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-
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