American Mineralogist, Volume 67, pages I-13, 19E2

Petrology and geochemistry of calc-alkaline andesite of presumed upper mantle originfrom ltinome-gata, Japan

KeN-IcHrno Aorr euo Hrnorezu Funuerr

[nstitute of Mineralogy, Petrology and Economic GeologyTohoku University, Sendai 980, Japan

Abstract

Itinome-gata and Sannome-gata volcanoes, northeastern Japan, are characterized by theeruption of mafic to salic andesite of the calc-alkaline series and high-alumina basalt, bothof which contain small amounts of high-pressure diopside and forsterite megacrysts,lherzolite and websterite derived from the upper mantle, and gabbro and amphibolitexenoliths from the lower crust (about 20 to 30 km in depth). It seems likely that this is thefirst discovery of upper mantle peridotite xenoliths in calc-alkaline andesite. This andesiteis characterized by presence of hornblende and biotite phenocrysts.

Three representative basalts and five andesites have been analyzed for major elementsby a conventional method, and for rare earth elements and Ba by the isotope dilutionmethod.

Major element variations of the basalt and andesite suite follow a typical calc-alkalinetrend with increasing fractionation. In addition, there are no essential diferences of Ba andREE concentrations and chondrite-normalized patterns between them. Furthermore,5tE775.80 ratios in basalt and the most salic andesite are nearly the same (0.7030 and 0.7033,respectively).

It is possible to conclude that the andesite magmas are produced by fractionalcrystallization of basalt magma based on mineral assemblages, major element variationsand strontium isotope ratios. However, REE and Ba concentrations and patterns are notconsistent with this hypothesis. Accordingly, it is more probable that andesite and basaltmagmas are formed independently by nearly the same degree of partial melting of hydrousand less hydrous parts of upper mantle peridotite, with increasing temperature, at thedepths of 40 to 60 km.

Introduction

Recently, much attention has been paid to theorigin of andesites of the calc-alkaline series, whichare widely distributed in time and space in islandarcs and continental margins. Several new hypothe-ses have been proposed on the basis ofthe results ofexperimental petrology and geochemistry (c/. Allenet al., 1972; Kushiro et a\.,1968; Green and Ring-wood, 1968; Boettcher, 1973; Ringwood, 1974).These include formation of andesite magma by (1)partial melting of hydrous mantle peridotite, (2)partial melting of subducted oceanic crust, (3) mag-ma mixing and (4) amphibole fractionation of basal-tic magma. However, it is very difficult to finddirect evidence to evaluate these proposals.

Itinome-gata volcano in northeastern Japan is oneof the most famous localities of upper mantle and0003-.004)o820102--m0l$02.00 I

lower crustal fragments in Japan (Fig. 1). A largevariety of xenolith types has been studied by manyinvestigators (Kuno, 1967; Kuno and Aoki, 1970;Aoki, l97l; Aoki and Shiba, 1973a, b, 1974a, b:Takahashi, 1978, 1980; Zashu et a\.,1980; Tanakaand Aoki, 1981), though detailed petrographic andgeochemical studies of host rocks have been ne-glected. Recently, Katsui and Nemoto (1977) andKatsui et al. (1979) carried out field work in theItinome-gata district and found that the xenolith-bearing volcanic rocks are calc-alkaline andesite atItinome-gata and silica-undersaturated basalt atSannome-gata.

Our purpose in this paper is to describe themineralogy, petrography and REE geochemistry ofhigh-alumina basalt and calc-alkaline andesite fromthe Itinome-gata district and to discuss the origin ofthese rocks.

AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE

Fig. l. Map showing the location of ltinome-gata and Sannome-gata, northeastern Japan.

Occurrence and petrography

Three maars or craters, Itinome-gata, Ninome-gata and Sannome-gata, all of which formed about10,000 years ago, iire located on the west coast ofnortheastern Japan (Fig. 1). The basement underly-ing the Itinome-gata district consists of a complexof Cretaceous to Paleogene salic plutonic and vol-canic rocks and Neogene volcanics and sediments.According to the field work of Katsui et al. (1979),eruption took place in three distinct stages separat-ed by intercalated soil zones. The first stage beganwith the destruction of near-surface rocks andopening of the Itinome-gata and Ninome-gata ventsby steam explosions. This activity was followed bythe eruption of gray- to white-colored andesiticpumices and abundant accidental upper crustal

fragments, which are represented by mud flow andbase-surge deposits. All xenoliths derived from theupper mantle and lower crust occur as ellipsoidalbombs or as loose blocks. within the andesiticpumices, up to 30 cm in diameter. Megacrysts ofdiopside (less than 5 cm in size), which may containeuhedral forsterite crystals, occur also as crystallapilli or as "xenocrysts" in the host rocks. In thethird stage, basaltic scoria falls were erupted fromthe Sannome-gata crater; these also contain smallamounts of deep-seated xenoliths.

Basalt scoria from Sannome-gata usually shows aporphyritic appearance due to the presence ofabun-dant quartz and feldspar xenocrysts derived fromgranitic basement rocks. Under the microscope, thecommonest phenocrysts are olivines, which occupyless than 5%by volume, and range up to 0.8 mm in

HOKKAIOO

s

AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE

length but are mostly less than 0.4 mm. They areeuhedral tabular or elongated parallel to the ccrystallographic axis, showing weak zoning at theperipheral margins of each crystal. Clinopyroxenephenocrysts are much less common than those ofolivine, being subhedral, short, and prismatic inform and up to 0.3 mm in length. Weak sectorzoning is always seen. The groundmass is ratherfine-grained and hypocrystalline with numerousvesicles. It consists of brown glass, plagioclaselaths, subhedral clinopyroxene, and euhedral oliv-ine, in decreasing order of abundance. Plagioclasexenocrysts have an irregular anhedral form, up to 3mm in size. All of the crystals of plagioclase showmottled appearance due to glass inclusions in themargins, and are always immediately surroundedby calcic rims of groundmass plagioclase composi-tion. Quartz xenocrysts are irregular in crystaloutline. They are surrounded by thin rims com-posed of minute acicular clinopyroxene. Olivinexenocrysts, presumably derived from dispersed pe-ridotite, are sometimes recognized. They are irregu-lar in form, less than 3 mm in grain size, andinvariably homogeneous. Accordingly, it is veryeasy to distinguish between phenocrystic and xeno-crystic olivines by their grain size and crystal form.

Andesite pumices are megascopically gray inmafic rocks to white in salic rocks and show aporphyritic appearance which is due to the presenceof abundant quartz and plagioclase xenocrysts.Phenocrystic minerals are plagioclase (up to 4 mmin size), olivine (0.6 mm), clinopyroxene (0.8 mm),green hornblende (1 mm), biotite (2 mm) and mag-netite (0.5 mm). The modal volume of olivine andclinopyroxene gradually decrease and these ofhornblende and biotite increase with increasingSiO2 content. In addition to these minerals, smallamounts of quartz (0.5 mm) and apatite are found asphenocrysts in salic rocks. All of the andesitescontain small to negligible amounts of mafic xeno-crysts derived from the upper mantle and lowercrust. The mafic phenocrysts can be distinguishedfrom xenocrysts by their crystal outline: the formerare euhedral and smaller in size, while the latter areirregular, broken and homogeneous. No phenocrys-tic or groundmass orthopyroxenes have been identi-fied in any basalts and andesites.

Chemistry

Analytical methods and results

Because all the rocks contain abundant xeno-crysts, it is difficult to interpret whole rock analy-

ses. Therefore, scoriaceous and pumiceous sampleswere carefully crushed to pass through a 48- or 100-mesh sieve. They were washed with distilled waterand dried. Most xenocrysts and phenocrysts sepa-rate easily from the soft and loose groundmass. Thegroundmass was separated in the Frantz isodynam-ic separator. Finally, the light fraction (specificgravity is less than 2.5) was floated on dilutedmethylene iodide. The most salic pumice with lessabundance of accidental fragments was purified byelimination of impurities under the binocular micro-scope.

Major element compositions were determined bywet chemical methods for silicate analysis com-bined with flame photometric and atomic absorp-tion spectroscopic techniques. Mineral analyseswere done on a Hitachi scanning electron-probemicroanalyzer, Model X-560S, using an energy-dispersive analytical system (Kevex Corporation).Synthetic and natural minerals were used as stan-dards. Detailed procedures and accuracy of analy-sis by the energy-dispersive analytical system aregiven by Fujimaki and Aoki (1980).

The determination of Ba and rare earth elements(REE) was made by the technique of stable isotopedilution using a JMS-05RB type, 30 cm radius massspectrometer of the Geological Survey of Japan.Precision of the analysis is better than 2 percent.Analytical data are shown in Tables 1 to 3.

Rock composition

The rocks show a restricted range in major ele-ment composition. SiO2 contents range from 51 to61 percent, and the other components graduallyincrease or decrease systematically with SiO2 varia-tion. Comparing the basalts from Itinome-gata dis-trict with those from other localities of Japan, theformer exhibit characteristics intermediate betweentypical tholeiites and alkali basalts and resemblehigh-alumina basalts from Izu-Hakone region(Kuno, 1960). They are characterized by rather highAl2O3, MgO, and total alkalies and low total FeO.In detail however, there are differences which maybe significant. Most high-alumina basalts have K2Ocontents of less than 0.8Vo. The average Itinome-gata basalt composition has a higher K2O content ofl.3Vo comparable to that of alkali basalts fromsouthwest Japan.

Although these basalts contain neither normativequartz nor nepheline (normalizing Fe2O3lFeO +Fe2O3 to 0.2-0.15), all of the andesites are saturatedin silica, and show many similarities with typical

AOKI AND FUTIMAKI: CALC-ALKALINE ANDESITE

Table l Chemical analyses and norms (water-free basis) ofxenolith-bearing basalts and andesites from Itinome-gata district

and calc-alkaline series of Nasu zone (outer volcan-ic zone), northeastern Japan. All the rocks plotbelow the fields of the calc-alkaline series of Nasuzone in spite of being in the same rock series, andconsequently trend from the MgO-(FeO + Fe2O3)side line toward the apex of alkalies, following thetypical course of island arc calc-alkaline series.

REE and Ba

Chondrite-normalized Ba and REE patterns forone whole rock and eight groundmass analysesfrom the Itinome-gata district are shown in Figure4. It is emphasized that none of the patterns arecorrelated with either SiO2 or K2O contents. Thevery marked enrichment of light REE (Sm to La) isnotable and the rocks are essentially indistinguish-able from one another, showing the typical abun-dances found in calc-alkaline rocks (Jakes and Gill,1970). Total REE abundances and LaNb ratios inthese rocks vary from 98 ppm to 118 ppm and 7.1 to9.1 respectively, with weak negative anomaliesevident in only three patterns. There is a slightdifference between patterns of the most salic andes-ite and its groundmass (Nos. 9 and 8, Fig. 4); theformer shows weak positive Eu anomaly due toaccumulation of some plagioclase phenocrysts.

Plagioclases

The compositional range of groundmass plagio-clase in the basalts is AnTe.aAbzo.rOro.r to An63Ab31Or1 whereas the cores and narrow margins ofplagioclase phenocrysts in the andesites are An65Ab33Or2 to An32Ab63Ors. It is clear that the mostsodic margins of the former are still less sodic thanthe cores of the latter. The compositions of thecores of phenocrysts tend to be more sodic withincrease in SiO2 content of the host rocks, but thesodic margins are not strikingly different from oneanother. The Or content increases from 0.3 to 6.9with increase in the Ab content (Fig. 5).

Olivines

Phenocrystic olivines in the basalts and andesitesrange in composition from cores of Fo6.3 and Foes 2to rims of Fosa.2 and Fos2.3 respectively, and euhe-dral olivine in diopside megacrysts from the andes-ite is Fos7.3. On the other hand, groundmass oliv-ines in basalts are Fos6.a to Fo7s.5. All the olivinesare characterized by very high Fo contents and anarrow compositional range, regardless of theiroccunence and bulk rock composition (Fig. 6). Aspreviously mentioned, basalts and andesites con-

s i 0 2 5 0 . 9 5 5 9 . 7 2

T i 0 2 0 . 9 6 I . 0 4

A 1 2 0 3 t 6 7 6 t 7 . 0 2F e 2 0 3 2 . 2 1 1 . 9 2

F e O 5 . 9 9 6 . 1 2

l ' ,1n0 0 .18 0 .18

l , fSo 8 .25 7 .84

Cao 9 .53 9 56

Nazo 2 .58 2 .66

K z o l . 3 l 1 . 3 0

HzO 0 .88 0 .99P z 0 s 0 . 2 0 0 . 1 9Tota l 99 .80 99 .54

5 0 . 5 5 5 3 . 8 7

l 06 0 .89

1 7 . 2 0 1 6 . 1 92 . 5 8 3 . 0 75 . 8 5 4 . 7 7

0 . t 9 0 . 1 3

6 82 6 ,41

9 . 5 9 I 4 4

2 . 4 6 2 5 2

1 . 4 3 1 . 9 3

l 8 0 0 . 9 6

0 . 2 1 0 . 3 199.74 99 .49

54.65 58 .84 59 .29

0 . 7 2 0 . 7 1 0 . 7 1

1 8 . 7 4 1 6 . 3 5 1 6 . 5 9

2 . 4 4 2 . 4 3 2 . 5 1

3 . 8 0 3 . 4 0 3 . 2 1

0 . 2 5 0 1 6 0 . 1 6

4 . 5 0 3 . 3 7 3 . 5 2

7 . 0 4 7 2 2 6 . 6 9

3 . 1 0 2 . 8 6 3 . r 32 . 0 t 2 . 7 0 2 . 5 8

2 . 2 7 1 . 3 8 1 . 3 3

0 . 2 4 0 . t 6 0 . I 6

9 9 . 7 6 9 9 . 5 8 9 9 . 8 8

59 73 60 .65

0 . 6 1 I . t 2

t6 68 ',|6 4l

1 . 6 5 I 7 3

3 . 0 1 2 . 8 1

0 . 1 5 0 . 1 5

3 . 4 5 3 . 3 5

5 . 2 7 5 . 5 7

3 . 1 9 3 3 8

2 . 9 7 2 . 2 9

2 . 3 4 2 . 0 5

0 . 3 1 0 . 2 9

99.40 99 8 l

a0r

Ab

An

l H ou r

i L nI t s

EnH v t -

f s

o , , ' oFa

Mt

I I

Ap

I G r o u n d n a s s o f a u g i t e - b e a r i n g o l i v i n e b a s a l t ( 7 7 7 2 9 0 1 )

2 G r o u n d n a s s o f a u g i t e - b e a r i n g o l i v i n e b a s a l t ( 7 7 7 2 9 0 1 ' J

3 . G r o u n d m s s o f a u g i t e - b e a r i n g o l i v i n e b a s a l t ( 7 7 9 0 3 0 2 )

4 . G r o u n d m s s o f h o r n b l e n d e b i o t i t e a u g i t e o l i v i n e a n d e s i t e ( 7 7 9 0 3 0 3 )

5 . G r o u n d r u s s o f o l i v i n e a u g i t e b i o t i t e h o r n b l e n d e a n d e s i t e ( 7 7 9 0 3 0 4 )

6 , G r o u n d m s s o f a u g i t e , o l i v i n e a n d q u a r t z - b e a r i n g b i o t i t e h o r n b l e n d ea n d e s i t e { 7 7 9 0 3 0 1 )

7 . G r o u n d m a s s o f a u g i t e , o l i v i n e a n d q u a r t z - b e a r i n g b i o t i t e h o r n b l e n d ea n d e s i t e ( 7 7 9 0 3 0 5 )

8 , G r o u n d m s s o f o l i v i n e , a u g i t e a n d q u a r t z - b e a r i n g h o r n b l e n d e b i o t i t ea n d e s i t e ( 7 7 7 2 7 1 0 )

9 . 0 l i v l n e , a u g ' i t e d n d q u a r t z - b e a r i n g h o r n b l e n d e b i o t i t e a n d e s i t e\ 7 7 7 2 7 1 0 ) , w h o l e r o c k a n a l y s i s

Nos I -3: Scoria fron Sannome-gata, Nos. 4 - 9: Pumice frcm l t inore-gata

calc-alkaline andesites from island arcs or continen-tal margins.

In Figure 2, the variation of main oxides againstSiO2 contents of basalts and andesites is plottedalong with the average variation curve of volcanicrocks of the Chokai zone (high-alumina basaltzone). The Itinome-gata volcano is a part of thisvolcanic zone of northeastern Japan (Fig. l). TotalFeO, MgO and CaO decrease gradually from basaltto salic andesite while Na2O and K2O increase, andit is noticeable that the basalt-andesite suite of theItinome-gata district is clearly higher in MgO andKzO and lower in total FeO and Na2O than theaverage curves.

The data are also plotted in a MFA [MgO-(FeO +FezOr)-(Na2O + K2O)l diagram (Fig. 3) togetherwith the fractionation trends of the tholeiitic series

0 . 5 4 5 . 6 0 6 . 5 0 1 2 . 9 4 1 2 . 7 6 1 3 . 4 7 1 5 . 1 3

7 . 7 9 8 . 6 3 1 t . 5 8 1 2 1 9 1 6 . 2 5 1 5 . 4 7 1 8 . 0 9 1 3 . 8 0

? 2 . 8 6 2 1 . 2 3 2 1 . 6 5 ? 6 . 9 0 2 4 . 5 9 2 6 . 9 0 2 7 . 4 4 2 9 . 2 6

3 1 . 1 0 3 2 . 3 5 2 7 . 5 6 3 2 . 0 7 2 4 . 2 5 2 3 . 9 3 2 3 . 0 9 2 3 . 3 9

6 . 6 1 6 . 1 9 5 . 3 9 0 . 8 8 4 . 6 8 3 . 6 6 0 . 7 2 1 . 2 3

4 . 3 3 4 - 2 8 3 . 7 6 0 . 5 9 3 . 1 0 2 . 5 2 0 4 8 0 . 8 8

1 - 8 2 I 4 i 1 . 1 7 0 . 2 2 1 . 2 4 0 . 8 4 0 . 1 8 0 . 2 4

1 0 . 9 4 1 3 . 0 5 1 2 . 4 5 t 0 9 1 5 . 4 4 6 . 3 6 8 . 3 8 7 . 6 8

4 . 6 3 5 . 9 5 3 . 8 9 4 . 1 3 2 1 9 2 1 5 3 . 3 4 l 9 9

3 l 91 4 8

2 . A 2 3 . 8 2 4 . 5 1 3 . 6 3 3 . 5 9 4 0 2 ? . 4 8 2 . 5 0

2 . 0 2 2 . 0 5 l . 7 l 1 . 4 1 1 . 3 7 1 . 3 7 1 . 2 0 2 . 1 9

0 . 4 4 0 . 5 0 0 . 7 4 0 . 5 1 0 . 3 7 0 . 3 7 0 . 7 7 0 . 7 1

7 . 7 9

22.0730.62

6 . 6 24 . 4 3

1 . 7 0

12.64

4 . 8 82 . 6 0

r . l l

3 . 2 4

I .84

o . 4 7

AOKI AND FUJIMAKI: CALC.ALKALINE ANDESITE

Table2. Ba and REE abundances (ppm) in xenolith-bearing basalts and andesites from ltinome-gata district

J B - I * Leedeychondr i te**

6l605954si02 %

l l 3 3 9 3 3

2 1 . 7 2 4 . 3

4 5 . I 4 8 . 4

2 0 . 5 2 2 . 9

4 . 4 6 4 . 6 6

0 . 9 8 0 I . 2 1 4

4 . 6 5 5 . 1 9

4 . 7 4 5 . 0 1

2 . 5 4 2 . 7 0

2 . 5 0 2 . 6 7

0.403 0 .383

8 9 1 l l 7 5'18 .

9 23 .2

43.5 46 .7

2 4 . 6 1 9 . 4

5 . 7 9 3 . 8 6

1 . 3 3 4 . l . 2 3 8

5 . 3 7 3 . 9 3

4 . 6 7 3 . 9 3

2 . 7 9 2 . 5 7

z . b o t . J s

0.388 0 .394

I 040 1243 I 754

19.2 20 .1 23 .4

3 9 . 0 4 0 . 3 5 0 . 0

2 1 . 3 2 1 . 4 2 2 . 0

4 . 6 0 4 . 6 1 4 . 2 0

1 . 3 2 1 I . 3 7 0 1 . 2 9 6

4 . 0 0 4 . 0 5 4 . ? 0

4 . r 4 4 . 5 8 4 . 7 Q

2 . 4 0 2 . 7 4 2 . 8 9

? . 2 1 2 . 4 0 ? . 5 6

0 . 3 4 ? 0 . 3 7 9 0 . 3 9 1

20.2 36 .9 0 .378

3 9 . 1 6 7 . 2 0 . 9 7 6

1 9 . 9 2 6 . 7 0 . 7 1 6

4 . 2 9 5 . 1 9 0 . 2 3 0'1 .639 . l .553 0 .0866

4 . 4 2 4 . 8 1 0 . 3 1 1

3 . 9 2 4 . 1 8 0 . 3 9 0

2 . 4 2 2 . 3 4 0 . 2 5 5

2 . 2 4 2 . 1 6 0 . 2 4 9

0.330 0 .307 0 .0387

5 l 6861 4 . 2 1Ba

LA

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

Rb***

Sr8 7 8 6

Sr /Sr

6 5 . 0

581

0. 7030

39

469

0 . 7 0 3 1

8 7 . I

428

0. 7033

* Geological Survey of ,rapan geochemical standard.* * Masuda e t aJ . (1973) fo r REE and Nakamura (1974) fo r Ba.

* * * Zashu e t a t . (1980) .

tain abundant xenocrystic olivines dispersed fromupper mantle peridotite xenoliths. Accordingly, it issuggested that Mg-rich cores of phenocrysts are ofxenocrystic origin, and the slightly Fe-rich marginsare overgrown around the former after capture inmagmas. However, there are some chemical differ.ences between such xenocrysts and phenocrysts;the latter are poor in NiO (0.40 to 0.28 and 0.33 to0.l8Vo, respectively) and rich in CaO (less than 0.15and 0.23 to 0.15%0, respectively) when Fo contentsare the same (Foeo to Fosa).

Clinopyroxenes

As is clear from Table 3 and Figure 6, clinopyrox-enes show a rather wide range of composition. Thecation ratios of Ca:Mg:Fe are 45.3:46.7:8.0 to48.1:41.2:10.7 in basalt phenocrysts and44.8:45.4:9.8 to 40.0:41.8:18.2 in andesite pheno-crysts, whereas those of basalt groundmass are47.3:41.9:10.8 to 46.9:39.2:13.8. Al l of the pyrox-enes, like the olivines, are highly magnesian.

The most striking feature of the clinopyroxenes isthe variation of Al2O3 Q.60-9.95%) and SiO2(42.78-46.93Vo), which are reciprocal in relation toeach other. Accordingly, calculated Tschermak'scomponents range from 5 to 18 mol percent, withthe majority falling within the range 1l to 15 per-cent. Clinopyroxene phenocrysts can be distin-

guished easily from dispersed xenocrysts ofperido-tites by higher AlzO: and total FeO and lower MgOand Cr2O3 in the former. High-pressure diopsidemegacrysts fall within the chemical range of clino-pyroxene phenocrysts, but there are clear differ-ences between them. The former are easily distin-guished also from the latter by being very homoge-neous, by having no perceptible zoning within asingle crystal, and by their slightly Mg-rich and Ti-poor nature.

Hornblendes

Hornblendes are characterized by a wide varia-tion of all major components except that CaO:SiO2and MgO tend to decrease, whereas Al2O3 and FeOincrease with increasing SiO2 content of the hostandesites. The Mg value (l00Mg/Mg + Fe + Mnratio) shows a range from 78 to 66 in mafic andesiteand 62 to 56 in salic andesite. Although these Mgvalues of hornblendes in andesites are clearly lowerthan those of pargasites in peridotite xenoliths (89to 73), there is an overlap between hornblendes inandesites and in gabbro xenoliths; the Mg values ofthe latter types range from74 to 59 (Fig. 6).

The relationships among calciferous amphibolesolid solutions are graphically expressed in a Alrv-(AlvI + Fe+3 * Cr * Ti) and Afv-(Na + K)diagram (Deer et al., 1962). The diagram (Fig. 7)

AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE

Olivires

Table 3. EPMA analyses of minerals shows that the hornblendes in the andesites plot inthe field of common hornblende, but are separatedfrom the pargasites in peridotite and hornblende ingabbro xenoliths by their lower contents of parga-site and tschermakite molecules.

Biotites

Biotites are comparable in composition to thosefrom common calc-alkaline andesites and dacitesbut have higher TiO2 contents. The Mg values ofbiotites range from 59 to 54, which are the lowestamong coexisting mafic silicates.

Magnetites

Homogeneous magnetite phenocrysts consistmainly of Fe2O3 and FeO associated with smallamounts of TiO2 and MnO. The magnetites showsignificant differences in composition when com-pared with those of calc-alkaline andesites withsimilar SiO content in the Nasu zone (TiO214.61,Al2O3 2.02, Cr2O3 0.45, V2O3 1.62, Fe2O3 36.69,FeO 42.67, MnO 0.66, MgO 1.05, total99.77). Theformer are characterized by higher Fe2O3 and MnOand lower TiO2, Al2O3,YzOz, FeO and MgO, beingcomposed of 90 moleVo magnetite, 87o ulvospineland 2%o spinel molecules; the latter magnetitesconsist of 55% magnetite, 4lVo ulvdspinel and 4Vospinel.

Discussion

Tremendous amounts of calc-alkaline andesitesare distributed in island arcs and continental mar-gins. To elucidate the origin of these rocks is one ofthe most important problems in igneous petrology,and various hypotheses have been proposed duringthe past five decades. For example, andesite mayrepresent (1) residual liquid from fractional crystal-lization of basalt magma (anhydrous mineral frac-tionation, magnetite fractionation and amphibolefractionation), (2) contamination of granitic rocksby basalt magma, (3) partial or complete melts fromthe lower crust, (4) basalt and salic magma mixing,(5) partial melts of oceanic crust in subductionzones, and (6) partial melts of hydrous upper man-tle. In view of the large variety of mineral assem-blages, mineral compositions, and bulk rock com-positions among andesites, any single hypothesiscannot apply to all calc-alkaline andesite magmas.Recently, amphibole fractionation, magma mixing,partial melts of oceanic crust and partial melts ofhydrous mantle peridotite have been favored bymany petrologists and geochemists.

4

Ph-c Ph-M Gr-c Gt-M ilega Ph-c Ph-M Ph-c Ph-M

9

s i02 41 .02

F e o * l l . l 8Mno 0 .21MSo 48.06

N i o 0 . 2 5

Cao 0 . , ] 9

Tota l 100.91

39.98 40 .24' 1 4 . 6 8

1 2 . 8 3

0 . 2 4 0 2 l44.51 46.45

0 . r 9 0 . r 9

0 . 2 3 0 . 2 1

99.89 99 .94

39.09 40 .24

1 7 . 1 7 1 1 . 9 4

0 . 4 4 0 . 2 142.80 47 .O2

0 . 2 5

0 . 2 0 0 1 5

99. 70 99 .81

4 0 l 0 3 9 . 9 2

I 3 . 0 7 I 5 . 0 4

0 . 2 0 0 2 846.23 44 .57

0 . 1 7 . 0 . 2 3

0 1 6 0 2 l

9 9 . 9 3 1 0 0 . 2 6

40.90 39 .94

9.43 ' , |5 .71

0.09 0 22

48.92 44 .31

0 . 2 9

0 . t 5 0 . 1 6

9 9 . 7 9 1 0 0 . 3 5

Fo rc l Z 88 .3 84 .2 86 4 81 .3 A7 4 86 .2 83 .8 90 .2 82 .3

Clinopyroxenes

Ph-C Ph-M Gr-C Gt-M Mega Ph-C Ph-M Ph-M Ph-C

s i02 52 .78 49 .45 50 .16 48 .46T i 0 2 0 . 2 7 0 . 6 6 0 . 6 9 l . t 04 1 2 0 3 2 . 6 0 5 . 6 3 6 . 0 1 7 . 0 6Cr2O3 0 41 0 .26 0 .42 0- '17Feo* 4 .91 6 36 5 .41 7 .72r ' 1 n 0 0 . 1 4 0 . 1 5 0 . 1 4 0 . 1 7t ' fg0 15 .14 13 .79 14 .22 13 .20Cao 21.76 22 .4o 22 .34 21-82Na20 0 .62 0 .34 0 .37 0 4 lT o t a l 9 9 . 6 3 1 0 0 . 0 4 1 0 0 . 7 4 1 0 0 l l

5 l . l l 5 2 . 5 1 4 6 9 3 5 1 . 3 4 4 9 . 1 00 . 3 6 0 . 3 0 1 . 2 7 0 6 1 0 . 7 35 . 3 1 4 . 3 1 9 . 9 5 3 . 7 0 6 0 70 . l t 0 . 1 3 0 . 1 45 . 2 7 5 . 8 8 7 . 6 7 1 0 . 7 2 7 . 5 60 . 1 0 0 . 1 2 0 . l t 0 . 3 6 0 . 1 7

1 5 1 3 1 5 . 5 6 1 2 . 7 1 ' t 4 . 2 9 1 4 . 2 6

2 2 . 0 1 2 1 . 3 4 2 1 . 5 9 1 9 . 0 1 2 t 6 90 . 5 7 0 . 4 9 0 . 3 6 0 s 0 0 . 4 0

9 9 9 7 1 0 0 . 6 4 1 0 0 . 7 3 1 0 0 . 5 3 1 0 0 . 1 4Ca a ton g 45 .2 48 .0 47 .3 47 . ' ll 4 s 4 6 . 6 4 l . l 4 1 9 3 9 . 6F e 8 . 2 1 0 . 9 1 0 . 8 i 3 . 3

46.6 44 .8 47 .6 40 .0 45 .74 4 . 5 4 5 . 4 3 9 . 0 4 t I 4 1 . 8

8 . 9 9 . 8 1 3 . 4 1 8 . 2 1 2 5

l 4

Gr-C Gr-M Ph-C Ph-M

9

S i 0 2 4 8 . 8 54 1 2 0 3 3 1 . 8 2Feo* 0.86c a o 1 6 . l l

Na20 2 .28

KzO 0 .05

Tota l 99 .79

50.40 54 . 5 129.42 28 .99

0 . 9 61 4 . s 6 1 r . 0 4

3 . 6 4 5 . 0 2

0 . 2 3 0 . 1 7

99.21 99 .73

6 l . 51 57 .822 3 . 9 9 2 5 . 4 7

0 . t 9

6 . 7 2 9 . 2 37 . 0 3 5 . 8 8

0 . 6 5 0 . 4 3

99.90 100 03

60.0024.62

0 . 1 76 . 7 5

7 .60

0 6 1

99 76

A n r c l Z 7 9 . 4 6 8 . 0 5 4 . 3 3 3 . 2 4 5 . 3 3 1 . 8

EortuTendes, bioxites and nagDexites

4

EO BA MX

s i02 44 .05

T i 0 2 l . 1 2At 203 I 3 . 76

Cr203 0 . I 7

VzogFe203*"

Feo 9 .45

l,tln0 0. 14

l |so 15 .14

Cao I I .80

N a 2 0 1 . 7 2

KzO 0 .71

Tota l 98.06

47.53 37 35

0 . 8 0 3 . 8 6 2 . 8 4

7 . 6 2 1 4 . 3 2 0 . 7 9

0 . 3 56 l . 7 1

1 4 . 7 4 I 7 . 0 5 3 1 . 3 7

1 . 3 2 0 . 7 8 I . 8 3

1 2 . 5 1 1 3 . 0 4 0 . 2 0' l

l .54 0 .20

t . l 3 0 . 3 9

0 . 6 2 8 . 7 4

9 7 . 8 1 9 5 . 7 3 9 9 . 0 9

3 7 . 3 0

3 . 7 1' t4 .5 l

I 7 . 0 0

0 . 7 5

1 3 . 2 1

0 . 1 6

0 . 6 78 . 7 4

9 6 . 1 5

2 . 4 9

0.840 . 2 0

0 . 3 8

61.923 t . 2 6

2 . 3 3

0 . 1 5

99.97

Ul vosp.r c l %

. 5 5 7 . 0 58. r 56 .68 . 28 . 3

* Total i rcn as Feo. ** Calculated as Feo and Fe203 assuming stoichionetryph, Phenocryst, 6rr grcundmass, c; core tr , margin, uega,. megacryst,

E vaJue; 100 Mg/Mg + Fe + liln.

AOKI AND FUJIMAIQ: CALC-ALKALINE ANDESITE

AlrO3 .

8- ---:- --- - -- -'--t--

--- -

o B a s a l to A n d e s i t e

First, the following petrographic and geochemicalfeatures of basalts and andesites of the Itinome-gatadistrict must be emphasized.

1. The order of beginning of crystallization ofphenocrystic minerals in the basalt-andesite suite,indicated from the mineral assemblage, crystalform, and Mg-Fe partitioning, is Mg-rich olivine --->

Mg-rich clinopyroxens + plagioclase --+ magnetite+ hornblende + biotite + quartz. This crystalliza-tion sequence agrees well with that commonlyobserved in the calc-alkaline series except thatthere is no appearance oforthopyroxene, which is acommon mineral in the hornblende- and biotite-freecalc-alkaline series. However, hornblende- or horn-blende and biotite-bearing andesite magmas, includ-ing Itinome-gata, sometimes do not have orthopy-roxene at their liquidus, the reasons for which arenot apparent.

2. Basalt and andesite contain upper mantle peri-dotite and websterite, lower crustal gabbro andamphibolite fragments, anhedral high-pressure di-opside, and euhedral forsterite megacrysts. Asshown by many petrologists, upper mantle perido-tite xenoliths composed of olivine, enstatite, diop-side and chromian spinel with or without pargasiteare usually incorporated in alkali basalts and related

CaO

rocks only. In extremely rare cases, olivine tholei-ites include such peridotite xenoliths. High-pres-sure megacrysts also occur with peridotite xeno-liths. Xenolith- and megacryst-bearing calc-alkaline

Fig. 3. MgO-Feo* (FeO + Fe2O3)-Na2O * K2O diagram. T:differentiation trend of the tholeiite series of Nasu zone,northeastem Japan, C: calc-alkaline series of Nasu zone(Kawano and Aoki, 1960; Kawano et al.,196l).

um

1 0

6

1 0

I

Io

ta

Mgo

50 60 65% 50 55 60 6Fv.sio, !s io,

Major oxides-silica variation diagram. Dashed lines are average variation curves of volcanic rocks of Chokai zone (Aoki'

5 5

Fis.2.1978).

C----------__. : - -____

FeO* o.

FeO'

o H i g h - a l u m i n a b a s a l t. C a l c - a l k a l i a n d e s i t e

AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE

o

!co3o

:ooc

Fig. 4. Chondrite-normalized (Leedey chondrite in Table 2)plot of Ba and REE contents. Numbers are the same as those forTable l.

andesites have only been found in Itinome-gatavolcano and nowhere else in the world. Accordingto Kuno (1967), Aoki and Shiba (1973a) and Araiand Saeki (1980), Itinome-gata and Sannome-gataperidotite xenoliths are characterized by the pres-ence of calcic plagioclase associated with spinel-pyroxene symplectite without exception, in addi-tion to the ordinary spinel peridotite mineral assem-blage, presumably because of the relatively thincrust in this district. Furthermore, Takahashi (1980)has demonstrated that some of the Itinome-gataperidotites do not show single PIT conditions ofequilibration but record complex thermal historiesbetween 800'C and 1,000" C from long residencetime in the upper mantle before transportation tothe surface. Estimated thickness of the crust basedon petrological and geophysical data is indicated tobe about 20 to 25 km. In addition, the depth of theWadati Benioff zone beneath this district is estimat-ed to be about tr50 km by the observation ofmicroearthquakes (Hasegawa et al., 1978).

3. Chemical compositions of basalts are transi-

tional between tholeiite and alkali basalt, and agreewell with the nature of high-alumina basalt (Kuno,1960), whereas the andesites are calc-alkaline. Ma-jor components vary rather smoothly from basalt tothe most salic,andesite without any abrupt changewith increasing SiO2. Chondrite-normalized REEand Ba abundances show no major differencesamong all the volcanic rocks. Sr87/Sf6 ratios areconstant at 0.7030-0.7033 for basalt and salic andes-ite, being lower than those of incorporated perido-tites (one peridotite is 0.7030 and three are 0.7044-0.7053), hornblende gabbro (0.7041-0.7048) and py-roxene gabbro (0.7035-0,7039) (Zashu et al.,1980).Furthermore, Tanaka and Aoki (1981) have report-ed the REE and Ba abundances of nine peridotites,two hornblende gabbros, two pyroxene gabbros,and two amphibolites from Itinome-gata. The REEabundances and normalized patterns of peridotitesreflect the degrees of basaltic component extrac-tions; namely, undepleted peridotites have the high-est concentration and a convex-upwards pattern,whereas Mg-rich and depleted ones have the lowestconcentration and a V-shaped pattern. The patternsof calculated liquid phases extracted from these

A b A b

Fig. 5. EPMA'analyses of feldspars in mole percent anorthite(An), albite (Ab) and orthoclase (Or). Gabbro: plagioclases inhomblendite-hornblende gabbro xenoliths from Itinome-gata(Aoki, l97l).

A n A n

AOKI AND FUJIMAIil: CALC-ALKALINE ANDESITE

Fig. 6. EPMA analyses of pyroxene, olivine and calciferous amphibole in atomic percent Ca, Mg and Fe. Peridotite:clinopyroxene, olivine and calciferous amphibole in peridotite xenoliths from Itinome-gata (Aoki and Shiba, 1973a, b, 1974b),Gabbro: clinopyroxene, olivine and calciferous amphibole in hornblendite-hornblende gabbro-pyroxene gabbro xenoliths (Aoki,

r97r).

peridotites difer from those of the host basalt andandesite and resemble those of island arc, low-alkalitholeiites, which are characteized by LREE-de-pleted and flat patterns, from the Pacific side ofJapan. All the mafic xenoliths have similar tholeiiticpatterns. These Sr isotope and REE data indicateno genetic relation between basalt and andesite andtheir incorporated ultramafic-mafic xenoliths.

Because they contain upper mantle xenoliths, thephenocryst assemblage and chemical compositionsof the basalts and salic andesites must have beendetermined in the uppermost mantle, correspondingto a depth at least 20 to 25 km from the surface.They must have ascended directly and rapidly tothe surface without compositional change by frac-tionation at shallower levels. Furthermore. there isno direct seismologic evidence of partial melting atthe subducted oceanic crust (Hasegawa et al.,1978). Accordingly, only two possibilities explainthe origin of these ltinome-gata magmas: one ishigh-pressure fractional crystallization includingamphibole fractionation from basalt magma to pro-duce andesite melt, and the other is partial meltingof mantle peridotite to produce basalt and andesitemagmas under dry and wet conditions, respective-ly, from the same source region.

Recent experimental work by Allen er al. (1975)and Allen and Boettcher (1978) has shown thatfractionation of amphibole-bearing silicate phasesfrom basalt magma could be a very effective mecha-nism for causing SiO2 increase in the residual melt,

without the marked iron concentration that charac-terizes the tholeiite series (Fig. 3). This concept hasbeen utilized by many petrologists to explain theorigin of hornblende-bearing calc-alkaline andesitemagmas. For example, the Oshima-Oshima volca-no, which is located about 180 km north of Itinome-gata, is composed of 70% alkali basalt and30Vo calc'alkaline andesite. Most of the andesites containabundant xenoliths of hornblende-bearing ultramaf-ic-mafic cumulates. Yamamoto et al. (1977) andMori (1977) have demonstrated that these andesitemagmas were formed by amphibole fractionation ofalkali basalt magmas at a shallow depth in the uppercrust, on the basis of geological and petrographicdata. During differentiation of high-alumina basaltmagma from the Itinome-gata district, hornblende,biotite, magnetite and plagioclase crystallized inaddition to olivine and clinopyroxene; thereforesuccessive liquids should have followed the calc-alkaline trend, as in the case of the Oshima-Oshimabasalt-andesite suite, because compositions of sub-tracted minerals are clearly higher in total FeO,MgO, and CaO and lower in SiO2, Na2O, and K2Othan coexisting liquids. Indeed, all the andesitescontain these minerals as phenocrysts. Also,Sr87/Sf6 ratios are consistent with fractional crys-tallization including amphibole fractionation.

In order to explain the observed major elementvariations of the Itinome-gata basalt:andesite suite,degrees of fractional crystallization were calculatedusing a least-squares mixing computer program

l 0 AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE

(A lv t1 Fe '+Cr+T i ) a toms

Fig. 7. Plots of calciferous amphibole in Al/v-(Na + K) and AlIvlAlvI * Fe3+ + Cr + Ti) diagram.

(Wright and Doherty, 1970) written by T. Miyata.When hornblende and biotite are included in thecalculation, however, the results show a large errorimplying that hornblende and/or biotite subtractionwas minor or negligible. The best-fitting calcula-tions are shown in Table 4. The most mafic and salicandesite liquids can be produced by about 40Vo and70% solidification of the least differentiated basaltmagma, respectively. Fractional crystallization in-volving amphibole fractionation cannot, moreover,explain similar REE concentration in all the rocks.This is because the partition coefficient of REE

between the sum of subtracted crystals composedof olivine, pyroxene, plagioclase and magnetitewith or without hornblende and basalt magma ismuch less than one (Schnetzler and Philpotts, 1968,1970). Therefore, it is impossible to subtract theseminerals from basalt magma without increasing theREE content of the residual liquids.

When a sequence of magmas is generated byvarying degrees of partial melting of peridotite, theREE concentration in magmas varies inversely withdegree of melting; namely, with increasing meltingof spinel peridotite, total REE concentrations de-

AOKI AND FU.IIMAKI: CALC-ALKALINE ANDESITE 1 l

1 2 3 4 5 6 7FeO'/llflgQ

Fig. E. FeO*/MgO-SiO, diagram. Dashed line is the fieldboundary between tholeiite series and calc-alkaline series(Miyashiro, 1974). Tholeiite series and calc-alkaline series arequotedfrom Kawano and Aoki (1960) and Kawano et al. (1961).

crease rapidly (Shimizu and Arculus, 1975). Ac-cordingly, the same degree of the REE concentra-tion in basalts and andesites can be interpreted asresulting from the same degree of partial melting ofthe same peridotitic upper mantle.

On the basis of experimental work, Kushiro(1972,1974), Mysen et al. (1974), Kushiro and Sato(1978), and Tatsumi (1981a, b) have demonstratedthat calc-alkaline andesite magma can be producedby partial melting of the upper mantle lherzoliteunder hydrous conditions at depths of 40 to 60 km.Such andesite magmas ought to have higherMgO/total FeO molecular ratio (MgO > FeO) thanandesite formed by fractional crystallization of ba-salt magma, because the former is produced inequilibrium with mantle peridotite which has veryhigh Mg value (about 88 to 90). Indeed, Itinome-gata basalt and andesite contain Mg-rich olivine(Fos0-80) as phenocrysts and also have high MgO/to-tal FeO (1.84-1.15), even in the groundmass com-position. According to Roedder and Emslie (1970),the Mg-Fe partition coefficient between olivine andliquid (D?];!is/a) is 0.3. The coefficient of thesebasalts and andesites approaches 0.3, and it issuggested that olivine phenocrysts are nearly inequilibrium with their groundmass. Figure 8 showsthe relationship between FeO*/MgO and SiO2 forItinome-gata basalt and andesite and for andesite ofthe tholeiitic series and calc-alkaline series of theNasu zone. Calc-alkaline andesites of the Nasu

zone plot along the boundary line between thetholeiitic series and calc-alkaline series (Miyashiro,1974); on the other hand, Itinome-gata basalt andandesite plot nearly parallel to the vertical axis,being clearly higher in MgO/FeO* than the calc-alkaline andesites of Nasu zone. Recently, Masudaand Aoki (1979\ have demonstrated that the mag-mas of the tholeiitic series and calc-alkaline seriesof Nasu zone appear to have been generated bydifferent degrees of partial melting (about 20Vo and3Vo respectively) of the same peridotite upper man-tle, based on the Ba, Hf, Th, U and REE contentvariations. However, there are conspicuous differ-ences between calc-alkaline andesites from Itin-ome-gata district and those of Nasu zone: theformer have clearly higher K2O, Ba and REEconcentrations and 116t+:7tr16taa and lower 5187/5186ratios, indicating that different source materials arerequired in these two regions (Kawano et al.,196l;Masuda and Aoki, 1979:' Zashu et al., 1980; Nohdaand Wasserburg, 1981; Tanaka and Aoki, 1981).

It may be reasonable to consider that calc-alka-line andesite and high-alumina basalt magmas ofItinome-gata district were generated independentlyby the same degree of partial melting of hydrousand less hydrous parts of the same peridotiticsource material which is enriched in LREE and/ordepleted in HREE (Lopez-Escobar et al., 1977:'Tanaka and Aoki, 1981), at depths greater thanthose of incorporated peridotite xenoliths, but farabove the Wadati-Benioffzone, or the order of 40 to60 km deep. When it is assumed that the major partof K2O, F and H2O are derived from phlogopite, aqualitative estimate of the original HzO in basaltmagmas can be inferred from both F and K2Ocontents (Aoki er al. , l98l) . Estimated H2O contentin the ltinome-gata basalts (K2O 1.3-1.4Vo and F280-290 ppm) is about 0.5 to 0.9Vo, and that of theandesites would be much higher. Recent experi-

Table 4. Representative results of computer calculation basedon l0 major elements for fractional crystallization model

B a s a ' 1 t ( N o . l ) *

S a l i c a n d e s i t e ( N o . 9 )

01 Fo6s

Cpx l loa5Ena2Fs 12Pl Ansq

l'lt Ul vosps

Sum of squares of

res idua l s

sio2 oo

,

Maf ic andes i te (No. 4 )

0 l Fo66

Cpx l,Joa5EnaTFsg

P' l An5a

Mt Ul vosps

Sum of squares of

res i dua l s

0. 60840.06030.08390 .22240. 0233

0.4869

0 . 0 5 1 5

0 . 1 4 6 3

0 . 2 5 9 6

0.0420

* Numbers are the sane as those o f Tab le l .

l 2 AOKI AND FUJIMAKI: CALC.ALKALINE ANDESITE

mental results (Tatsumi, 1981a, b) indicate that themelt of the high magnesian andesite can be inequilibrium with magnesian olivine and pyroxene inthe temperature range between 1,000' and 1,100' Cand in the pressure range between 10 and 15 kbar inthe presence of about 5-20 percent H2O in theliquid, being presumably produced by 10 percentpartial melting of peridotite containing less than Ipercent H2O. Accordingly, the source materialswould be a pargasite-bearing spinel peridotite inwhich phlogopite is present as the primary or as ametasomatized phase. Some Mg-rich olivine andclinopyroxene, which are sometimes recognized asmegacrysts, would be separated from these basaltand andesite magmas (less than l0%) during theirupward movement before they reached the top ofmantle. Furthermore, the andesite magmas crystal-lized plagioclase, hornblende, biotite and magnetitewith or without quartz in addition to olivine andclinopyroxene, but effective fractionation of theseminerals to yield drastic change of residual liquiddid not take place at depths of about 30 km.Subsequently, the magmas captured fragments ofwall rocks in the upper mantle and lower crustduring their rapid ascent to the surface. This hy-pothesis can explain all the petrographic, minera-logic, and major and trace element geochemicalevidence of basalt and andesite from Itinome-gatadistrict.

AcknowledgmentsThe authors are grateful to Prof. R. Brousse for his helpful

discussions. This petrographic and geochemical study was car-ried out at the Tohoku University, and the manuscript completedat the Universit€ de Paris-Sud during the senior author's tenureof a Fellowship from Ministere des Universites, France. A partof expenses of this study was defrayed with a Grant-in-Aid forSpecial Project Research (Nos. 420901 and 520102) from theMinistry of Education, Science and Culture of Japan.

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AOKI AND FUJIMAKI : CALC-ALKALINE ANDESITE 13

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Manuscript received, January 12, 1981;accepted for publication, August 21, 1981.