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 ap- pearanceof characteristicmineral assemblagesfrom 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 increasingmetamorphic grade. As smectite transforms to chlorite, the percentageof smectitelayers in interstraiified chlo- rite/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 trans- formation 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 corren- site 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 interestingthat 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