Magma mixing process of calc-alkalic andesites from Funagata volcano

17 Pages·2007·1.01 MB·English

Magma process mixing calc-alkalic of andesites
from Funagata volcano
KEUI WADA*
Department of and Geology 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 content. SiO2 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.
while they gradually decrease in number
Introduction toward the back-arc side. The volcanic rocks
A number of authors have discussed the along the volcanic front comprise the low-
origin of voluminous andesites of the calc-al- alkali tholeiitic series and the calc-alkalic
kalic series which characteristically occur in series. Distinct contrasts in the whole-rock
island arcs and continental margins. More chemical compositions between the two rock
recently, on the basis of petrographic and chem series have been described, i.e. the tholeiitic
ical evidence an increasing effort has been series show a higher degree of iron-enrichment,
extended toward magma mixing that plays an lower level of incompatible elements such as K,
important role in the genesis of calc-alkalic Th and U and lower SiO2 mode than the calc-
volcanic rocks (Anderson, 1976; Eichelberger, alkalic series (Kuno, 1950; Kawano et al.,
1975, 1978; Sakuyama, 1979, 1981 ; Gerlach 1961; Katsui, 1961; Aoki, 1978; Masuda and
and Grove, 1982 and others). The present Aoki, and 1979 others). It has been considered
study is concerned with mixed andesites from that the whole-rock chemical variations of the
the Funagata volcano group situated on the calc-alkalic series represent either (a) a liquid
volcanic front of northeast Japan. line of descent by fractional crystallization
Quaternary volcanoes in northeast Japan from primary tholeiitic or calc-alkalic mag-
are densely distributed along the volcanic front, 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, Funagata is composed mainly of talc-alkalic
disequilibrium petrographic features such as andesite and intercalates tholeiitic basalt and
coexistence of phenocrystic olivine and quartz, andesite. The first product of the Kita-
and bimodal composition and reverse zoning of izumigatake is tholeiitic basalt and the rocks of
phenocrystic plagioclase and pyroxenes in the latter stages are calc-alkalic andesite with
talc-alkalic andesites can be best explained by subordinate tholeiitic andesite. The rocks of
magma mixing (Sakuyama, 1981; 1979, Wada, the Ushiro-shirahige are dominantly calc-al-
1981). Sakuyama has (1981) documented from kalic andesite with subordinate tholeiitic an-
detailed phase petrological study that the chem- desite.
ical variations of the talcalkalic andesites
from Myoko and Kurohime volcanoes repre- Analytical method
sent mixing lines developed by the mixing of
Chemical analyses of minerals were perfor
high-alumina basalt magma and dacite mag-
med with electronprobe microanalyzers both of
mas from different stages of fractional crystal
Hitachi Model 5A of the Department of Earth
lization. He has also suggested that most of
Science, Kanazawa University and JEOL
the calc-alkalic andesites in northeast Japan
Model 50A of the Faculty of Engineering,
are products of "internal magma mixing.
Hokkaido University, using data reduction and
However, some calc-alkalic dacites along the
matrix correction procedures of Bence and
volcanic front which represent possible silicic
Albee (1968) and Albee and Ray (1970). Ana
end-member magmas before mixing may not
lyses of glass inclusions in phenocryst were
be formed by fractional crystallization of
made under a specimen current of 0.01-0.015
tholeiitic magma as inferred from their erup-
microampere and an electron beam of 5ƒÊm in
tive volume ratios, chemical relations and Sr
diameter. Major element chemical analyses of
isotopic data.
rocks were also carried out with electronprobe
The purpose of this paper is to clarify the microanalyzer on fused glass by the method of
magma mixing process involved in the forma Fukuyama and Sakuyama (1976). Analytical
tion of talc-alkalic andesites from the precision and reproducibility of this method are
Funagata volcano group and to deduce the
given in Wada (1981). Representative chemi
characteristics of the end-member magmas. cal analyses are listed in Table 1.
Sr-isotopic ratios of 9 samples from calc
Geological setting alkalic andesites and of the same number from
Funagata volcano group is situated about tholeiitic basalts and andesites were deter-
km 35 northwest of Sendai city, and it is on the mined by Dr. H. Kurasawa, Geological Survey
central portion of the volcanic front of north- of Japan.
east Japan arc. The volcano group overlies
the Plio-Pleistocene Pre-Funagata volcanic Petrography
rocks, and consists of three composite vol- Outline of petrography of the tholeiitic and
canoes : Mt. Funagata, Ushiro-shirahige and calc-alkalic rocks was already described by
Kita-izumigatake (Wada, 1981). These vol- Wada (1981), only significant features of the
canoes have been highly dissected and neither analysed samples are given here. Since the
craters nor fumaroles are found. Among three tholeiitic rock series show no petrographic
volcanoes Mt. Funagata seems to be the young- evidence of magma mixing, the petrography of
est, judging from the degree of erosion. Mt. 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. vol.%) and show various zoning pattern : nor
1) Calc-alkalic andesites mal and reverse, oscillatory and other complex
Calc-alkalic andesite lavas contain vari zoning. The Ca/(Ca+Na) ratio of the broad
able amounts of discrete phenocrysts, glomero core of plagioclase phenocrysts attains a maxi-
porphyritic clumps and sometimes small mum range from 0.90 to 0.42 (sample No. JA-
basaltic inclusions in hand specimen. The but 37), differs from specimen to specimen (Fig.
andesitic rocks have phenocryst assemblages of 9). Plagioclase phenocrysts frequently contain
plagioclase, orthopyroxene, augite, titano- glass inclusions in certain zones and/or dust
magnetite with or without olivine, quartz and inclusion zones of fine-grained opaque minerals
hornblende (Fig. 1 and Table 2). Plagioclase and alkali feldspar. Olivine phenocrysts are
phenocrysts are most dominant phase (20-30 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 2) -a Tholeiitic basalts from Mt. Funagata
rock MgO content. Olivine phenocrysts show The basaltic rocks (FG-II-1(cid:129)`3) are por
commonly euhedral to subhedral and normal
phyritic and contain abundant plagioclase
zoning without reaction rim. Olivine pheno
phenocrysts with subordinate olivine, augite
crysts in the lavas of YH-29 and SJ-35 have and orthopyroxene phenocrysts set in an inter-
maximum Fo contents of Fo86-80, but coexist
granular groundmass. Plagioclase pheno
with quartz phenocrysts in these lavas. crysts have compositional range from An93 to
Chromian spinels are frequently included in An,e and show normal zoning from core to rim.
olivine phenocrysts even in low Fo contents Olivine phenocrysts are euhedral to subhedral.
(Fo75-70, in FG-I-13). Orthopyroxene pheno and vary from 2 to 8 vol.% in modal content.
crysts are commonly euhedral and usually both Individual olivine phenocryst mantled by thin
normally zoned and reversely zoned. The reaction corona of pigeonites is normally zoned
sharp boundary between the iron-rich inner and ranges from Fo86 to Fo70, (FG-II-1 and 2),
core and the magnesian outer core is sometimes and from Fo,fi to For,, (FG-II-3). Inclusions of
recognized by difference in interference color or minute euhedral spinel are common in olivine
Beck's line. Microphenocrysts of orthopyrox
phenocrysts. Augite and orthopyroxene
ene commonly display euhedral form and are
phenocrysts are euhedral to subhedral and vary
generally high in Mg/(Mg+Fe) ratio. Augite in Mg/(Mg+Fe) ratio from 0.82 to 0.73 and
phenocrysts commonly have a compositionally from 0.78 to 0.65, respectively. Orthopyroxene
homogeneous core, whereas some of them have
phenocrysts are always surrounded by reaction
outer core of reverse zoning. Augite micro corona of pigeonites.
phenocrysts commonly display secter zoning 2)-b Tholeiitic basalts from Kita-izumigatake
and are considerably higher in Mg/(Mg+Fe) The basalts (KH-604 and KH-820) from
ratio, Al and Ti contents than the core of Kurohana lavas in Kita-izumigatake are
phenocrysts. Some Fe-rich orthopyroxene plagioclase-phyric rocks with phenocrysts of
and augite phenocrysts contain subspherical olivine, orthopyroxene, augite and pigeonite.
glass inclusions of dacitic to rhyolitic composi Plagioclase phenocrysts are normally zoned
tion with a bubble. and range from An90 to An82. Olivine pheno
crysts are commonly euhedral and rimmed with
Magma mixing process, Funagata volcano 471
iddingsite. Inclusions of spinels are rarely reaction corona of coarse-grained pigeonite
olivine. found in Individual olivine phenocryst and augite, and range in Mg/(Mg+Fe) ratio
normally is zoned and ranges to Fo74. from Fo66 from 0.80 to 0.76. Augite and pigeonite pheno
Orthopyroxene phenocrysts are mantled by 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 Fig. 3. versus Cr2O3 Mg/(Mg+Fe) ratio of augite
phenocrysts from FG-I-13, KI-20, YH-29
magmas have a mixing origin. In this section, and JA-37. Solid circles show inner core
the mineralogical evidence of magma mixing is composition and open circles outer core
composition.
examined and the phenocrysts are divided into
those derived from either mafic or silicic mag
toward the outer core, where attain a maxi-
mas.
1) Pyroxenes mum, and finally decrease at the rim. The
The compositional distribution of 100 Mg/ zonal structure of orthopyroxene from is KI-20
(Mg +Fe) ratio (reffered to Mg-value hereafter) not always developed concordantly with crystal
and Wo content in reverse-zoned orthopyrox- shape and shows rather irregular compositional
ene phenocrysts were determined by about 100 border. In Fig. 3, outer cores of the reverse-
point analyses (Fig. 2). The Mg-value and Wo zoned augite phenocrysts have the highest
content increase from the broad inner core content. Cr2O3 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.
Magma mixing process, Funagata volcano 473
chemical variations in reverse-zoned pyroxenes continuous mixing of an injected mafic magma
do not represent those predicted from frac (Sakuyama, 1979; Sato, 1982 and others).
tional crystallization. These phenomena can Compositional zoning profiles of pyroxene
be ascribed to temperature increase and phenocrysts from 10 specimens of the calc
alkalic andesites are shown in Figs. 4, 5 and 6.
compositional change of host liquid to mafic
composition, which were possibly caused by 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.
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-
Fig. 7. Schematic illustration of representative zo
value of pyroxene phenocrysts derived from the
ning profiles of pyroxene phenocrysts.
Reverse triangle indicates outer core, S,, magma mafic expected is to be than higher that
higher semi-critical value ; S2, lower semi-
of the outer core of reverse-zoned phenocrysts
critical value ; Mafic., Mg-value range of
from the silicic magma. Hence, a minimum
phenocrysts derived from mafic magma
(higher than S,) ; Silicic., those from silicic Mg-value of the pyroxene phenocrysts derived
magma (lower than S,); Mix., those from
from mafic end-member magma before mixing
mixed magma (between S, and S2).
was fixed to a value slightly higher a , than
value of reverse zoning from inner core to outer maximum Mg-value of the outer core of pheno
core varies among different phenocrysts within crysts from silicic end-member magma. Simi-
one thin section ; the maximum change of Mg- larly, the compositional range of Mg-value of
value of the reverse zoning amounts to 20 pyroxene phenocrysts derived from silicic end-
mol.% in orthopyroxene and 15 mol.% in member magma can be represented by that of
augite. The degree of reverse zoning from inner cores of the reverse- and flat-type zoned
inner core to outer core may depend principally phenocrysts. Here, the minimum Mg-value of
on the extent of difference chemical in composi the core of phenocrysts from mafic magma and
tion between the mafic and silicic end-member the maximum Mg-value of them from silicic
magmas. The zonal pattern can be classified magma are designated by the term "semi-criti-
mainly into three types : 1) normal-type with cal values" 7).' (Fig. Then, the Mg-values of
inner core of high Mg-value, and 2) reverse- pyroxene phenocrysts in end-member magmas
type and 3) flat-type with that of low Mg- 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-value=10OMg/(Mg+Fe)Mg:
Magma mixing process, Funagata volcano 475
critical values ; higher semi-critical value rep- Mg-value (Fig. 6). Such step zoning profiles
resents the lower limit of the Mg-value of may have been formed by complex mixing of
pyroxene phenocrysts in mafic end-member magmas (Sato, 1982) suggesting at least two
magma and lower semi-critical value the upper events of mixing. Orthopyroxene phenocrysts
that of limit end-member silicic in magma (Fig. having inner core of high Mg-value (70 to 76)
7). Semi-critical values of Mg-value of pyrox- display normal zoning followed by outward
phenocrysts ene each in andesite can be deter- reverse zoning. This suggests that these
mined from the compositional zoning profiles phenocrysts is derived from basaltic magma
4, 5 and (Figs. 6 and Table 3). and later affected by injection of more mafic
The pyroxenes having Mg-value between magma.
two semi-critical values may be crystallized 2) Plagioclase
from magmas in various advanced stage of Compositional profiles of plagioclase
mixing of mafic and silicic end-member mag- phenocrysts display step-like variation of Ca/
mas. Microphenocrysts and some phenocrysts (Ca+Na) ratio. Both normal- and reverse-
with relatively high Mg-value at the inner core zoned plagioclase phenocrysts are always found
may be grown from the mixed magma. within one specimen. In Fig. 8, Ca/(Ca+Na)
In the mafic andesites (FG-I-13 and YH- ratio of the inner and outer core pair of a
two 29), steps of reverse zoning are found in the plagioclase phenocryst are given. These data
pyroxene phenocrysts with inner core of low 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- Fig. 9. Frequency distribution of Ca/(Ca+Na)
ing of the mafic and silicic end-member mag- ratio of the cores of plagioclase pheno
crysts. Solid symbols indicate phena
mas. trysts derived from mafic magma, open
symbols those from mixed magma, and
Mixing line of calc-alkalic andesites dotted those symbols magma. from silicic
Numbers are the same as Nos. of sample
Mineralogical evidence such as compo- Tables 1, 2 and 3.
sitional zoning profiles indicates that the pheno
cryst phases in talc-alkalic andesites can obvi- out quartz and/or hornblende (Table 3).
ously be divided into two end-members. Figs. 9 and 10 display frequency distribu
Phenocrysts derived from the mafic end-mem- tion of chemical composition of the cores of
ber magma are olivine (except for JA-37), An- plagioclase, olivine and pyroxenes against the
rich plagioclase, with or without Mg-rich semi-critical` Ca/(Ca+Na) and Mg/(Mg+Fe)
augite and/or Mg-rich orthopyroxene, whereas ratios which distinguish two end-member
those from the silicic end-member magma are phenocrysts. Assuming that these histograms
An-poor plagioclase, Mg-poor augite, Mg-poor approximate the compositional distributions of
orthopyroxene, titanomagnetite, with or with- all the phenocrysts of plagioclase and pyrox-