contain abundant plagioclase

phenocrysts with subordinate olivine, augite

and orthopyroxene phenocrysts set in an inter-

granular groundmass. Plagioclase pheno

crysts have compositional range from An93 to

An,e and show normal zoning from core to rim.

Olivine phenocrysts are euhedral to subhedral.

and vary from 2 to 8 vol.% in modal content.

Individual olivine phenocryst mantled by thin

reaction corona of pigeonites is normally zoned

and ranges from Fo86 to Fo70, (FG-II-1 and 2),

and from Fo,fi to For,, (FG-II-3). Inclusions of

minute euhedral spinel are common in olivine

phenocrysts. Augite and orthopyroxene

phenocrysts are euhedral to subhedral and vary

in Mg/(Mg+Fe) ratio from 0.82 to 0.73 and

from 0.78 to 0.65, respectively. Orthopyroxene

phenocrysts are always surrounded by reaction

corona of pigeonites.

2)-b Tholeiitic basalts from Kita-izumigatake

The basalts (KH-604 and KH-820) from

Kurohana lavas in Kita-izumigatake are

plagioclase-phyric rocks with phenocrysts of olivine, orthopyroxene, augite and pigeonite.

Plagioclase phenocrysts are normally zoned

and range from An90 to An82. Olivine pheno

crysts are commonly euhedral and rimmed with


iddingsite. Inclusions of spinels are rarely

found in olivine. Individual olivine phenocryst is normally zoned and ranges from Fo66 to Fo74. Orthopyroxene phenocrysts are mantled by

reaction corona of coarse-grained pigeonite

and augite, and range in Mg/(Mg+Fe) ratio

from 0.80 to 0.76. Augite and pigeonite pheno

crysts vary in Mg/(Mg+Fe) ratio from 0.82 to

Fig. 2. A. Zonal distribution of Mg/(Mg+Fe) ratio in orthopyroxene phenocrysts from JA-37 and KI-20.

B. Zonal distribution of Wo content in the same phenocrysts.

472 Keiji Wada

0.73 and from 0.76 to 0.70, respectively.

Mineralogical evidence of magma mixing

Petrographic characteristics of pheno

crysts in all of the calc-alkalic andesites

analysed indicate that they constitute disequili-

brium assemblages and show reverse trends in

chemical variation, and suggest that the host

magmas have a mixing origin. In this section,

the mineralogical evidence of magma mixing is

examined and the phenocrysts are divided into

those derived from either mafic or silicic mag

mas.

1) Pyroxenes

The compositional distribution of 100 Mg/

(Mg +Fe) ratio (reffered to Mg-value hereafter) and Wo content in reverse-zoned orthopyrox-

ene phenocrysts were determined by about 100

point analyses (Fig. 2). The Mg-value and Wo content increase from the broad inner core

Fig. 3. Cr2O3 versus Mg/(Mg+Fe) ratio of augite phenocrysts from FG-I-13, KI-20, YH-29

and JA-37. Solid circles show inner core composition and open circles outer core composition.

toward the outer core, where attain a maxi-mum, and finally decrease at the rim. The

zonal structure of orthopyroxene from KI-20 is not always developed concordantly with crystal shape and shows rather irregular compositional

border. In Fig. 3, outer cores of the reverse-zoned augite phenocrysts have the highest Cr2O3 content. These results indicate that the

Fig. 4. Compositional zoning profiles of orthopyroxene phenocrysts. Dotted lines' show zoning

profiles of microphenocrysts. Numbers are the same as sample Nos. of Tables 1, 2 and 3.


chemical variations in reverse-zoned pyroxenes

do not represent those predicted from frac

tional crystallization. These phenomena can

be ascribed to temperature increase and

compositional change of host liquid to mafic

composition, which were possibly caused by

continuous mixing of an injected mafic magma

(Sakuyama, 1979; Sato, 1982 and others).Compositional zoning profiles of pyroxene

phenocrysts from 10 specimens of the calcalkalic andesites are shown in Figs. 4, 5 and 6.

As illustrated in these figures, the range of Mg-

Fig. 5. Compositional zoning profiles of augite phenocrysts.

Fig. 6. Compositional zoning profiles of pyroxene phenocrysts from mafic andesites (FG-I-13 (No. 1)

and YH-29 (No. 5)).

474 Keiji Wada

value.

These zoning profiles can be used to deduce

the range of Mg-value of pyroxene phenocrysts

crystallized from the end-member magmas

prior to mixing (Fig. 7). This model is present-ed as follows.

Fig. 7. Schematic illustration of representative zo

ning profiles of pyroxene phenocrysts.

Reverse triangle indicates outer core, S,, higher semi-critical value ; S2, lower semi-

critical value ; Mafic., Mg-value range of

phenocrysts derived from mafic magma (higher than S,) ; Silicic., those from silicic magma (lower than S,); Mix., those from mixed magma (between S, and S2).

value of reverse zoning from inner core to outer

core varies among different phenocrysts within one thin section ; the maximum change of Mg-value of the reverse zoning amounts to 20 mol.% in orthopyroxene and 15 mol.% in augite. The degree of reverse zoning from

inner core to outer core may depend principally on the extent of difference in chemical composi

tion between the mafic and silicic end-member magmas. The zonal pattern can be classified mainly into three types : 1) normal-type with inner core of high Mg-value, and 2) reverse-

type and 3) flat-type with that of low Mg-

The normal-zoned phenocrysts with inner core of high Mg-value must be derived from a mafic magma, whereas the reverse- and flat-

type zoned phenocrysts with that of the low Mg-value from a silicic magma. The Mg-value of pyroxene phenocrysts derived from the

mafic magma is expected to be higher than that of the outer core of reverse-zoned phenocrysts from the silicic magma. Hence, a minimum Mg-value of the pyroxene phenocrysts derived

from mafic end-member magma before mixing was fixed to a value slightly higher than , a maximum Mg-value of the outer core of phenocrysts from silicic end-member magma. Simi-

larly, the compositional range of Mg-value of

pyroxene phenocrysts derived from silicic end-member magma can be represented by that of inner cores of the reverse- and flat-type zoned

phenocrysts. Here, the minimum Mg-value of the core of phenocrysts from mafic magma and the maximum Mg-value of them from silicic magma are designated by the term "semi-criti-

cal values" (Fig. 7).' Then, the Mg-values of

pyroxene phenocrysts in end-member magmas before mixing may be prescribed by the semi-

Table 3. Inffered phenocryst assemblages in end-member magmas

P1: plagioclase 01:olivine Aug: augite Opx:orthopyroxene Mt: titanomagnetite

Qz :quartz Hor:hornblende Mg: Mg-value=10OMg/(Mg+Fe)


critical values ; higher semi-critical value rep-resents the lower limit of the Mg-value of

pyroxene phenocrysts in mafic end-member magma and lower semi-critical value the upper limit of that in silicic end-member magma (Fig.

7). Semi-critical values of Mg-value of pyrox-ene phenocrysts in each andesite can be deter-mined from the compositional zoning profiles

(Figs. 4, 5 and 6 and Table 3).The pyroxenes having Mg-value between

two semi-critical values may be crystallized

from magmas in various advanced stage of

mixing of mafic and silicic end-member mag-

mas. Microphenocrysts and some phenocrysts

with relatively high Mg-value at the inner core

may be grown from the mixed magma.

In the mafic andesites (FG-I-13 and YH-

29), two steps of reverse zoning are found in the

pyroxene phenocrysts with inner core of low

Mg-value (Fig. 6). Such step zoning profiles

may have been formed by complex mixing of

magmas (Sato, 1982) suggesting at least two

events of mixing. Orthopyroxene phenocrysts

having inner core of high Mg-value (70 to 76)

display normal zoning followed by outward

reverse zoning. This suggests that these

phenocrysts is derived from basaltic magma and later affected by injection of more mafic

magma.

2) Plagioclase

Compositional profiles of plagioclase

phenocrysts display step-like variation of Ca/

(Ca+Na) ratio. Both normal- and reverse-zoned plagioclase phenocrysts are always found

within one specimen. In Fig. 8, Ca/(Ca+Na)

ratio of the inner and outer core pair of a

plagioclase phenocryst are given. These data were determined by line scanning or plural

Fig. 8. Ca/(Ca+Na) ratios of the inner and outer core pair of plagioclase phenocryst. Solid circles represent inner cores and open circles outer cores. Numbers are the same as sample Nos. of Tables 1, 2 and 3. Dotted lines indicate the semi-critical values which distinguish between the plagioclase phenocrysts derived from mafic magma and those from silicic magma. See text for details.

476 Keiji Wada

point analyses. Some outer cores of the reverse-zoned phenocrysts do not always repre

sent exactly the peak position of An-content

because of point analyses. It can be seen that

plagioclase phenocrysts with calcic inner core

show normal zoning, whereas those with sodic

inner core show reverse zoning.

Semi-critical values of An-content of

plagioclase phenocrysts derived from two end-member magmas can be determined by the

same method as pyroxene model of Fig. 7, using

the compositional data of Fig. 8 (Table 3).

The An - contents of plagioclase pheno

crysts derived from the mafic magma are

expected to be higher than that of the outer

core of reverse-zoned phenocrysts from the

silicic magma. Therefore, a minimum An-

content of phenocrysts from the mafic end-

member magma (i.e. the higher semi-critical

value) was set up by a value slightly higher than

maximum An -content of the outer core of

reverse-zoned phenocrysts. On the other hand,

the lower semi-critical value was fixed to the

maximum An-content of the inner core of the

reverse-zoned phenocrysts. The plagioclase

phenocrysts having An-contents between the two semi-critical values at the core may be

crystallized from the mixed magma after mix-

ing of the mafic and silicic end-member mag-

mas.

Mixing line of calc-alkalic andesites

Mineralogical evidence such as compo-

sitional zoning profiles indicates that the pheno

cryst phases in talc-alkalic andesites can obvi-

ously be divided into two end-members.

Phenocrysts derived from the mafic end-mem-

ber magma are olivine (except for JA-37), An-

rich plagioclase, with or without Mg-rich

augite and/or Mg-rich orthopyroxene, whereas

those from the silicic end-member magma are

An-poor plagioclase, Mg-poor augite, Mg-poor

orthopyroxene, titanomagnetite, with or with-

Fig. 9. Frequency distribution of Ca/(Ca+Na) ratio of the cores of plagioclase phenocrysts. Solid symbols indicate phena trysts derived from mafic magma, open symbols those from mixed magma, and dotted symbols those from silicic magma. Numbers are the same as sample Nos. of Tables 1, 2 and 3.

out quartz and/or hornblende (Table 3).

Figs. 9 and 10 display frequency distribution of chemical composition of the cores of

plagioclase, olivine and pyroxenes against the semi-critical` Ca/(Ca+Na) and Mg/(Mg+Fe) ratios which distinguish two end-member

phenocrysts. Assuming that these histograms approximate the compositional distributions of all the phenocrysts of plagioclase and pyrox-


Table 4. Mixing index of calc-alkalic

andesites

M: modal abundance of phenocrysts derived from mafic magma

S: that from silicic magma

Pm: modal abundance of plagioclase

phenocrysts derived from mafic magmaPs: that from silicic magma

Fig. 10. Frequency distribution of Mg/(Mg+Fe)

ratio of the cores of olivine, augite and

orthopyroxene phenocrysts. Symbols

are the same as those of Fig. 9. No ana-lyses of olivine from Nos. 4, 6 and 7 have

determined.

enes in a given sample, they represent the ratio

of phenocrysts derived from the two end-mem-

ber magmas. Thus, content of two kinds of

phenocrysts from the mafic and silicic magmas within one thin section can be calculated by

modal analyses (Table 2) and phenocryst

assemblages (Table 3), using the phenocryst

ratio of Figs. 9 and 10.

Here, the ratio of modal abundance of

phenocrysts crystallized from mafic magma to the total phenocryst contents without those

from mixed magma is called a "mixing index"

(Table 4). The modal abundance ratio of cal

cic plagioclase to sodic plagioclase based on the

idea of Sakuyama (1981) are also listed in the

table. If there is no significant difference in

total phenocryst contents between the two end-

member magmas before mixing, then the mix

ing index indicates only mixing proportion of

mafic magma to silicic magma. Sakuyama

and Koyaguchi (1984) estimated the total pheno-

cryst contents in end-member magmas before

mixing on the basis of the negative correlation

between the volume abundances of groundmass

and whole-rock SiO2 content. In the Funagata

volcano, groundmass content (55-75 vol.%)

remains almost constant from tholeiitic basalts

(possible mafic end-member) to calc-alkalic andesites. This implies that the modal abun-

dance of phenocrysts is crudely similar to pre-

mixing end-member magmas.

The relationship of oxide contents versus

mixing index for 10 calc-alkalic andesites is

shown in Fig. 11. The oxide contents show

systematic correlation with mixing index : the

more abundant phenocryst contents from mafic

magma display the more mafic whole-rock

chemical composition. These chemical varia

tions represent the mixing line of mafic and

silicic magmas. It is considered that the rela-

tively dispersive data along the mixing line in

Fig. 11 is due to the low accuracy of calculated

mixing index (MF-15 and US-33) and variabil-

ity of chemical compositions of end-member

magmas.

In a MgO-K2O diagram (Fig. 12), most of

the calc-alkalic andesites from Funagata vol-

478 Keiji Wada

Fig. 11. Oxide contents versus mixing index. The pointed end of connected line with solid circle

indicate abundance ratio of calcic plagioclase to sodic plagioclase, i.e. mixing ratio by Sakuyama (1981)'s method (Table 4).

Fig. 12. MgO-K2O diagram. Double circles indicate calculated composition of silicic end-member

magma by mixing index (Table 6). TH, tholeiitic series ; CA, calc-alkalic series. Dacite

to rhyolite from several volcanoes along the volcanic front of northeast Japan are plotted.

cano group appear to plot on two mixing lines :

mixing-A and -B. The two mixing lines are

constricted at the MgO-rich tholeiitic basalts

which represent the mafic end-member magma.

On the other hand, the two mixing lines toward

silicic composition are separated, which sug-

gests that silicic end-member magmas are

different from each other in K2O contents. As

a test of two mixing lines, the chemical linear-

ity of oxides on the Si02 variation diagram was

ascertained by normal linear regression method

(Table 5). The calculated linear correlation coefficients of oxides seem to be consistent with

mixing line except for TiO2 and A1203 on


Table 5. SiO2-normalized straight line regression (y=a+bx)

Table 6. Calculated chemical com

position of silicic endmember magma

mixing line-A.

Characteristics of end-member magmas

1) Mafic end-member magma

Chemical compositions of end-member

magmas of the mixed andesites are expected to

be the extension of the mixing lines such as

oxides-mixing index (Fig. 11) and MgO-K2O

diagram (Fig. 12). The basalts of the tholeiitic

series represent a mafic end-member magma

from the following reasons : (1) the tholeiitic

lavas are closely associated with the mixed

andesitic lavas in time and space, (2) the

assemblage and chemical composition of mafic

end-member phenocrysts in the mixed an-

desites are similar to those from tholeiitic

basalts, and (3) the mixing lines of whole-rock

composition of the andesites are convergent to

the tholeiitic basalts.

The chemical composition of the silicic

end-member magma will be deduced as follows.

2) Silicic end-member magma estimated by

mixing index

Firstly, the chemical composition of silicic

end-member magma was estimated by using

the mixing index, assuming that (1) the mixing

index is identical with mixing proportion of

mafic magma to silicic magma and (2) the

major chemical composition of mafic end-mem

- ber magma is represented by the tholeiitic

basalts. In the case of the mixed andesites of

YH-29 and FG-I-13, however, the mafic end-

member cannot be fixed in the calculation

because of their complex mixing origin. The

calculated results are listed in Table 6.

Deduced silicic end-member magmas are felsic

andesite to dacite composition.

Assuming from the internal mixing of the

tholeiitic magmas, these end-member magmas

should be differentiated end products of

tholeiitic magmas by substantial amounts of

crystal fractionation (Fig. 12). However, this

model of silicic magma generation by fractional

crystallization of the tholeiitic magma tends to

be denied by Sr-isotopic data (unpublished data

of Kurasawa). The Sr-isotopic ratio of some

calc-alkalic andesites (B7Sr/86Sr=0.704360 to

0.704603, Nos. FG-I-13, YH-29, MF-31 and SJ-

35 which are lying on mixing line-B except for

SJ-35) is slightly higher than that of the

tholeiitic series (B7Sr/86Sr=0.704296 to 0.704362

with exception of 0.704463 from YS-I-7).

Therefore, the silicic end-member magma of

these andesites is expected to be higher Sr-

isotopic ratio than the tholeiitic basalts. This

suggests that the dacitic end-member magma

of the mixed andesites mainly lying on mixing

line-B is not formed by fractional crystal

lization of the tholeiitic magmas. However,

480 Keiji Wada

Sr-isotopic ratio of the mixed andesites lying

on mixing line-A (Nos. KI-20, YS-II-32, US-33

and JA-37) and on mixing line-B (MF-15) is

lower than that of the tholeiitic series. Genesis

of silicic end-member magma of these an

desites is not clear. 87Sr/86Sr ratio from the

Funagata volcano will be discussed in a sepa

rate paper (Wada and Kurasawa).

3) Silicic end-member magma estimated by

glass inclusionSecondly, end-member magmas were

deduced from analyses of glass inclusions in the

phenocrysts. Glass inclusions in liquidus mineral phases can provide important constraint on

the compositions of the magma at the time of

melt entrapment (Anderson, 1974, 1976; Wat-

son, 1976; Dungan and Rhodes, 1978). The

chemical composition of glass inclusions in the

core of reverse-zoned phenocrysts may repre

sent the silicic end component, whereas that

from normal-zoned phenocrysts does the mafic

end component. Glass inclusions in plagio

clase, orthopyroxene, augite and quartz pheno

crysts derived from silicic magma exhibit high-

ly evolved rhyolitic composition, and rare glass

inclusions in mafic end-member phenocrysts

show a basaltic composition (Table 7).

It is noted that the crystallization of the

host phenocryst phase from the trapped melt

Table 7. Selected chemical compo

sition of glass inclusion in

phenocrysts

Fig. 13. Frequency distribution of K2O in glasses included in phenocrysts. Solid reverse

triangles indicate K2O content of host

rocks.

modifies the composition of primary glass inclu

sion (Anderson, 1974). However, selective

enrichment in K2O during entrapment of melt

by growing crystals is shown to be negligible

(Anderson, 1976). K2O contents in the glass

inclusions larger than 10ƒÊm in diameter are

shown in Fig. 13, along with those from dacites

in Usu and Narugo volcanoes on the volcanic

front of northeast Japan. Most of the glass

inclusions range 1 to 2 wt.% K2O, whereas

those from YH-29 on mixing line-B and in

quartz phenocrysts have much higher K2O con-

tent. In US-33, bimodal distribution of K2O

content is recognized by the glass derived from

mafic magma at one extreme (0.1 wt.% K2O)

and that from silicic magma at the other (1.5

wt.% K2O). K2O contents of the glass inclu-

sion except for basaltic glass from US-33 are

always higher than whole-rock K2O contents.

In the case of the dacites from Usu and

Narugo volcanoes, there is no difference in K2O


contents between whole-rock and glass inclu

sion. This implies that the whole-rock compo

sition represents the equilibrium liquid with

phenocrysts. Consequently, these dacites

appear to be no magma mixing origin and

representative of the silicic end-members in the

volcano.

4) Variety of silicic end-member magma

In Fig. 12, silicic rocks of the calc-alkalic

series (SiO2>68wt.%) along the volcanic front

of northeast Japan are plotted in a large

compositional range of K2O content because

probably of a logarithmic concentration of K2O in later stage differentiation (McIntire, 1963) or

difference in degree of partial melting of lower

crust. These silicic rocks occur as large pyro

clastic flow deposits such as Shikotsu, Towada,

etc. or lava domes from Usu, Narugo, Takahara, etc. The compositions of these silicic rocks

lie on the extension of the mixing lines from the

Funagata mixed andesites. In the light of the

foregoing discussion of glass inclusion from

Usu and Narugo, these silicic rocks may repre

sent a possible silicic end-member magma prior

to mixing involved in the formation of calc-

alkalic andesites. As shown in Fig. 14, in the

case of Towada volcano which is composed of

tholeiitic basalts through calc-alkalic andesites

to dacites, if calc-alkalic andesites are magma

mixing origin, the dacites which are a possible

silicic end-member do not lie on crystallization

trend of tholeiitic series but on mixing lines of

the calc-alkalic andesites. This indicates that

the dacites are no end products of fractional

crystallization of the tholeiitic magmas.

If the calc-alkalic andesites from northeast

Japan result from mixing of magmas (Sakuyama, 1981), there are two mixing cases of

end-member magmas: (1)"internal" mixing

by Sakuyama (1981) such as Myoko and Kuro-

hime, and (2) mixing between genetically

different magmas by Eichelberger (1978) such

as Towada and some andesites from Funagata.

Fig. 14. Na20/CaO versus FeOt/MgO relationship of Towada volcano. Solid circles indi

cate tholeiitic rocks, open circles calc

alkalic andesites, and open squars calc

alkalic dacites (Si02>68wt.%). Solid line with arrow represents crystallization

trend of olivine, orthopyroxene, augite

and plagioclase. Solid line with divide

represents possible mixing line of tholeiitic basalt and calcalkalic dacite.

Numerical values are mixing ratio of

basalt to dacite. Whole-rock data are taken from Kawano (1939), Chiba (1966)

and Taniguchi (1972).

Acknowledgements: The author is grate-

ful to Prof. Y. Katsui of Hokkaido University

for his helpful advice and valuable suggestions

in improving manuscript. He is also indebted

to Dr. H. Sato of Kanazawa University for

critical reading of the manuscript and for use of

EPMA. Special thanks are due to Mr. T.

Tohara of Hokkaido University for valuable

discussions and Dr. H. Kurasawa of Geological

Survey of Japan for analyses of Srisotopic

ratio. Thanks are also due to Messrs. K.

Moribayashi and T. Kuwajima of Hokkaido

University for preparing a part of thin sections.

A part of the expense of this study was de-

frayed by the Scientific Grant of the Ministry of

Education of Japan (No. 59740411).

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船形火山カルク・アルカリ安山岩のマグマ混合過程

和田恵治

船形火山カルク・アルカリ安山岩は,① カンラン石と石英が共存する非平衡な斑晶組舎せ,② 斜長石

と輝石の逆累帯構造,③ 斜長石内核の広い組成範囲,④:玄武岩質と流紋岩質ガラス包右物の共存等に

よって特徴づけられ,苦鉄質マグマと珪長質マグマの混合によって形成された。安山岩は,全岩化学組成

においても,MgO-K2O図では2つの混合線上にのり,ハーカー図で直線的な関係にある。混合前の苦鉄質

端成分マグマは,混合線の延長にあるソレアイト系列玄武岩である。一方,珪長質端成分マグマの化学組

成は,安山岩の混合比(珪長質マグマに由来する斑晶量に対する苦鉄質マグマに由来する斑晶量の割合)

から推定され,SiO2=64%と66%, K2O=1.1%と1.8%の珪長質安山岩とデーサイトである。このデーサ

イト質端成分マグは,Sr同位体比から考えると,ソレアイトマグマの結晶分別作用によって形成され得

ないだろう。

magma mixing process of calc-alkalic andesites from

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