fractionation and magma mixing within intruding … and magma mixing within intruding dike swarm:...

Fractionation and magma mixing within intruding dike swarm:evidence from the Miocene Shitara-Otoge igneous complex,

central Japan

Nobuo Geshi*

Geological Institute, Faculty of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received 8 March 1999; received in revised form 4 November 1999; accepted 4 November 1999

Abstract

The analysis of intrusion pattern and petrological character of the central dike swarm in the Miocene igneous complex of theShitara district, central Japan clarified that magma mixing between a strongly differentiated magma and a less-differentiatedmagma occurred within a dike swarm. The dike rocks have a wide compositional variation ranging from 5.5 to 0.7 wt.% MgO.They are divided into P1- and P2-types. The P2-type rocks provide many lines of evidence for magma mixing such as reverselyzoned phenocrysts, bimodal composition distribution, and dissolution texture, whereas P1-type rocks do not. Phenocrystcompositions of P2-type suggest that the magma mixing occurred between a less-fractionated phenocryst-poor magma anda strongly fractionated crystal-rich magma. Concentration ratios among incompatible elements show that the mixing endcomponents were derived from a similar parental magma common to P1-type by fractional crystallization in a near closedsystem. The dikes with evidence for the magma mixing (P2-type) are distributed only in the southern marginal part of the dikeswarm, whereas P1-type dikes do not show any such localization. The distribution and the intrusion direction of the dikesindicate a nearly horizontal outward flow of magmas in the southern part of the dike swarm and accompanied magma mixing inthe dike during intrusion. The fractionated end component is inferred to be a product of crystal fractionation within small andephemeral magma pockets in the dike swarm. Magma mixing is thought to have occurred when a newly intruded dike rupturedthe magma reservoir. The frequency of magma mixing was controlled mainly by competition between the lifetime of ephemeralmagma reservoir and frequency of dike intrusions. The condition of magma mixing was satisfied only in the southern part of thedike swarm affected by the preceding volcanic activities.q 2000 Elsevier Science B.V. All rights reserved.

Keywords: dike swarm; magma reservoir; fractionation; magma mixing

1. Introduction

During ascent through the crust, magma changes itscomposition by fractional crystallization, mixing withother magmas, and interaction with crustal materials.Mixing of magmas with different compositions, inparticular, causes a wide compositional variation

(Eichelberger, 1975; Sakuyama, 1979; Koyaguchi,1986). Magma mixing could have taken place in amagma chamber and in a conduit. End members ofmixed magmas are either related or unrelated in theirorigin. Magma mixing in a dike system has beenintensively studied in mid-ocean ridge systems(Dungan and Rhodes, 1978; Kuo and Kirkpatrick,1982; Sinton and Detrick, 1992) and the Hawaiianrift zone (Wright and Fiske, 1971; Wright andTilling, 1980; Ho and Garcia, 1988; Rhodes,

Journal of Volcanology and Geothermal Research 98 (2000) 127–152

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* Fax: 181-3-815-9490.E-mail address:[email protected] (N. Geshi).

1988; Garcia et al., 1989; Parfitt, 1991; Helz andWright, 1992; Tilling and Dvorak, 1993; Yang etal., 1999). Although magma mixing between anewly supplied magma and remnant magma havebeen reported in convergent-margin volcanoes, theyhave been explained by a mixing within isolatedmagma reservoirs (Nakamura, 1995; Umino andHorio, 1998; Nakagawa et al., 1999). There are fewstudies about the magmatic process in dike systems ofthe volcanoes of subduction zone although theirmagma plumbing system mainly consists of a dikeand sheet complex. The purpose of this paper is todescribe an example of the process of fractionationand mixing in a dike system of polygenetic volcanoin a subduction zone.

The spatial relation between sites of fractionationand magma mixing as well as conditions of thoseprocesses is a key to understand the process in thedike system beneath a volcano. It is difficult toknow the magmatic process in an active volcanobecause their plumbing system is hidden beneath thevolcano and because of their long interval of eruptionsin the subduction zone volcano, there is very littlechance to observe the behavior of magmas by geophy-sical methods. In order to address this problem, astudy of dissected polygenetic volcanoes is needed,where the structure of dike swarm can be directlyobserved. Dike swarm exposed in the dissected volca-nic field is a part of the magma plumbing system(Nakamura, 1977) and it is only a case that one canobserve the structure of conduit directly.

The Shitara central dike swarm of the Otoge volca-nic complex, central Japan, shows systematic spatialvariations regarding petrologic characters and intru-sion directions. Such spatial variation serves a criticalobservation in understanding the magmatic processestook place in the dike swarm, because magma motionand petrologic characters of dike rocks can becombined to reconstruct the process of the dike intru-sion and magma mixing. This paper documents frac-tionation and mixing of magmas during formation ofthe dike swarm on the basis of its structural and petro-logic characters of the Shitara central dike swarm.

2. Geological outline

The Shitara central dike swarm is one of the

members of the middle Miocene Shitara igneouscomplex in central Japan (Fig. 1). Sugihara and Fuji-maki (1998) reported K–Ar ages for the dike rocksranging from 12.6 to 13.6 Ma. The stratigraphy andthickness of pyroclastic flow deposits of the Shitaraigneous complex suggest that the present exposurelevel of the dike swarm is about 500–1000 m belowthe original surface of the pyroclastic flow deposit inthe southern part of the dike swarm. No clear evidencefor the inclination of the erosion along the dike swarmis observed.

The Miocene volcanic rocks in the Shitara districtare divided into three igneous units (Takada, 1987a,b,1988): Shitara igneous complex distributed in thesouthern part, Otoge igneous complex (the Otogering complex of Takada (1987a)) in the north, andTsugu volcanic rocks in the north-northwestern sidesof the Otoge igneous complex (Fig. 1). The Shitaraigneous complex is composed mainly of felsic extru-sive rocks and a subordinate amount of andesiticintrusive rocks. The Otoge igneous complex iscomposed of cauldron-filling mafic—intermediatepyroclastic rocks (Otoge pyroclastic rocks) and intru-sive rocks of the post-cauldron stages (Kamoyama-gawa trachyte dikes, Otoge cone sheets, Otogestocks, and Shitara central dike swarm). The volcaniccenter of the Otoge igneous complex is estimated tobe beneath Mt. Otoge based on the concentric struc-ture of the cauldron and distribution of the cone sheets(Takada, 1987a, 1988).

Petrological studies on the Otoge igneous complexare limited to those by Kuno (1960), who clarified thealkaline nature of the complex, and by Sawai andShimazu (1979) who described the extremely frac-tionated trachyte from the Kamoyamagawa area.The Otoge igneous complex ranges in compositionfrom alkali basalt to trachyte except for the Otogestocks, which consist mainly of quartz-bearing biotitehornblende dacite. The essential fragments of theOtoge pyroclastic rocks consist of olivine-bearingclinopyroxene basaltic andesite to andesite. Kamoya-magwa trachyte dikes consist of porphyritic trachytewith phenocryst of fayalite, hedenbergite, andanorthoclase. Otoge cone sheets consist of olivine-bearing clinopyroxene alkali basalt to trachyande-site and the petrological character is common tothat of the alkaline rocks of the Shitara centraldike swarm. The rocks of the Otoge cone sheets and

N. Geshi / Journal of Volcanology and Geothermal Research 98 (2000) 127–152128

the Shitara central dike swarm are generally poor inphenocrysts.

The Shitara central dike swarm (Takada, 1987a,1988) extends for,35 km through the center of the

Otoge cauldron and the Shitara cauldron with 1–5 kmwidth (Fig. 1). Numerous vertical dikes form asubparallel dike swarm in this zone. The centraldike swarm cuts all the igneous units of the Shitara

N. Geshi / Journal of Volcanology and Geothermal Research 98 (2000) 127–152 129

Fig. 1. Generalized geological map of the Shitara district. The outline of distribution area of the Shitara central dike swarm and the Otoge conesheets are shown by broken lines. Distribution of the Shitara igneous complex is after Takada (1987b). Distribution of one of the largest dike(Anataki dike) is after Urakawa and Yokoyama (1981). Locations of samples for detailed petrographical and mineralogical studies are alsoshown.

and Otoge igneous complexes, including the Otogepyroclastic rocks, the Kamoyamagawa trachyte, andthe Otoge cone sheets. The Kamoyamagawa trachyteis also cut by the cone sheets, and thus predated theformation of the cone sheets and dike swarm.Although no direct intrusive relationship with theOtoge stocks is observed, the activity of the centraldike swarm is inferred to be latest in the Shitaradistrict following the activity of the Otoge conesheets.

Most of the dike rocks in the Otoge volcaniccomplex have undergone hydrothermal alteration tovarious degrees and limited samples preserve theoriginal igneous characters. In the southern part ofthe dike swarm and the Otoge cauldron area, mostof the dike rocks are replaced by chlorite, calcite,sericite, and quartz. In the northeastern part of thedike swarm, the effect of the hydrothermal alterationis limited.

3. Structure and intrusion directions of the Shitaracentral dike swarm

The Shitara central dike swarm is divided intonorthern and southern parts at the center of theOtoge cauldron, which is the structural center of theOtoge volcanic complex, and shows structuralcontrast between both parts (Fig. 1). Variations ofnumber density, which is defined as the number ofdikes in 1 km width across the dike swarm, and aver-age thickness of dikes along the central dike swarmshow contrast in both areas (Fig. 2). The maximumnumber density is observed at the southern end of theOtoge cauldron (Kami-Awashiro area) and the densitydecreases southward with increasing distance fromKami-Awashiro. The number density in the northeast-ern part of the dike swarm is far lower than that in thesouthern part although the limited exposure hindersthe exact estimation.

The average thickness of dikes is smallest (about2 m) at Kami-Awashiro and increases southward andnorthward. At Lake Horai, 12 km south from thecenter of the Otoge cauldron, the average thicknessreaches 5 m. Dikes more than 5 m thick developmainly in the southern part of the dike swarm morethan 10 km from the center of the Otoge cauldron.More than 25 dikes reach 5 m thickness at the Lake

Horai and the largest dike is about 20 m in thickness.In contrast, the most dikes are less than 5 m in thick-ness in the northern part.

Urakawa and Yokoyama (1981) traced some dikesin the southern part of the central dike swarm, andfound that one dike is continually exposed for about8.5 km length (Fig. 1, Anataki dike, sample 60316D).They also traced several dikes, which are exposed formore than 5 km. In the Otoge cauldron area and north-eastern part of the dike swarm, the length of individualdikes is difficult to be estimated due to the incompleteexposure.

The northern part of the dike swarm intruded in thegneiss and granites of the Cretaceous Ryoke meta-morphic rocks. Hydrothermal alteration of the dikesand their host rock in this area is weak and most rockspreserve the original mineral assemblages. The south-ern part of the dike swarm intruded into the pyroclas-tic flow deposits and intrusives of the Shitara igneous


Fig. 2. Variation of number density (a) and thickness (b) of dikesalong the Shitara central dike swarm. The cardinal point of thedistance is Mt. Otoge, the structural center of the Otoge volcaniccomplex. Numbers in (a) represent that of the observed dikes alongeach route. The number density is the dike numbers per 1 km acrossthe dike swarm. Average and maximum thickness of dike (b) areshown for each route. Horizontal bars in (a) and (b) indicate therange of distance for the measurement.


Fig. 3. Photographs of dikes showing the initial intrusion directions. Arrows indicate the inferred intrusion direction. (a) Lineation on the dikewall exhibited by elongated bubbles. The photograph was taken from the direction perpendicular to the dike wall. The right side of the dike wallis covered by the host rock. (b) Imbrication of elongated bubbles near the dike wall. Both outcrops are at Iname, 3 km south to the center of theOtoge cauldron.

complex, which were formed just before the activityof the dike swarm. Both dikes and their host rocks aregenerally suffered from severe hydrothermal altera-tion and lost their original mineral assemblages.Hydrothermal alteration is not limited to the dikeswarm but took place extensively in the Shitaraigneous complex.

Many dikes display alignments and imbrication ofelongated bubbles and aphyric enclaves near the dikewall (Fig. 3). Some dikes have slickensides on their

contact surfaces, from which the direction of magmamotion in a dike before solidification can be estimated(Smith, 1978; Delaney and Pollard, 1981; Ui et al.,1984; Walker, 1987). The sense of shear is determinedby imbrication of elongated bubbles, platy pheno-crysts, and aphyric enclaves along dike walls, whichshow the direction of the relative movement of themagma against dike wall. Lineation and imbricationof bubbles, enclaves, and phenocrysts only near thedike wall were used because the initial direction ofmagma injection should be recorded only in thechilled margin (Knight and Walker, 1988; Wada,1992). Inside of a dike solidifies slowly and theymay suffer from secondary disturbance such asconvective flow and drain back.

The intrusion directions are estimated at 76 outcropsof the central dike swarm (Fig. 4). Most of the intrusiondirections of the dike swarm are radial from the center ofthe Otoge igneous complex. The dominant intrusiondirection is from SW to NE in the northeastern part ofthe dike swarm, and from NNE to SSW in the southernpart. Thedipsof intrusiondirectionsare steep within andaround the Otoge cauldron (area B) and relatively shal-low at the southern and northeastern part of the centraldike swarm (areas A, C, and D). These intrusion direc-tions indicate that the magmas filling the central dikeswarm were mainly supplied from the lower level ofthe Otoge cauldron nearly vertically to the presenterosion level and flowed both northward and south-ward with low angles. These observations indicatethat the magmas of Shitara central dike swarm weresupplied from the magma reservoir beneath the Otogecauldron and intruded laterally to north and south.

4. Petrography and classification of dike rocks

The central dike swarm consists mainly of mafic tointermediate alkaline rocks with small amounts offelsic subalkaline rocks (,5%) in the southern part.These rock types of the Shitara central dike swarm arecommon to those of the other members of the Otogeigneous complex. The alkaline rocks of the Shitaracentral dike swarm are divided into P1-type and P2-type by petrographical characteristics of their pheno-crysts.

The P1-type rocks are alkaline basalt to trachyan-desite with small amounts of euhedral phenocrysts of


Fig. 4. The intrusion directions of the Shitara central dike swarminferred from lineations and oblique alignments of elongatedbubbles, phenocrysts, and aphyric inclusions at the marginal partof the dikes. Left: horizontal distribution of intrusion directions.Arrows show the horizontal component of intrusion direction andthe length represents the dip of intrusion direction. P1-type dike:dike without magma mixing, P2-type dike: dike with the evidencesfor magma mixing, Type unknown: dikes difficult to distinguishtheir type because of the alteration. Distance from the center ofthe Otoge cauldron are shown. Right: frequency of the intrusiondirections projected to the vertical plane parallel to the axis of thedike swarm. Upper half area is illustrated because no dike showsdownward intrusion direction.


Fig. 5. Photomicrographs of phenocrysts in the Shitara central dike swarm. (a) An aggregate of euhedral plagioclase and magnesian clinopyr-oxene in a P1-type rock (960604X). cpx; clinopyroxene, pl; plagioclase, ol; olivine (replaced by secondary minerals), usp; ulvo¨spinel. Planepolarized light. (b) Euhedral clinopyroxene with sector zoning in a P1-type sample (60518C). usp; ulvo¨spinel. Crossed polarized nicols. (c)Plagioclase mantled by a thick dusty zone in a P2-type sample (60316D). Plane polarized light. (d) Backscattered electron image of thedissolved edge of an iron-rich clinopyroxene phenocryst in a P2-type sample (960518C). cpx; clinopyroxene, usp; ulvo¨spinel. (e) Backscatteredelectron image of deeply embayed olivine phenocryst in a P2-type rock (60316C). The rounded morphology suggests the phenocryst experi-enced dissolution. Dark-colored part within the olivine crystal is iddingsite.

plagioclase, augite, olivine (), and Fe–Ti oxides(ulvospinel and ilmenite) (Fig. 5). Although a veryrare dikes have more than 30 vol.% phenocrysts,most of the rocks of this type have less than1 vol.%. Phenocrysts in the P1-type rocks sometimesform aggregates and show glomeroporphyritic texture(Fig. 5a). The euhedral plagioclase phenocrysts showremarkable oscillatory zoning in their core and rela-tively steep normal zoning in the rim. Clinopyroxenephenocrysts are euhedral and pale brown in color, andexhibit concentric and/or sector zoning (Fig. 5b). Fe–Ti oxides and apatite inclusions are common in clin-opyroxene phenocrysts. Some P1-type rocks haveeuhedral olivine phenocrysts, which are usuallyaltered into secondary minerals. Olivine phenocrystshave euhedral shape and form aggregates with euhe-dral plagioclase, clinopyroxene, and oxide minerals.Some microphenocrysts of plagioclase, augite, andoxides are skeletal and have long slender shape.Groundmass is intersertal in texture and sometimesexhibits flow structure.

The P2-type rocks are trachyandesites and arecharacterized by the presence of phenocrysts showingdissolved textures, such as sodic plagioclase with dustyzone, Fe-rich clinopyroxene with vermicular rim, anddeeply embayed olivine coexisting with euhedralcalcic plagioclase and magnesian clinopyroxene.The modal abundance of phenocrysts in this type ishigher than that in P1-type on the average, and isusually less than 5 vol.%. Phenocrysts of sodic plagi-oclase and alkali feldspar commonly show euhedraloutlines and have a dusty zone surrounded by a clearrim with thickness of several tens to 100mm (Fig. 5c).Clinopyroxene phenocrysts with greenish core havevermicular rim with brownish color (Fig. 5d). Theseplagioclase and clinopyroxene sometimes form crys-tal clots or occur in gabbroic xenoliths less thanseveral cm in diameter. Some P2-type rocks haveolivine phenocrysts with a deeply embayed shape(Fig. 5e).

Enclaves of the P1-type rocks are sometimescontained in the P2-type dikes. The enclaves are ellip-soidal with length ranging from several mm to 30 cmcommonly with fine-grained rims with thickness lessthan 1 mm to several mm.

Kamoyamagawa trachyte occurs as minor intru-sions at the southwestern margin of the Otoge caul-dron (Sawai and Shimazu, 1979). The activity of the

Kamoyamagawa trachyte was just after the eruptionof the Otoge pyroclastic rocks and prior to the activ-ities of the Otoge cone sheets and the Shitara centraldike swarm. The trachyte has about 20–30 modal% ofeuhedral phenocrysts ofanorthoclase, hedenbergite, andfayalite. Anorthoclase phenocrysts are 1–1.5 mm inlength and show weak normal zoning. Hedenbergitephenocrysts are about 1–1.5 mm in length, and aregreen in the core and deep green in the margin. Pheno-crysts of anorthoclase and hedenbergite form aggre-gates. Phenocrysts of fayalite, 1 mm in length, occurrarely in the trachyte.


Fig. 6. Whole-rock major element contents plotted against the MgOcontents of the Shitara central dike swarm. P1-type: rocks withoutdissolved phenocrysts, P2-type: rocks with dissolved phenocrysts.The Kamoyamagawa trachytes of the Otoge volcanic complex arealso plotted (open triangles).

5. Whole-rock compositions

5.1. Analytical method

Whole-rock compositions of 10 major oxides and10 trace elements were determined with an X-rayfluorescence analyzer (PHILIPS PW-1480) at theGeological Institute of the University of Tokyo. Theanalytical procedure is the same as that described byYoshida and Takahashi (1997). The least alteredsamples were selected to avoid the effect of hydro-thermal alteration with the procedure described in

Appendix A. The representative compositions arelisted in Table 1.

5.2. Whole-rock composition

The whole-rock analytical data of major elementsof the central dike swarm are plotted against the MgOcontents in Fig. 6. They show wide compositionalvariations ranging from 0.7 to 5.5 wt.% in MgOcontent. The SiO2 content increases from 50 to63 wt.%, and FeOp/MgO (weight ratio) increasesfrom 1.9 to 11.2 with the decrease in the MgO content.


Fig. 7. Whole-rock trace element contents plotted against the MgO content (a) and Nb and Y contents against the Zr contents (b) of the Shitaracentral dike swarm. Symbols are the same as those in Fig. 6.


Table 1Representative whole-rock compositionsa

Sample no.: 60317F 60519F 51219A 606042A 60929P2B 60608S 50524Y 61123B2 61123NRock type: P1 P1 P1 P1 P1 P1 P2 P2 P1Distance from thevolcanic center (km)

10.0(NE) 3.7(NE) 1.6 2.0 2.8 3.5 7.5 7.5 8.2

Major elements(wt. %)SiO2 59.0 53.4 51.9 52.3 52.6 53.0 55.7 56.5 52.5TiO2 1.7 2.0 2.5 2.6 2.0 2.5 1.6 1.5 2.4Al2O3 14.9 16.2 16.4 15.5 17.3 16.1 15.3 15.2 15.2FeOp 8.7 8.4 10.1 10.8 8.7 10.0 11.0 11.2 11.8MnO 0.2 0.2 0.2 0.2 0.1 0.2 0.3 0.3 0.2MgO 2.0 4.4 4.1 3.6 4.5 3.8 1.7 1.4 3.0CaO 4.9 8.9 8.4 7.8 8.9 8.2 5.4 5.3 7.1Na2O 4.9 3.8 3.5 3.9 3.1 2.9 5.2 4.6 4.4K2O 2.3 1.4 1.4 1.5 1.3 1.7 1.9 2.4 1.5P2O5 0.5 0.4 0.5 0.5 0.5 0.5 0.6 0.6 0.8Total 99.1 99.1 98.9 98.8 99.0 98.9 98.8 99.0 98.9Trace elements(ppm)Ba 171 200 265 168 159 205 219 296 172Cr 53 77 67 33 101 46 6 7 17Nb 10 11 9 9 8 11 13 13 10Ni 18 34 6 3 41 4 0 0 0Rb 26 26 32 56 33 33 35 40 30Sr 463 452 465 417 552 502 381 505 414Y 31 26 27 31 25 29 39 40 33Zr 218 223 175 232 187 204 272 293 210FeOp/MgO wt. ratio 4.4 1.9 2.5 3.0 1.9 2.7 6.4 7.8 3.9

Sample no.: 60518B 60518C 60518L 60316C 60517B 60316D 71103Q2 60928B KTRock type: P1 P2 P2 P2 P2 P2 P2 P2Distance from thevolcanic center (km)

9.2 9.3 9.4 12.0 12.0 12.7 12.8 16.5 2.5

Major elements(wt. %)SiO2 51.8 58.1 59.0 56.7 60.7 57.1 63.0 55.3 68.5TiO2 2.7 1.3 1.2 1.4 1.0 1.7 0.9 2.3 0.4Al2O3 16.1 15.2 15.4 15.2 14.9 15.1 15.0 14.5 14.6FeOp 10.3 10.4 10.1 11.0 9.2 10.0 8.0 10.6 4.6MnO 0.2 0.2 0.2 0.3 0.2 0.3 0.2 0.2 0.1MgO 3.9 1.3 1.0 1.4 1.1 1.8 0.7 2.4 0.0CaO 8.2 4.9 4.4 5.2 4.1 5.1 3.8 6.2 1.0Na2O 3.4 5.0 4.4 5.3 4.8 4.9 5.4 4.3 6.9K2O 1.7 2.0 2.9 1.8 2.4 2.2 2.7 2.2 3.4P2O5 0.6 0.5 0.4 0.5 0.4 0.7 0.3 0.9 0.1Total 98.8 98.8 98.9 98.8 99.0 98.9 99.8 98.8 99.5Trace elements(ppm)Ba 280 278 296 224 271 509 288 199 434Cr 52 5 7 7 6 12 10 21 0Nb 10 14 14 13 14 14 15 12 27Ni 4 0 0 0 0 0 12 0 16Rb 30 35 67 33 59 102 67 51 72Sr 480 402 484 353 312 403 368 390 77Y 28 38 38 39 40 43 41 37 58Zr 186 269 284 284 287 291 250 221 574FeOp/MgO wt. ratio 2.6 7.9 10.5 7.7 8.6 5.7 11.4 4.5 –

a See text for explanations of rock type and distance from the volcanic center. FeOp; Total Fe as FeO. NE: sample collected from thenortheastern part of the dike swarm. KT: Kamoyamagawa trachyte.

The CaO and Al2O3 contents show positive, and theNa2O and K2O contents show good negative correla-tions with the MgO content. The FeO, TiO2, and P2O5

contents increase as the MgO content decreases from 5to 3 wt.%, and then decrease with decreasing MgO(Fig. 6). The Nb, Y, and Zr contents, which are lesssusceptible to alteration (see Appendix A), show nega-tive correlation with the MgO content. The concentra-tions of Nb, Y, and Zr are positively correlated, showingthat the dike rocks keep almost constant concentrationratios for these trace elements (Fig. 7).

The MgO content of the P1-type rocks is ranging

from 1.6 to 5.5 wt.% and more than 80% of them haveMgO content ranging from 2 to 4 wt.%. The MgOcontent of P2-type samples ranges from 0.7 to3.6 wt.% and the range shifts to the lower value thanthat of the P1-type rocks (Fig. 6).

Whole-rock compositions of the Kamoyamagawatrachyte, which is the most extensively fractionatedmagma in the Otoge igneous complex intruded inthe early stage of the igneous activity, are also plottedin Figs. 6 and 7. They are plotted on the extension ofthe trends of the central dike swarm, except for alkalisand some incompatible trace elements.


Fig. 8. Compositional variations of the feldspar, clinopyroxene, and olivine phenocrysts in a P1-type sample (a: 60604X; solid squares) and aP2-type sample (b: 60518L; open and solid squares). Ranges of core compositions of phenocrysts in each type are also shown. P1: phenocrystsin P1-type samples, N: euhedral and normally zoned phenocrysts in P2-type samples, R: reversely zoned phenocrysts in P2-type samples, KT:phenocrysts in the Kamoyamagawa trachyte.


Table 2Representative compositions of phenocryst mineralsa

Clinopyroxene

Sample no.: 60608S (P1-type) 60604X1(P1-type)

cpx1core cpx1rim cpx2core cpx2rim cpx1core cpx1rim cpx2core cpx2rim

SiO2 49.3 45.6 50.1 51.7 49.8 50.8 50.3 51.8TiO2 1.7 0.8 1.6 0.9 1.6 1.3 1.3 1.3Al2O3 4.1 6.8 3.9 2.2 4.4 2.4 3.5 2.3FeO 10.6 18.5 7.9 9.3 7.7 11.3 8.8 9.2MnO 0.3 0.2 0.2 0.2 0.1 0.3 0.2 0.3MgO 13.6 14.2 14.1 15.4 14.2 14.8 14.2 14.9CaO 19.8 13.2 21.4 19.9 21.4 18.5 20.9 19.7Na2O 0.4 0.2 0.4 0.3 0.4 0.3 0.4 0.3Total wt.% 99.8 99.5 99.6 99.9 99.6 99.7 99.6 99.7Mg# 0.70 0.58 0.76 0.75 0.77 0.70 0.74 0.74

Clinopyroxene

Sample no.: 60518L(P2-type) 60518C(P2-type)

cpx1core cpx1rim cpx2core cpx2rim cpx1core cpx1rim cpx2core cpx2rim

SiO2 47.8 48.4 50.7 50.7 48.0 48.2 48.7 50.9TiO2 0.5 0.5 0.7 0.6 0.5 0.5 0.5 0.4Al2O3 0.5 0.6 1.5 1.4 0.6 0.6 0.5 1.1FeO 29.0 28.4 16.7 15.7 28.8 28.2 28.2 20.6MnO 1.0 1.0 0.6 0.3 1.1 1.0 1.0 1.2MgO 1.0 1.1 10.3 10.7 0.7 1.1 0.9 6.5CaO 19.7 19.6 19.2 20.1 19.8 19.8 19.7 18.8Na2O 0.4 0.4 0.3 0.3 0.5 0.4 0.4 0.5Total wt.% 100.0 100.0 100.0 99.8 100.0 99.9 100.0 99.9Mg# 0.06 0.07 0.52 0.55 0.04 0.07 0.05 0.36

Olivine Fe–Ti oxide

Sample no.: 60604X(P1-type) 60518C(P2-type) 60604X 60518L

ol1 ol2 ol1 ol2 ilm uls uls

SiO2 37.4 37.1 35.2 31.2 SiO2 0.3 0.1 1.0TiO2 0.0 0.0 0.1 0.0 TiO2 47.1 21.0 25.8Al2O3 0.0 0.0 0.1 0.0 Al2O3 0.5 4.1 1.2FeO 27.0 26.2 45.4 58.9 FeO 50.2 72.3 67.0MnO 0.5 0.5 1.0 2.1 MnO 1.2 0.9 0.3MgO 35.1 36.1 17.9 7.0 MgO 0.1 0.1 0.0CaO 0.3 0.2 0.3 0.5 CaO 0.1 0.0 0.0Na2O 0.0 0.0 0.0 0.0 Na2O 0.0 0.0 0.1K2O 0.0 0.0 0.0 0.0 K2O 0.0 0.0 0.1Total wt.% 100.2 100.2 100.0 100.0 Total wt.% 99.5 98.5 95.5Fo mol.% 0.70 0.71 0.41 0.18 #TiO2 0.46 0.21 0.26

#FeO 0.53 0.77 0.74

6. Mineralogy of phenocrysts

6.1. Analytical methods

Chemical compositions of plagioclase, clinopyrox-ene, olivine, and oxide phenocrysts were determinedwith an electron microprobe analyzer (JEOL JCMA733 Mk-II) at the Geological Institute, the Universityof Tokyo, using 15 keV accelerating voltage, 12 nAbeam current and a beam size 1mm. The data werereduced using Bence and Albee (1968) matrix correc-tions and the correction factors are after Nakamuraand Kushiro (1970). Representative phenocrystcompositions are listed in Table 2.

6.2. Phenocrysts in P1-type

The compositional variation among plagioclasephenocrysts within individual P1-type sample isgenerally less than 15 mol.% in the An content[100× Ca/(Ca1 Na) molar ratio] (Fig. 8a). Theplagioclase phenocrysts are homogeneous or oscil-lated in An number in their inner portion and arenormally zoned at the rim. The An content of homo-geneous inner portion of the plagioclase phenocrystsfor one of the least fractionated samples (specimen606042A) ranges from 42 to 55.

Clinopyroxene phenocrysts of P1-type arecharacterized by relatively higher Mg# [Mg/(Mg 1 Fe) molar ratio] than that of P2-type rocks.

The Mg# of the clinopyroxene phenocryst showslimited variation, which is generally less than 0.2 inindividual samples (Fig. 8b), and the average valueshows good correlation with their whole-rock compo-sitions. The Mg# for one of the most primitivesamples (specimen 606042A) ranges from 0.62 to0.72. Clinopyroxene phenocrysts are generally homo-geneous in Mg# except for their rims. Remarkableoscillatory zoning of Al and Ti contents is developedin some clinopyroxene phenocrysts and is superim-posed on sector zoning.

Although olivine is contained in many dikes, mostof them are altered to secondary minerals, and theoriginal chemical properties cannot be determinedfor most olivine phenocrysts. Olivine phenocrysts inone of the P1-type samples (specimen 606042A) haveMg# of 0.70–0.74, which is similar to that of clino-pyroxene in the same sample (average Mg#� 0.73,Fig. 8a). Phenocrysts of ulvo¨spinel and ilmenite arerarely observed. They usually form a glomerocrystwith plagioclase, olivine, and clinopyroxene.

6.3. Phenocrysts in P2-type

The compositional variation of plagioclase cores inP2-type rocks is wide and often bimodal within indi-vidual samples. Plagioclase with high An content,which usually has an euhedral shape, is homogeneousin An content with margin showing steep normalzoning. This type of plagioclase phenocryst does not


Feldspar

Sample no.: 606042A(P1-type) 60518L(P2-type)

pl1-core pl1-rim pl2-core pl2-rim pl1-core pl1-rim pl2-core pl2-rim

SiO2 54.1 61.1 55.8 57.0 63.5 58.5 65.3 59.1TiO2 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1Al2O3 27.8 24.7 27.3 26.1 22.6 25.7 21.2 24.8FeO 0.8 0.8 0.6 0.8 0.2 0.6 0.2 0.4MnO 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0MgO 0.1 0.0 0.1 0.1 0.0 0.1 0.0 0.1CaO 11.7 5.3 9.9 8.7 4.3 8.1 2.6 7.5Na2O 5.1 7.3 5.8 6.6 8.6 6.5 9.5 7.3K2O 0.3 0.7 0.4 0.4 0.6 0.4 1.0 0.7Total wt.% 100.1 100.0 100.0 99.8 99.9 99.9 99.9 100.0An mol.% 55.00 27.00 47.00 41.00 21.00 40.00 12.00 35.00

a pl:plagioclase, cpx:clinopyroxene, ol:olivine, ilm:ilmenite, uls:ulvo¨spinel.

Table 2 (continued)

have a dusty zone. In contrast, phenocrysts with lowAn content have a dusty zone in their margin (Fig. 5c),which sometimes cuts the oscillated zoning pattern ofthe interior part. The An content is constant and low intheir inner portion, and clear margin with high Ancontent and thickness of several tens of micrometersdevelops outside the dusty zone (Fig. 9a).

Clinopyroxene phenocrysts of P2-type are charac-terized by wide bimodal compositional variationwithin a sample (Figs. 8b and 10). They are dividedinto high-Mg# core and low-Mg# core groups. Clin-opyroxene phenocrysts of the high-Mg# core groupshow euhedral shape and are homogeneous in Mg#except for their rim. Their Mg# are similar to or alittle higher than the values in equilibrium with thewhole rock (Fig. 10). Some of them show remarkableoscillatory and sector zoning regarding Al and Ticontents. Clinopyroxene phenocrysts of the low-Mg# core group show euhedral shape, sometimeswith a vermicular rim (Fig. 5d). The Mg# abruptlyincreases at the inner limit of the vermicular rimand decreases outward (Fig. 9b). The inner portionof these clinopyroxene phenocrysts sometimesshows oscillatory zoning in Al and Ti contents. Thevermicular texture of the high Mg# rim clearly cutsthe zoning pattern of the inner portion. The composi-

tion of the rim is similar to that of the euhedral crystalsand groundmass clinopyroxenes (Fig. 10).

The composition of the olivine phenocrysts in mostof the P2-type rocks is poorly known because ofalteration. Partially preserved olivine phenocrystswith the embayed shape in a P2-type rock areextremely rich in iron (Mg#t 0.1; Figs. 5e and 8b).Ulvospinel crystals are sometimes accompaniedby low Mg# clinopyroxenes (Fig. 5d). The cal-culated ferric–ferrousratios,assumingmineral stoichio-metry, are extremely low [Fe31/(Fe21 1 Fe31) , 0.05;Table 2].

7. Spatial distribution of the rock types andcompositional variation in the dike swarm

Systematic spatial variations of the rock types andchemical compositions are observed along the Shitaracentral dike swarm (Figs. 11 and 12). The P1-type


Fig. 9. Representative compositional profiles of the An content ofplagioclase (a) and the Mg# of clinopyroxene (b) phenocrysts in aP2-type sample (60517B).

Fig. 10. Compositional variation of phenocryst clinopyroxenes inindividual P2-type rock. The stippled areas show the range of calcu-lated Mg# values of clinopyroxene that would crystallize from aliquid with the groundmass composition of each sample. The distri-bution coefficient for Fe–Mg exchange between clinopyroxene andliquid is 0.23–0.27 (Groove and Bryan, 1983; Tormey et al., 1987;Groove et al., 1992; Kinzler and Grove, 1992), assuming that alliron is Fe21. Distance means the distance from the center of theOtoge cauldron.

dikes are distributed over the whole area, and theP2-type dikes do not appear within 6 km from thecenter of the Otoge cauldron, which are distributedonly in the southern part of the dike swarm (Fig. 11).More than half of the dikes are P2-type around LakeHorai, which is about 12 km south from the cauldroncenter. No P2-type dike is found, on the other hand,in the northeastern part of the central dike swarm(Fig. 12a).

The P1-type rocks show a range of FeOp/MgO(� 2–5) along the entire dike swarm and no correla-tion against the distance from the Otoge cauldron isobserved. The P2-type rocks have higher FeOp/MgO(generally larger than 4) than that of the P1-type rock.The Mg# of clinopyroxene phenocrysts in the P1-type

rocks ranges from 0.5 to 0.8. The clinopyroxenephenocrysts with normal zoning in the P2-type rocksshow lower Mg# (0.5–0.7) without distinct relation-ship to the distance. The low Mg# clinopyroxeneswith reverse zoning in the P2-type rocks show widecompositional variation (Mg# 0.01–0.4) and arerestricted to the southern part of the dike swarm(Fig. 12b). No crosscutting relationship between P1-type and P2-type is observed.

8. Differentiation of the magma of Shitara centraldike swarm

Although the compositional diversity of the Shitaracentral dike swarm is partly due to this mixingprocess, a wide compositional variation is alsoobserved for the rocks without evidence for magma


Fig. 11. Distribution of the two types of dikes in the Shitara centraldike swarm. P1-type dikes do not contain clinopyroxene pheno-crysts with vermicular rim, and P2-type dikes do.

Fig. 12. Variation of the whole-rock FeOp/MgO ratio (a), and aver-age Mg# value at the core part of clinopyroxene phenocrysts (b)with the distance from the Otoge volcanic center. Bars in (b) showthe range of Mg# in one specimen.

mixing (P1-type). In this section, the origin of thecompositional variation of the P1-type is discussed.

The compositional variation of the Shitara centraldike swarm was examined in terms of fractional crys-tallization by the least-squares analysis (Bryan et al.,1969; Chambers and Brown, 1995). The calculationwas performed with averaged six groundmass compo-sitions of the P1-type rocks (G1–G6) and theKamoyamagawa trachyte (Table 3). The groundmasscompositions were calculated from their whole-rockcomposition, averaged phenocrysts composition, andmodal compositions of phenocrysts. The corecompositions of the euhedral phenocrysts of plagio-clase, clinopyroxene, and Fe–Ti oxides (ulvo¨spineland ilmenite) in the corresponding parental samples

were used to represent the composition of the fractio-nating phases. The Mg# of olivine is assumed to bethe same as that of coexisting clinopyroxene in thesame sample (Kinzler and Grove, 1992).

The result (Table 4) shows that the late stage of thefractionation of the Otoge volcanic complex producedthe residual melt similar to the Kamoyamagawatrachyte. Plagioclase, clinopyroxene, and olivine arethe major extracted phases in the early stage, and theratio between plagioclase and mafic minerals does notchange largely. The fractionation of clinopyroxeneand olivine lowers the MgO content of the magma,which causes the rapid increase of the whole-rockFeOp/MgO. This is a characteristic feature of thecompositional variations of the Otoge igneous


Table 3Whole-rock average compositions of the selected samples for the least square calculation

Group name G1 G2 G3 G4 G5 G6 Kamoyamagawa trachyteRange of MgO (wt.%) 5.1–4.7 4.5–4.2 4.0–3.8 3.2–2.6 2.2–1.5 1.1–0.9 0.0Averaged number 4 4 5 5 5 2 1

Whole-rockSiO2 52.9 53.0 52.8 54.1 56.0 63.5 68.9TiO2 2.1 2.3 2.7 2.4 1.8 0.9 0.4Al2O3 16.5 16.1 16.1 15.5 15.4 15.0 14.7FeO 9.4 9.9 10.6 11.3 11.2 7.2 4.6MgO 4.9 4.3 3.9 3.0 2.0 1.0 0.0CaO 8.7 8.6 8.1 6.9 5.9 4.1 1.0Na2O 3.9 4.0 3.6 4.6 5.1 5.4 6.9K2O 1.2 1.3 1.7 1.6 1.8 2.5 3.4Y (ppm) 27 25 29 34 42 44 58Zr (ppm) 188 196 209 223 298 313 584

Calculated groundmass composition(wt.%)SiO2 53.1 53.0 52.8 54.1 56.0 65.1 70.0TiO2 2.1 2.4 2.7 2.4 1.8 0.8 0.4Al2O3 15.9 15.8 16.1 15.5 15.4 14.8 13.1FeO 9.8 10.3 10.6 11.3 11.2 6.3 5.5MgO 5.0 4.4 3.9 3.0 2.0 0.7 0.0CaO 8.3 8.4 8.1 6.9 5.9 3.9 0.4Na2O 3.9 4.0 3.5 4.6 5.1 5.4 6.4K2O 1.3 1.4 1.7 1.6 1.8 2.8 4.2Y (ppm) 29 27 30 36 45 51 74Zr (ppm) 204 207 213 242 323 366 750

Modal composition of phenocrysts(vol.%)Plagioclase 6.1 3.9 1.4 Tr. Tr. 10.0 21.0Clinopyroxene 0.8 0.8 0.3 Tr. Tr. 1.4 1.1Olivine 0.4 Tr. 0.1 0.0 0.0 1.9 Tr.Oxides 0.2 0.2 0.1 0.0 0.0 0.8 0.0Groundmass 92.2 94.9 97.9 100.0 100.0 85.6 77.9Modal % of crystals 7.8 5.1 2.1 0.0 0.0 14.4 22.1

Tr.: Trace, FeOp: Total Fe as FeO.

complex including the Shitara central dike swarm.The extraction of Fe–Ti oxides in the more fractio-nated stage causes the decrease of FeOp and TiO2 andsignificant increase of SiO2. Fractionation of plagio-

clase causes the decrease of CaO and the increase ofK2O. The compositional relationship between ilme-nite and ulvospinel in the P1-type rocks indicatesthe extremely low oxygen fugacity (below FMQ


Table 4Results of major element modelling

Parent G1 G2 G3to daughter G2 G3 G4

Residual melt comp. Residual melt comp. Residual melt comp.

Obs. Calc. Diff. Obs. Calc. Diff. Obs. Calc. Diff.

SiO2 53.0 52.9 0.0 52.8 52.8 0.0 54.1 53.6 0.5TiO2 2.4 2.4 0.0 2.7 2.7 0.0 2.4 2.4 0.0Al2O3 15.8 15.8 20.1 16.1 16.0 0.1 15.5 16.3 20.8FeO 10.3 10.3 0.0 10.6 10.6 0.0 11.3 11.3 0.0MgO 4.4 4.4 0.0 3.9 3.9 0.0 3.0 3.5 20.5CaO 8.4 8.5 20.1 8.1 8.1 0.0 6.9 7.8 20.8Na2O 4.0 3.9 0.1 3.5 4.1 20.5 4.6 3.7 1.0K2O 1.4 1.3 0.0 1.7 1.4 0.3 1.6 1.9 20.4

Y (ppm) 27 29 22 30 28 2 36 34 2Zr (ppm) 207 207 0 213 214 21 242 242 0

Calculatedextractedphases: wt.%

Pl 32 Pl 25 Pl 49Cpx 0 Cpx 56 Cpx 30Ol 69 Ol 18 Ol 9Il 0 Il 0 Il 12Usp 0 Usp 0 Usp 0

% Crystallized 1.3 3.2 12

Parent G4 G5 G6to daughter G5 G6 Kamoyamagawa trachyte

Residual melt comp. Residual melt comp. Residual melt comp.

Obs. Calc. Diff. Obs. Calc. Diff. Obs. Calc. Diff.

SiO2 56.0 55.8 0.2 65.1 64.9 0.2 70.0 70.0 0.0TiO2 1.8 1.8 0.0 0.8 0.8 0.0 0.4 0.4 0.0Al2O3 15.4 15.0 20.6 14.8 14.9 20.1 13.1 13.6 20.5FeO 11.2 11.2 0.0 6.3 6.3 0.0 5.5 5.5 0.0MgO 2.0 2.7 20.7 0.7 1.1 20.4 0.0 0.8 20.8CaO 5.9 6.1 20.2 3.9 4.0 20.1 0.4 0.4 20.1Na2O 5.1 4.9 0.2 5.4 6.2 20.8 6.4 5.2 1.2K2O 1.8 2.0 20.1 2.8 3.5 20.7 4.2 5.2 21.0

Y (ppm) 45 48 23 51 51 0 74 104 230Zr (ppm) 323 323 0 366 366 0 750 746 4

Calculatedextractedphases: wt.%

Pl 58 Pl 66 Pl 53Cpx 31 Cpx 17 Cpx 13Ol 0 Ol 7 Ol 22Il 6 Il 2 Il 11Usp 5 Usp 8 Usp 0

% Crystallized 25 57 51

Pl:plagioclase, Cpx: clinopyroxene, Ol: olivine, Usp: ulvo¨spinel, Il: ilmenite.

buffer by 3–4 log units; Ghiorso and Sack, 1991) forseveral P1-type rocks, of which whole-rock MgOcontents ranging from 3.0 to 4.6. The significantincrease of the whole-rock FeOp/MgO duringfractionation (Fig. 6) is consistent with the lowoxygen fugacity.

Variation of the degree of fractionation of the P1-type rocks (FeOp /MgO� 2–6) is constant along thedike swarm and shows no correlation against thedistance from the center of the Otoge cauldron. There-fore, their variation was not formed furing the intru-sion of dikes but was already formed when magmasintruded to the dike swarm. The variation in the indi-vidual dikes is much smaller (less than 1 wt% inMgO) than the variation of the whole P1-type dikes.This suggests that magmas with various degrees offractionation were supplied from the main magmareservoir to each dike intrusion event. Compositionalvariation of the P1-type dikes would reflect the spatialheterogeneity of the degree of fractionation within amagma reservoir or temporal evolution of the magmareservoir.

The low concentration of Cr and Ni in the severaldike rocks with highest value of MgO (,5 wt.%)suggests fractionation of olivine, clinopyroxene, andpossibly Cr-rich spinel in the earlier stage of differ-entiation. The magmas might have resided in a deeperpart of the magma plumbing system of the Otogeigneous complex, where fractionation of the maficphases took place before they intruded into the shal-low level to form the dike swarm.

9. Magma mixing

Bimodal textual and compositional variations of thephenocrysts within P2-type rocks are clear evidencefor magma mixing. The low Mg# clinopyroxene andlow-An plagioclase in the P2-type rocks have reversezoning, whereas the high-Mg# clinopyroxene andhigh An plagioclase in the same samples have normalzoning in their rim (Figs. 8–10). The dusty zone of thelow-An plagioclase with reverse zoning (Fig. 5c) issimilar to partially dissolved crystals that were experi-mentally reproduced by Tsuchiyama (1985), Johannes(1989), and Nakamura and Shimakita (1998). Thevermicular rim of clinopyroxene with low-Mg# core(Fig. 5d) is texturally similar to the dusty zone of

plagioclase. These features indicate that the sodicplagioclase and low-Mg# clinopyroxene phenocrystshave undergone partial dissolution by the change ofmelt composition, which was induced by mixingbetween less-fractionated and strongly fractionatedmagmas. Because the compositions of plagioclase,clinopyroxene, and olivine with dissolved textureare very similar to those in the strongly fractionatedKamoyamagawa trachyte, it is inferred that thesephenocrysts are not xenocrysts but were derivedfrom fractionated magmas through mixing which arerelated to the activity of the Otoge volcano. The P1-and P2-type rocks and the Kamoyamagawa trachyteshow linearly correlated concentration ratios amongincompatible elements, although the whole-rock MgOcontents show wide variation from 1 to 5 wt.% (Fig.7). This shows an internal mixing for P2-type rocks;the end components of the magma mixing of theShitara central dike swarm were derived from similarparental magmas by a fractional crystallizationprocess with different degrees.

The steep compositional profile between core andovergrown rim of the clinopyroxene phenocrysts inthe P2-type rocks (Fig. 9) indicates that the coreretains Mg# before magma mixing without significantFe–Mg diffusion. Thus, the Mg# of clinopyroxenecore of each group represents those of the mixingend components. The shaded areas in Fig. 10 repre-sent the range of Mg# that are expected for clinopyr-oxenes in equilibrium with the melt having thegroundmass composition of individual samples. Thedistribution coefficient of Fe and Mg between clino-pyroxene and melt (KD) for this estimation rangesfrom 0.23 to 0.27 (Groove and Bryan, 1983; Tormeyet al., 1987; Groove et al. 1992; Kinzler and Grove,1992). Because of the very reduced condition, Fe31 isassumed to be negligible. The whole-rock FeOp/MgOis nearly equal to that of the groundmass becausethese samples contain less than 5 vol.% of pheno-crysts and most of them are plagioclase. The ground-mass is homogeneous in a handspecimen scale andthus the groundmass FeOp/MgO can be regarded asthe mixed melt composition.

The Mg# of the normally zoned clinopyroxenes isslightly higher than the calculated values (Fig. 10 andTable 5), whereas those of the cores of reverselyzoned clinopyroxenes are much lower. One of theP2-type samples (specimen 60518C) has whole-rock


FeOp/MgO of 7.9 (Fe/Mg� 4.4). The representativeMg# of the normally zoned clinopyroxenes is 0.58 incore and are in equilibrium with a melt having Fe/Mgof 2.7–3.1, which is slightly smaller than the whole-rock value (Table 5). The representative Mg# of thereversely zoned clinopyroxenes is 0.05 in core andthey are in equilibrium with a melt having Fe/Mg of70–83, which is markedly greater than the whole-rockvalue. This tendency is common to other P2-typerocks listed in Fig. 10 and Table 5.

The relationship between the whole-rock Fe/Mgvalue and Mg# of clinopyroxene phenocrysts in theP2-type rocks (Table 5) shows that the volume ratio ofthe strongly fractionated magma against the less-frac-tionated magma (F/P on Table 5) is estimated to bevery small. The whole-rock compositions of P2-typerocks are similar to their less-fractionated end compo-nent. Therefore, the P2-type rocks are plotted aroundthe fractionation trend of P1-type rocks withoutmixing (Fig. 13).

The P2-type rocks form no clear mixing trend and

are scattered on the variation diagram of whole rockcomposition (Fig. 6); nevertheless, the P2-type rocksprovide clear evidence for magma mixing discussedabove. Compositional variations of both high andlow Mg# clinopyroxenes in individual P2-typesamples are different among the dikes (Fig. 10).These observations indicate that the mixing endcomponents of P2-type rocks were not fixed butwere various. In other words, different combinationof magmas were mixed in each intrusion event ofP2-type dike. Variation of the end components ofP2-type rocks reflects the degree of fractionation ofmagmas (Fig. 13).

The phenocryst abundance of each mixing endcomponent also can be calculated from the mixingratio and their modal abundance of phenocrysts(Table 5). The phenocryst abundance is obtained bythe point-counting method for about 2000–4000points in 10 cm2 area of thin sections for individualsamples. The effect of overgrowth of phenocrystsafter mixing is negligible (less than 10% against the


Table 5Calculated composition and phenocryst abundance of mixing end componentsObserved values

Sample no. Whole-rock Representative Mg# of cpx Phenocryst abundance (%)

FeOp/MgOweight ratio

Fe/Mgmolar ratio

Normallyzoned

Reverselyzoned

Normallyzoned

Reversely zoned

60518C 7.9 4.4 0.58 0.05 0.7 0.7(0.56–0.59)a (0.03–0.13)

60518L 10.5 5.9 0.56 0.09 , 0.1 1.8(0.54–0.58) (0.07–0.19)

60517B 8.6 4.8 0.51 0.05 0.4 1.0(0.35–0.60) (0.03–0.07)

60316D 5.7 3.2 0.58(0.54–0.64)

0.08(0.08–0.18)

, 0.1 0.5

Results of calculation

Sample no. Mixing ratioF/Pb

Fe/Mg molar ratio Phenocryst abundance (%)

Less-fractionatedend comp.

Fractionatedend comp.

Less-fractionatedend comp.

Fractionated endcomp.

60518C 0.02 2.7–3.1 70–83 0.7 3560518L 0.06–0.08 2.9–3.4 37–44 , 0.1 24–3260517B 0.01–0.02 3.6–4.2 70–83 0.4 . 5060316D 0.01 2.7–3.1 43–50 , 0.1 . 50

a Compositional range within a sample is shown in parenthesis.b F/P: mixing volume ratio of fractionated end component against less-fractionated end component.

volume of crystal). The fractionated end componenthas relatively high (24–50 vol.%) and the primitiveend component has low (,0.7 vol.%) abundance ofphenocrysts. Such a high crystal abundance in thefractionated end component suggests that themagma was a crystal mush with high viscosity andyield strength (Marsh, 1988). The existence of thecrystal clots or small gabbroic xenolith in a P2-typesample, which consist of sodic plagioclase andferroaugite with composition similar to the dissolvedphenocrysts in their host P2-type rock, supports thehigh crystal abundance in the fractionated end compo-nent. Magma intruded into the crust cooled slowlybecause high viscosity of the crystal mush prevents

thermal convection (Huppert and Sparks, 1988).During this slow cooling stage, the highly fractionatedremnant magma in a small magma batch at the shal-low depth separated effectively and crystallizedsuccessively. When a new magma injected into sucha mushy magma body, euhedral crystals and clots ofcrystals were disrupted by shearing between the eject-ing magma and the mush (de Silva, 1989; Nakada etal., 1994; Umino and Horio, 1998), and wereentrained into the injecting magma. Because mixingratio of the fractionated magma against the less-frac-tionated magma is small as shown in Fig. 10 andTable 5, the decrease of temperature of less-fractio-nated hot magma was small. Therefore, the hot andless-fractionated magma could mix with the cold andstrongly fractionated magma without quenching.

The less-fractionated end components of the P2-type rocks are similar to the magmas of the P1-typemagmas. Distribution of the whole-rock compositionsof the P2-type rocks in variation diagram (Figs. 6 and13) suggests that the “less-fractionated” end compo-nents of P2-type rock were magmas withMgO , 4 wt.% and most primitive magmas of thedike swarm (MgO. 4 wt.%) did not participate inthe formation of P2-type dike. Compositional rangeof the normally zoned and “high” Mg# clinopyrox-enes of the P2-type rocks is ranging from 0.39 to 0.66in Mg# and higher Mg# clinopyroxenes (Mg# 0.66–0.80) are observed only in the P1-type rocks. This alsosuggests that the most primitive magma of the dikeswarm did not form the P2-type rocks.

10. Magma plumbing system of the Shitara centraldike swarm

Magma mixing by tapping of magmas from astratified magma reservoir has been proposed(Eichelberger, 1975; Sakuyama, 1979; Koyaguchi,1986). Fractionated magma in a single magma reser-voir may accumulate beneath the roof as a roof bound-ary layer (McBirney et al., 1985). The magma densityshould lower during the fractionation in the Shitaracentral dike swarm owing to the increase of SiO2

content and could form a stratified reservoir. Tappingof such a vertically stratified magma reservoir couldhave formed mixed magma to form dikes in the vici-nity of the supply center.


Fig. 13. Compositional relationship among P2-type rocks and theirmixing end components on the variation diagrams of whole-rockSiO2 and TiO2 against MgO content. Position of fractionation trendand mixing lines between the assumed mixing end members alsoshown. Fractionation trend calculated by least-square analysis(Table 4) is shown with broad broken line. Variation of the less-fractionated end components is shown with broad solid line on thefractionation trend. Estimated compositions of mixing end compo-nent of four P2-type dikes (Table 5) are shown by solid circle (less-fractionated ends) and open circle (fractionated ends). Kamoyama-gawa trachyte is also shown with open triangle. Representativemixing lines are shown with narrow dashed lines. Equivalent linesof mixing ratio of the fractionated end component (F=P� 0:10 and0.25) are shown with dotted lines. Other symbols are same to Fig. 6.

Absence of P2-type dikes in the Otoge cauldronarea is, however, not consistent with the stratifiedmagma chamber model. Therefore, magma mixingby rupturing of a stratified magma reservoir beneaththe Otoge cauldron to produce P2-type dikes was notthe case. The limited distribution of the P2-type rocksin the southern part of the dike swarm (Figs. 11 and12) suggests that the fractionated end components ofthe P2-type rocks were not stored beneath the Otogecauldron but existed only in the southern part of thedike swarm. Petrological characters of the P2-typerocks show that both mixing end components of theP2-type rocks are originated from the Otoge volcaniccomplex although the P2-type rocks distribute only inthe distant area from the Otoge cauldron.

The fractionated end component of the P2-typedikes can be formed as a remnant magma within thedikes, which changed their composition by in situfractionation. Intrusion of a dike to the southern partof the dike swarm formed an isolated pocket of aremnant magma in a distance from the main magmareservoir beneath the Otoge cauldron and fractionalcrystallization in the magma pocket formed stronglyfractionated magma. When a newly intruded dike ofless-fractionated P1-type magma ruptured the magmapocket, magmas with different degree of fractionationmixed and formed P2-type dike. The low intrusionangle (less than 458) of the P2-type dikes in the south-ern part of the dike swarm (Fig. 4) suggests that themagma pockets existed at a shallow depth (several kmfrom the present surface). In summary, rupturing of aremnant magma pocket by a new dike is a cause of themagma mixing forming the P2-type dikes.

A vertical settled, tabular shaped, and small magmapocket was suitable for formation of an evolvedmagma body because of rapid cooling, in which thefractionated residual melt with low-density accumu-lates at the upper tip of the pocket. Temperature of thehost rocks in the marginal part of the dike swarmmight be lower than the vicinity of the main magmareservoir and this might accelerate the fractional crys-tallization. The fractionated magma stored in anisolated pocket could not re-intrude as a new dikebecause of high crystallinity. When another newdike ruptures such a magma pocket, the storedmagma mixed with the new magma and formed anew dike as a mixed magma. Intermittent intrusionof a new dike from the main magma reservoir with

slightly variable degree of fractionation offered achance to mix with magmas with various degree offractionation.

The asymmetric distribution of the P2-type dikes inthe dike swarm (Figs. 11 and 12) suggests that thecondition for the magma mixing was satisfied onlyin the southern part of the dike swarm. The southernand northern parts show contrasts in the numberdensity of dikes, maximum thickness of dikes, andlithology and physical conditions of the host rocks(Figs. 1 and 2). In the southern part of the dikeswarm, the subalkaline Shitara igneous complex isdistributed (Fig. 1), where large-scale pyroclasticflows erupted repeatedly and two cauldrons wereformed just before the activity of the Shitara centraldike swarm (Takada, 1987a,b). Existence of thecomposite dikes with the Shitara magma nature indi-cates that the magma reservoirs of the Shitara igneouscomplex were still alive when the central dike swarmwas formed. The survived magma reservoir(s)beneath the southern part of the dike swarm shouldhave caused a large gradient of geotherm. In the north-eastern part, on the contrary, dikes intruded into coolmetamorphic rocks and granitic rocks of CretaceousRyoke metamorphic belt. Weak alteration in thenortheastern part of the dike swarm shows that thegradient of geotherm was small and that the tempera-ture of the host rocks might have been below 1008C atthe present eroded level.

The effect of the temperature of host rocks and dikethickness on the solidification time of the dikes isestimated by the method of Jaeger (1968). Solidifica-tion time is defined as the time for the center of a diketo lower the temperature from 11008C (liquidus of thedike rock with 55 wt.% SiO2) to 7008C (solidus of theKamoyamagawa trachyte). The liquidus temperatureswere determined by melting experiments at 1 atmon the less-fractionated P1-type dike rocks(MgO . 4 wt.%) and Kamoyamagawa trachyte. Thecalculated solidification time for a 5-m-thick dike is195 days for 5008C of the host temperature which isthe representative temperature of the southern part atthe intrusion depth, and 51 days for 1008C, northeast-ern part. This result shows that the solidification timeat the southern part was several times longer than thatin the northeastern part. The thickness of a dike alsocontrols the lifetime of a magma pocket. Althoughaverage dike thickness is similar in southern and


northeastern parts, the maximum thickness is larger inthe southern part (Fig. 2b). The solidification time isabout 8 days for a 1-m-thick dike, 195 days for a 5-m-thick dike, and 780 days for a 10-m-thick dike. Thecalculation shows that the higher ambient temperatureand thicker width of dikes prolong the solidificationtime at the southern part by several times compared tothe northeastern part.

Frequency of intrusion should be also an importantfactor to control the magma mixing of this typebecause high frequency of intrusion increases thechance of rupture of a magma pocket in a solidifying

dike by a newly intruding dike. The number density ofdikes in the southern part of the dike swarm is higherthan that in the northeastern part (Fig. 2a). Thissuggests that frequency of intrusion in the southernpart was higher than that of the northeastern part, ifthe temporal span of the dike swarm activity wassimilar in both parts. Longer lifetime of the fractio-nated magma in a pocket and more frequent intrusionof dikes resulted in formation of mixed magma prefer-entially in the southern part.

The possible magma plumbing system in the Shitaracentral dike swarm is schematically summarized in


Fig. 14. A schematic model for the internal magma mixing in the Shitara central dike swarm. (a) In the southern part of the dike swarm,solidification time of dike was long because of the high temperature of host rocks and large thickness of dikes. Highly fractionated trachyticmagma formed a small magma pocket inside a solidifying dike. When a new dike ruptured the fractionated magma pocket, magma mixingbetween a newly intruded less-fractionated magma and the highly fractionated remnant magma occurred. High frequency of intrusion enhancedthe chance of magma mixing. (b) In the northeastern part, dike was completely solidified before the intrusion of a next dike because of lowtemperature, small thickness of dike, and low frequency of intrusion.

Fig. 14. Magmas intruded intermittently from themagma reservoir beneath the Otoge cauldron intothe dike swarm formed a new dike. The dikes intrudedfrom the magma reservoir were fractionated tovarious degree acquired in the reservoir. Storedmagma within a dike underwent cooling and frac-tional crystallization. A magma pocket filled withstrongly fractionated magma may formed in thedike. When a newly intruding dike ruptured themagma pocket, mixing between the magmas withdifferent degree of fractionation occurred and P2-type dike was formed. The P2-type magmas intrudedto a shallower level and were chilled quickly beforesignificant Fe–Mg diffusion in clinopyroxenephenocrysts. When a newly intruding dike did notrupture the fractionated magma pocket, it resulted ina P1-type dike. Asymmetrical distribution of the P2-type dikes shows that the condition for magma mixingwas satisfied only in the southern part of the dikeswarm. High frequency of intrusion, larger thicknessof dikes, and higher temperature of the host rock in thesouthern part of the dike swarm generated the condi-tion for magma mixing in the dike swarm. The exis-tence of the preceding volcanic activities (Shitaraigneous complex) may have caused the difference inthe geotherm, which affected the chance for formationof the P2-type dikes.

11. Conclusions

A magma plumbing system was reconstructed onthe basis of geology and petrology of the Shitaracentral dike swarm in central Japan. Intrusion direc-tions of the dikes were estimated from the flow linea-tion on the dike wall and oblique alignment ofelongated bubbles against the dike wall. The magmasfilled the central dike swarm were supplied verticallyfrom underneath the Otoge cauldron, then intrudednorthward and southward with low angles.

The rocks of Shitara central dike swarm are dividedinto P1- and P2-types by the petrologic characters ofphenocrysts; P1-type rocks have only euhedral pheno-crysts without reverse zoning and dissolution texture,and P2-type rocks are characterized by the presence ofreversely zoned and dissolved phenocrysts. Thecompositional variation of P-1 type rocks wereformed by fractional crystallization in the magma

reservoir beneath the Otoge cauldron. The disequili-brium assemblage, reverse zoning, and dissolutiontexture of the phenocrysts in the P2-type rocks indi-cate internal mixing of magmas with different degreeof fractionation. The whole-rock compositions anddisequilibrium clinopyroxene phenocrysts suggestthat a small amount of fractionated magma mixedwith a large amount of less-fractionated magma.Both mixing end components of the P2-type dikesare formed from the similar parental magmas of theOtoge volcano by fractional crystallization.

Compositional variation of the phenocrysts amongthe P2-type rocks shows that both mixing endcomponents of the P2-type rocks were different foreach P2-type dike. The compositional variation ofthe strongly fractionated mixing end components ofthe P2-type dikes suggests that the end componentswere formed within isolated reservoirs in the dikeswarm. The spatial distribution and intrusiondirection of the dikes of P2-type rock indicatethat magma mixing took place not in the magmareservoir beneath the Otoge volcanic center butwithin the dike swarm at a shallower level.When dikes with less-fractionated P1-typemagma ruptured a small magma pocket in apreceding dike, magmas with different degree offractionation mixed and formed P2-type dike. Thevariation of the less-fractionated end componentsreflects the difference in the degree of fractiona-tion in the magma reservoir beneath the Otogecauldron as P1-type rocks.

The asymmetrical structure of the dike swarm(length of the dike swarm, number density of dikes,thickness of dikes, and distribution of P2-type dikes)reflects the geological contrast between the northernand southern parts of the dike swarm. The existence ofthe proceeded volcanic activities of the Shitaraigneous complex in the southern part of the dikeswarm caused the asymmetrical structure of the dikeswarm and gave appropriate conditions for the frac-tionation and internal magma mixing only in thesouthern part of the dike swarm.

Acknowledgements

I thank Hiroko Nagahara and Kazuhito Ozawa forvaluable discussions and encouragement during this


work. I also thank Akira Takada for his valuablesuggestions and helpful information for the fieldsurvey. Hideto Yoshida is thanked for XRF andEPMA analysis, and helpful discussion during thiswork. Critical reviews by Michael Garcia and AkiraTakada greatly improved the manuscript. I am alsoindebted to Akira Imai, Hikaru Iwamori, SetsuyaNakada, Michihiko Nakamura, Takamitsu Sugihara,Eiichi Takahashi, Atsushi Toramaru, and MitsuhiroToriumi for their important advises. This work wassupported by the Japanese Society for the Promotionof Science for Japan Junior Scientists.

Appendix A. Sample selection procedure tominimize the effects of hydrothermal alteration

The evaluation of the effects of hydrothermalalteration on the whole-rock compositions and selec-tion of least-altered samples are crucial becausevolcanic rocks in a dissected volcano are usuallysuffered from hydrothermal alteration in variousdegrees and rarely keep the original composition.Dike rocks of the Shitara central dikes are not anexception. To evaluate the effects of alteration, aseries of samples (V1–V5) with different degrees of


Table A1Effect of alteration on the whole-rock composition

Groundmass minerals:W, not altered;K, partly altered;× , completely altered. Secondary minerals:2, absent;1, present (minor);1 1 ,abundant.

alteration were collected from an aphyric basalticandesite block in the Otoge pyroclastic rocks withabout 2 m in diameter and showing concentric altera-tion, and were analyzed with same procedure forwhole-rock analysis. Judging from the structural andtextural characters, it is assumed that the block had ahomogeneous composition before alteration and thealteration type is same to the dike rocks.

The contents of SiO2, TiO2, Al2O3 and FeOp arerelatively constant, whereas the MgO, CaO, Na2O,K2O and P2O5 contents systematically increase ordecrease as the degree of alteration changes (TableA1). Particularly, remarkable enrichment of K2Oand depletion of Na2O is observed in the alteredsamples. The differences of the Nb, Y, and Zr contentsbetween the least altered sample (V1) and the weaklyaltered sample (V2) are small.

On the basis of the combined mineralogical andchemical analysis, discussion on the whole-rockanalysis is limited in the samples with the alterationdegree as V1 and V2 samples with minor chlorite andclay minerals. Most of the olivine crystals are alteredin these samples in V1 sample. All the samples for thewhole-rock analysis were observed under an opticalmicroscope and checked the degree of alteration. Thislimits the effect of hydrothermal alteration on theNa2O and K2O contents in less than 4% and on theoxides less than 2% of original values. The effect onNb, Y, and Zr contents are limited to less than 1–2%.This value is comparable to the analytical error ofXRF for these elements (Yoshida and Takahashi,1997). About 90% of the dike samples were excludedfrom the whole-rock analysis with this selection.

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