Smectite-To-Chlorite Transformation in Thermally Metamorphosed Yolcanoclastic Rocks in the Kamikita Area, Northern Honshu, Japan

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

Smectite-to-chloritetransformation in thermally metamorphosedyolcanoclastic rocks in the Kamikita area, northern Honshu, Japan Arsuyuxr lNour Geological Institute, Collegeof 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 effects of an intensive episodeof thermal metamorphism causedby a hornblende quartz diorite intrusion. 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 appearanceof characteristicmineral assemblagesfrom smectite (Zane I), to smectite * heulandite + stilbite (Zone II), to corrensite + laumontite (Zone III), to chlorite + epidote (Zone IV), and finally to biotite + actinolite (Zone V) with increasingmetamorphic grade. As smectite transforms to chlorite, the percentageof smectitelayers in interstraiified chlorite/smectite (C/S) (the intermediate products) decreasesdiscontinuously with increasing metamorphic grades,with stepsat 100-800/0,50-40o/o (corrensite), and 10-00/0.The transformation of smectiteto chlorite through corrensiteis characterizedchemically by decreas- es in Ca and Si, an increase in Al, and a constant Fel(Fe + Mg) ratio. The mineral paragenesis,structural variation, and compositions all support the hypothesisthat corrensite is a thermodynamically stable C/S phase.Corrensite formed at temperaturesbetween approximately 100 and 200 "C. The Kamikita metamorphic zonation was developed in responseto a thermal gradient of approximately 70 .C/km, and the secondaryminerals crystallized with near-equilibrium compositions.

fNrnooucrroN rite transformation, e.g., from smectite to corrensite or from corrensite to smectite. Furthermore, Helmold and Mafic phyllosilicates such as saponite, interstratified van der Kamp (1984) noted that there is a continuous chlorite/smectite (C/S), and chlorite are abundant in low- decreasein expandability over the entire range of pro- grade metabasites.The presenceof these minerals has portion of smectite layers (o/osmectite)in C/S. phase re- been reported from diageneticenvironments (Hoffman lationsofC/SwerediscussedbyPeterson(1961)andVelde and Hower, 1979;Chang et al., 1986),active and fossil (1977).They inferred,on the basisof thephaserule, that geothermal 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 mafic Inoue' 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 massesat Ka- anic crust (Alt et al., 1986). Interstratified c/S generally mikita, northern Honshu, Japan. Detailed X-ray powder occurs as an intermediate product during the smectite- diffraction and microprobe analyseshave beenpe*ormed to-chlorite transformation. It is interesting that the nature on the phyllosilicates and assoiiated calcium aluminum of 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 processof smectite to chlorite in Hower, 1979; Srodori and Eberl, 1984; Horton, 1985). the thermal metamorphicenvironment and the thermo- Earlier work reported, for the processof transformation dynamic status of corrensite as an intermediate product. of smectite to chlorite, that the expandability of smectite decreaseddiscontinuously with increasingtemperature in diagenetic environments and hydrothermal systems(In- oue et al., 1984a,1984b; Inoue, 1987).Only a few types Gnor,ocrcAl, SETTTNG of ordered structures,Reichweite : 0 and l, appearedin The Kamikita area is located in the northern part of intermediate C/S products (Reynolds, 1988). However, Honshu, Japan. As shown in Figure l, Miocene and Chang et al. (1986) and Schultz (1963) indicated that the Quaternary sequencesoccur, as does the Kamikita Ku- expandability ofC/S could decreasecontinuously in lim- roko-type ore deposit. The geology ofthe area has been ited portions of the entire range of the smectite-to-chlo- described in detail by many reseirchers (Miyajima and 0003-004x/9l/0304-0628$02.00 628 INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION 629

LEGEND Lake Deposits [ElOuatetnary Andesites ffiQuaternary @Quaternary f;lwaoaoa*a Formation Formation flvotsuza*a Formation @lrane9"""wa lFloiorite fTlruroto ore Deposits fflo'itt not""

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

Mizumoto, 1965, 1968; Lee, 1970:'Lee et al., 1974', MBrnoos MMAJ, 1974, 1975, 1986). The Miocene group consistsof three formations in or- More than 200 sampleswere collected from outcrops der of decreasingage, as follows: the Kanegasawafor- and five drill holes. X-ray powder diffraction (XRD) and mation is composed of massive or brecciated basaltic to thin section studies were used to determine the distri- andesitic lava flows. The Yotsuzawa formation is com- bution and textural variation of secondaryminerals. The posed principally of andesitic to dacitic lava flows and clay size fraction (< I pm) was obtained by ultrasonic volcanoclastic rocks and some mudstone layers. The disaggregationand centrifugation of clay-HrO suspen- Wadagawa formation is composed of felsic and basaltic sions. Oriented specimenswith an approximate area of volcanics.It contains intercalatedmarine mudstones.The 20 x 15 mm were prepared by placing the suspensions Kamikita Kuroko deposit occurs in the lowermost hori- on a glass slide. XRD patterns of the air-dried and eth- zon. Several small diorite bodies have intruded the Ka- ylene-glycol-saturatedspecimens were obtained using a negasawaand Yotsuzawa formations but hornfelsic rocks Rigaku RAD I-B diffractometer (40 kV, 20 mA) equipped are not observed in the field. The Kanegasawa,Yotsu- with a Cu tube, graphite-monochrometer, and 0.5" di- zawa, and Wadagawaformations dip moderately(10-30') vergenceand scattering slits. Percentageof smectite lay- on both sides of an anticlinorium. which has a north- ers (o/oS)and ordering type (Reichweite) of interstratified south direction in the study area. C/S and I/S were determined by comparing the observed The Quaternary group is divided into three formations: XRD patterns with computer-simulated patterns using Tashirodai welded tuffs, andesitelava flows, and lake de- the program Newmod (developed by R. C. Reynolds, posits. They unconformably overlie the Miocene sedi- Dartmouth College,Hanover). XRD patternsofrandom- ments and dip gently (0-10'). ly oriented specimenswere also obtained in order to de- A simplified geologicmap with the locations of the drill termine the d(060) value of mafic phyllosilicates. holes utilized in this study is shown as Figure l, and a Energy-dispersive microprobe analyses of phyllosili- geologiccross section including the drill holes is given as cates and calcium-aluminum silicates in polished thin Figure 2. A diorite intrusive mass is found at a depth sectionwere conducted on a Hitachi 5-550 scanningelec- below 400 m of drill hole no. 15, as shown in Figure 2. tron microscopefitted with a Kevex 7000 A-75 solid state 630 INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION

E E c c .9 o G 6 9 +oo g IIJ ul

(km) Dlstancefrom Dlorlte lntruslve Mass (km) Dlstancefrom DloriteIntrusive Mass

Ewetded Tutts ffi Sandstones Fig. 3. Crosssection showing the distributionofzones. B Basaltic Rocks ffiMudstones Rocks ElAndesitic E Diorite EDacitic Rocks detector. Analyseswere caried out using an accelerating voltage of 20 kV, a beam current of 200 pA, a beam Fig. 2. Geologic cross section along the line of Figure I . Qwt diameter of 2 pm, and a counting time of 200 s. Analyses : Quaternary welded tutrs; Wg : Wadagawa formation; Yz : were obtained on 3- 15 points of each mineral in a thin Yotsuzawa formation; Kn : Kanegasawa formation. section. EDS data were reduced by a ZAF scheme (QANTX software)which was modified by Mori and Ka- nehira (1984). The standardsincluded quartz (Si), corun- dum (Al), periclase(Mg), metals (Fe, Ti, and Mn), calcite Tlau 1. Secondaryminerals occurring in zones (Ca), albite (Na), and potassium chromate crystals (K) (Mori and Kanehira, 1984).The bulk composition of rocks Zones was determined with a JEOL JSX-60PX X-ray fluores- Sub- Sub- cencespectrometer using fused glassdisks. Secondary Zone Zone zone zone Zone Zone minerals I ll llla lllb lV V Rocx lr-rnnarroN d-cristobalite + Six mineralogical zones were defined on the basis of Quartz + + Montmorillonite + + the assemblagesof 22 secondary minerals identified in Saponite + + the drill cores, as listed in Table l. These zones are dis- lllite/smectite + tributed from the contact ofthe diorite massto a distance Corrensite TA Phengite + of approximately 6 km, as shown in Figure 3. The pet- Chlorite +++ rographic relations ofeach zone are describedbriefly be- Biotite + Mordenite + low. Heulandite + Zone I is defined by the presenceof smectite and the Stilbite + minerals, except calcite and Chabazite + absenceof other secondary Laumontite + silica minerals. The rocks from the Wadagawaformation Wairakite + and the upper-most part of the Yotsuzawa formation be- Albite +++ Sphene ++ long to this zone. Brown-colored saponite is common in Epidote +++ mafic to intermediate rocks,whereas montmorillonite oc- Actinolite + curs in felsic rocks. Saponiteusually occurs as a replace- Calcite + + ++ Hematite + + +++ ment of primary orthopyroxene phenocrysts and glassy Pyrite + groundmassin mafic rocks. Montmorillonite principally Note: Zone | : smectite zone, Zone ll : smectite-zeolitezone, SUb. replacesthe glassygroundmass of felsic rocks. Clinopy- zones llla and lllb: corrensita.laumontitezone, Zone lV : chlorite-epi- roxene and plagioclasephenocrysts are unaltered in both : dote zone, Zone V biotite-actinolitezone. rock types except for occasionalreplacement by saponite INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION 631 and calcite, respectively. Cristobalite is associatedwith itive elongations.Grains of epidote in this zone are less smectite in the part of this zone of lowest grade,whereas than 0.3 mm in length but are larger than those found in secondaryquartz occurs in the part with higher grade. subzone IIIb. Plagioclasephenocrysts are almost com- ZoneII is defined by the presenceof both smectite and pletely replaced by calcite or epidote or both. The re- zeolites.The rocks from the upper part ofthe Yotsuzawa maining plagioclaseis strongly albitized. In felsic rocks, formation belong to this zone. The mode of occurrence phengitic mica coexists with chlorite. Mudstones of this of smectite in this zone is the same as that of Zone l. zone are usually composed of illite, chlorite, and quartz. That is, saponiteis a common variety of smectitein mafic A wairakite-like mineral was occasionally identified in to intermediate rocks, and montmorillonite is a common tuffaceousmudstones (samples l6-280-m and l6-300-m). variety in felsic rocks. Where both saponite and mont- A mudstone sample (16-560-m) is composedof regularly morillonite were identified (in the upper part of drill hole interstratified I/S (rectorite) with hematite and very small no. 18) by XRD, each was observedto replace only one amounts of illite, chlorite, and quartz. Veinlets composed kind of rock fragment, as observed in thin section. Stil- ofepidote, andradite, and anhydrite are found in an an- bite, heulandite, and chabaziteoccur in mafic rocks; mor- desitesample (l 6-576-m). denite is dominant in felsic rocks. These zeolites replace ZoneY is defined by the presenceofbiotite and actin- glassygroundmass and fill cavities. Weakly albitized pla- olite. This zone is recognized only within the diorite mass. gioclaseis partially replaced by calcite or saponite. Sec- Primary hornblende is replaced by fine-grained biotite, ondary quartz occurs as fine-grained aggregates with actinolite, chlorite, hematite, and pyrite. Thesesecondary smectite in the glassy groundmass.Celadonite and ver- minerals have crystallized along the cleavagesof horn- miculite, which were clearly distinguished from saponite blende grains. Biotite usually shows strong pleochroism by optical, microprobe, and XRD analyses,were recog- from reddish brown to yellow. In the upper parts of the nized as vesicle fillings in sample l8-320-m, which was diorite mass (413-420 m depths of drill hole no. l5), located near the boundary between Zones II and III. A secondarybiotite is further altered to interstratified bio- precipitation sequencewas observedwithin vesiclesfrom titelvermiculite or vermiculite. In this zone, epidote is the wall to the center as follows: (l) a transparent thin sporadicallyfound as fine-grainedaggregates, and chlorite rim consisting offine-grained zeolite, quartz, or both (2) is characterizedby anomalous brown interferencecolors fine-grained brown vermiculite, (3) fine-grained bluish- and negativeelongation. Although hematite is commonly green celadonite, (4) flaky vermiculite and zeolites. The found in the rocks of Zones II-Y, pyrite is rare in the last assemblageis absent in small vesicles. other zones. Zone lll is defined by the presenceof corrensite and As already reported by Inoue and Utada (1989), dick- laumontite. The rocks from the middle part of the Yotsu- ite, pyrophyllite, zunyite, topaz, alunite, rectorite, su- zawa formation belong to this zone. Corrensite generally doite, and tosudite are recognizedin the upper parts of occurs as a replacement of mafic phenocrystsand glassy drill hole no. 15 above the diorite mass. They are prob- groundmass, and as a pore filling identical in mode of ably the products of an acidic hydrothermal alteration occurrenceto that of saponite in Zones I and II. It also that prevailed at a late stage.The original metamorphic occurs as inclusions within albitized plagioclasepheno- minerals and textures of the rocks in the upper parts of crysts,associated with micaceousclays. Flakes of corren- drill hole no. l5 were completely destroyedby the super- site are generally less than 0.1 mm in length and show imposed effectsof the acidic hydrothermal alteration. pleochroism from green to pale green. In higher grade parts of the Zone III, however, corrensite is deeper in color and gtains are larger. Such corrensite often coexists Srnuctuur, vARrATroN oF TNTERSTRATTFTED with spherulites of fine-grained spheneand epidote that CLAY MINERALS are less than 0.1 mm in diameter. Znne III is further Structural variation of interstratified minerals can be divided into two subzones:subzone IIIa (epidote-absent clarified by examining changesin position and intensity subzone) and subzone IIIb (epidote-present subzone). of the 001 reflections (Reynolds, 1980). Figure 4a shows Laumontite with corrensite occurs commonly in both representativeXRD patterns of samplesfrom the sapo- subzonesas pore fillings. Interstratified I/S having < 150/o nite-corrensite-chlorite seriesfrom the study area. Sapo- smectite is present in mudstonesand sandstonesin sub- nite is recognized by the expansion of the 001 spacing zone IIIb. from 14 to approximately 17 A upon ethyleneglycol sat- Tnne IY is defined by the generalpresence of chlorite uration. The d(060) value is 1.532-1.535 A Glg. +t). and epidote and the absenceof corrensiteand laumontite. Corrensite shows expansion from 29 to 3l A with eth- The rocks from the lower part of the Yotsuzawa forma- ylene glycol saturation. The value ofd(060) ofcorrensite tion and the upper part of the Kanegasawaformation ranges from 1.540 to 1.542 L. Chlorite generally has belong to this zone. Chlorite occurs as a replacement of d(001): 14.2A andd(060) : r.541-1.545A. The value mafic phenocrystsand glassygroundmass, and as a pore ofd(001) doesnot changewith ethyleneglycol saturation. filling. It shows strong pleochroism from dark green to XRD patterns of examination, some of the C/S contained pale green. It is also characterizedby isotropic-anoma- broad peaks with intermediate d(001) values of approx- lous blue interference colors and both negative and pos- imately 16.5 A or 15.1 A, correspondingto a value be- 632 INOUE AND UTADA: SMECTITE-TO-CHLORITETRANSFORMATION

23-250-m 23-250-m

100%s

18-349-m

eo%s+50%s 7'7,2 o g.gs, t.tg 1.541A

f 18-465-m .= 18-465-m 0 tr o o 45 %S tr F o c sf, o 1.541A 17-438-m 14.52 40%s+o%s

16-570-m

2468101214 59.O 60.0 61.0 62.0 o2O Cu Ka radiation "29 Cu Kc.radiation Fig. 4. (a) X-ray powder diffraction patternsofsamples ofthe saponite-corrensite-chloriteseries. They wererecorded on oriented, ethylene-glycol-saturatedspecimens using step-scanning.The d values of peaks are given in A. O) The (060) peaks of the same samplesas in a. tween those for saponite and corrensite or betweenthose those of the above two pairs of minerals. Recording of for corrensite and chlorite. The intensities of the reflec- the XRD patterns by means of a step-scanningmethod tion with d: 3l A were variable. The relative intensities revealed that the intermediate d(001) value and intensi- of the l4-, 7-, and 4.7-A reflections were intermediate to ties were not due to some specificinterlayering in C/S but INOUE AND UTADA: SMECTITE-TO-CHLORITETRANSFORMATION 633

No.23 No.iz No.16 LEGEND

'@&60 &;:.:J","I 6040 20 o :'::;:""':T"".""1:;J ""JT,"j Fig. 5. Variationsof percentagesof smectitelayers in chlo- Fig. 6. Compositionsofchlorite, corrensite, and saponitefrom rite/smectite(solid circles) and illite/smectite(open circles) as a variousdrill holes.Solid circles and asterisks: drill holeno. 16; functionofdepth ofdrill holes.The tie linesindicate the pres- opencircles and stars: drill hole no. 17;open triangles : drill enceof more than two chlorite/smectitesamples having different hole no. 15;solid starsand triangles: drill hole no. l8; open smectitelayer percentages in a rock specimen.Abbreviations as boxesand solidboxes : outcropsamples. in Figure2. were caused by more than two phases,as clearly illus- not clearly defined in the Kamikita areabecause diocta- trated by Figure 4a. hedral clays are rare in core nos. 17 and 18. The variation of the relative amount of smectite in C/S and in I/S is illustrated in Figure 5 as a function ofdepth. VlnrlnoNs rN coMPosrrroN oF The value of o/osmectitein C/S decreasesdiscontinuously SECONDARY MINERALS from 1000/osmectite (saponite) to 00/osmectite (chlorite) Representativeanalyses of selectedminerals are listed with intermediate values of 50-40o/osmectite (corrensite) in Tables 2-4. The total Fe for phyllosilicates and actin- with increasingmetamorphic grade. The o/osmectite val- olite was calculated as Fe2+and as Fe3+for epidote and ue in saponiteis > 800/0,whereas that in chlorite is < 100/0. garnet. The existenceof C/S having intermediate 0/osmectite values other than 50-400/osmectite is not certain in the Ka- Saponite mikita area.The intimate associationof saponite,corren- The Si content of saponite ranges from 3.36 to 3.56, site, or chlorite as separatephases is implied by XRD and the Fe/(Fe + Mg) ratio from 0.351 to 0.361 (Table data in samplesof drill hole nos. 16, 17, and 18. The C/ 2 and Fig. 6). The interlayer cation is principally Ca. The S having 5o-40o/osmectite (termed corrensite in this saponite composition rangesare essentiallyidentical for study), which yields a superlatticereflection with d : 3l all modes of occurrence. A after ethylene glycol saturation, has a Reichweite (R) value of l The C/S that is more or less expandablethan Corrensite corrensite may have R : 0. The C/S in drill hole no. 18 Empirical forrnulas of corrensite were norlnalized to shows irregular variation in the proportion of smectite Oro(OH)ro(Newman and Brown, 1987). The Si content with depth. These data imply that less expandable C/S was variable from grain to grain but was relatively con- occurscommonly in porous rocks like tuffaceousbreccias stant within a grain. The Fe/(Fe + Mg) ratio rangesfrom and hyaloclastites,and more expandableC/S dominates 0.3 to 0.4 (Table 2 and Fig. 6). The corrensitein samples in massive rocks like lavas. l7-438-m and 17-343-mhas ratios lessthan 0.2, how- The variation of proportion of smectite in I/S shown ever. The generalpetrographic characteristics ofthe latter in Figure 5 varies discontinuously. The I/S from core nos. corrensite samples are nearly the same as those of the 18 and 23 generally is pure smectite. The I/S in mud- others, except that the latter are larger in size and deeper stones and sandstonesin the upper part of core no. 16 in color. (near the lower end of subzoneIIIa) contains l5olosmec- Figure 7 shows a plot of lolAl, vs. the total number of tite (Fig. 5) and the I/S has R : 3 ordering. The propor- octahedral cations for corrensite. Inoue (1985) demon- tion of smectite in I/S decreasescontinuously from I 5 to strated that there is a linear relation between those two 0olowith depth in drill hole no. 16. The proportion of variables for Fe-rich corrensite (0.52 < Fe/(Fe + Mg) < smectite layers in I/S is zero approximately where cor- 0.61). This relationship is shown by the solid line in Fig- rensite no longer occurs. The trends in variation ofpro- ure 7. The data for Kamikita corrensite also fall along portion of smectite in I/S at intermediate I/S values is the line. Ideally, corrensite has nine octahedral ions per 634 INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION

TmLe2. Representativeanalyses of saponite(1), corrensite (2-4), and chlorite (5-1 1)

(1) (21 (3) (4) Samde 18-240 1&360 1&465 16-60 sio, 44.18 33.27 3ial.76 31.88 Tio, Alros 7.86 13.12 13.10 15.46 FeO- 15.34 16.2'l 17.77 20.25 MnO 0.08 0.34 0.19 0.08 Mgo 15.04 20.71 18.24 17.69 CaO 2.68 0.76 't.12 1.15 Naro l(,O 0.30 0.08 0.04 Total 85.95 84.40 84.25 86.55 Numbers of O atoms 11 25 25 25 Si 3.43 6.15 6.29 5.87 rltAl o.57 1.85 1.71 2.13 I4lTotal 4.00 8.00 8.00 8.00 Ti rorAl 0.15 1.01 1.17 1.23 Fe 1.00 2.51 2.77 3.',12 Mn 0.01 0.05 0.03 0.01 Mg 1.81 5.71 5.07 4.86 I6lTotal 2.97 9.28 9.04 9.22 Ca 0.23 0.15 0.22 0.23 Na K 0.03 0.02 0.01 Fe/(Fe + Mg) 0.356 0.305 0.353 0.391 Range in Fel(Fe + Mg) 0.351-0.361 0.290-0.317 0.341-0.378 0.370-0.398 Range in Si 3.36-3.56 6.15-6.38 6.20-6.36 5.79-5.87 Range in numbers of oct. cations 2.82J.06 9.01-9.28 8.99-9.16 9.22-9.38 'Total Fe as FeO

Oro(OH)ro.The number of octahedralcations rangesfrom 8.85 to 9.7. Thesevalues correspond to 52-38o/osmectite if the number of octahedral cations is assumedto be six per Oro(OH)ofor saponite,nine for corrensite,and 12 per Or0(OH)r6for chlorite. The proportions of smectite inter- layeredwith corrensiteestimated from the chemical composition are consistent with those determined by XRD, o as described above. Consequently,if the linear relation c of the numbers of octahedral cations vs. the tetrahedral .9 Al content as shown in Figure 6 is a generalrelation for a corrensite, we infer that the C/S, which provided a su- perlattice reflection at 3l A after ethylene glycol satura- a2 tion, contains approximately SiroAlru - SiuoAl,u per t o Oro(OH),. in the tetrahedral sheetand 8.8-9.8 cations in E values in o the octahedral sheet. The estimated are satis- factory agreementwith analyzed values for corrensite from o hydrothermal systems(Seki et al., 1983), diagenetic environments (Chang et al., 1986), and ophiolites (Evarts and Schiffman,1983; Bettison and Schiffman,1988).

Chlorite 8910 The compositional data for chlorite are plotted in a Numbersof OctahedralCations Foster-typediagram in Figure 6 (Foster, 1962).The number of Si atoms ranges from 2.75 to 3.2, and the Fel(Fe Fig. 7. Plot of totAlcontent vs. total of octahedralcations per + Mg) ratio from 0.24 to 0.40. For the chlorite of sample Oro(OH)roin corrensite. Bars indicate rangesof data. The solid l6-546-m, which filled pore spacesof andesitic volcanics, line indicates the correlation of Fe-rich corrensite as noted bv however, the number of Si atoms ranges from 2.68 ro Inoue(1985). 2.77, and,the Fel(Fe + Mg) ratio rangesfrom 0.525 to INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION 635

TaEte 2-Continued

(5) (6) (7) (8) (s) (10) 01) | 7-459 16-140 16-242 I 6-480 1&570 15-480 outcrop75 28.26 29.40 27.93 26.96 26.43 27.88 28.90 0.12 16.97 15.65 20.09 20.20 18.87 16.84 17.O4 19.40 18.61 19.52 18.01 16.86 17.00 16.00 0.46 0.34 0.40 0.34 0.41 0.s9 0.16 20.21 20.35 18.35 19.56 20.75 21.56 22.75 0.14 0.13 0.30 0.15 0.11 0.06 0.11 0.04 0.06 0.42 85.50 84.48 86.59 85.22 83.43 84.49 84.97 14 14 14 14 14 14 14 2.96 3.10 2.88 2.81 2,81 2.94 2.99 1.04 0.90 1.',t2 1.19 1.19 1.06 1.01 4.00 4.00 4.00 4.00 4.00 4.00 4.00 0.01 1.06 1.04 1.32 1.29 1.17 1.03 1.07 1.70 1.64 1.68 1.57 1.50 1.50 1.38 0.04 0.03 0.03 0.03 0.04 0.0s 0.01 3.16 3.20 2.82 3.04 3.29 3.38 3.s0 5.96 5.91 5.85 5.93 6.00 5.97 5.96 0.02 0.01 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.06 0.350 0.339 0.373 0.341 0.313 0.307 0.283 0.350-0.399 0.310-0.345 0.364-0.379 0.337-0.350 0.302-0.325 o.299-0.327 0.238-0.296 2.90-2.97 2.90-3.12 2.86-2.91 2.81-2.85 2.79-3.02 2.83-3.10 2.80-2.99

5.96-5.98 5.70-5.92 5.83-5.88 5.93-5.95 5.93-6.04 5.8M.04 5.96-6.03

0.558. Except for this one specimen,however, the Si con- Figure 9 is a plot of r4lAl - I vs. t6lAll+ 2Ti - I for tent and the Fe/(Fe + Mg) ratio of Kamikita chlorite tend Kamikita chlorite. The l: I line in Figure 9 illustrates the to be smaller with increasingmetamorphic grade (Figs. 6 Tschermak substitution (Al + Al + Si + Mg). Analyses and 8). plotting above the l: I line show dioctahedralsubstitution (AlrMg-,) and correspondto octahedralvacancies (Laird, 1988). Analytical data for most chlorite from Zone IV plot above the l:l line, suggestingthe existenceofocta- Tleu 3. Representativeanalyses of biotite(1-2) and phengite (3-4) hedral vacancies.Data for chlorite in Zane V appearsto approach the l:l line, implying a smaller proportion of (1) (21 (3) (4) octahedral vacancies.The total Al content also tends to Samole 15440 15-500 1&498 16-520 decreasein chlorite from Zone IV to that from Zone V. sio, 36.84 38.38 50.38 49.64 Tio, 4.11 2.62 0.23 0.20 Alro3 12.77 11.42 28.66 27.O7 FeO' 11.00 12.02 3.83 5.42 MnO 0.15 0.20 Mgo 17.36 19.46 2.78 3.42 CaO 0.71 0.02 0.13 0.14 GO 9.73 8.46 10.24 9.22 Total 92.6s 92.52 96.25 95.11 o:11 Si 2.80 2.90 3.34 3.35 rarAl 1.19 1.O2 0.66 0.65 Fe 0.01 0.08 t4iTotal 4.00 4.00 4.00 4.00 Ti 0.24 0.15 0.01 0.01 r6rAl 0.00 0.00 1.58 1.50 Fe 0.69 0.68 0.21 0.31 Mn 0.01 0.01 ll Mg 1.97 2.19 o.27 0.34 -o 4 r6rTotal 2.91 2.95 2.07 2.',t6 Ca 04 02 0.06 0.00 0.01 0.01 Feo K 0.94 o.82 0.87 0.79 FeO + HgO Fe(Fe + Mg) o.262 0.258 Range in Fe/ phengite, (Fe + Mg) 0.239-0.309 0.222-0.268 Fig. 8. Compositions of chlorite, and biotite. Open Range in Si 2.80-2.88 2.89-2.98 and solid circles indicate chlorite compositions (averages)from Zones IV and V, respectively.The tie lines correspondto coex- t Total Fe as FeO. isting chlorite and biotite, and chlorite and phengite. 636 INOUE AND UTADA: SMECTITE-TO-CHLORITETRANSFORMATION

TABLE4. Representativeanalyses of epidote (1-6), actinolite (7), andradite (8), heulandite(9), and laumontite (10) (1) (21 (3) (4) (5) (6) (7t (8) (s) (10) Sample 17-228 17-438 16-498 16-520 16-546 16-570 15-500 16-570 18-231 18-465 sio, 37.41 38.03 37.39 37.36 37.41 37.37 53.30 35.70 58.14 52.27 Tio, 0.M 0.24 0.07 Alros 24.60 24.68 25.8til 23.83 23.24 22j0 1.28 17.06 20.82 FerO. 11.56 11.67 10.51 12.30 't3.28 15.16 30.82 FeO 9.11 0.49 0.21 MnO 0.30 0.13 o.22 0.14 0.2',1 0.24 0.88 0.59 Mgo 18.12 0.48 CaO 23.18 23.42 23.71 23.41 23.35 23.16 10.82 33.20 6.75 10.30 Naro 0.25 0.72 0.60 KrO 1.80 0.36 Total 97.05 97.93 97.70 97.28 97.49 98.03 93.75 100.31 85.51 84.54 Numbers of O atoms 25 25 25 25 25 25 23 12 72 72 Si s.98 6.01 5.92 5.97 5.99 5.99 7.85 3.01 26.72 24.54 Ti 0.01 0.03 0.04 AI 4.63 4.60 4.82 4.49 4.39 4.18 0.22 9.24 11.52 Fe3* 1.39 1.39 1.25 1.48 1.60 1.8!! 1.96 Fe. '1.12 0.16 0.06 Mn 0.04 0.02 0.03 o.o2 0.03 0.03 0.11 0.04 Mg 3.98 0.32 Ca 3.97 3.97 4.02 4.01 4.01 3.98 1.71 3.00 3.32 5.22 Na 0.07 0.64 0.54 K 1.04 0.24 Total 16.01 15.99 16.03 16.00 16.01 16.01 15.07 8.01 41.52 42.06 xw 0.23 0.23 0.21 0.25 0.27 0.31 Range in XF 0.23-0.27 0.214.23 0.214.26 0.20-4.29 0.21-0.32 0.224.31

Biotite The brown-colored phyllosilicate present in Zone V showsa wide variation in K content from 0.18 to 0.94 K o.6 per Or.(OH)r. The low K content conespondsto vermiculitized biotite. The grains with more than 0.7 K are classifiedas biotite in this study. The Kamikita biotite is o.s characterizedby high TiO, contents, ranging from 1.96 to 5.31 wto/o(3o/o on average).The averageTiO, content of vermiculitized biotite is smaller (0.6 wt0/0).The apparent distribution coefficientsof Mg and Fe for biotite and o.4 chlorite, K" : (Mg/Fe)BV(Mg/Fe)Chll, range from 0.99 r6-546 o to 1.15. These values are slightly larger than those reported from rocks of higher metamorphic grade (Ernst et I o.3 (se al., l98l; Lang and Rice, 1985;Holdaway et al., 1988). i: N + o.2 16-140 o a o a

o.1 G c

o_ 5 o g, o ll E 3 z o o.1 0.2 0.3 0.4 Ar(tv)-1

Xps (= Fe3*/ Fe3+*Al) Fig. 9. Plot of I4rAl - 1 vs. t6rAl + 2Ti - I in chlorite. Individual points indicate averagecompositions ofchlorite. The Fig. 10. Distribution of X* (: Fe3+/Fe3++ Al3*) in epidote numbers correspondto drill-hole depths of the samples. samplesfrom Zones IIIb and IV. INOUE AND UTADA: SMECTITE-TO-CHLORITETRANSFORMATION 637

A= Al2o3+Fe2O3- K2O - Na2O C= CaO f= FeO+MgO

Zone ll Zone lll (a) (b)

Zone lV Zone V (c) (d)

Fig. 11. ACF diagrams illustrating progressivechanges in mineral assemblageswith increasing metamorphic grade. Heu : heulandite;St:stilbite;Lau:laumontite;Ep:epidote;Act:actinolite;Sp:saponite;Cor:corrensite; Chl:chlorite.(a)- (d) Relations for Zones II, III, IV, and V, respectively.(e) Bulk rock compositions plot within the gtay area.

Epidote Pnocnnssrvr PHASE RELATToNS Phaserelations of the mafic phyllosilicates in the Ka- The distribution of the pistacitecomponent, Xo,: Fe3+/ mikita low-grade metamorphic rocks can be determined (Fe3++ Al), is given in Figure 10. Epidote grains in Zone by the compositional variations and paragenesesof the IV generally exhibit smaller values of Xo" in cores and secondaryminerals describedabove. Textural and chem- larger values in rims. Epidote (X*: 0.224.30) associ- ical equilibria are rarely attained, and only local equilib- ated with andradite in a veinlet of sample l6-576-m is rium may be realized in low-grade metamorphism (Bish- apparently not in equilibrium with the andradite because op, 1972; Znn, 19741'Coombs et al., 1976). Although it the andradite contains no Al (Table 4). is not possible to prove that the studied samplesformed under equilibrium conditions, the orderly, systematic, and predictable changesin mineral occurrences,as shown in Actinolite Table I and Figure 3, indicate that stablemineral assem- The Si content of actinolite rangesfrom 7.82 to 7.85 blagescan be identified. per Orr(OH), and the Al and alkali contents are corre- The inferred stable mineral assemblageswithin the Ifu- spondinglylow. The Fe/(Fe + Mg) ratio rangesfrom 0.215 mikita metabasitescan be illustrated using ACF diagrams to 0.219, and the apparent distribution coefficient, Ko : (Fig. I l). The secondarymineral assemblagein basic to (Fe/Mg)Chl/(Fe/Mg)Actl, : I .30- 1.34. intermediate rocks of the lowest grade(Zonel) is saponite + calcite. Primary plagioclaseremains almost intact in rocks of Zone I. In Zone II, the secondarymineral assem- Other minerals blage is saponite + heulandite + stilbite + calcite (Fig. Analyses of phengite coexisting with chlorite in sand- I la). In the higher gtade Zone III, this assemblageis re- stonesand felsic volcanics ofZone IV are given in Table placed by corrensite + laumontite * calcite. In the ACF 3 and plotted in Figure 8. Analyses of heulandite and diagram, the corrensiteJaumontite tie line intersectsthe laumontite are given in Table 4. saponite-heulandite(or stilbite) tie line (Fig. I le). Tran- 638 INOUE AND UTADA: SMECTITE-TO-CHLORITE TRANSFORMATION

TABLE5, Bulk compositions of altered rocks

Sample sio, Tio, Alro3 Mgo CaO Naro KrO Prou Zone No. 18 240 55.42 0.75 15.72 10.04 0.10 7.04 8.75 1.88 0.15 0.15 tl 251 55.41 0.81 17.46 8.98 0.08 6.66 8.26 1.78 0.45 0.11 tl 308 64.32 1.09 13.94 7.96 0.10 3.35 3.70 4.28 1.10 0.16 llla 360 57.01 0.92 16.22 9.42 0.11 5.89 4.64 5.65 0.05 0.09 llla 405 58.27 0.74 15.50 10.43 0.13 4.66 3.88 6.14 0.16 0.08 llla 420 56.24 0.90 14.96 10.03 0.11 6.62 8.57 2.21 0.25 0.09 llla 459 53.02 0.85 15.58 9.61 0.20 7.O1 11.44 2.09 0.07 0.12 llla 465 60.68 1.10 16.25 7.44 0.09 3.24 3.37 7.64 0.04 0.15 llla 480 54.28 0.86 16.23 9.60 0.12 7.05 9.57 1.75 0.43 0.10 llla No. 17 228 51.90 0.83 17.93 8.96 0.14 6.38 7.59 5.31 0.83 0.13 ilb 343 51.55 0.89 14.56 9.46 0.11 12.08 6.07 3.88 1.28 0.11 iltb 438 52.27 1.03 16.23 9.90 0.16 10.82 2.94 5.26 't.27 0.12 iltb 459 56.29 0.70 18.20 8.52 0.14 4.62 7.05 2.90 1.48 0.10 iltb 529 54.55 0.81 16.38 9.04 0.13 9.3s 4.32 3.98 1.32 0.11 IV No.16 '17.27 60 52.02 1.02 10.55 0.04 8.08 9.02 1.82 0.09 0.07 iltb 140 57.91 0.73 17.78 7.94 0.12 5.81 4.51 2.94 2j9 0.08 iltb 220 55.67 0.85 16.84 8.99 0.14 7.46 1.48 5.40 1.92 0.10 iltb 242 54.46 0.94 17.O2 10.14 0.17 8.41 3.74 3.17 1.79 0.15 IV 460 54.66 1.08 16.84 10.06 0.14 7.16 5.18 3.10 1.68 0.10 IV 480 64.30 0.76 13.67 7.25 0.12 5.86 1.99 3.29 1.64 0.11 IV 498 75.95 0.17 12.91 1.66 0.05 1.73 1.57 3.28 2.6s 0.03 IV 546 58.05 1.08 16.88 10.07 0.19 4.27 3.08 5.60 0.68 0.10 IV 570 54.17 1.03 16.70 10.38 0.18 7.02 7.25 3.13 0.02 0.11 IV No. 15 480 63.49 o.82 14.36 7.38 0.20 4.05 5.48 2.68 1.43 0.10 V 500 62.s9 0.79 13.98 7.91 0.17 4.03 5.91 2.96 1.58 0.08 V Outcrop 78 51.81 0.88 16.59 10.91 0.19 6.26 11.38 1.74 0.15 0.10 V 'Total Fe as FeO

sition from Znne Il to Tnne III is marked by transfor- DrscussroN mation of saponite to corrensite;the corrensite has more Al, less Ca and Si, and a nearly constant Fe/(Fe + Mg) As describedabove, saponite and chlorite do not show ratio as compared to saponite. Heulandite and stilbite a continuous change in proportion of layers of chlorite were also replaced by laumontite. during thermal metamorphism in the Kamikita area. Only The characteristicassemblage of Zone IV is chlorite + corrensite having 50-40o/osmectite layers was formed at epidote + calcite (Fig. I lc). In the transition from Zone intermediate stagesof the smectite-to-chlorite transfor- III to Zone IV, the chlorite-epidote tie line intersectsthat mation. The corrensite coexistswith saponite or chlorite of corrensite-laumontite(Fig. lle). Fe3+is accommodat- or both. Each of these minerals is interpreted to be a ed preferentially in epidote and hematite, and as a result distinct, separatephase. Each is quite distinct in chemical the Fe'z+/(Fe,*+ Mg) ratios of corrensiteand chlorite are composition. The phase relations and structure varia- equal, as shown in Figure 7. However, since the Al con- tions of the smectite-corrensite-chlorite seriesindicate that tent of chlorite is larger than that of corrensite and epi- corrensite should be regarded as a single interstratified dote contains much more Ca than laumontite, the chlo- phasefrom a thermodynamic point of view. This conclu- rite-epidote tie line intersectsthat of corrensite-laumontite. sion supports previous inferencesconcerning thermody- SubzoneIIIb may be a transition zone between subzone namic relations for corrensite (Peterson, 196l; Yelde, IIIa and ZoneIY becausethe coexistenceofchlorite, cor- 1977;EvartsandSchiffman, 1983; Reynolds, 1988). More rensite, laumontite, and epidote were demonstrated in recently, Shau et al. (1990) also concluded from their de- subzone IIIb. The bulk compositions of the mafic phyl- tailed TEM/AEM study of corrensite from the Taiwan losilicate-bearingrocks (Table 5) plot near the areawhere ophiolite that corrensite should be treated as a unique the three tie lines in the ACF diagram intersecteach other phaserather than as a l: I ordered mixed-layer C/S. (Fig. l le). This implies that the assemblagesof secondary A discontinuous decreasein proportion of smectite in minerals changed from Zone II to Zone IV in response C/S during the smectite-to-chlorite transformation has to an increase in temperature under nearly isochemical been observed in hydrothermal systems and diagenetic conditions. environments in previous studies (Inoue et al., 1984a, The highest grade assemblageis biotite + actinolite + 1984b;Inoue, 1987),as in the presentstudy. On the other chlorite. Chlorite contains progressively more Mg and less hand, Chang et al. (1986) described a burial sequence Al as a result of the formation of Mg-rich actinolite and with continuous decreasein proportion of smectite layers biotite in rocks with scarceepidote and abundant albite. from saponite to corrensite in Brazilian offshore sedi- INOUE AND UTADA: SMECTITE-TO-CHLORITETRANSFORMATION 639 ments. Schultz (1963) noted that C/S could rangecontin- creaseddiscontinuously, with stepsat 100-800/o(sap- uously from 500/osmectite to 00/osmectite with transfor- onite), 50-400/o(corrensite), and l0-00/o(chlorite). C/S mation of smectite to chlorite, although he noted the having intermediate o/osmectite values other than those absenceof C/S that contains more than 500/osmectite. ofthe above three rangeswas not found. Such a dis- The detailed examination of XRD patterns of C/S having continuous transformation of smectite to chlorite is intermediated(001) valuesin this study indicate that they probably a general relation in various geologic envi- are mixtures of more than two discrete phases. These ronments. relations suggestthat a discontinuous decreasein pro- 2. The saponite-to-chlorite transformation at Kamikita portion of smectite layers in C/S occurscommonly in the took place with decreasesin Si and Ca, increasein Al, entire compositional range of the smectite-to-chlorite and a nearly constant Fel(Fe + Mg) ratio. Saponite, transformation. corrensite, and chlorite were distinctly different in The physical conditions of corrensite formation and composition. metamorphism in the Kamikita area can be approximat- 3. In addition to the structural and compositional vari- ed by analogy with observationsfrom hydrothermally al- ations in the saponite-corrensite-chloriteseries, the tered rocks in geothermal fields. The transition between mineral paragenesisimplies that corrensite should be Zones II and III is characterizedby the heulandite- (or regardedas a single interstratified phasefrom a ther- stilbite- ) laumontite transition as well as by the saponite- modynamic point of view. The formation of C/S hav- to-corrensite transformation. Liou (197 l) experimentally ing other intermediate o/osmectite values, except that determined the maximum temperature of stilbite stabil- of corrensite, may be restricted during the smectite- ity to be 100 "C at 300 bars. Heulandite is structurally to-chlorite transformation. similar to stilbite (Gottardi and Gali, 1985). Therefore 4. Corrensite formed at temperatures of approximately the boundary between Zones II and III is thought to be 100-200'C. The formation of corrensitemay be fa- at approximately 100'C in the Kamikita area. The tem- cilitated in more permeablerocks, which are more af- perature of the first appearanceof discrete chlorite with fected by hydrothermal fluids. or without C/S has been documented to be 150-240 'C from many gebthermal systems(Kristmannsdottir, 1979; AcxNowr,nncMENTs McDowell and Elders, 1980, 1983; Keith and Bargar, The authors gratefully thank B. Velde, A. Meunier, and D. Beaufort 1988). Laumontite is present at temperaturesbelow 200 for their helpfirl reviews ofan initial version ofthe manuscript. They also "C and epidote usually occurs at temperatures greater than acknowledge the help ofH. Tatematsu, Japan Railway Research Institute, publication of this study. 200-250 "C in geothermal fields (Bird et al., 1984). The who supplied the core samples and allowed interstratified R : 3 I/S near the lower end of subzone IIIa contained 150/osmectite and the o/osmectite in I/S approacheszero in ZoneIY. The correspondingtemper- RnrnnnNcns crrED ature is usually 200-230 'C (Srodoi and Eberl, 1984). It can reasonably be inferred that the temperature at the Alt, J.C., Honnorez, J., Iaveme, C., and Emmermann, R. (1986) Hydro- boundary between Zones III and IV, where corrensite thermal alteration ofa I km section through the upper oceanic crust, DSDP Hole 504B: Mineralogy, chemistry, and evolution of seawater- disappeared,was approximately 200'C. Accordingly, it basaltinteractions. Joumal of GeophysicalResearch, 91, 10309-10335. is inferred that corrensite occurred at temperatures of ap- Bettison, L., and Schiffrnan,P. (1988) Compositional and structural vari- proximately 100-200'C in the Kamikita area. The Ka- ations ofphyllosilicates from the Point Sd ophiolite, California. Amer- mikita metamorphic zonation (Zones I-IV) was formed ican Mineralogist, 73, 62-7 6. P., Elders, W.A., Williams, A.E., and McDowell, in responseto a thermal gradient of approximately 70'C/ Bird, D.K., Schiffman, S.D. (1984) Calc-silicatemineralization of active geothermal systems. km and relatively high /", conditions, as deduced from Economic Geology, 79, 67 1-695. the presenceof hematite. The diorite mass, Zone V, at- Bishop, D.G. (1972) Progressivemetamorphism from prehnite-pumpel- tained a temperatureof approximately 300 "C, as inferred lyite to greenschist facies in the Dansey Passarea, Otago, New Zealand. -3198. from the temperature of the first appearanceof biotite in Geological Society of America Bulletin, 83, 3177 Chang,H.K., Mackenzie,F.T., and Schoonrnaker,J. (1986) Comparisons the Salton Sea geothermal field (Cho et al., 1988). The between the diagenesisofdioctahedral and trioctahedral smectite, Bra- isolated presenceofthe epidote + andradite * anhydrite zilian offshorebasins. Clays and Clay Minerals, 34,407-423. assemblageas a vein in the l6-576-m sampleand the pres- Cho, M., Liou, J.G., and Bird, D.K. (1988) Progradephase relations in ence of pyrite in the diorite mass may suggestlocal cir- the State 2-14 well metasandstones, Salton Sea geothermal field, Cali- Research,93, 13081-13103. culation of hydrothermal fluids in a hydrothermal stage fornia. Joumal ofGeophysical Coornbs, D.S., Nakamura, Y., and Vuagtat, M. (1976) Pumpellyite-ac- following the diorite intrusion. tinolite facies schists ofthe Taveyanne Formation near Loeche, Valais, Switerland. Journal of Petrology, 17, 440-47 l. Suvrvr.a,ny AND coNcLUsroNS Ernst, W.G., Liou, J.G., and Moore, D.E. (1981) Multiple melamorphic rocks of the The results of this study can be summarized as follows: events recorded in Tailuko amphibolites and associated Suao-Nanao area, Taiwan. Geological Society of China Memoir, 4, l. During thermal metamorphism in the Kamikita area, 39r-441. 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