smectite-to-chlorite transformation in thermally ... · smectite-to-chlorite transformation in...

American Mineralogist, Volume 76, pages 628-640, l99l

Smectite-to-chlorite transformation in thermally metamorphosed yolcanoclastic rocksin the Kamikita area, northern Honshu, Japan

Arsuyuxr lNourGeological Institute, College of Ans and Sciences, Chiba University, Chiba 260, Japan

MrNonu Uuoa.The University Museum, University of Tokyo, Tokyo I 13, Japan

Ansrnlcr

Miocene volcanoclastic rocks at Kamikita, northern Honshu, Japan, exhibit the effectsof an intensive episode of thermal metamorphism caused by a hornblende quartz dioriteintrusion. Metamorphic zones with a total thickness of approximately 6 km are concen-trically developed around the diorite mass. The zones are defined by the sequential ap-pearance of characteristic mineral assemblages from smectite (Zane I), to smectite * heu-landite + stilbite (Zone II), to corrensite + laumontite (Zone III), to chlorite + epidote(Zone IV), and finally to biotite + actinolite (Zone V) with increasing metamorphic grade.As smectite transforms to chlorite, the percentage of smectite layers in interstraiified chlo-rite/smectite (C/S) (the intermediate products) decreases discontinuously with increasingmetamorphic grades, with steps at 100-800/0, 50-40o/o (corrensite), and 10-00/0. The trans-formation of smectite to chlorite through corrensite is characterized chemically by decreas-es in Ca and Si, an increase in Al, and a constant Fel(Fe + Mg) ratio. The mineralparagenesis, structural variation, and compositions all support the hypothesis that corren-site is a thermodynamically stable C/S phase. Corrensite formed at temperatures betweenapproximately 100 and 200 "C. The Kamikita metamorphic zonation was developed inresponse to a thermal gradient of approximately 70 .C/km, and the secondary mineralscrystallized with near-equilibrium compositions.

fNrnooucrroN rite transformation, e.g., from smectite to corrensite orfrom corrensite to smectite. Furthermore, Helmold and

Mafic phyllosilicates such as saponite, interstratified van der Kamp (1984) noted that there is a continuouschlorite/smectite (C/S), and chlorite are abundant in low- decrease in expandability over the entire range of pro-grade metabasites. The presence of these minerals has portion of smectite layers (o/osmectite) in C/S. phase re-been reported from diagenetic environments (Hoffman lationsofC/SwerediscussedbyPeterson(1961)andVeldeand Hower, 1979; Chang et al., 1986), active and fossil (1977). They inferred, on the basis of thephase rule, thatgeothermal systems (Tomasson and Kristmannsdottir, c/S is a stable phase.1972; Kristmannsdottir, 1983; Inoue et al., 1984a, 1984b; In this study, we describe the relations for the maficInoue' 1987; Liou et al., 1985), ophiolites (Evarts and phyllosilicates from thermally metamorphosed volcano-Schiffman, 1983; Bettison and Schiffman, 1988), and oce- clastic rocks peripheral to diorite intrusive masses at Ka-anic crust (Alt et al., 1986). Interstratified c/S generally mikita, northern Honshu, Japan. Detailed X-ray powderoccurs as an intermediate product during the smectite- diffraction and microprobe analyses have been pe*ormedto-chlorite transformation. It is interesting that the nature on the phyllosilicates and assoiiated calcium aluminumof interlayering in C/S is temperature sensitive in a fash- silicate minerals. The objectives of this paper are to clar-ion similar to illite/smectite (I/S) (e.g., Hoffman and ify the transformation process of smectite to chlorite inHower, 1979; Srodori and Eberl, 1984; Horton, 1985). the thermal metamorphic environment and the thermo-Earlier work reported, for the process of transformation dynamic status of corrensite as an intermediate product.of smectite to chlorite, that the expandability of smectitedecreased discontinuously with increasing temperature indiagenetic environments and hydrothermal systems (In-oue et al., 1984a, 1984b; Inoue, 1987). Only a few types Gnor,ocrcAl, SETTTNGof ordered structures, Reichweite : 0 and l, appeared in The Kamikita area is located in the northern part ofintermediate C/S products (Reynolds, 1988). However, Honshu, Japan. As shown in Figure l, Miocene andChang et al. (1986) and Schultz (1963) indicated that the Quaternary sequences occur, as does the Kamikita Ku-expandability ofC/S could decrease continuously in lim- roko-type ore deposit. The geology ofthe area has beenited portions of the entire range of the smectite-to-chlo- described in detail by many reseirchers (Miyajima and0003-004x/9 l/0304-0628$02.00 628

Mizumoto, 1965, 1968; Lee, 1970:' Lee et al., 1974',MMAJ, 197 4, 197 5, 1986).

The Miocene group consists of three formations in or-der of decreasing age, as follows: the Kanegasawa for-mation is composed of massive or brecciated basaltic toandesitic lava flows. The Yotsuzawa formation is com-posed principally of andesitic to dacitic lava flows andvolcanoclastic rocks and some mudstone layers. TheWadagawa formation is composed of felsic and basalticvolcanics. It contains intercalated marine mudstones. TheKamikita Kuroko deposit occurs in the lowermost hori-zon. Several small diorite bodies have intruded the Ka-negasawa and Yotsuzawa formations but hornfelsic rocksare not observed in the field. The Kanegasawa, Yotsu-zawa, and Wadagawa formations dip moderately (10-30')on both sides of an anticlinorium. which has a north-south direction in the study area.

The Quaternary group is divided into three formations:Tashirodai welded tuffs, andesite lava flows, and lake de-posits. They unconformably overlie the Miocene sedi-ments and dip gently (0-10').

A simplified geologic map with the locations of the drillholes utilized in this study is shown as Figure l, and ageologic cross section including the drill holes is given asFigure 2. A diorite intrusive mass is found at a depthbelow 400 m of drill hole no. 15, as shown in Figure 2.

629

MBrnoos

More than 200 samples were collected from outcropsand five drill holes. X-ray powder diffraction (XRD) andthin section studies were used to determine the distri-bution and textural variation of secondary minerals. Theclay size fraction (< I pm) was obtained by ultrasonicdisaggregation and centrifugation of clay-HrO suspen-sions. Oriented specimens with an approximate area of20 x 15 mm were prepared by placing the suspensionson a glass slide. XRD patterns of the air-dried and eth-ylene-glycol-saturated specimens were obtained using aRigaku RAD I-B diffractometer (40 kV, 20 mA) equippedwith a Cu tube, graphite-monochrometer, and 0.5" di-vergence and scattering slits. Percentage of smectite lay-ers (o/oS) and ordering type (Reichweite) of interstratifiedC/S and I/S were determined by comparing the observedXRD patterns with computer-simulated patterns usingthe program Newmod (developed by R. C. Reynolds,Dartmouth College, Hanover). XRD patterns ofrandom-ly oriented specimens were also obtained in order to de-termine the d(060) value of mafic phyllosilicates.

Energy-dispersive microprobe analyses of phyllosili-cates and calcium-aluminum silicates in polished thinsection were conducted on a Hitachi 5-550 scanning elec-tron microscope fitted with a Kevex 7000 A-75 solid state

INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION

Fig. 1. Geologic map of the Kamikita area indicating localities of drill holes and outcrop samples used in this study.

L E G E N D

[ElOuatetnary Lake Deposits

ff iQuaternary Andesites

@Quaternary

f;lwaoaoa*a Formation

f lvotsuza*a Formation

@lrane9"""wa Formation

l F l o i o r i t e

fTlruroto ore Deposits

f f lo ' i t t not""

630 INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION

E

co

6

gul

Ec.9G

9 +ooIIJ

Dlstance f rom Dlor l te ln t rus lve Mass (km)

Ewetded Tut ts f f i Sandstones

B Basa l t i c Rocks f f iMudstones

ElAndes i t i c Rocks E D ior i te

E D a c i t i c R o c k s

Fig. 2. Geologic cross section along the line of Figure I . Qwt: Quaternary welded tutrs; Wg : Wadagawa formation; Yz :Yotsuzawa formation; Kn : Kanegasawa formation.

Tlau 1. Secondary minerals occurring in zones

Zones

Secondaryminerals

Dlstance f rom Dlor i te Int rus ive Mass (km)

Fig. 3. Cross section showing the distribution ofzones.

detector. Analyses were caried out using an acceleratingvoltage of 20 kV, a beam current of 200 pA, a beamdiameter of 2 pm, and a counting time of 200 s. Analyseswere obtained on 3- 15 points of each mineral in a thinsection. EDS data were reduced by a ZAF scheme(QANTX software) which was modified by Mori and Ka-nehira (1984). The standards included quartz (Si), corun-dum (Al), periclase (Mg), metals (Fe, Ti, and Mn), calcite(Ca), albite (Na), and potassium chromate crystals (K)(Mori and Kanehira, 1984). The bulk composition of rockswas determined with a JEOL JSX-60PX X-ray fluores-cence spectrometer using fused glass disks.

Rocx lr-rnnarroN

Six mineralogical zones were defined on the basis ofthe assemblages of 22 secondary minerals identified inthe drill cores, as listed in Table l. These zones are dis-tributed from the contact ofthe diorite mass to a distanceof approximately 6 km, as shown in Figure 3. The pet-rographic relations ofeach zone are described briefly be-low.

Zone I is defined by the presence of smectite and theabsence of other secondary minerals, except calcite andsilica minerals. The rocks from the Wadagawa formationand the upper-most part of the Yotsuzawa formation be-long to this zone. Brown-colored saponite is common inmafic to intermediate rocks, whereas montmorillonite oc-curs in felsic rocks. Saponite usually occurs as a replace-ment of primary orthopyroxene phenocrysts and glassygroundmass in mafic rocks. Montmorillonite principallyreplaces the glassy groundmass of felsic rocks. Clinopy-roxene and plagioclase phenocrysts are unaltered in bothrock types except for occasional replacement by saponite

d-cristobaliteQuartzMontmorilloniteSaponitelllite/smectiteCorrensitePhengiteChloriteBiotiteMordeniteHeulanditeStilbiteChabaziteLaumontiteWairakiteAlbiteSpheneEpidoteActinoliteCalciteHematitePyrite

++++

+++

++++

++

++

Note: Zone | : smectite zone, Zone ll : smectite-zeolite zone, SUb.zones llla and lllb: corrensita.laumontite zone, Zone lV : chlorite-epi-dote zone, Zone V : biotite-actinolite zone.

Sub- Sub-Zone Zone zone zone Zone Zone

I l l l l la l l lb lV V

+T A

++ + +

+

++

+ + ++ ++ + +

++ ++ + +

+

INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION 631

and calcite, respectively. Cristobalite is associated withsmectite in the part of this zone of lowest grade, whereassecondary quartz occurs in the part with higher grade.

ZoneII is defined by the presence of both smectite andzeolites. The rocks from the upper part ofthe Yotsuzawaformation belong to this zone. The mode of occurrenceof smectite in this zone is the same as that of Zone l.That is, saponite is a common variety of smectite in maficto intermediate rocks, and montmorillonite is a commonvariety in felsic rocks. Where both saponite and mont-morillonite were identified (in the upper part of drill holeno. 18) by XRD, each was observed to replace only onekind of rock fragment, as observed in thin section. Stil-bite, heulandite, and chabazite occur in mafic rocks; mor-denite is dominant in felsic rocks. These zeolites replaceglassy groundmass and fill cavities. Weakly albitized pla-gioclase is partially replaced by calcite or saponite. Sec-ondary quartz occurs as fine-grained aggregates withsmectite in the glassy groundmass. Celadonite and ver-miculite, which were clearly distinguished from saponiteby optical, microprobe, and XRD analyses, were recog-nized as vesicle fillings in sample l8-320-m, which waslocated near the boundary between Zones II and III. Aprecipitation sequence was observed within vesicles fromthe wall to the center as follows: (l) a transparent thinrim consisting offine-grained zeolite, quartz, or both (2)fine-grained brown vermiculite, (3) fine-grained bluish-green celadonite, (4) flaky vermiculite and zeolites. Thelast assemblage is absent in small vesicles.

Zone lll is defined by the presence of corrensite andlaumontite. The rocks from the middle part of the Yotsu-zawa formation belong to this zone. Corrensite generallyoccurs as a replacement of mafic phenocrysts and glassygroundmass, and as a pore filling identical in mode ofoccurrence to that of saponite in Zones I and II. It alsooccurs as inclusions within albitized plagioclase pheno-crysts, associated with micaceous clays. Flakes of corren-site are generally less than 0.1 mm in length and showpleochroism from green to pale green. In higher gradeparts of the Zone III, however, corrensite is deeper incolor and gtains are larger. Such corrensite often coexistswith spherulites of fine-grained sphene and epidote thatare less than 0.1 mm in diameter. Znne III is furtherdivided into two subzones: subzone IIIa (epidote-absentsubzone) and subzone IIIb (epidote-present subzone).Laumontite with corrensite occurs commonly in bothsubzones as pore fillings. Interstratified I/S having < 150/osmectite is present in mudstones and sandstones in sub-zone IIIb.

Tnne IY is defined by the general presence of chloriteand epidote and the absence of corrensite and laumontite.The rocks from the lower part of the Yotsuzawa forma-tion and the upper part of the Kanegasawa formationbelong to this zone. Chlorite occurs as a replacement ofmafic phenocrysts and glassy groundmass, and as a porefilling. It shows strong pleochroism from dark green topale green. It is also characterized by isotropic-anoma-lous blue interference colors and both negative and pos-

itive elongations. Grains of epidote in this zone are lessthan 0.3 mm in length but are larger than those found insubzone IIIb. Plagioclase phenocrysts are almost com-pletely replaced by calcite or epidote or both. The re-maining plagioclase is strongly albitized. In felsic rocks,phengitic mica coexists with chlorite. Mudstones of thiszone are usually composed of illite, chlorite, and quartz.A wairakite-like mineral was occasionally identified intuffaceous mudstones (samples l6-280-m and l6-300-m).A mudstone sample (16-560-m) is composed of regularlyinterstratified I/S (rectorite) with hematite and very smallamounts of illite, chlorite, and quartz. Veinlets composedofepidote, andradite, and anhydrite are found in an an-desite sample (l 6-576-m).

ZoneY is defined by the presence ofbiotite and actin-olite. This zone is recognized only within the diorite mass.Primary hornblende is replaced by fine-grained biotite,actinolite, chlorite, hematite, and pyrite. These secondaryminerals have crystallized along the cleavages of horn-blende grains. Biotite usually shows strong pleochroismfrom reddish brown to yellow. In the upper parts of thediorite mass (413-420 m depths of drill hole no. l5),secondary biotite is further altered to interstratified bio-titelvermiculite or vermiculite. In this zone, epidote issporadically found as fine-grained aggregates, and chloriteis characterized by anomalous brown interference colorsand negative elongation. Although hematite is commonlyfound in the rocks of Zones II-Y, pyrite is rare in theother zones.

As already reported by Inoue and Utada (1989), dick-ite, pyrophyllite, zunyite, topaz, alunite, rectorite, su-doite, and tosudite are recognized in the upper parts ofdrill hole no. 15 above the diorite mass. They are prob-ably the products of an acidic hydrothermal alterationthat prevailed at a late stage. The original metamorphicminerals and textures of the rocks in the upper parts ofdrill hole no. l5 were completely destroyed by the super-imposed effects of the acidic hydrothermal alteration.

Srnuctuur, vARrATroN oF TNTERSTRATTFTEDCLAY MINERALS

Structural variation of interstratified minerals can beclarified by examining changes in position and intensityof the 001 reflections (Reynolds, 1980). Figure 4a showsrepresentative XRD patterns of samples from the sapo-nite-corrensite-chlorite series from the study area. Sapo-nite is recognized by the expansion of the 001 spacingfrom 14 to approximately 17 A upon ethylene glycol sat-uration. The d(060) value is 1.532-1.535 A Glg. +t).Corrensite shows expansion from 29 to 3l A with eth-ylene glycol saturation. The value ofd(060) ofcorrensiteranges from 1.540 to 1.542 L. Chlorite generally hasd(001) : 14.2 A and d(060) : r.541-1.545 A. The valueofd(001) does not change with ethylene glycol saturation.XRD patterns of examination, some of the C/S containedbroad peaks with intermediate d(001) values of approx-imately 16.5 A or 15.1 A, corresponding to a value be-

23-250 -m

100 %s

eo%s +50%sg.gs , 7 '7 ,2 t . tg

18-465 -m

45 %S

14.52 17-438-m

40%s+o%s

632

2 4 6 8 1 0 1 2 1 4o2O Cu Ka rad ia t ion

tween those for saponite and corrensite or between thosefor corrensite and chlorite. The intensities of the reflec-tion with d: 3l A were variable. The relative intensitiesof the l4-, 7-, and 4.7-A reflections were intermediate to


.=otro

f,s

f

0tro

F

c

Fig. 4. (a) X-ray powder diffraction patterns ofsamples ofthe saponite-corrensite-chlorite series. They were recorded on oriented,ethylene-glycol-saturated specimens using step-scanning. The d values of peaks are given in A. O) The (060) peaks of the samesamples as in a.

59 .O 60 .0 61 .0 62 .0

"29 Cu Kc. radiat ion

those of the above two pairs of minerals. Recording ofthe XRD patterns by means of a step-scanning methodrevealed that the intermediate d(001) value and intensi-ties were not due to some specific interlayering in C/S but

b

23-250-m

18-349-m

o1 .541A

18 -465 -m

o1 .541A

16-570-m


N o . 1 6

633

N o . i zNo.23

'@& 60 &;:.:J","I :'::;:""':T"".""1:;J

""JT,"j

60 40 20 o

Fig. 5. Variations of percentages of smectite layers in chlo-rite/smectite (solid circles) and illite/smectite (open circles) as afunction ofdepth ofdrill holes. The tie lines indicate the pres-ence of more than two chlorite/smectite samples having differentsmectite layer percentages in a rock specimen. Abbreviations asin Figure 2.

were caused by more than two phases, as clearly illus-trated by Figure 4a.

The variation of the relative amount of smectite in C/Sand in I/S is illustrated in Figure 5 as a function ofdepth.The value of o/osmectite in C/S decreases discontinuouslyfrom 1000/o smectite (saponite) to 00/o smectite (chlorite)with intermediate values of 50-40o/o smectite (corrensite)with increasing metamorphic grade. The o/o smectite val-ue in saponite is > 800/0, whereas that in chlorite is < 100/0.The existence of C/S having intermediate 0/o smectite val-ues other than 50-400/o smectite is not certain in the Ka-mikita area. The intimate association of saponite, corren-site, or chlorite as separate phases is implied by XRDdata in samples of drill hole nos. 16, 17 , and 18. The C/S having 5o-40o/o smectite (termed corrensite in thisstudy), which yields a superlattice reflection with d : 3lA after ethylene glycol saturation, has a Reichweite (R)value of l The C/S that is more or less expandable thancorrensite may have R : 0. The C/S in drill hole no. 18shows irregular variation in the proportion of smectitewith depth. These data imply that less expandable C/Soccurs commonly in porous rocks like tuffaceous brecciasand hyaloclastites, and more expandable C/S dominatesin massive rocks like lavas.

The variation of proportion of smectite in I/S shownin Figure 5 varies discontinuously. The I/S from core nos.18 and 23 generally is pure smectite. The I/S in mud-stones and sandstones in the upper part of core no. 16(near the lower end of subzone IIIa) contains l5olo smec-tite (Fig. 5) and the I/S has R : 3 ordering. The propor-tion of smectite in I/S decreases continuously from I 5 to0olo with depth in drill hole no. 16. The proportion ofsmectite layers in I/S is zero approximately where cor-rensite no longer occurs. The trends in variation ofpro-portion of smectite in I/S at intermediate I/S values is

Fig. 6. Compositions ofchlorite, corrensite, and saponite fromvarious drill holes. Solid circles and asterisks: drill hole no. 16;open circles and stars : drill hole no. 17; open triangles : drillhole no. 15; solid stars and triangles : drill hole no. l8; openboxes and solid boxes : outcrop samples.

not clearly defined in the Kamikita area because diocta-hedral clays are rare in core nos. 17 and 18.

VlnrlnoNs rN coMPosrrroN oFSECONDARY MINERALS

Representative analyses of selected minerals are listedin Tables 2-4. The total Fe for phyllosilicates and actin-olite was calculated as Fe2+ and as Fe3+ for epidote andgarnet.

Saponite

The Si content of saponite ranges from 3.36 to 3.56,and the Fe/(Fe + Mg) ratio from 0.351 to 0.361 (Table2 and Fig. 6). The interlayer cation is principally Ca. Thesaponite composition ranges are essentially identical forall modes of occurrence.

Corrensite

Empirical forrnulas of corrensite were norlnalized toOro(OH)ro (Newman and Brown, 1987). The Si contentwas variable from grain to grain but was relatively con-stant within a grain. The Fe/(Fe + Mg) ratio ranges from0.3 to 0.4 (Table 2 and Fig. 6). The corrensite in samplesl7-438-m and 17-343-m has ratios less than 0.2, how-ever. The general petrographic characteristics ofthe lattercorrensite samples are nearly the same as those of theothers, except that the latter are larger in size and deeperin color.

Figure 7 shows a plot of lolAl, vs. the total number ofoctahedral cations for corrensite. Inoue (1985) demon-strated that there is a linear relation between those twovariables for Fe-rich corrensite (0.52 < Fe/(Fe + Mg) <0.61). This relationship is shown by the solid line in Fig-ure 7. The data for Kamikita corrensite also fall alongthe line. Ideally, corrensite has nine octahedral ions per

LEGEND


TmLe 2. Representative analyses of saponite (1), corrensite (2-4), and chlorite (5-1 1)

Samde(1 )

18-240(21

1&360(4)

16-60(3)

1&465

sio,Tio,AlrosFeO-MnOMgoCaONarol(,O

TotalNumbers of O atomsSirltAlI4lTotalTirorAlFeMnMgI6lTotalCaNaKFe/(Fe + Mg)Range in Fel(Fe + Mg)Range in SiRange in numbers of oct.

cations

44.18

7.8615.340.08

15.042.68

0.3085.951 13.43o.574.00

0.151.000.011.812.970.23

0.030.356

0.351-0.3613.36-3.56

2.82J.06

33.27

13.1216.2'l0.34

20.710.76

84.40256.151.858.00

1.012.510.055.719.280.15

0.3050.290-0.3176.15-6.38

9.01-9.28

3ial.76

13.1017.770.19

18.24't .12

0.0884.25256.291.718.00

1 . 1 72.770.035.079.040.22

0.020.353

0.341-0.3786.20-6.36

8.99-9.16

31.88

15.4620.250.08

17.691 . 1 5

0.0486.55255.872.138.00

1.233.',120.014.869.220.23

0.010.391

0.370-0.3985.79-5.87

9.22-9.38'Total Fe as FeO

oc.9aa2toEo

o

8910Numbers of Octahedra l Cat ions

Fig. 7 . Plot of totAl content vs. total of octahedral cations perOro(OH)ro in corrensite. Bars indicate ranges of data. The solidline indicates the correlation of Fe-rich corrensite as noted bvInoue (1985).

Oro(OH)ro. The number of octahedral cations ranges from8.85 to 9.7. These values correspond to 52-38o/o smectiteif the number of octahedral cations is assumed to be sixper Oro(OH)o for saponite, nine for corrensite, and 12 perOr0(OH)r6 for chlorite. The proportions of smectite inter-layered with corrensite estimated from the chemical com-position are consistent with those determined by XRD,as described above. Consequently, if the linear relationof the numbers of octahedral cations vs. the tetrahedralAl content as shown in Figure 6 is a general relation forcorrensite, we infer that the C/S, which provided a su-perlattice reflection at 3l A after ethylene glycol satura-tion, contains approximately SiroAlru - SiuoAl,u perOro(OH),. in the tetrahedral sheet and 8.8-9.8 cations inthe octahedral sheet. The estimated values are in satis-factory agreement with analyzed values for corrensite fromhydrothermal systems (Seki et al., 1983), diagenetic en-vironments (Chang et al., 1986), and ophiolites (Evartsand Schiffman, 1983; Bettison and Schiffman, 1988).

Chlorite

The compositional data for chlorite are plotted in aFoster-type diagram in Figure 6 (Foster, 1962). The num-ber of Si atoms ranges from 2.75 to 3.2, and the Fel(Fe+ Mg) ratio from 0.24 to 0.40. For the chlorite of samplel6-546-m, which filled pore spaces of andesitic volcanics,however, the number of Si atoms ranges from 2.68 ro2.77, and, the Fel(Fe + Mg) ratio ranges from 0.525 to


TaEte 2-Continued

(5)| 7-459

(6)16-140

(7)16-242

(8)I 6-480

(s)1&570

(10)1 5-480

0 1 )outcrop 75

28.26

16.9719.400.46

20.210.14

0.0685.501 42.961.044.00

1.061.700.043.165.960.02

0.010.350

0.350-0.3992.90-2.97

5.96-5.98

29.40

15.6518.610.34

20.350.13

84.481 43.100.904.00

1.041.640.033.205.910.01

0.3390.310-0.3452.90-3.12

5.70-5.92

27.93

20.0919.520.40

18.350.30

86.591 42.881. ' , t24.00

1.321.680.032.825.850.03

0.3730.364-0.3792.86-2.91

5.83-5.88

26.96

20.2018.010.34

19.560.15

85.221 42.811 . 1 94.00

1.291.570.033.045.930.02

0.3410.337-0.3502.81-2.85

5.93-5.95

83.431 42,811 . 1 94.00

1 . 1 71.500.043.296.000.01

0.3130.302-0.3252.79-3.02

5.93-6.04

27.880.12

16.8417.000.s9

21.560.060.040.42

84.491 42.941.064.000.011.031.500.0s3.385.970.010.010.060.307

o.299-0.3272.83-3.10

5.8M.04

28.90

17.O416.000.16

22.750.11

84.971 42.991.014.00

1.071.380.013.s05.960.01

0.2830.238-0.2962.80-2.99

5.96-6.03

26.43

18.8716.860.41

20.750.11

0.558. Except for this one specimen, however, the Si con-tent and the Fe/(Fe + Mg) ratio of Kamikita chlorite tendto be smaller with increasing metamorphic grade (Figs. 6and 8).

Tleu 3. Representative analyses of biotite (1-2) and phengite(3-4)

(1) (21 (3) (4)Samole 15440 15-500 1&498 16-520

Figure 9 is a plot of r4lAl - I vs. t6lAll + 2Ti - I forKamikita chlorite. The l: I line in Figure 9 illustrates theTschermak substitution (Al + Al + Si + Mg). Analysesplotting above the l: I line show dioctahedral substitution(AlrMg-,) and correspond to octahedral vacancies (Laird,1988). Analytical data for most chlorite from Zone IVplot above the l:l line, suggesting the existence ofocta-hedral vacancies. Data for chlorite in Zane V appears toapproach the l:l line, implying a smaller proportion ofoctahedral vacancies. The total Al content also tends todecrease in chlorite from Zone IV to that from Zone V.

0 4 0 2Feo

FeO + HgO

Fig. 8. Compositions of chlorite, phengite, and biotite. Openand solid circles indicate chlorite compositions (averages) fromZones IV and V, respectively. The tie lines correspond to coex-isting chlorite and biotite, and chlorite and phengite.

50.380.23

28.663.83

2.780.13

10.2496.25

3.340.66

4.000.011.580.21

o.272.070.010.87

sio,Tio,Alro3FeO'MnOMgoCaOGO

Total

36.84 38.384.11 2.62

12.77 11.4211 .00 12.020.15 0.20

17.36 19.460.71 0.029.73 8.46

92.6s 92.52o : 1 1

49.640.20

27.O75.42

3.420.149.22

95.11

4.000.011.500.31

0.342.',t60.010.79

SirarAlFet4iTotalTir6rAlFeMnMgr6rTotalCaKFe(Fe + Mg)Range in Fe/

(Fe + Mg)Range in Si

2.801 . 1 90.014.000.240.000.690.011.972.910.060.94o.262

0.239-0.3092.80-2.88

2.901.O20.084.000.150.000.680.012.192.950.00o.820.258

0.222-0.2682.89-2.98

l l

- o 4

3.350.65

t Total Fe as FeO.


TABLE 4. Representative analyses of epidote (1-6), actinolite (7), andradite (8), heulandite (9), and laumontite (10)

Sample(1) (21 (3) (4) (5) (6) (7t (8) (s) (10)

17-228 17-438 16-498 16-520 16-546 16-570 15-500 16-570 18-231 18-465

sio,Tio,AlrosFerO.FeOMnOMgoCaONaroKrO

TotalNumbers of O atomsSiTiAIFe3*Fe.MnMgCaNaK

TotalxwRange in XF

15.99 16.030.23 0.21

0.214.23 0.214.26

16.00 16.01 16.010.25 0.27 0.31

0.20-4.29 0.21-0.32 0.224.31

37.41

24.6011.56

0.30

23.18

97.0525s.98

4.631.39

0.04

3.97

16.010.23

0.23-0.27

38.03

24.6811.67

0.13

23.42

97.93256.01

4.601.39

0.02

3.97

37.390.M

25.8til10 .51

o.22

23.71

97.70255.920.014.821.25

0.03

4.02

37.36 37.410.24

23.83 23.2412.30 't3.28

0.14 0.2',1

23.41

97.28255.970.034.491.48

o.o2

4.01

23.35

97.49 98.0325 255.99 5.99

37.37 53.30

22j0 1.281 5 .16

9 . 1 10.24 0.88

1 8 . 1 223.16 10.82

0.25

93.75237.85

0.22

'1.12

0 . 1 13.981.710.07

15.07

58.14 52.270.07

17.06 20.82

0.49 0.21

0.486.75 10.300.72 0.601.80 0.36

85.51 84.5472 7226.72 24.540.049.24 11.52

0.16 0.06

0.323.32 5.220.64 0.541.04 0.24

41.52 42.06

35.70

30.82

0.59

33.20

100.311 23.01

1.96

0.04

3.00

8.01

4.39 4.181.60 1.8!!

0.03 0.03

4.01 3.98

I

i:N+

o o.1 0.2 0.3 0.4Ar( tv) -1

Fig. 9. Plot of I4rAl - 1 vs. t6rAl + 2Ti - I in chlorite.Individual points indicate average compositions ofchlorite. Thenumbers correspond to drill-hole depths of the samples.

Biotite

The brown-colored phyllosilicate present in Zone Vshows a wide variation in K content from 0.18 to 0.94 Kper Or.(OH)r. The low K content conesponds to vermic-ulitized biotite. The grains with more than 0.7 K areclassified as biotite in this study. The Kamikita biotite ischaracterized by high TiO, contents, ranging from 1.96to 5.31 wto/o (3o/o on average). The average TiO, contentof vermiculitized biotite is smaller (0.6 wt0/0). The appar-ent distribution coefficients of Mg and Fe for biotite andchlorite, K" : (Mg/Fe)BV(Mg/Fe)Chll, range from 0.99to 1.15. These values are slightly larger than those re-ported from rocks of higher metamorphic grade (Ernst etal., l98l; Lang and Rice, 1985; Holdaway et al., 1988).

Xps (= Fe3*/ Fe3+*Al)

Fig. 10. Distribution of X* (: Fe3+/Fe3+ + Al3*) in epidotesamples from Zones IIIb and IV.

o.6

o.s

o.4

o.3

o.2

o.1

o

16-140o

r6-546o

(se

aoa

Gc

o _5

g,

ollE3z


Zone lV(c )

Zone V(d )

Epidote

The distribution of the pistacite component, Xo,: Fe3+ /(Fe3+ + Al), is given in Figure 10. Epidote grains in ZoneIV generally exhibit smaller values of Xo" in cores andlarger values in rims. Epidote (X*: 0.224.30) associ-ated with andradite in a veinlet of sample l6-576-m isapparently not in equilibrium with the andradite becausethe andradite contains no Al (Table 4).

Actinolite

The Si content of actinolite ranges from 7.82 to 7.85per Orr(OH), and the Al and alkali contents are corre-spondingly low. The Fe/(Fe + Mg) ratio ranges from 0.215to 0.219, and the apparent distribution coefficient, Ko :(Fe/Mg)Chl/(Fe/Mg)Actl, : I .30- 1.34.

Other minerals

Analyses of phengite coexisting with chlorite in sand-stones and felsic volcanics ofZone IV are given in Table3 and plotted in Figure 8. Analyses of heulandite andlaumontite are given in Table 4.

A= Al2o3+ Fe2O3- K2O - Na2O

C= CaO

f= FeO+ MgO

c-

Pnocnnssrvr PHASE RELATToNS

Phase relations of the mafic phyllosilicates in the Ka-mikita low-grade metamorphic rocks can be determinedby the compositional variations and parageneses of thesecondary minerals described above. Textural and chem-ical equilibria are rarely attained, and only local equilib-rium may be realized in low-grade metamorphism (Bish-op, 1972; Znn, 19741' Coombs et al., 1976). Although itis not possible to prove that the studied samples formedunder equilibrium conditions, the orderly, systematic, andpredictable changes in mineral occurrences, as shown inTable I and Figure 3, indicate that stable mineral assem-blages can be identified.

The inferred stable mineral assemblages within the Ifu-mikita metabasites can be illustrated using ACF diagrams(Fig. I l). The secondary mineral assemblage in basic tointermediate rocks of the lowest grade (Zonel) is saponite+ calcite. Primary plagioclase remains almost intact inrocks of Zone I. In Zone II, the secondary mineral assem-blage is saponite + heulandite + stilbite + calcite (Fig.I la). In the higher gtade Zone III, this assemblage is re-placed by corrensite + laumontite * calcite. In the ACFdiagram, the corrensiteJaumontite tie line intersects thesaponite-heulandite (or stilbite) tie line (Fig. I le). Tran-

Zone l l(a)

Z o n e l l l(b )

Fig. 11. ACF diagrams illustrating progressive changes in mineral assemblages with increasing metamorphic grade. Heu :

heulandite; St:st i lbi te;Lau:laumontite;Ep:epidote;Act:act inol i te;Sp:saponite;Cor:corrensite; Chl:chlori te.(a)-(d) Relations for Zones II, III, IV, and V, respectively. (e) Bulk rock compositions plot within the gtay area.


TABLE 5, Bulk compositions of altered rocks

Sample sio, Tio, Alro3 Mgo CaO Naro KrO Prou Zone

No. 18240 55.42251 55.41308 64.32360 57.01405 58.27420 56.24459 53.02465 60.68480 54.28

No. 17228 51.90343 51.55438 52.27459 56.29529 54.55

N o . 1 660 52.02

140 57.91220 55.67242 54.46460 54.66480 64.30498 75.95546 58.05570 54.17

No. 15480 63.49500 62.s9

Outcrop78 51.81

0.750.811.090.920.740.900.851 . 1 00.86

0.830.891.030.700.81

1.020.730.850.941.080.760.171.081.03

o.820.79

0.88

15.7217.4613.9416.2215.5014.9615.5816.2516.23

17.9314.5616.2318.2016.38

'17.2717.7816.8417.O216.8413.6712.9116.8816.70

14.3613.98

16.59

10.048.987.969.42

10.4310.039.617.449.60

8.969.469.908.529.04

10.557.948.99

10.1410.067.251.66

10.0710.38

7.387.91

10.91

0.100.080.100.110.130.110.200.090.12

0.140.110.160.140.13

0.040.120.140.170.140.120.050.190.18

0.200.17

0.19

7.046.663.355.894.666.627.O13.247.05

6.3812.0810.824.629.3s

8.085.817.468.417.165.861.734.277.02

4.054.03

6.26

8.758.263.704.643.888.57

11.443.379.57

7.596.072.947.054.32

9.024.511.483.745 .181 .991 .573.087.25

5.485.91

1 1 . 3 8

1.881.784.285.656.142.212.097.641.75

5.313.885.262.903.98

1.822.945.403.173.103.293.285.603.13

t lt l

l l lal l lal l lal l lal l lal l lal l la

ilbiltbiltbiltbIV

iltbiltbiltbIVIVIVIVIVIV

VV

V

0.15 0 .150.45 0.111 . 1 0 0 . 1 60.05 0.090.16 0.080.25 0.090.07 0.120.04 0.150.43 0.10

0.83 0.131.28 0.1 1't .27 0.121.48 0 .101.32 0.1 1

0.09 0.072j9 0.081.92 0 .101 .79 0.151 .68 0.101.64 0.1 12.6s 0.030.68 0.100.02 0.11

1 .43 0.101.58 0.08

0.15 0 .10

2.682.96

1.74'Total Fe as FeO

sition from Znne Il to Tnne III is marked by transfor-mation of saponite to corrensite; the corrensite has moreAl, less Ca and Si, and a nearly constant Fe/(Fe + Mg)ratio as compared to saponite. Heulandite and stilbitewere also replaced by laumontite.

The characteristic assemblage of Zone IV is chlorite +epidote + calcite (Fig. I lc). In the transition from ZoneIII to Zone IV, the chlorite-epidote tie line intersects thatof corrensite-laumontite (Fig. lle). Fe3+ is accommodat-ed preferentially in epidote and hematite, and as a resultthe Fe'z+/(Fe,* + Mg) ratios of corrensite and chlorite areequal, as shown in Figure 7. However, since the Al con-tent of chlorite is larger than that of corrensite and epi-dote contains much more Ca than laumontite, the chlo-rite-epidote tie line intersects that of corrensite-laumontite.Subzone IIIb may be a transition zone between subzoneIIIa and ZoneIY because the coexistence ofchlorite, cor-rensite, laumontite, and epidote were demonstrated insubzone IIIb. The bulk compositions of the mafic phyl-losilicate-bearing rocks (Table 5) plot near the area wherethe three tie lines in the ACF diagram intersect each other(Fig. l le). This implies that the assemblages of secondaryminerals changed from Zone II to Zone IV in responseto an increase in temperature under nearly isochemicalconditions.

The highest grade assemblage is biotite + actinolite +chlorite. Chlorite contains progressively more Mg and lessAl as a result of the formation of Mg-rich actinolite andbiotite in rocks with scarce epidote and abundant albite.

DrscussroN

As described above, saponite and chlorite do not showa continuous change in proportion of layers of chloriteduring thermal metamorphism in the Kamikita area. Onlycorrensite having 50-40o/o smectite layers was formed atintermediate stages of the smectite-to-chlorite transfor-mation. The corrensite coexists with saponite or chloriteor both. Each of these minerals is interpreted to be adistinct, separate phase. Each is quite distinct in chemicalcomposition. The phase relations and structure varia-tions of the smectite-corrensite-chlorite series indicate thatcorrensite should be regarded as a single interstratifiedphase from a thermodynamic point of view. This conclu-sion supports previous inferences concerning thermody-namic relations for corrensite (Peterson, 196l; Yelde,1977;Evartsand Schiffman, 1983; Reynolds, 1988). Morerecently, Shau et al. (1990) also concluded from their de-tailed TEM/AEM study of corrensite from the Taiwanophiolite that corrensite should be treated as a uniquephase rather than as a l: I ordered mixed-layer C/S.

A discontinuous decrease in proportion of smectite inC/S during the smectite-to-chlorite transformation hasbeen observed in hydrothermal systems and diageneticenvironments in previous studies (Inoue et al., 1984a,1984b; Inoue, 1987), as in the present study. On the otherhand, Chang et al. (1986) described a burial sequencewith continuous decrease in proportion of smectite layersfrom saponite to corrensite in Brazilian offshore sedi-


ments. Schultz (1963) noted that C/S could range contin-uously from 500/o smectite to 00/o smectite with transfor-mation of smectite to chlorite, although he noted theabsence of C/S that contains more than 500/o smectite.The detailed examination of XRD patterns of C/S havingintermediate d(001) values in this study indicate that theyare mixtures of more than two discrete phases. Theserelations suggest that a discontinuous decrease in pro-portion of smectite layers in C/S occurs commonly in theentire compositional range of the smectite-to-chloritetransformation.

The physical conditions of corrensite formation andmetamorphism in the Kamikita area can be approximat-ed by analogy with observations from hydrothermally al-tered rocks in geothermal fields. The transition betweenZones II and III is characterized by the heulandite- (orstilbite- ) laumontite transition as well as by the saponite-to-corrensite transformation. Liou (197 l) experimentallydetermined the maximum temperature of stilbite stabil-ity to be 100 "C at 300 bars. Heulandite is structurallysimilar to stilbite (Gottardi and Gali, 1985). Thereforethe boundary between Zones II and III is thought to beat approximately 100'C in the Kamikita area. The tem-perature of the first appearance of discrete chlorite withor without C/S has been documented to be 150-240 'C

from many gebthermal systems (Kristmannsdottir, 1979;McDowell and Elders, 1980, 1983; Keith and Bargar,1988). Laumontite is present at temperatures below 200"C and epidote usually occurs at temperatures greater than200-250 "C in geothermal fields (Bird et al., 1984). Theinterstratified R : 3 I/S near the lower end of subzoneIIIa contained 150/o smectite and the o/o smectite in I/Sapproaches zero in ZoneIY. The corresponding temper-ature is usually 200-230 'C (Srodoi and Eberl, 1984). Itcan reasonably be inferred that the temperature at theboundary between Zones III and IV, where corrensitedisappeared, was approximately 200'C. Accordingly, itis inferred that corrensite occurred at temperatures of ap-proximately 100-200'C in the Kamikita area. The Ka-mikita metamorphic zonation (Zones I-IV) was formedin response to a thermal gradient of approximately 70'C/km and relatively high /", conditions, as deduced fromthe presence of hematite. The diorite mass, Zone V, at-tained a temperature of approximately 300 "C, as inferredfrom the temperature of the first appearance of biotite inthe Salton Sea geothermal field (Cho et al., 1988). Theisolated presence ofthe epidote + andradite * anhydriteassemblage as a vein in the l6-576-m sample and the pres-ence of pyrite in the diorite mass may suggest local cir-culation of hydrothermal fluids in a hydrothermal stagefollowing the diorite intrusion.

Suvrvr.a,ny AND coNcLUsroNS

The results of this study can be summarized as follows:

l. During thermal metamorphism in the Kamikita area,saponite transformed to chlorite through corrensitewith increasing m€tamorphic grade so that the pro-portion of smectite layers in intermediate C/S de-

creased discontinuously, with steps at 100-800/o (sap-onite), 50-400/o (corrensite), and l0-00/o (chlorite). C/Shaving intermediate o/o smectite values other than thoseofthe above three ranges was not found. Such a dis-continuous transformation of smectite to chlorite isprobably a general relation in various geologic envi-ronments.

2. The saponite-to-chlorite transformation at Kamikitatook place with decreases in Si and Ca, increase in Al,and a nearly constant Fel(Fe + Mg) ratio. Saponite,corrensite, and chlorite were distinctly different incomposition.

3. In addition to the structural and compositional vari-ations in the saponite-corrensite-chlorite series, themineral paragenesis implies that corrensite should beregarded as a single interstratified phase from a ther-modynamic point of view. The formation of C/S hav-ing other intermediate o/o smectite values, except thatof corrensite, may be restricted during the smectite-to-chlorite transformation.

4. Corrensite formed at temperatures of approximately100-200'C. The formation of corrensite may be fa-cilitated in more permeable rocks, which are more af-fected by hydrothermal fluids.

AcxNowr,nncMENTs

The authors gratefully thank B. Velde, A. Meunier, and D. Beaufortfor their helpfirl reviews ofan initial version ofthe manuscript. They alsoacknowledge the help ofH. Tatematsu, Japan Railway Research Institute,who supplied the core samples and allowed publication of this study.

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