American Mineralogist, Volume 67, pages I-13, 19E2
Petrology and geochemistryof calc-alkalineandesiteof presumedupper mantle origin
from ltinome-gata, Japan
KeN-IcHrno Aorr euo Hrnorezu Funuerr
[nstitute of Mineralogy, Petrology and Economic Geology
Tohoku University,Sendai 980,Japan
Abstract
Itinome-gataand Sannome-gatavolcanoes,northeasternJapan, are characterizedby the
eruption of mafic to salic andesiteof the calc-alkalineseriesand high-aluminabasalt, both
of which contain small amounts of high-pressure diopside and forsterite megacrysts,
lherzolite and websterite derived from the upper mantle, and gabbro and amphibolite
xenoliths from the lower crust (about 20 to 30 km in depth). It seemslikely that this is the
first discovery of upper mantle peridotite xenoliths in calc-alkalineandesite.This andesite
is characterizedby presenceof hornblendeand biotite phenocrysts.
Three representativebasalts and five andesiteshave been analyzedfor major elements
by a conventional method, and for rare earth elements and Ba by the isotope dilution
method.
Major element variations of the basalt and andesite suite follow a typical calc-alkaline
trend with increasingfractionation. In addition, there are no essentialdiferences of Ba and
REE concentrations and chondrite-normalized patterns between them. Furthermore,
5tE775.80
ratios in basaltand the most salicandesiteare nearlythe same(0.7030and 0.7033,
respectively).
It is possible to conclude that the andesite magmas are produced by fractional
crystallization of basalt magma based on mineral assemblages,major element variations
and strontium isotope ratios. However, REE and Ba concentrationsand patterns are not
consistentwith this hypothesis.Accordingly, it is more probablethat andesiteand basalt
magmasare formed independentlyby nearly the samedegreeof partial melting of hydrous
and less hydrous parts of upper mantle peridotite, with increasing temperature, at the
depthsof 40 to 60 km.
lower crustal fragments in Japan (Fig. 1). A large
variety of xenolith types has been studied by many
Recently, much attention has been paid to the investigators(Kuno, 1967;Kuno and Aoki, 1970;
origin of andesitesof the calc-alkalineseries,which Aoki, l97l; Aoki and Shiba, 1973a,b, 1974a,b:
are widely distributed in time and space in island Takahashi,1978,1980;Zashuet a\.,1980; Tanaka
arcsand continentalmargins.Severalnew hypothe- and Aoki, 1981),though detailedpetrographicand
seshavebeenproposedon the basisofthe resultsof geochemicalstudies of host rocks have been neexperimentalpetrology and geochemistry(c/. Allen glected.Recently, Katsui and Nemoto (1977) and
et al., 1972;Kushiro et a\.,1968; Greenand Ring- Katsui et al. (1979) carried out field work in the
wood, 1968; Boettcher, 1973; Ringwood, 1974). Itinome-gata district and found that the xenolithThese include formation of andesite magma by (1) bearing volcanic rocks are calc-alkaline andesiteat
partial melting of hydrous mantle peridotite, (2) Itinome-gata and silica-undersaturated basalt at
partial melting of subductedoceanic crust, (3) mag- Sannome-gata.
ma mixing and (4) amphibole fractionation of basalOur purpose in this paper is to describe the
tic magma. However, it is very difficult to find mineralogy, petrography and REE geochemistryof
direct evidenceto evaluatetheseproposals.
high-aluminabasalt and calc-alkaline andesitefrom
Itinome-gatavolcano in northeasternJapanis one the Itinome-gatadistrict and to discussthe origin of
of the most famous localities of upper mantle and theserocks.
I
0003-.004)o820102--m0l$02.00
Introduction
AOKI AND FUJIMAKI:
CALC-ALKALINE
ANDESITE
HOKKAIOO
s
Fig. l. Map showing the location of ltinome-gata and Sannome-gata,northeasternJapan.
fragments, which are representedby mud flow and
base-surgedeposits.All xenoliths derived from the
Three maars or craters, Itinome-gata, Ninome- upper mantle and lower crust occur as ellipsoidal
gata and Sannome-gata,all of which formed about bombs or as loose blocks. within the andesitic
10,000years ago, iire located on the west coast of pumices, up to 30 cm in diameter. Megacrystsof
northeasternJapan(Fig. 1). The basementunderly- diopside(lessthan 5 cm in size),which may contain
ing the Itinome-gatadistrict consistsof a complex euhedral forsterite crystals, occur also as crystal
of Cretaceousto Paleogenesalic plutonic and vollapilli or as "xenocrysts" in the host rocks. In the
canic rocks and Neogenevolcanicsand sediments. third stage, basaltic scoria falls were erupted from
According to the field work of Katsui et al. (1979), the Sannome-gatacrater; these also contain small
eruptiontook place in three distinct stagesseparat- amountsof deep-seatedxenoliths.
ed by intercalated soil zones. The first stagebegan
Basalt scoria from Sannome-gatausually shows a
with the destruction of near-surface rocks and porphyritic appearancedue to the presenceofabunopening of the Itinome-gata and Ninome-gatavents dant quartz and feldspar xenocrysts derived from
by steamexplosions.This activity was followed by graniticbasementrocks. Under the microscope,the
the eruption of gray- to white-colored andesitic commonestphenocrystsare olivines,which occupy
pumices and abundant accidental upper crustal lessthan 5%by volume, and rangeup to 0.8 mm in
Occurrenceand petrography
AOKI AND FUJIMAKI:
CALC-ALKALINE
length but are mostly less than 0.4 mm. They are
euhedral tabular or elongated parallel to the c
crystallographicaxis, showing weak zoning at the
peripheralmargins of each crystal. Clinopyroxene
phenocrystsare much less common than those of
olivine, being subhedral, short, and prismatic in
form and up to 0.3 mm in length. Weak sector
zoning is always seen. The groundmassis rather
fine-grained and hypocrystalline with numerous
vesicles. It consists of brown glass, plagioclase
laths, subhedralclinopyroxene,and euhedralolivine, in decreasingorder of abundance.Plagioclase
xenocrysts have an irregular anhedralform, up to 3
mm in size. All of the crystals of plagioclaseshow
mottled appearancedue to glass inclusionsin the
margins, and are always immediately surrounded
by calcic rims of groundmassplagioclasecomposition. Quartz xenocrysts are irregular in crystal
outline. They are surrounded by thin rims composed of minute acicular clinopyroxene. Olivine
xenocrysts,presumablyderived from dispersedperidotite, are sometimesrecognized.They are irregular in form, less than 3 mm in grain size, and
invariably homogeneous.Accordingly, it is very
easyto distinguishbetweenphenocrysticand xenocrystic olivinesby their grain size and crystal form.
Andesite pumices are megascopicallygray in
mafic rocks to white in salic rocks and show a
porphyritic appearancewhich is due to the presence
of abundant quartz and plagioclase xenocrysts.
Phenocrysticmineralsare plagioclase(up to 4 mm
in size), olivine (0.6 mm), clinopyroxene(0.8 mm),
greenhornblende(1 mm), biotite (2 mm) and magnetite (0.5 mm). The modal volume of olivine and
clinopyroxene gradually decrease and these of
hornblende and biotite increase with increasing
SiO2 content. In addition to these minerals, small
amountsof quartz (0.5mm) and apatiteare found as
phenocrysts in salic rocks. All of the andesites
contain small to negligible amounts of mafic xenocrysts derived from the upper mantle and lower
crust. The mafic phenocrystscan be distinguished
from xenocrystsby their crystal outline: the former
are euhedraland smallerin size,while the latter are
irregular,broken and homogeneous.No phenocrystic or groundmassorthopyroxeneshavebeenidentified in any basaltsand andesites.
ANDESITE
ses.Therefore,scoriaceousand pumiceoussamples
were carefully crushed to passthrough a 48- or 100mesh sieve.They were washedwith distilled water
and dried. Most xenocrystsand phenocrystsseparate easilyfrom the soft and loosegroundmass.The
groundmasswas separatedin the Frantz isodynamic separator. Finally, the light fraction (specific
gravity is less than 2.5) was floated on diluted
methyleneiodide. The most salic pumice with less
abundanceof accidental fragments was purified by
elimination of impurities under the binocular microscope.
Major elementcompositionswere determinedby
wet chemical methods for silicate analysis combined with flame photometric and atomic absorption spectroscopic techniques. Mineral analyses
were done on a Hitachi scanning electron-probe
microanalyzer, Model X-560S, using an energydispersiveanalytical system (Kevex Corporation).
Synthetic and natural minerals were used as standards. Detailed procedures and accuracy of analysis by the energy-dispersiveanalytical system are
given by Fujimaki and Aoki (1980).
The determination of Ba and rare earth elements
(REE) was madeby the techniqueof stableisotope
dilution using a JMS-05RBtype, 30 cm radius mass
spectrometer 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 element composition.SiO2contentsrangefrom 51 to
61 percent, and the other componentsgradually
increaseor decreasesystematicallywith SiO2variation. Comparingthe basaltsfrom Itinome-gatadistrict with those from other localities of Japan,the
former exhibit characteristicsintermediatebetween
typical tholeiites and alkali basalts and resemble
high-alumina basalts from Izu-Hakone region
(Kuno, 1960).They are characterizedby ratherhigh
Al2O3,MgO, and total alkaliesand low total FeO.
In detail however, there are differenceswhich may
be significant.Most high-aluminabasaltshave K2O
contents of less than 0.8Vo.The averageItinomegatabasalt composition has a higher K2O content of
l.3Vo comparable to that of alkali basalts from
Chemistry
southwestJapan.
Although these basalts contain neither normative
Analytical methodsand results
quartz nor nepheline (normalizing Fe2O3lFeO +
Because all the rocks contain abundant xeno- Fe2O3to 0.2-0.15),all of the andesitesare saturated
crysts, it is difficult to interpret whole rock analy- in silica, and show many similarities with typical
AOKI AND FUTIMAKI: CALC-ALKALINE
Table l Chemical analyses and norms (water-free basis) of
xenolith-bearingbasaltsand andesitesfrom Itinome-gatadistrict
5 3 . 8 7 54.65 58.84 59.29 59 73
0.89 0 . 7 2 0 . 7 1 0 . 7 1 0 . 6 1
16 . 19 1 8 . 7 4 1 6 . 3 5 1 6 . 5 9 t6 68
3.07 2.44 2.43 2.51 1.65
4.77 3.80 3.40 3.21 3.01
0.13 0.25 0 16 0.16 0.15
6,41 4 . 5 0 3 . 3 7 3 . 5 2 3 . 4 5
I 44 7.04 7 22 6.69 5.27
2 52 3.10 2.86 3.r3 3.19
1. 9 3 2 . 0 t 2 . 7 0 2 . 5 8 2 . 9 7
0.88 0.99 l 8 0 0 . 9 6 2 . 2 7 1 . 3 8 1 . 3 3 2 . 3 4
0 . 2 0 0 . 1 9 0 . 2 1 0 . 3 1 0 . 2 4 0 . t 6 0 .I 6 0 . 3 1
99.80 99.54 99.74 99.49 9 9 . 7 6 9 9 . 5 8 9 9 . 8 8 99.40
50.95
0.96
t6 76
2.21
5.99
0.18
8.25
9.53
2.58
l.3l
a
0.54 5.60 6.50 12.94 12.76 13.47 15.13
7. 7 9 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
22.07 ? 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
30.62 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
6.62 6.61 6.19 5.39 0.88 4.68 3.66 0.72 1.23
4.43 4.33 4-28 3.76 0.59 3.10 2.52 0 48 0.88
1. 7 0 1 - 8 2 I 4 i
1.17 0.22 1.24 0.84 0.18 0.24
12.64 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
0r
Ab
An
lHo
ur
iLn
Its
Hvt-
En
fs
o,,'o
Fa
Mt
II
Ap
4.88
2.60
r.ll
3.24
I .84
o.47
59.72 50.55
I . 0 4 l 06
t 7 . 0 2 17 . 2 0
1.92 2.58
6.12 5.85
0.18 0 . t 9
7.84 6 82
9 56 9 . 5 9
2.66 2 . 4 6
1 . 3 0 1. 4 3
60.65
I. t2
',|6
4l
si02
Ti02
A 12 0 3
Fe203
FeO
l',1n0
l,fSo
Cao
Nazo
Kzo
HzO
Pz0s
Total
4.63 5.95 3.89
3 l9
148
2.A2 3.82 4.51
2.02 2.05 l.7l
0.44
0.50
0.74
4.13
2 19
2 15
G r o u n d n a s so f a u g i t e - b e a r i n g o l i v i n e
basalt (7772901)
2
G r o u n d n a s so f a u g i t e - b e a r i n g o l i v i n e
basalt (7772901'J
4. Groundmss of hornblendebiotite
5 . G r o u n d r u s so f o l i v i n e
l 99
basalt (7790302)
augite olivine
augite biotite
0.15
3.35
5.57
3 38
2.29
2.05
0.29
99 8l
3.63 3.59 4 02 ?.48 2.50
1.41 1.37 1.37 1.20 2.19
0.51 0.37 0.37 0.77 0.71
I
3. Groundmssof augite-bearing olivine
3.34
I 73
2.81
ANDESITE
and calc-alkalineseriesof Nasu zone (outer volcanic zone), northeasternJapan. All the rocks plot
below the fields of the calc-alkalineseriesof Nasu
zone in spite of being in the same rock series, and
consequently trend from the MgO-(FeO + Fe2O3)
side line toward the apex of alkalies, following the
typical course of island arc calc-alkaline series.
REE and Ba
Chondrite-normalized Ba and REE patterns for
one whole rock and eight groundmassanalyses
from the Itinome-gata district are shown in Figure
4. It is emphasized that none of the patterns are
correlated with either SiO2 or K2O contents. The
very markedenrichmentof light REE (Sm to La) is
notableand the rocks are essentiallyindistinguishable from one another, showing the typical abundancesfound in calc-alkaline rocks (Jakesand Gill,
1970).Total REE abundancesand LaNb ratios in
theserocks vary from 98 ppm to 118ppm and 7.1 to
9.1 respectively, with weak negative anomalies
evident in only three patterns. There is a slight
differencebetweenpatterns of the most salic andesite and its groundmass(Nos. 9 and 8, Fig. 4); the
former shows weak positive Eu anomaly due to
accumulationof someplagioclasephenocrysts.
andesite (7790303)
hornblendeandesite (7790304)
6, Groundmssof augite, olivine
andesite { 7790301
)
and quartz-bearing biotite
hornblende
7 . G r o u n d m a so
sf augite, olivine
andesite (7790305)
and quartz-bearing biotite
hornblende
Plagioclases
The compositional range of groundmass plagioclase in the basalts is AnTe.aAbzo.rOro.r
to An63
8, Groundmss of olivine, augite and quartz-bearing hornblendebiotite
Ab31Or1
whereas
the
and
narrow
margins
cores
of
andesite (7772710)
9. 0livlne, aug'ite dnd quartz-bearing hornblende biotite andesite
plagioclasephenocrystsin the andesitesare An65
\7772710), whole rock analysis
Ab33Or2to An32Ab63Ors.
It is clear that the most
Nos I -3: Scoria fron Sannome-gata, Nos. 4 - 9: Pumice frcm ltinore-gata
sodic margins of the former are still less sodic than
the cores of the latter. The compositionsof the
calc-alkalineandesitesfrom islandarcs or continen- cores of phenocryststend to be more sodic with
tal margins.
increasein SiO2content of the host rocks, but the
In Figure 2, the variation of main oxides against sodic margins are not strikingly different from one
SiO2 contents of basalts and andesitesis plotted another. The Or content increasesfrom 0.3 to 6.9
along with the average variation curve of volcanic with increasein the Ab content (Fig. 5).
rocks of the Chokai zone (high-alumina basalt
zone). The Itinome-gatavolcano is a part of this Olivines
volcanic zone of northeasternJapan(Fig. l). Total
Phenocrysticolivinesin the basaltsand andesites
FeO, MgO and CaO decreasegraduallyfrom basalt rangein compositionfrom coresof Fo6.3 and Foes2
to salic andesitewhile Na2Oand K2O increase,and to rims of Fosa.2and Fos2.3respectively,and euheit is noticeable that the basalt-andesitesuite of the dral olivine in diopsidemegacrystsfrom the andesItinome-gata district is clearly higher in MgO and ite is Fos7.3.On the other hand, groundmassolivKzO and lower in total FeO and Na2O than the ines in basaltsare Fos6.ato Fo7s.5.All the olivines
averagecurves.
are characterizedby very high Fo contents and a
The data are also plotted in a MFA [MgO-(FeO + narrow compositional range, regardlessof their
FezOr)-(Na2O+ K2O)l diagram (Fig. 3) together occunenceand bulk rock composition(Fig. 6). As
with the fractionationtrends of the tholeiitic series previously mentioned, basalts and andesitescon-
AOKI AND FUJIMAKI:
CALC.ALKALINE
ANDESITE
basaltsand andesitesfrom ltinome-gatadistrict
Table2. Ba and REE abundances(ppm) in xenolith-bearing
JB-I*
54
si02 %
Ba
LA
Nd
Sm
Eu
Gd
Dy
Er
Yb
Lu
ll33
2 1. 7
4 5 .I
20.5
4.46
0.980
4.65
4 . 74
2.54
2.50
0.403
60
59
6l
5 l6
861
I 754
1243
I 040
ll75
891
'18.
36.9
20.2
23.4
20.1
19.2
2 3. 2
9
24.3
6 7. 2
39.1
50.0
40.3
39.0
46.7
43.5
48.4
26.7
1
9
.
9
22.0
2 1. 4
2 1. 3
1 9 .4
24.6
22.9
5.19
4
.
2
9
4.20
4.61
4.60
3.86
5.79
4.66
.l.553
'1.639
.l.238
1.296
I .370
1.321
1.334
I.214
4.81
4.42
4.?0
4.05
4.00
3.93
5.37
5.19
4.18
3
.
9
2
4.7Q
4.58
4.r4
3.93
4.67
5 .0 1
2.34
2
.
4
2
2.89
2.74
2.40
2.57
2.79
2.70
2.16
2
.
2
4
?.56
2.40
?.21
t.Js
z.bo
2.67
0.307
0
.
3
3
0
0.391
0.379
0.34?
0.394
0.388
0.383
933
Leedey
chondrite**
4.21
0.378
0.976
0.716
0.230
0.0866
0 . 3 11
0.390
0.255
0.249
0.0387
8 7 .I
39
65.0
469
581
Sr
87
86
0.7031
0. 7030
Sr /Sr
Rb***
428
0 .7 0 3 3
* GeologicalSurveyof ,rapangeochemical
standard.
* * M a s u d ea t a J . ( 1 9 7 3 )f o r R E Ea n d N a k a m u r(a1 9 7 4 )f o r B a .
* * * Z a s h ue t a t . ( 1 9 8 0 ) .
tain abundantxenocrystic olivines dispersedfrom
uppermantleperidotitexenoliths.Accordingly,it is
suggestedthat Mg-rich cores of phenocrystsare of
xenocrysticorigin, and the slightly Fe-rich margins
are overgrown around the former after capture in
magmas.However, there are some chemical differ.
ences between such xenocrysts and phenocrysts;
the latter are poor in NiO (0.40to 0.28 and 0.33 to
0.l8Vo,respectively)and rich in CaO (lessthan 0.15
and 0.23 to 0.15%0,
respectively)when Fo contents
are the same (Foeoto Fosa).
guishedeasilyfrom dispersedxenocrystsofperidotites by higher AlzO: and total FeO and lower MgO
and Cr2O3in the former. High-pressurediopside
megacrystsfall within the chemical range of clinopyroxene phenocrysts, but there are clear differencesbetweenthem. The former are easily distinguishedalso from the latter by being very homogeneous, by having no perceptible zoning within a
single crystal, and by their slightly Mg-rich and Tipoor nature.
Clinopyroxenes
Hornblendes are characterized by a wide variation of all major componentsexcept that CaO:SiO2
and MgO tend to decrease,whereasAl2O3and FeO
increasewith increasingSiO2 content of the host
andesites.The Mg value (l00Mg/Mg + Fe + Mn
ratio) shows a range from 78 to 66 in mafic andesite
and 62 to 56 in salic andesite. Although these Mg
values of hornblendesin andesitesare clearly lower
than those of pargasitesin peridotite xenoliths (89
to 73), there is an overlap between hornblendes in
andesitesand in gabbro xenoliths; the Mg values of
the latter types range from74 to 59 (Fig. 6).
The relationships among calciferous amphibole
solid solutions are graphically expressedin a Alrv
-(AlvI + Fe+3 * Cr * Ti) and Afv-(Na + K)
diagram (Deer et al., 1962).The diagram (Fig. 7)
As is clear from Table 3 and Figure 6, clinopyroxenesshow a rather wide rangeof composition.The
cation ratios of Ca:Mg:Fe are 45.3:46.7:8.0to
48.1:41.2:10.7 in basalt phenocrysts and
44.8:45.4:9.8
to 40.0:41.8:18.2
in andesitephenocrysts, whereas those of basalt groundmassare
47.3:41.9:10.8
to 46.9:39.2:13.8.
All of the pyroxenes,like the olivines, are highly magnesian.
The most strikingfeatureof the clinopyroxenesis
the variation of Al2O3 Q.60-9.95%) and SiO2
(42.78-46.93Vo),which are reciprocal in relation to
each other. Accordingly, calculated Tschermak's
componentsrange from 5 to 18 mol percent, with
the majority falling within the range 1l to 15 percent. Clinopyroxene phenocrysts can be distin-
Hornblendes
AOKI AND FUJIMAKI:
Table 3. EPMA analysesof minerals
Olivires
9
4
Ph-c
si02
Feo*
Mno
MSo
Nio
Cao
Total
Ph-M
Gr-c
Gt-M
ilega
Ph-c
Ph-M
Ph-c
Ph-M
41.02 39.98 40.24 39.09 40.24 4 0 l 0 3 9 . 9 2 40.90 39.94
'14.68
',|5.71
ll.l8
1 2 . 8 3 1 7 . 1 7 1 1 . 9 4 I 3 . 0 7 I 5 . 0 4 9.43
0.21 0 . 2 4 0 2 l
0 . 4 4 0 . 2 1 0 . 2 0 0 2 8 0.09 0 22
48.06 44.51 46.45 4 2 . 8 0 4 7 . O 2 4 6 . 2 3 4 4 . 5 7 4 8 . 9 2 4 4 . 3 1
0.25 0.r9 0.r9
0.25 0 . 1 7 . 0 . 2 3 0.29
0 . , ]9 0 . 2 3 0 . 2 1 0 . 2 0 0 1 5 0 1 6 0 2 l
0.t5 0.16
1 0 0 . 9 1 9 9 . 8 9 9 9 . 9 4 9 9 .7 0 9 9 . 8 1 9 9 . 9 31 0 0 . 2 69 9 . 7 91 0 0 . 3 5
Forcl Z 88.3
84.2
86 4
81.3
A74
86.2
83.8
90.2
82.3
Ph-M
Gr-C
Gt-M
Mega
Ph-C
Ph-M
Ph-M
Ph-C
CALC-ALKALINE
ANDESITE
showsthat the hornblendesin the andesitesplot in
the field of common hornblende, but are separated
from the pargasitesin peridotite and hornblende in
gabbro xenoliths by their lower contents of pargasite and tschermakitemolecules.
Biotites
Biotites are comparablein compositionto those
from common calc-alkalineandesitesand dacites
but have higher TiO2 contents. The Mg values of
biotites range from 59 to 54, which are the lowest
amongcoexistingmafic silicates.
Clinopyroxenes
Magnetites
Ph-C
si02
Ti02
41203
Cr2O3
Feo*
r'1n0
t'fg0
52.78
0.27
2.60
0 41
4.91
0.14
15.14
21.76
49.45 50.16 48.46 5 l . l l 5 2 . 5 1 4 6 9 3 5 1 . 3 4 4 9 . 1 0
0.66 0.69 l.t0
0.36 0.30 1.27 0 61 0.73
5.63 6.01 7.06 5.31 4.31 9.95 3.70 6 07
0.26 0.42 0-'17 0 . l t
0.13 0.14
6 36 5.41 7.72 5 . 2 7 5 . 8 8 7 . 6 7 1 0 . 7 2 7 . 5 6
0.15 0.14 0.17 0.10 0.12 0.lt
0.36 0.17
13.79 14.22 13.20 1 5 1 3 1 5 . 5 6 1 2 . 7 1 ' t 4 . 2 9 1 4 . 2 6
Cao
22.4o 22.34 21-82 2 2 . 0 1 2 1 . 3 4 2 1 . 5 9 1 9 . 0 1 2 t 6 9
Na20
0.62 0.34 0.37 0 4l
0.57 0.49 0.36 0 s0 0.40
Total
9 9 . 6 31 0 0 . 0 41 0 0 . 7 41 0 0l l 9 9 9 7 1 0 0 . 6 41 0 0 . 7 31 0 0 . 5 31 0 0 . 1 4
C aa t o n g 4 5 . 2 4 8 . 0 4 7 . 3 4 7 . ' l 4 6 . 6 4 4 . 8 4 7 . 6 4 0 . 0 4 5 . 7
l4s
46.6 4l.l
41 9 39.6 44.5 45.4 39.0 4t I
41.8
Fe
8.2 10.9 10.8 i3.3
8.9
9.8 13.4 18.2 125
l4
Gr-C
Si02
41203
Feo*
cao
9
Gr-M
Ph-C
4 8 . 8 5 50.40
3 1 . 8 2 29.42
0.86 0 . 9 6
1 6 . l l 1 4 s. 6
2.28 3 . 6 4
Ph-M
5 4 .51 6 l . 5 1 5 7 . 8 2 6 0 . 0 0
28.99 2 3 . 9 9 2 5 . 4 7 24.62
0 . t 9 0 .1 7
1r.04 6.72 9.23 6.75
5 . 0 2 7 . 0 3 5 . 8 8 7. 6 0
Na20
KzO
0.05 0 . 2 3 0 . 1 7 0 . 6 5 0 . 4 3 0 6 1
Total
9 9 . 7 9 9 9 . 2 1 9 9 . 7 3 9 9 . 9 0 1 0 00 3 9 9 7 6
A nr 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
si02
Ti02
A t2 0 3
Cr203
Vzog
Fe203*"
Feo
l,tln0
l|so
Cao
Na20
KzO
Total
Ulvosp.
rcl%
BA
MX
44.05 3 7 . 3 0
l. 12 3.71 2.49
't4.5l
I 3 .7 6
0.84
0.I 7
0.20
0.38
61.92
9 . 4 5 I 7 . 0 0 3 t. 2 6
0. 14 0 . 7 5 2 . 3 3
1 5 . 1 4 1 3 . 2 1 0 .1 5
I I . 8 0 0 .1 6
1.72 0.67
0.71 8 . 7 4
98.06 9 6 . 1 5 9 9 . 9 7
.5 57.0
8.3
47.53 37 35
0.80 3.86 2.84
7.62 14.32 0.79
0.35
6l.71
14.74 I7.05 31.37
1. 3 2 0 . 7 8 I . 8 3
12.5113.04 0.20
'l
l .54 0.20
t.l3 0.39
0.62 8.74
97.81 95.73 99.09
5 8 .r
56.6
8.2
* Total ircn as Feo. ** Calculated as Feo and Fe203 assuming stoichionetry
ph, Phenocryst, 6rr grcundmass, c; core tr, margin, uega,. megacryst,
E vaJue; 100 Mg/Mg + Fe + liln.
Homogeneous magnetite phenocrysts consist
mainly of Fe2O3and FeO associatedwith small
amountsof TiO2 and MnO. The magnetitesshow
significant differences in composition when compared with those of calc-alkaline andesiteswith
similar SiO content in the Nasu zone (TiO214.61,
Al2O32.02, Cr2O30.45, V2O3 1.62, Fe2O336.69,
FeO 42.67,MnO 0.66,MgO 1.05,total99.77).The
former are characterizedby higher Fe2O3and MnO
and lower TiO2, Al2O3,YzOz,FeO and MgO, being
composedof 90 moleVomagnetite, 87o ulvospinel
and 2%ospinel molecules; the latter magnetites
consist of 55% magnetite, 4lVo ulvdspinel and 4Vo
spinel.
Discussion
Tremendousamounts of calc-alkalineandesites
are distributed in island arcs and continental margins. To elucidatethe origin of theserocks is one of
the most important problemsin igneouspetrology,
and varioushypotheseshave beenproposedduring
the past five decades.For example, andesitemay
represent(1) residualliquid from fractionalcrystallization of basalt magma (anhydrous mineral fractionation, magnetite fractionation and amphibole
fractionation), (2) contamination of granitic rocks
by basalt magma, (3) partial or complete melts from
the lower crust, (4) basaltand salic magmamixing,
(5) partial melts of oceanic crust in subduction
zones,and (6) partial melts of hydrous upper mantle. In view of the large variety of mineral assemblages,mineral compositions,and bulk rock compositions among andesites,any single hypothesis
cannot apply to all calc-alkaline andesite magmas.
Recently, amphibolefractionation,magmamixing,
partial melts of oceanic crust and partial melts of
hydrous mantle peridotite have been favored by
many petrologistsand geochemists.
AOKI AND FUJIMAIQ: CALC-ALKALINE
AlrO3
8-
u
m
.
---:- --- - -- -'--t--
ANDESITE
--- - 1 0
6
o Basalt
o Andesite
CaO
10
I
C----------__
.
:--____
o.
FeO*
I
o
t
a
Mgo
50
55
60
s io,
65%
50
55
sio,
60
6Fv.
!
Fis.2. Major oxides-silica variation diagram. Dashedlines are averagevariation curves of volcanic rocks of Chokai zone (Aoki'
1978).
First, the following petrographicand geochemical
featuresof basaltsand andesitesof the Itinome-gata
district must be emphasized.
1. The order of beginning of crystallization of
phenocrystic minerals in the basalt-andesitesuite,
indicated from the mineral assemblage, crystal
form, and Mg-Fe partitioning, is Mg-rich olivine --->
Mg-rich clinopyroxens + plagioclase--+ magnetite
+ hornblende+ biotite + quartz. This crystallization sequence agrees well with that commonly
observed in the calc-alkaline series except that
there is no appearanceoforthopyroxene, which is a
common mineral in the hornblende-and biotite-free
calc-alkalineseries. However, hornblende-or hornblendeand biotite-bearingandesitemagmas,including Itinome-gata, sometimes do not have orthopyroxene at their liquidus, the reasonsfor which are
not apparent.
2. Basalt and andesitecontain upper mantle peridotite and websterite, lower crustal gabbro and
amphibolite fragments, anhedral high-pressure diopside, and euhedral forsterite megacrysts. As
shown by many petrologists, upper mantle peridotite xenoliths composedof olivine, enstatite,diopside and chromian spinel with or without pargasite
are usually incorporated in alkali basaltsand related
rocks only. In extremely rare cases,olivine tholeiites include such peridotite xenoliths. High-pressure megacrysts also occur with peridotite xenoliths. Xenolith- and megacryst-bearingcalc-alkaline
FeO'
o High-alumina
basalt
. Calc-alkali
andesite
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).
AOKI AND FUJIMAKI: CALC-ALKALINE ANDESITE
o
!
c
o
3
o
:o
o
c
Fig. 4. Chondrite-normalized (Leedey chondrite in Table 2)
plot of Ba and REE contents. Numbers are the sameas those for
Table l.
tional between tholeiite and alkali basalt, and agree
well with the nature of high-alumina basalt (Kuno,
1960),whereas the andesitesare calc-alkaline. Major componentsvary rather smoothly from basalt to
the most salic,andesitewithout any abrupt change
with increasing SiO2. Chondrite-normalizedREE
and Ba abundances show no major differences
among all the volcanic rocks. Sr87/Sf6 ratios are
constantat 0.7030-0.7033
for basaltand salicandesite, being lower than those of incorporated peridotites (one peridotite is 0.7030and three are 0.70440.7053),hornblendegabbro(0.7041-0.7048)
and py(Zashuet al.,1980).
roxenegabbro(0.7035-0,7039)
Furthermore,Tanakaand Aoki (1981)have reported the REE and Ba abundancesof nine peridotites,
two hornblende gabbros, two pyroxene gabbros,
and two amphibolites from Itinome-gata. The REE
abundancesand normalized patterns of peridotites
reflect the degrees of basaltic component extractions; namely,undepletedperidotiteshavethe highest concentration and a convex-upwards pattern,
whereasMg-rich and depletedones have the lowest
concentrationand a V-shapedpattern. The patterns
of calculated liquid phases extracted from these
An
An
andesites have only been found in Itinome-gata
volcano and nowhere else in the world. According
to Kuno (1967), Aoki and Shiba (1973a)and Arai
and Saeki (1980),Itinome-gataand Sannome-gata
peridotite xenoliths are characterized by the presence of calcic plagioclase associated with spinelpyroxene symplectite without exception, in addition to the ordinary spinel peridotite mineral assemblage, presumably because of the relatively thin
crust in this district. Furthermore, Takahashi(1980)
has demonstrated that some of the Itinome-gata
peridotites do not show single PIT conditions of
equilibration but record complex thermal histories
between 800'C and 1,000"C from long residence
time in the upper mantle before transportation to
the surface. Estimated thickness of the crust based
on petrological and geophysicaldata is indicated to
be about 20 to 25 km. In addition, the depth of the A b
Ab
Wadati Benioff zone beneath this district is estimatFig. 5. EPMA'analyses of feldsparsin mole percent anorthite
ed to be about tr50 km by the observation of (An),
albite (Ab) and orthoclase (Or). Gabbro: plagioclasesin
microearthquakes(Hasegawaet al., 1978).
homblendite-hornblende gabbro xenoliths from Itinome-gata
3. Chemical compositions of basalts are transi- (Aoki, l97l).
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-hornblendegabbro-pyroxenegabbro xenoliths (Aoki,
r97r).
peridotites difer from those of the host basalt and
andesiteand resemblethose of island arc, low-alkali
tholeiites, which are characteized by LREE-depleted and flat patterns, from the Pacific side of
Japan.All the mafic xenoliths have similar tholeiitic
patterns. These Sr isotope and REE data indicate
no genetic relation between basalt and andesiteand
their incorporated ultramafic-mafic xenoliths.
Becausethey containupper mantlexenoliths,the
phenocrystassemblageand chemicalcompositions
of the basaltsand salic andesitesmust have been
determinedin the uppermostmantle, corresponding
to a depth at least 20 to 25 km from the surface.
They must have ascended directly and rapidly to
the surface without compositional change by fractionation at shallower levels. Furthermore. there is
no direct seismologicevidence of partial melting at
the subducted oceanic crust (Hasegawa et al.,
1978).Accordingly, only two possibilitiesexplain
the origin of these ltinome-gata magmas: one is
high-pressure fractional crystallization including
amphibolefractionation from basalt magmato produce andesitemelt, and the other is partial melting
of mantle peridotite to produce basalt and andesite
magmasunder dry and wet conditions,respectively, from the same source region.
Recentexperimentalwork by Allen er al. (1975)
and Allen and Boettcher (1978) has shown that
fractionation of amphibole-bearingsilicate phases
from basalt magmacould be a very effective mechanism for causing SiO2increasein the residual melt,
without the marked iron concentration that characterizesthe tholeiite series(Fig. 3). This concepthas
been utilized by many petrologists to explain the
origin of hornblende-bearingcalc-alkaline andesite
magmas.For example,the Oshima-Oshimavolcano, which is locatedabout 180km north of Itinomegata, is composedof 70% alkali basalt and30Vocalc'
alkaline andesite. Most of the andesites contain
abundantxenoliths of hornblende-bearingultramafic-mafic cumulates. Yamamoto et al. (1977) and
Mori (1977)have demonstratedthat theseandesite
magmaswere formed by amphibole fractionation of
alkali basalt magmasat a shallow depth in the upper
crust, on the basis of geological and petrographic
data. During differentiation of high-alumina basalt
magmafrom the Itinome-gata district, hornblende,
biotite, magnetite and plagioclase crystallized in
addition to olivine and clinopyroxene; therefore
successiveliquids should have followed the calcalkaline trend, as in the case of the Oshima-Oshima
basalt-andesitesuite, becausecompositionsof subtracted minerals are clearly higher in total FeO,
MgO, and CaO and lower in SiO2,Na2O, and K2O
than coexisting liquids. Indeed, all the andesites
contain these minerals as phenocrysts. Also,
Sr87/Sf6ratios are consistent with fractional crystallization including amphibole fractionation.
In order to explain the observed major element
variations of the Itinome-gatabasalt:andesitesuite,
degreesof fractional crystallization were calculated
using a least-squares mixing computer program
l0
AOKI AND FUJIMAKI:
CALC-ALKALINE ANDESITE
( A l v t 1F e ' + C r + T i) a t o m s
Fig. 7. Plots of calciferous amphibole in Al/v-(Na + K) and AlIvlAlvI
(Wright and Doherty, 1970)written by T. Miyata.
When hornblende and biotite are included in the
calculation, however, the results show a large error
implying that hornblende and/or biotite subtraction
was minor or negligible. The best-fitting calculations are shown in Table 4. The most mafic and salic
andesiteliquids can be produced by about 40Voand
70% solidification of the least differentiated basalt
magma,respectively.Fractional crystallizationinvolving amphibole fractionation cannot, moreover,
explain similar REE concentration in all the rocks.
This is because the partition coefficient of REE
* Fe3+ + Cr + Ti) diagram.
betweenthe sum of subtractedcrystals composed
of olivine, pyroxene, plagioclase and magnetite
with or without hornblende and basalt magma is
much lessthan one (Schnetzlerand Philpotts, 1968,
1970).Therefore,it is impossibleto subtract these
minerals from basalt magmawithout increasingthe
REE content of the residualliquids.
When a sequence of magmas is generated by
varying degreesof partial melting of peridotite, the
REE concentrationin magmasvaries inversely with
degreeof melting; namely, with increasingmelting
of spinel peridotite, total REE concentrations de-
AOKI AND FU.IIMAKI:
CALC-ALKALINE
ANDESITE
1l
zone plot along the boundary line between the
tholeiitic seriesand calc-alkaline series(Miyashiro,
oo
sio2
1974);on the other hand, Itinome-gata basalt and
,
andesite plot nearly parallel to the vertical axis,
being clearly higher in MgO/FeO* than the calcalkalineandesitesof Nasu zone. Recently,Masuda
and Aoki (1979\ have demonstrated that the magmas of the tholeiitic series and calc-alkaline series
of Nasu zone appear to have been generated by
different degreesof partial melting (about 20Voand
3Vorespectively)of the sameperidotite upper mantle, basedon the Ba, Hf, Th, U and REE content
variations. However, there are conspicuous differences between calc-alkaline andesites from Itinome-gata district and those of Nasu zone: the
former have clearly higher K2O, Ba and REE
1234567
FeO'/llflgQ
and lower 5187/5186
concentrationsand 116t+:7tr16taa
ratios, indicating that different source materials are
Fig. E. FeO*/MgO-SiO, diagram. Dashed line is the field
boundary between tholeiite series and calc-alkaline series requiredin thesetwo regions(Kawano et al.,196l;
(Miyashiro, 1974).Tholeiite series and calc-alkaline series are
Masuda and Aoki, 1979:'Zashu et al., 1980;Nohda
quotedfrom Kawano and Aoki (1960)and Kawano et al. (1961).
and Wasserburg,1981;Tanaka and Aoki, 1981).
It may be reasonableto consider that calc-alkacrease rapidly (Shimizu and Arculus, 1975).Ac- line andesite and high-alumina basalt magmas of
cordingly, the same degree of the REE concentra- Itinome-gata district were generatedindependently
tion in basalts and andesitescan be interpreted as by the same degree of partial melting of hydrous
resulting from the samedegreeof partial melting of and less hydrous parts of the same peridotitic
the sameperidotitic upper mantle.
source material which is enriched in LREE and/or
On the basis of experimental work, Kushiro depleted in HREE (Lopez-Escobar et al., 1977:'
(1972,1974),Mysen et al. (1974),Kushiro and Sato Tanaka and Aoki, 1981), at depths greater than
(1978),and Tatsumi (1981a,b) have demonstrated those of incorporated peridotite xenoliths, but far
that calc-alkaline andesite magma can be produced above the Wadati-Benioffzone, or the order of 40 to
by partial melting of the upper mantle lherzolite 60 km deep.When it is assumedthat the major part
under hydrous conditions at depths of 40 to 60 km. of K2O, F and H2O are derived from phlogopite,a
Such andesite magmas ought to have higher qualitative estimate of the original HzO in basalt
MgO/total FeO molecular ratio (MgO > FeO) than magmas can be inferred from both F and K2O
andesiteformed by fractional crystallization of ba- contents (Aoki er al. , l98l) . Estimated H2O content
salt magma, because the former is produced in in the ltinome-gatabasalts (K2O 1.3-1.4Voand F
equilibrium with mantle peridotite which has very 280-290ppm) is about 0.5 to 0.9Vo,and that of the
high Mg value (about 88 to 90). Indeed, Itinome- andesiteswould be much higher. Recent experigata basalt and andesite contain Mg-rich olivine
(Fos0-80)
as phenocrystsand also have high MgO/total FeO (1.84-1.15),even in the groundmasscom- Table 4. Representativeresults of computer calculation based
on l0 major elementsfor fractional crystallization model
position. According to Roedderand Emslie (1970),
the Mg-Fe partition coefficient between olivine and
B a s a ' 1 (t N o . l ) *
liquid (D?];!is/a) is 0.3. The coefficient of these
0.4869
S a l i c a n d e s i t e( N o . 9 )
Mafic andesite (No. 4)
0. 6084
basalts and andesites approaches0.3, and it is 0 l F o 6 6
0 . 0 5 15
0.0603
01 Fo6s
suggestedthat olivine phenocrysts are nearly in Cpx l,Joa5EnaTFsg
C p x l l o a 5 E n a 2 F1 s2
0.1463
0.0839
equilibrium with their groundmass.Figure 8 shows P ' l A n 5 a
Pl Ansq
0.2596
0.2224
l'lt Ul vosps
0.0420
0 .0 2 3 3
the relationship between FeO*/MgO and SiO2 for Mt Ul vosps
Sumof squares of
Sumof squares of
Itinome-gatabasalt and andesiteand for andesiteof
r e s i d u a ls
r e s i d u a ls
the tholeiitic series and calc-alkaline series of the
* N u m b e r sa r e t h e s a n e a s t h o s e o f T a b l e l .
Nasu zone. Calc-alkaline andesites of the Nasu
l2
AOKI AND FUJIMAKI: CALC.ALKALINE ANDESITE
mentalresults (Tatsumi, 1981a,b) indicate that the
melt of the high magnesian andesite can be in
equilibrium with magnesianolivine and pyroxene in
the temperaturerangebetween 1,000'and 1,100'C
andin the pressurerangebetween10and 15kbar in
the presence of about 5-20 percent H2O in the
liquid, being presumablyproduced by 10 percent
partial melting of peridotite containing less than I
percent H2O. Accordingly, the source materials
would be a pargasite-bearingspinel peridotite in
which phlogopiteis presentas the primary or as a
metasomatizedphase. Some Mg-rich olivine and
clinopyroxene,which are sometimesrecognizedas
megacrysts,would be separatedfrom these basalt
and andesitemagmas(less than l0%) during their
upward movement before they reached the top of
mantle.Furthermore,the andesitemagmascrystallized plagioclase,hornblende,biotite and magnetite
with or without quartz in addition to olivine and
clinopyroxene, but effective fractionation of these
minerals to yield drastic changeof residual liquid
did not take place at depths of about 30 km.
Subsequently, the magmas captured fragments of
wall rocks in the upper mantle and lower crust
during their rapid ascent to the surface. This hypothesis can explain all the petrographic, mineralogic, and major and trace element geochemical
evidence of basalt and andesite from Itinome-gata
district.
Acknowledgments
The authors are grateful to Prof. R. Brousse for his helpful
discussions.This petrographic and geochemicalstudy was carried out at the Tohoku University, and the manuscriptcompleted
at the Universit€ de Paris-Sudduring the senior author's tenure
of a Fellowship from Ministere des Universites, France. A part
of expensesof this study was defrayed with a Grant-in-Aid for
Special Project Research (Nos. 420901 and 520102)from the
Ministry of Education, Scienceand Culture of Japan.
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