sign in sign up
<<
Home , Heulandite, Laumontite

Reactions in Zeolitic Triassic , Hokonui Hills, New Zealand

JAMES R. BOLES ' I Department of Geology, University of Otaeo, Dunedin, New Zealand DOUGLAS S. COOMBS Y S 7 6

ABSTRACT inadequately understood as has the of several of the phases encountered. Later studies in thick sedimentary sequences Textural evidence and other considerations indicate the follow- (for example, Seki and others, 1969; Otalora, 1964) also recog- ing paragenetic sequence of reactions in marine Triassic tuff beds of nized mineral zonation apparently related to depth of burial. rhyolitic to andesitic parentage that are scattered through a 4.8 to Others (Dickinson, 1962) have recognized a definite order of zeol- 8.5 km thickness of the Murihiku Supergroup, Hokonui Hills, ite replacements unrelated to depth of burial. In thin discontinuous Southland, New Zealand: (1) Glass—* montmorillonite ± illite; (2) zeolitic beds of Cenozoic and Holocene saline-alkaline lake de- glass -^ + chlorite and ; and (3) heulandite —» posits (Sheppard and Gude, 1973), assemblages have been , or , or calcite or analcime, or albite. successfully related to patterns of ionic activities in lake waters. Chemical analyses of the altered tuff indicate that Ca and Na We here document chemical and mineralogical data on zeolitic ions have been relatively mobile. Heulandite and laumontite al- ash beds in a 4.8 to 8.5 km thickness of Triassic volcanogenic tered tuffs are Ca-enriched, whereas analcime tuff is Na-enriched marine sediments'in the Hokonui Hills, Southland, New Zealand relative to unaltered volcanic rocks. Heulandite, chlorite, and (Boles, 1971b, 1974). The area is contiguous along strike with the celadonite have been analyzed by electron microprobe. Heulan- Taringatura Hills (Fig. 1). Our emphasis will be on mineral reac- dite with high Si/Al ratio, sometimes in the range, is tions that have taken place in these beds and on factors controlling associated with calcium-poor pyroclastic feldspar, whereas distribution of individual species. Laumontite and heulandite occur heulandite with low Si/Al ratio is associated with calcium-rich pyroclastic plagioclase. Such data indicate that the Si/Al ratio in the heulandite was controlled by the Si/Al ratio of the glass precursor. Chlorite and celadonite have high Fe/Mg ratios and variable A1 contents. Some celadonite appears to form interlayered structures with chlorite. Distribution patterns and stability relations of analcime with quartz and of laumontite show that average temperature gradients did not exceed about 25°C/km. The breakdown of heulandite to Na-aluminosilicates (analcime or albite) or to Ca-aluminosilicates (laumontite or prehnite) over a wide stratigraphic interval suggests

that such factors as PH2o and activity of various ions in stratal wa- ters played a more significant role than depth of burial in control- ling distribution of the diagenetic and very low grade metamorphic phases in the Hokonui Hills. Key words: metamorphic- sedimentary petrology, diagenesis, geochemistry, volcanic ash, mineralogy.

INTRODUCTION

The zeolite facies was defined by Fyfe and others (1958) mainly on the basis of observations of Coombs (1954) in the Taringatura Hills, Southland, New Zealand. The definition and validity of the facies has been explored by Coombs and others (1959), Zen (1961), Seki (1969), and Coombs, (1971). In the original studies of the Taringatura area, an apparent se- quence of mineral assemblages was related to depth of burial. Gross overlap of mineral ranges was recognized but has remained Figure 1. Map showing location of Hokonui Hills, Taringatura Hills, " Present address: (Boles) Atlantic Richfield Company, P.O. Box 2819 Dallas, Texas and outcrop of Triassic rocks in the Southland Syncline, South Island, New 75221. Zealand.

Geological Society of America Bulletin, v. 86, p. 163-173, 11 figs., February 1975, Doc. no. 50204.

163

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 164 BOLES AND COOMBS

TABLE 1. CHEMICAL AND PARTIAL MODAL ANALYSES OF ALTERED VITRIC AND CRYSTAL TUFFS OF TRIASSIC AGE FROM THE SOUTHLAND SYNCLINE

Sample 1 2 34 5678 9 10

Si02 64.71 64. .89 61.72 65.59 62.00 77.97 60. 90 62.91 55.90 59.32 65.15 60. 00 55.00 66.19 64.90 65.72 70.40 A120, 15.22 14. .80 14.54 13.76 17.00 10.50 16. 30 13,85 19.50 17.88 14.37 16. 10 17.40 13.11 13.13 15.71 14.20 Ti02 0.41 0..5 2 0.73 0.38 0.70 0.41 0. 21 0.37 0.55 0.71 1.02 1. 30 0.70 0.84 0.63 0.97 0.30 Fe203 2.85 1..7 4 1.27 1.05 5.29 0.75 2. 18 1.01 1.75 1.54 0.52 6. 43 5.86 1.65 0.82 0.75 1.70 FeO 1.00 0. 92 1.86 2.43 1.02 0. 55 2.28 2.50 5.61 2.31 0.71 0.56 2.19 0.80 MnO 0.02 0. 03 0.07 0.04 0.04 0. 03 0.08 0.05 0.15 0.06 0.05 0.02 0.07 tr. MgO 0.82 0. 51 1.28 1.18 1.41 0.45 0. 28 1.42 2.30 1.62 1.27 1. 87 2.84 1.31 1.11 0.99 0.50 CaO 3.00 5. 25 4.59 2.63 1.97 0.57 5. 45 2.84 0.75 1.77 3.34 1. 64 1.08 2.92 3.27 2.04 0.60 SrO 0.01 0. 02 0.11 0.50 tr. tr. 0.35 n.d. n.f. 0.11 0.02 0.07 tr. n.d. Na20 2.45 1. 60 1.39 1.76 1.97 4.85 0. 42 1.24 2.20 7.19 2.39 2. 24 0.54 1.44 1.42 5.26 5.70 K20 4.46 1. 19 1.13 3.25 2.17 0.87 2. 54 2.71 3.30 0.65 1.51 4. 58 4.89 2.90 2.12 3.01 1.60 P20s tr. 0. 09 0.06 0.07 0.08 0. 05 0.12 1.00 0.14 0.09 0.13 0.03 0.09 0.07 C02 n.f. 0. 02 0.04 0.05 0.01 "n. f. n.f. n.f. 0.05 0.01 tr. n.f. 0.02 n.d. HjO 4.93 7. 58 10.74 7.05 7.72 2.33 10. 92 10.61 9.68 3.15 7.29 6. 01 12.34 9.21 10.78 2.93 4.00 Total 99.99* W.1 6 99753 99796s 100.23 99785 W.8 3 100.13* 99748 99.89** 99744 100. 17 100.65 100.48 98786 99775 99787 Analyst"'* DSC JRB JRB DSC JUM JRB DSC DSC JAR FTS JRB JUM JUM JRB JRB JRB XKU

Partial modal analyses

Plagioclase55 n.d. n. d. 6.00 n.d. n.d. 8.60 n. d. n.d. n.d. n.d. 17.60 n. d. n.d. 9.20 7.80 22.10 n.d. Quartz" 1.20 1.70 3.40 3.10 2.20 3.10

Specific gravity 2.55 2. 36 2.36 2.49 n.d. 2.49 2. 38 2.40 n.d. n.d. 2.42 n. d. n.d. 2.37 2.35 2.53

* Analyst: DSC = D. S. Coombs; JEW - j. w. Murphy; JRB = J. P. Boles; JAR = J. A. Ritchie; FTS = F. T. Seelye; XKK = analysis compiled and in part averaged from partial analyses by X. K. Williams and w. Kitt. t Includes 0.11 BaO. 5 Includes 0.22 BaO. tt Includes 0.34 BaO. ** Includes 0.03 Zr02 and 0.08 S. tt 600 to 700 counts on a 4mm x 0.3 to 0.4 mm grid. 55 Detrital, untwinned, fresh grains without visible , or shape characteristic of pyroclastic feldspar. Some grains counted as quartz may be untwinned oligoclase. tfft Detrital grains only. Note; Sample identification. 1. 8800, coarse-grained laumontitized vitric-crystal tuff, Gavenwood Tuffs, Taringatura Hills (Coombs, 1954). 2. 30119, fine-grained laumontitized vitrie tuff, Gavenwood Tuffs, S160/498603, Hokonui Hills. 3. 30120, fine-grained heulanditized vitric tuff, Gavenwood Tuffs, S160/498603, Hokonui Hills. 4. 8768, coarse-grained heulanditized vitric-crystal tuff,, Gavenwood Tuffs, Taringatura Hills (Coombs, 1954). 5. 30336, silty bentonite, Fairplace Formation, S160/500605, Hokonui Hills. 6. 30305, fine-grained analcimized vitric tuff, Crosshill Gully Siltstone, S160/673528, Hokonui Hills. 7. 8791, fine-grained laumontitized vitric tuff, North Peak Formation, Taringatura Hills (Coombs, 1954). 8. 8776, fine-grained heulanditized vitric tuff. Stag Siltstone, Taringatura Hills (Coombs, 1954). 9. P24451 (New Zealand Geol. Survey colln.), bentonitic tuff with clinoptilollte, S179/56S068, Kaka Point. 10. Crystal tuff with albitized feldspar, "Parks cutting," S179/572016, Nugget Point. 11. 30234, coarse-grained helanditized vitric-crystal tuff, Taringatura Group, "Caroline cuttings," S160/352708, Hokonui Hills. 12. 30348, silty bentonite, Taringatura Group, "Caroline cuttings," S160/849703, Hokonui Hills. 13. 30349, bentonite, Taringatura Group, "Caroline cuttings," S160/349703, Hokonui Hills. 14. 26042, coarse-grained helanditized (clinoptilolite) vitric tuff. Bare Hill Tuff zone, S169/527527, Hokonui Hills. 15. 26043, coarse-grained helanditized (clinoptilolite) vitric tuff, Taringatura Group (Oretian), S160/303733, Taringatura Hills. 16. 30141, coarse-grained impure analcimized vitric-crystal tuff. Bare Hill Tuff zone, S169/527527, Hokonui Hills. 17. 18615, coarse-grained impure analcimized crystal-vitric tuff, Taringatura Group (Otamitan), Taringatura Hills (coombs, 1965).

throughout the section studied, although heulandite appears to be zeolite-rich tuff. Most tuff altered to heulandite or laumontite is missing in about 2 km of conformably underlying laumontite-rich relatively poor in , whereas analcimized tuff is relatively strata in which vitric tuffs have not been found. rich in sodium. Numerous vitric and vitric-crystal ash beds range from a few mil- As concluded by Coombs (1954) for the Taringatura area, com- limeters to 15 m thick throughout the section here described. Glass positions vary from andesitic to rhyolitic. If the tuff was initially is absent and its space is now occupied by one or more of the fol- similar in composition to unaltered rhyolite, dacite, or andesite (see lowing: heulandite (including the Si-rich variety clinoptilolite), Nockolds, 1954), then calcium and sodium in particular must have analcime, laumontite, montmorillonite (used for members of the been relatively mobile as will be discussed later. smectite mineral group), or less commonly prehnite, calcite, albite, celadonite, or chlorite. Glassy tuff has reacted readily to yield ALTERATION OF GLASS TO MONTMORILLONITE simple zeolitic assemblages. WITH OR WITHOUT ILLITE Wet chemical and electron microprobe analytical procedures used are described by Boles (1971b, 1972). In electron microprobe The petrology of clay-rich tuff or bentonite has not been studied analyses, HaO is reported as the difference between oxides found in detail. Relict shard outlines are usually poorly preserved; pyro- and 100 percent and has qualitative significance only. The true val- clastic feldspar, if present, is fresh and some of the samples contain ues may be greater because of water loss under vacuum or less be- trace amounts of heulandite or laumontite. cause of oxidation of iron, reported as FeO, volatilization of al- The clay in 17 bentonite samples were identified using kalies, or nondetermination of some components. Specimen num- the procedure outlined by Carroll (1970). Montmorillonite pre- bers used here refer to the catalog collection of the Geology De- dominates in most samples, illite (used for a white mica of 1 M or 1 partment, University of Otago. Md polytype) being subordinate. Glycolation and heating tests in- dicate that most of the montmorillonite is free of appreciable in- CHEMICAL COMPOSITION OF TUFFS terstratified chlorite. However, regular mixed-layer montmorillo- nite and chlorite, or less commonly, montmorillonite and illite, Chemical analyses of tuff of the Murihiku Supergroup are shown have been found. Random mixed-layer montmorillonite and chlo- in Table 1 in approximate stratigraphic order. Each analysis rite occur rarely. We have detected no systematic changes in clay reflects the composition of the dominant authigenic mineral and minerals with depth of burial in the present study, whereas in the the proportion of pyroclastic feldspar. Anhydrous Si02 and A1203 Gulf Coast Tertiary, Perry and Hower (1970) showed a system- values are in the range 62 to 75 and 14 to 17 weight percent, re- atic decrease in expandable layers within interlayered mont- spectively. Bentonite (Table 1, samples 5, 9,12,13) has lower Si/Al, morillonite-illite over a 5.5 km stratigraphic interval. higher total iron and magnesium, and lower calcium than most The mean refractive index of the montmorillonite is about 1.550,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 ZEOLITIC TRIASSIC TUFFS, NEW ZEALAND 165

indicating a relatively low Fe203 content of about seven percent (Ross and Hendricks, 1945). The bulk chemical analyses of bento- nite in Table 1 (samples 5, 9, 12, 13) also suggest low iron content in the montmorillonite. Alteration of volcanic glass to montmorillonite was probably one of the earliest reactions. The association of montmorillonite with volcanic glass is well known in modern marine environments (see Griffin and others, 1968). A comparison of the average com- position of the bentonite in Table 1 with the average composition of rhyodacite and andesite given by Nockolds (1954) shows that the alteration of volcanic glass to an equivalent weight of bentonite involves about a ten-fold increase in water, a 20 to 70 percent in- crease in , a loss of 60 to 70 percent in both calcium and sodium, and relatively constant Si, AI, Fe, and Mg.

ALTERATION OF GLASS TO HEULANDITE + CHLORITE AND CELADONITE

Relict shard outlines in heulanditized tuffs are occupied by Figure 2. Photomicrograph of large lath-shaped heulandite crystals em- fibrolamellär or tabular-shaped heulandite crystals. Some samples bedded in chlorite. Heulanditized vitric tuff. 26045, plain light. contain subspherical aggregates with heulandite crystals up to 200 ;u.m long (Fig. 2). Thin rims of pale-green phyllosilicate, probably Or

Samples plotted in triangle 1 are 26044 (Gavenwood Tuffs), 26045 (Otapirian), 26049 (Gavenwood Tuffs), 30070 (North Peak Formation), 30234 (Etalian), and 30261 (Etalian). Samples plotted in triangle 2 are 26042, 26043, and 26047, all of Oretian age.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 166 BOLES AND COOMBS

Fe-rich chlorite,1 outline the former position of the shards. Large Figure 4. Compositional ratios of coexist- pools of chlorite and celadonite occur within relict shards, mi- ing heulandite group minerals in four altered crolapilli, or the matrix. Pyroclastic feldspar (usually fresh), tuffs. Solid circles refer to large crystals in clinopyroxene, hornblende, and biotite are present in varying cavities or vesicles; open circles refer to small crystals in altered shards. Sample A = 30070, proportions. 2.5- B = 30234, C = 26047, and D = 30216. Data from Table 2. Mineralogy \

Pyroclastic Feldspar. Electron microprobe analyses of Na20, K20, and CaO contents of feldspar in nine tuffs (Fig. 3) allow in- ferences on initial glass composition. A U-stage check of 25 clear 2.0- \ feldspar grains (Slemmons, 1962) indicated that they have high temperature optics, although all grains analyzed by microprobe v \ Ca+Mq \ \ could not be checked by this technique. The following conclusions aD Na+K \ \ can be drawn: o- \ \ c ^^ 1. Compositions of feldspar in the tuff range from An to Or ; 62 98 ratio \ \ plagioclase predominates but about 10 percent of the grains are al- \ \ kali feldspar 1.5- AX S B 2. A substantial orthoclase content is present in plagioclase, A \ \ progressively increasing with increasing albite content. \ \ 3. In tuff of Oretian age (local stage, early Upper Triassic), 41 \ \ \ percent of all grains are unalbitized plagioclase in the An]5Or15 re- \ gion, whereas only 2.5 percent of the total grains in tuff from other \ \ horizons have such compositions. Hornblende and (or) biotite are \ i.o- common in Oretian tuff, whereas augite is more common in tuff fc from other horizons. ~l— 4. The compositions of the feldspar and assorted phenocryst in 3.0 3.5 4.0 7* Oretian tuffs suggest more highly evolved magmas during Oretian Si/Al ratio volcanism than during earlier and later times. 5. Feldspar close to the Ab-An or Ab-Or join (about seven per- contents of the silica-rich phase (Table 2, samples 7, 9) are unique cent of total) have possibly formed under plutonic conditions at for heulandite in the Hokonui area. lower temperatures and (or) higher water pressures than the A plot of Ca + Mg/Na + K ratio against Si/Al ratio (Fig. 4) for feldspar of more markedly ternary composition (Tuttle and Bowen, coexisting heulandite group minerals shows that Ca + Mg/Na + K 1958; Carmichael, 1963). decreases with increasing Si/Al. Heulandite. Boles (1972) showed that heulandite from eight Chlorite. Electron microprobe analyses of two chlorites from a tuffs in the Hokonui and Taringatura Hills has a wide range of heulanditized tuff and two from a volcarenite are shown in Table 3. Si/Al ratios (3.0 to 4.3), those with Si/Al ratios of 4.0 or greater The chlorite from the heulandite tuff has a significantly higher being regarded as clinoptilolite. Calcium is almost always domin- Al/Mg ratio than in the arenite. Relatively high birefringence and ant over Na and K, and there is as much as 1.0 Mg per unit cell. (-) optic sign (Deer and others, 1963) suggest that most Hokonui The thermal stability test described by Boles (1972, p. 1486) indi- chlorite is oxidized. cates that except in the Oretian and Otamitan (Upper Triassic) The 14 A reflection of most chlorite from Hokonui rocks shows stages, most heulandite in forty tuffs has a Si/Al ratio <3.75, and intensity losses up to 50 percent after heating for 1 hr at 325° to that there is no correlation between composition and depth of 500°C, and diffuse chlorite diffraction patterns appear to be de- burial. However, most Oretian and Otamitan heulandite has a stroyed with this treatment. The low thermal stability of this chlo- Si/Al ratio in excess of 3.75. rite is attributed to high Fe content and (or) poor crystallinity (see Several tuffs contain coexisting heulandite of different composi- Carroll, 1970). Chlorite with diffuse basal reflections and with low tion. Thus subspherical aggregates and sometimes veins of rela- thermal stability has been reported in marine bottom sediments off tively large clinoptilolite and silica-rich heulandite crystals have the coast of Peru and Chile (Zen, 1959). higher Si/Al ratios and lower Ca + Mg/Na + K ratios (Table 2, Celadonite. Electron microprobe analyses (EMA) of celado- samples 1, 3, 5) than the finer-grained heulandite replacing glass nite from Hokonui tuff (Table 4; Fig. 5) indicate that the Al/(Mg + shards in the same rock (Table 2, samples 2, 4, 6). Heulandites or Fe) ratio varies considerably, magnesium being relatively constant. heulandite and clinoptilolite with different compositions occasion- In addition, samples with lower Al/(Mg + Fe) ratios tend to have ally coexist as discreet phases within the same relict shard (Table 2, higher potassium contents. There are insufficient data to test statis- samples 7 to 10). tically for any systematic variation in composition with depth of The latter type of occurrence has been recognized in only two burial, but the preliminary results indicate that both types occur samples collected 4 km apart, probably from the same ash bed. The over a wide stratigraphic interval. Furthermore, there is no relation silica-poor phases in both samples have higher relief and bire- between composition of celadonite and composition of coexisting fringence than the silica-rich phases. In addition, the silica-poor heulandite. phase sometimes has subparallel concentric extinction zones with In some tuff samples, greenish micaceous minerals have absorp-

no discernible difference in relief between zones. The high Na20 tion tints intermediate between the typical glaucous green of cela- donite and the paler green of chlorite. EMA analyses show that 1 An x-ray diffraction study of the <10 /xm fraction of ten tuffs altered to zeolite these minerals are often inhomogeneous but have compositions in- indicates that the phyllosilicates in most cases have weak 14 A basal spacings, are termediate between chlorite and celadonite (Table 4, sample 9; nonexpanding, and are destroyed after heating at 325°C for 1 hr, suggesting a poorly Fig. 6). These minerals of "intermediate" compositions probably crystalline Fe-rich chlorite (Bradley, 1954; Carroll, 1970). Weak reflections in the 19 to 28 A region suggest interlayered structures in several samples, but the poor quality represent interlayered chlorite-celadonite. Celadonite samples 4 of the diffractograms prevented conclusive identification. through 7 of Table 4 also plot between ideal end-member chlorite

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 ZEOLITIC TRIASSIC TUFFS, NEW ZEALAND 167

to Fe+Mg+AI 30 Figure 6. Chorite (solid triangle), celadonite (solid circle) and probable Pg L If If M % X If u U x i interlayered chlorite-celadonite (open circle) analyses from the Hokonui Hills plotted in terms of K to (Fe,,,, + Mg + Al) to Sr. Data from Tables 3 Figure 5. Celadonite from the Hokonui Hills and from other areas plot- and 4 and Boles (1971b). End-member celadonite of formula ted in terms of Mg to Fe total to Al. Solid circles represent Hokonui +2 +3 KR R Si4O,0(OH)2 shown as open square. analyses; numbers correspond to analysis numbers in Table 5. Open circles are analyses from other areas summarized by Wise and Eugster (1964). been controlled by the composition of the altering glass rather than and celadonite, though close to the latter. This indicates some de- by depth of burial. The predominant heulandite phases associated parture from the ideal dioctahedral, tetrasilicic condition of end with the pyroclastic feldspar in triangle 1 (Fig. 3) have Si/Al ratios member celadonite or, conceivably, minor amounts of interlayered ranging between 2.98 and 3.52, whereas those associated with the chlorite. generally more alkalic feldspar in triangle 2 (Fig. 3) range from Cell contents given in Table 4 have been calculated on the basis 3.82 to 4.31. Chlorite, celadonite, and interlayered chlorite- of 20 atoms, all iron being treated as Fe+2. In most celado- celadonite appear to have crystallized penecontemporaneously

nite, Fe203 exceeds FeO (Wise and Eugster, 1964). As a result, the with heulandite as alteration products of glass. It is not known to number of cations given in the table will exceed the actual numbers what extent the Fe/Mg ratios in these minerals reflect the composi- by about one to four percent. Excess Al over that required with Si tion of the original glass in the tuff. to complete eight tetrahedral sites is ascribed to octahedral sites. Comparing Tables 2, 3, and 4 with Nockolds (1954) data on un- Hence the octahedral occupancy reported in the table, in excess of altered volcanic rocks shows that heulanditization of a range of the figure of four required for a strictly dioctahedral mica, will be andesitic and rhyolitic glasses may cause little change to Si, Al, Ca, significantly greater than the excess actually present. Unfortunate- and K contents but results in marked loss of Na, Mg, and Fe. ly, the nature of the material precludes determination of FeO and Chloritization is accompanied by a major loss of Si, Ca, Na, and K Fe203 by conventional methods. and substantial gains in Fe and Mg. The effects of replacement by celadonite are similar (though less marked) to those of chloritiza- Discussion tion, except that K increases drastically. While chlorite and cela- donite undoubtedly replace glass in part, a substantial portion of Clinoptilolite in deep-sea sediments usually occurs in Miocene or these minerals are precipitated in voids. It is concluded that local older sediments (Rad and Rosch, 1972), which suggests that migration of most major ions has occurred, and that for the heulandite minerals do not form from glass at the sediment-water heulandite-chlorite-celadonite type of alteration, there is a marked interface. In the Hokonius, there is no evidence of a mineral precur- net loss of Na. sor to the heulandite that probably formed directly from volcanic glass after burial. ALTERATION OF HEULANDITE TO LAUMONTITE The Si/Al ratios of the heulandite in Hokonui tuff appear to have The alteration of heulandite to laumontite is a zeolite facies reac- ion suggested by Coombs (1954, p. 96) and discussed by Fyfe and ithers (1958), Coombs and others (1959), and Coombs (1971).

Figure 7. Contact between heulanditized tuff (below arrows) and laumontitized tuff (above arrows). Bedding is vertical. The contact between the heulandite and laumontite alteration appears to be controlled by Figure 8. Photomicrograph of the matrix of the heulanditized tuff show- jointing. ing relict shard texture. 30120, plain light.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 168 BOLES AND COOMBS

TABLE 2. COMPOSITION AND SIGN OF ELONGATION FOR SOME HEULANDITE GROUP MINERALS FROM THE SOUTHLAND SYNCLINE

Sample 1 2 3 4 5 6 7 8 9 10

S10! 64.04 61.04 67.00 64.36 66.74 64.61 65.48 57.65 68.49 58.90 AlaOj 13.78 15.57 13.37 14.30 13.52 14.22 13.68 16.43 13.79 16.34 Fe20,* 0.68 0.15 0.13 1.12 0.39 0.40 <0.06 <0.06 0.13 0.11 MgO 0.34 1.26 0.39 0.83 0.10 0.22 <0.05 0.16 0.25 1.54 CaO 3.55 4.57 4.46 4.56 4.77 5.46 5.21 4.58 4.07 5.05 SrO 0.11 0.48 0.07 0.21 <0.09 <0.09 <0.09 <0.09 0.13 0.60 NJ¡0 0.32 0.25 0.15 0.21 0.24 0.30 1.19 0.76 2.10 0.60 M 3.04 2.00 2.21 2.40 2.12 2.18 1.63 1.47 0.70 1.31 Hj0+ 14.14 14.68 12.22 12.01 12.12 12.61 12.81 18.95 10.34 15.55 Total 100.00 1Û0.Û0 1ÙO.0O 100.00 ÏTÎÛU5 100.00 100.00 100.00 100.00 100.00

Optical orientation (mounted 1n Lakeside 70) slow fast slow Slow slow slow slow • fast slow fast

Anhydrous 72 oxygen cell S1 28.85 27.77 29.28 28.39 29.18 28.60 28.94 27.45 29.21 27.14 A1 7.31 8.35 6.88 7.43 6.97 7.40 7.12 9.22 6.93 8.87 +Ï Fe 0.07 0.05 0.04 0.37 0.13 0.13 0.04 0.04 Mg 0.23 0.85 0.25 0.55 0.06 0.15 0.11 0.16 1.06 Ca 1.71 2.23 2.09 2.15 2.23 2.59 2.47 2.38 1.86 2.49 Sf- 0.03 0.13 0.02 0.05 0.03 0.16 Na 0.28 0.22 0.13 0.18 0.20 0.26 1.02 0.70 1.74 0.54 K' 1.75 1.16 1.23 1.35 1.18 1.23 0.92 0.89 0.38 0.77 S1/A1 3.95 3.33 4.26 3.82 4.19 3.87 4.06 2.98 4.22 3.06 S1/Al+Fe 3.90 3.30 4.23 3.64 4.11 3.80 4.06 2.98 4.19 3.05 E2(Ca+Mg+ Sr)+Na+K 5.97 7.80 6.08 7.03 5.96 6.97 6.88 7.57 6.22 8.73

Note: Election microprobe analyses of heulnndltes. 1 and 2. 30070, altered vitzic tuff, north Peak Formation, S169/595548, Hokonui Hills. 3 and 4. 26047, altered vitric tuff, Taringatura Group (Bare Hill Tuff zone), S169/667496, Hokonui Hills. 5 and 6. 30216, altered vitric tuff, Taringatura Group (Bare Hill Tuff zone), S160/408614, Hokonui Hills. 7 and 8. 30261, altered vitric tuff, Taringatura Group {upper Etalian) , S160/380673, Hokonui Hills. 9 and 10. 30234, altered vitric tuff, Taringatura Group (upper Etalian), S160/3S2708, Hokonui Hills. * All Fe assumed to be present as FeiOi. t By difference. Dickinson (1962) reported that heulandite replacing glass is itself tion of quartz in the laumontite rock. The alumina contents also replaced by laumontite in Jurassic rocks from central Oregon. In are essentially the same. (2) The laumontitized tuff has a markedly

the Hokonuis, laumontite replacement of heulanditized glass is un- higher Fe203/Fe0 ratio. (3) Significant losses in MgO and total Fe equivocal in the Gavenwood Tuffs, where a vitric tuff is rep- oxides reflect the reduced content of phyllosilicate in the

resented in part by a dark greenish-gray heulandite-rich rock laumontite-rich specimen. (4) Na20, K20, and CaO are relatively (30120) and in part by a light yellowish-gray laumontite-rich rock constant, slight gains in Na20 and CaO in the laumontite rock (30119). The contact between the two cuts across the vertical- being doubtfully significant. The sodium in the laumontite rock is standing beds at a steep angle and appears to follow the joint pat- present mostly as albitized plagioclase. Potassium released from the tern (Fig. 7). Rock 30120 consists mainly of heulanditized shards heulandite forms K-feldspar in laumontitized tuff. with fresh pyroclastic plagioclase (six percent), detrital quartz (ab- From the above data it appears that, during laumontitization, out one percent), chlorite phyllosilicate, and trace amounts of part of the chloritic phyllosilicate content of the heulanditized tuff green-brown hornblende, clinopyroxene, biotite, and magnetite. has been destroyed. Probably some has been replaced by laumon- The laumontite-rich portion (30119) consists of a mosaic of large tite with the addition of Ca and Si and removal of Mg; some may laumontite crystals each of which replaces several shards. Other have been removed in solution. Iron oxide stains on fracture sur- areas, subequal in size to the laumontite crystals, are rich in au- faces may have formed from the release of iron during such reac- thigenic quartz and K-feldspar. Plagioclase is albitized, relatively tions. However, the main reaction has been approximately

inclusion free, and sometimes is replaced in part by laumontite. As Ca,KAl9Si27072- 26H20 noted by Coombs (1954) for the Taringatura district, plagioclase heulandite associated with laumontitized tuff in the Hokonui Hills is invari- ably albitized. A striking difference between the two rocks is the = 4CaAl2Si4012-4H20i laumontite smaller chloritic content in the laumontitized specimens. In 30120, large patches or scattered grains of phyllosilicate are common in + 8Si02 + KAlSi3Oe + 10H20 . the matrix and as continuous rims about zeolite-replaced shards quartz K-feldspar (Fig. 8). In contrast, the matrix of 30119 contains much less fine- Most or all of the laumontite tuff in the Hokonui area is believed grained phyllosilicate; relict shards are only faintly outlined by dis- to have formed from heulandite tuff. The partial destruction of continuous rims of phyllosilicate, and pools of chlorite are rare shard outlines as well as the consistently more altered nature of de- (Fig. 9). The contact between the heulanditized and laumontitized trital feldspar in the laumontite rocks demonstrates that the portions (30118) is gradational over a distance of 1 to 3 mm and is heulanditized tuff has not formed from a laumontite precursor. sometimes a joint plane filled with laumontite crystals. In some places near the contact, large isolated crystals of laumontite replace ALTERATION OF HEULANDITE TO ANALCIME several relict shards on the heulandite side. Veinlets of iron oxide (hematite and (or) goethite) and highly oxidized phyllosilicate also Analcime tuff is petrographically similar to heulandite tuff ex- occur in 30118. cept that analcime rather than heulandite forms the pseudomorphs Samples 2 and 3 of Table 1 are from regions of laumontite and after glass shards. Pyroclastic feldspar is generally fresh and has heulandite, respectively, taken about 30 cm apart along strike. The similar compositions to that in heulanditized tuff occurring at simi- anhydrous contents of the two samples are surprisingly similar, lar stratigraphic horizons (Boles, 1971b). Fine-grained authigenic considering the difference in handspecimen appearance. The data quartz is significantly more abundant in analcimized tuff and is show the following: (1) Although the heulandite to laumontite reaction involves release of silica (Coombs and others, 1959), the — EMA analyses of laumontite in the Hokonuis indicates about 0.1 sodium plus silica contents of the two rocks are similar, due to the crystalliza- potassium ions/12 oxygen unit cell (Boles, 1971b).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 ZEOLITIC TRIASSIC TUFFS, NEW ZEALAND 169

would be required. However, from the evidence given above, wide- spread heulanditization of glass apparently released substantial quantities of Na+ to stratal waters at an early stage of diagenesis. The abundance of authigenic quartz in analcimized tuff as com-

pared with heulanditized tuff is in part due to the release of Si02 during the alteration of heulandite to analcime. Authigenic K-feldspar is present in several analcimized tuff samples. The cal- cium and potassium released are presumably removed in solution. The heulandite or clinoptilolite to analcime reaction has been studied experimentally (Boles, 1971a). The reaction is favored by high [Na+]/[H+] ratios, and the Si/Al ratio of the analcime was found to be directly correlatable with the Si/Al ratio of the zeolite precursor. Analcime found in the Hokonui-Taringatura area at stratigraphic horizons that contain high Si/Al ratio heulandite is no more siliceous than analcime found at horizons where heulandite has a low Si/Al ratio (Coombs and Whetten, 1967; Boles, 1971b), indicating that the heulandite reactant has here not controlled the Si/Al ratio of the analcime product. As suggested by Coombs and Whetten (1967), the uniform composition of analcime in the Figure 9. Photomicrograph of the matrix of the laumontitized tuff, 30119, plain light. Note the lower proportion of phyllosilicates (dark min- Hokonuis may be due to equilibration of analcime with quartz. erals) in the lamontitized tuff. Photomicrographs Figures 10 and 11 were taken about 3 cm apart across the contact shown here. ALTERATION OF HEULANDITE TO ALBITE

commonly intergrown with analcime, K-feldspar, or sometimes Some vitric and vitric-crystal tuff in the study area is mainly re- segregated into concentric bands. placed by albite or less commonly by K-feldspar. Authigenic quartz Lath-shaped pseudomorphs of analcime embedded in chlorite is abundant in either type. Coombs (1954) suggested the reaction

show that, at least locally, analcime has replaced heulandite (Fig. analcime + quartz = albite + H20 to account for albite-rich tuff in 10) and in several cases better preservation of shard texture in the Taringatura. Some albite-rich tuff in the Hokonuis may be a direct heulanditized rock shows that the analcime rocks cannot have been alteration product of heulandite. Albite- and quartz-filled glass precursors for heulandite. Sheppard and Gude (1969) have recog- shards were found in numerous heulanditized tuffs from the nized pseudomorphs of analcime after clinoptilolite in lacustrine Gavenwood Tuffs. Furthermore in the Bare Hill Tuff zone, a 10- rocks of Miocene age from California. The alteration of heulandite cm-thick heulanditized tuff passes within several centimeters along to analcime involves a considerable influx of sodium as shown in strike to albite- and quartz-rich tuff. Quartz and albite occupy re- the following reaction: lict shards and form hematite-rimmed pseudomorphs after large heulandite crystals in cavities. There is no evidence in these cases to 8Na+ + Ca K Al Si 0 • 23H 0 3 2 8 28 72 2 suggest that the albite has been derived from an analcime precur- heulandite sor, although pseudomorphs of feldspar after analcime have been = 0.57NaHAl14Si340,16l- 14H20 reported from tuff in the North Range, Taringatura district analcime (Coombs, 1954). As in the heulandite to analcime reaction, the ++ + 8.6Si02 + 15H,0 -t-3Ca + 2K+. transition of heulandite to albite required considerable influx of quartz sodium. A typical reaction, compatible with the observed mineral- ogy, is About 90 cc of sea water containing 10,550 ppm Na+ would be 8Na+ + Ca K Al Si 0 • 23H 0 sufficient to change 6.5 cc of heulandite to 4.5 cc of analcime and 3 2 8 28 72 2 heulandite 1.0 cc of quartz by this reaction. If trapped sea water were the sole ++ supplier of sodium for the reaction, a considerable volume of fluid -> 8NaAlSi.,0„ + 4Si02 + 23H20 + 2K+ + 3Ca . albite quartz ¿EMA analysis of analcime in Hokonui tuff indicates about 0.17 calcium plus potassium/96 oxygen unit cell (Boles, 1971b).

Figure 10. Photomicrographs of analcime pseudomorphs after heulandite. The lath-shaped analcime is embedded in chlorite. 30159, plain light.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 170 BOLES AND COOMBS

ALTERATION OF HEULANDITE TO PREHNITE laumontite-wairakite and laumontite-lawsonite equilibria for Ptot = PH2O (Fig- lU- In a sedimentary sequence of this sort, fluid pressure On general petrographic grounds as well as from experimental was probably less than load pressure, especially if joints or faults data, the P-T stability of prehnite has sometimes been shown to be communicated from permeable strata to higher horizons. If so, separated from that of heulandite by laumontite (Coombs, 1960; maximum temperatures were significantly less than those indicated Liou, 1971b). However, overlap of the stability field of prehnite by the laumontite-wairakite curve in Figure 11. The experimentally with those of laumontite and heulandite has been emphasized by determined laumontite-lawsonite equilibrium at about 3 kb Coombs (1961, 1971) by the reaction suggests that load pressure did not exceed this value and hence that the maximum depth of burial did not exceed about 11 km. heulandite or laumontite + calcite Analcime has not been found more than 4.3 km and 5.1 km —> prehnite + HzO + C02. below the top of the Triassic in the east and west of the Hokonui In the Hokonuis, prehnite has not been found in laumontite tuff. Hills, respectively. The experimentally determined analcime-albite Apparently, P-T conditions have not been sufficiently severe for curve in Figure 11 indicates that the temperature at these points did widespread breakdown of laumontite to prehnite-rich assemblages rot exceed 200°C. Allowing for 3.0 km of exposed Jurassic strata, as has occurred in tuff of the Torlesse terrane of New Zealand or possibly a total of 4.5 km of Jurassic deposit before erosion, the (Coombs, in prep.) and in sandstone of the Taveyanne Formation thermal gradient in the eastern part of the section evidently did not of the European Alps (Martini and Vuagnat, 1968). exceed 23° to 29°C/km. Similarly, allowing for 1.9 km of exposed Prehnite occurs frequently in small quantities in heulanditized Jurassic and as much as 1.5 km removed by erosion, the thermal tuff in the Hokonuis, suggesting heulandite replacement by prehn- gradient in the western part of the section did not exceed 23° to ite. There is no textural evidence to rule out the reaction glass —» 27°C/km. The analcime + quartz = albite + H20 equilibrium is heulandite + prehnite. However, prehnite has not been reported strongly displaced to lower temperatures and pressures by in- with clinoptilolite as a glass alteration product in modern marine creased salinity and (or) by decreased PH20 (Liou, 1971a), which ac- basins. Furthermore, a tuffaceous siltstone from the upper part of counts for the spasmodic occurrence of authigenic albite in the the section contains large heulandite-filled cavities, which have been partially replaced by prehnite, quartz, and K-feldspar. A pos- sible reaction is 4-1 ++ 4Ca + Ca4KAl„Si27072-26H20 heulandite

-» 4Ca2Al2Si3O10(OH)2 prehnite + + KAlSi3Os + 12Si02 + 18H20 + 8H . K-feldspar quartz This reaction, unlike reactions described above, requires addition 3- of Ca for the conversion of heulandite.

ALTERATION OF HEULANDITE TO CALCITE

A few heulanditized tuff samples contain some shards that have ANALCIME been replaced by calcite. As shown in the following reaction, this Fluid change required considerable mobility of components: pressure (Kb)

4C02 + Ca4KAl9Si2707! 26H,0 2- heulandite

+ 4CaC03 + 27Si02 + K calcite quartz 3 + 9A1+ + 12HzO + 280H". Quartz and K-feldspar are not found in the calcite-replaced shards, suggesting removal of Si, Al, and K by solution and intro-

duction of C02.

SUMMARY OF PARAGENETIC SEQUENCE

Evidence presented in the previous sections suggests the follow- ing sequence of alteration in the tuff: (1) glass —» montmorillonite ± illite; (2) glass —» heulandite + chlorite and celadonite; and (3) heulandite —> analcime, or prehnite, or calcite, or laumontite, or albite. 1 1 1 REACTION CONTROLS 100 200 300 Temperature (°C) P-T Conditions Figure 11. Pnukt-T diagram for the equilibrium reactions: (1) laumontite = lawsonite + 2 quartz + fluid (Liou, 1971b), (2) laumontite = wairakite Because wairakite and lawsonite are absent in the Hokonui * fluid (Liou, 1971b), and (3) analcime + quartz = albite + fluid (Liou, Triassic section, temperatures and pressures are limited by the 1971a).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 ZEOLITIC TRIASSIC TUFFS, NEW ZEALAND 171

TABLE 3. COMPOSITION OF CHLORITES FROM HOKONUI been made. It has not even been proved that they have true stability ARENITE AND HEULANDITE TUFF fields relative to laumontite plus alkali feldspar and analcime plus Sample quartz. However, the apparent absence of heulandite and abun- dance of laumontite in 2 km of volcanogenic sandstone conform- SiOi 32.70 31.07 30.49 30.53 AUO, 13.48 12.69 17.10 17.40 ably underlying the lowest of the vitric tuff here reported is com- Fe20j FeO* 20.76 28.34 31.77 31.41 patible with the thermal gradients calculated with a maximum MgO 19.21 11.56 8.89. 8.19 temperature for heulandite stability slightly more than 200°C. CaO 0.62 0.27 0.31 0.38 Na20 0.10 0.09 0.12 0.09 Apart from the initial replacement of glass by heulandite and (or) K20 0.35 0.36 0.13 0.26 H2O+ 12.78 15.62 11.19 11.74 clay minerals, the observed reactions in the Hokonui Triassic tuff Total 100.00 100.00 100.00 lÛfl.ÙÛ are dehydration reactions that would be favored by rise in tempera- ture along any simple thermal gradient (see discussions in Fyfe and Anhydrous 28 oxygen unit cell

Si 6.72 6.90 6.49 6.53 others, 1958; Coombs and others, 1959). Nevertheless it is clear Al 1.28 1.10 1.51 1.47 that, with the probable exception of the absence of analcime from E OB Off O0 Off Al 1.99 2.22 2.78 2.91 the lowermost parts of the section and perhaps the absence of Fe+2 3.57 5.26 5.66 5.62 heulandite in immediately underlying rocks, temperature does not Mg 5.89 3.83 • 2.82 2.61 Ca 0.14 0.06 0.07 0.09 control the detailed distribution of the secondary minerals de- Na 0.04 0.04 0.05 0.04 K 0.09 0.10 0.04 0.07 scribed in this paper. E 1T77? 1T75T ITTÎ? 1T73T

Note: Ideal chlorite formula = (Hg, Al, Fe)12(Si, Al)8O20(OH)i6. Role of C02 Sample identification: 1 and 2. 30004, chlorltes in fine~grained volcarenite containing heulandite and prehnite. North Peak Formation, S160/415665, Hokonui Hills. 3 and 4. 30234, chlorltes in vitric tuff altered to heulandite, Taringatura Group, Following the work of Zen (1961) and subsequent discussions, it "Caroline cuttings," S160/352708, Hokonui Hills. is well established that at sufficiently high values of /uC02, Ca- * Total Fe expressed as FeO. zeolite assemblages will be inhibited, clay-carbonate facies being t By difference. favored. Thompson (1971) has shown by calculation that the reac- tions zone of analcime; and because the stratal waters were not pure

H20, the actual temperatures and temperature gradients were less laumontite + C02 = calcite than the figures given. + kaolinite + 2 quartz + 2 H20 (A) In this argument it is assumed that the nonappearance of waira- and kite from laumontite and the persistence of both albite and anal- cime within the indicated stratigraphie limit are not the result of laumontite + calcite = prehnite unfavorable kinetics of nucleation and growth. Wairakite is known + quartz + 3 H20 + C02 (B)

to nucleate readily in synthesis experiments, and albite has nu- are in equilibrium with an H20 — C02 fluid having cleated and grown readily in tuff and sandstone higher in the se- quence than the lowest observed analcime. Unequivocal determi- XC02 = 0.0070 for Pnuid = Plot = 2kb at 270°C. nations of the stability fields of the heulandite minerals have not Highly localized partial replacement of heulandite by calcite is re-

C0MP0SITI0N OF CELADONITES FROM HOKONUI TUFFS AND FROM OTHER AREAS

Sample 1 2 3 4 5 6 7 average range

SIO2 53.89 55.63 53.46 53.49 53.66 52.08 54.47 53.44 49.05-57.72 38.66 0.33-18.17 15.61 Al 20j 6.47 6.25 9.84 18.73 19.61 12.08 17.03 7.06 20.46- 1.14 Fe20i 13.67 FeO* 18.11 17.08 14.81 9.38 8.02 13.27 11.84 3.70 2.00- 8.16 23.14 MgO 4.43 4.94 4.87 3.90 4.03 4.88 2.61 5.32 0.00- 9.32 10.46 MnO n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.75 0.00- 4.71 n.d. CaO 0.13 0.05 0.05 0.50 0.13 0.20 0.16 0.85 0.00- 1.45 0.22 0.16 Na20 0.04 0.06 0.06 0.10 0.06 0.07 0.10 0.57 0.00- 3.82 3.25 K20 9.23 9.48 9.51 7.17 7.86 7.33 5.85 7.61 3.33-10.03 4.88-13.47 8.50 H2O 7.70 6.51 7.40 6.70 6.63 10.09 7.94 7.53 Total 100.00 100.00 100.00 100.00 100.00 100.00 loû.Oô T50Ö

Cell contentson basis of 22 oxygen anhydrous oeil (20 oxygens3 4 0B-) 7.70 7.64 \ Si 8.06 8.15 7.83 7.40 7.38 tetrahedral layer Al 0.17 0.60 0.62 0.30 0.36 1 ï 8.06 6.15 8.00 8.00 8.00 8.00 8.00 Al 1.14 1.07 1.52 2.45 2.55 1.80 2.45 Ì +S Fe " L Fe+2 2.26 2.09 1.81 1.08 0.92 1.63 1.38 ? octahedral layer Mg 0.99 1.07 1.06 0.80 0.82 1.07 0.55 Mn • • t E 4.39 4.23 4.39 4.33 4.29 4. SO 4.38 Ca 0.02 0.01 0.01 0.07 0.02 0.03 0.02) Na 0.01 0.02 0.02 0.02 0.02 0.02 0.02 } Interi ayer cations K 1.76 1.77 1.78 1.26 1.37 1.38 1.04 1 "tr T735-

JVote: Ideal celadonite formula = KR R Si4o,0(OH)2 or (K, Na, Ca)1.2 - i.0(Mg, Fe, Al}^(Si, Al) e02o(OH)u. Sample identification: 1 and 3. 26049. Altered vitric tuff, Gavenwood Tuffs, S160/499603, Hokonui Hills. Analyses 2, 3 represent extreme variants in a separate aggregate from analysis 1. 4 and 5. 30234. Separate aggregates in altered vitric tuff, Taringatura Group (Etalian), "Caroline cuttings," S160/352708, Hokonui Hills. 6. 26047 (average of rims of two spheres). Altered vitric tuffs, Taringatura Group, Bare Hill Tuff zone, S169/667496, Hokonui Hills. 7. 30134. Altered vitric tuff, Taringatura Group, Bare Hill Tuff zone, S169/496S45, Hokonui Hills. 8. Average and range of compositions for 18 celadonite analyses listed in vise and Eugster (1964). 9. Interlayered chlorite-celadonite? 26049 in Gavenwood Tuffs, S160/499603, Hokonui Hills. * Total Fe in analyses 1 through 7 and 11 expressed as FeO. t By difference in analyses 1 through 7 and 11.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 172 BOLES AND COOMBS

ported above, but in view of the prevalence of zeolite and sparsity cite, and occasionally pumpellyite and chlorite. veins cut

of carbonate in the Hokonui tuff, it can be concluded that XC02 in laumontite veins and presumably indicate late-stage events under the fluid remained very low. In fact, in associated sandstone in the conditions of declining temperature (Coombs and others, 1959; Taringatura Group, partial replacement of shell material by Liou, 1971c). laumontite is not uncommon, presumably as a result of reaction of shell carbonate and sandstone matrix by a reaction analogous to Influence of Parent Glass Composition (A) above. Initial values of XC02 were clearly below the equilib- rium values for reactions analogous to (A) in such cases. Evidence has been given that the composition of parent glass controlled the initial composition of heulandite group minerals that Role of Fluid and Activity of Ions replaced it. Thus, high silica activity resulting from the presence of tne most siliceous volcanic glasses has favored the early appearance As pointed out by Helgeson (1967) and by Eugster (1970), if a cf the most siliceous members of the heulandite group (see, for solution is involved in a reaction, the phases must be in ionic example, Read and Eisbacher, 1974). Laumontite and prehnite equilibrium with that solution as well as with each other under the occur mainly in the stratigraphic intervals in which more andesitic prevailing P-T conditions in order that equilibrium be attained. glasses were prevalent, suggesting a bulk composition control on Stationary fluids will tend to equilibrate with the existing mineral the presence or absence of laumontite and prehnite in Hokonui assemblage in a rock. The equilibrium composition of such fluids tuff. will vary as P-T conditions change and, in particular, will vary if It is to be emphasized, however, that although laumontite, the mineral assemblage should change. The new mineral as- heulandite, and prehnite all exist, either stably or as metastable rel- semblage may be the result of a reaction induced either under icts throughout a 10-km section in the Hokonui Hills where their equilibrium conditions in response to changing P-T conditions, or occurrence is in part controlled by whole-rock chemistry, heulan- under nonequilibrium conditions, in response to kinetic factors. dite does not persist to the higher grade limits of the zeolite facies. The composition of stationary fluids in a closed system will not, in Thus, in an area transitional to prehnite-pumpellyite facies, felsic general, control the mineral assemblage except perhaps at the ear- tuff of the Torlesse Supergroup at Benmore, Waitaki Valley, New liest stages of diagenesis; rather it should be controlled by the min- Zealand (Coombs, in prep.), contain laumontite partially replaced eral assemblage. by prehnite with no trace of any heulandite precursor preserved. If, however, fluids equilibrated with one mineral assemblage under one set of P-T conditions should move to a region of differ- CONCLUSION ent mineral assemblage, or of different P-T conditions, they may well be capable of inducing a change in the mineral assemblage. In all hydrothermal and burial or regional metamorphic terranes The situation here is analogous to that in hydrothermal systems in with which we are familiar, laumontite or sometimes wairakite which solution-controlled reactions involving zeolite, clay miner- persists to higher grades with advancing metamorphism than als, and feldspar have been described (for example, Browne and El- heulandite or other Ca-bearing . Such extreme zeolite facies lis, 1970). conditions were not reached in the Hokonui vitric tuffs described in

For the reactions given in this paper, lowering of aH20 or asl02 this paper, and except for the apparent disappearance of analcime should favor the replacement of heulandite by analcime, albite, low in the section and perhaps heulandite below the base of the sec- prehnite, laumontite, or calcite, assuming the activities of other tion as here described, reactions in the vitric tuffs were controlled components in solution are constant. All of these products are to a large extent by composition of parent materials and of stratal probably equilibrated with quartz. On the other hand, the initial waters. replacement of glass by heulandite presumably occurred under

conditions of relatively high asl02 due to the presence of volcanic ACKNOWLEDGMENTS glass. At a late stage of heulandite growth, fluids of still higher asl02/aAl203 may have been generated, which led to the formation of We thank the technical staff of the Geology Department, Univer- vug- and vein-filling clinoptilolite with high Si/Al ratios. The effect sity of Otago for their help. Use of the electron microprobe of pH on silica activity in solution is well known (Jones and others, analyzer at Australian National University, Canberra, Australia, is 1967); hence, pH can play a significant role in determining reaction also acknowledged. R. C. Surdam, University of Wyoming, criti- products. cally read an earlier draft of this manuscript and made helpful + ++ + ++ Activities of Na and Ca ions and aNa /aCil ratios are impor- suggestions. The research has been supported by National Science tant in considering alternative assemblages of heulandite, analcime, Foundation, Grant GA 754 (Coombs). albite, laumontite, and prehnite. Alteration of Ca-rich heulandite to laumontite and prehnite should be favored by a relatively low + ++ REFERENCES CITED aXa /aCa ratio, whereas alteration of the heulandite to analcime or albite would be favored by a higher a +/a ++ ratio, if P, T, and Na Ca Boles, J. R., 1971a, Synthesis of analcime from natural heulandite and activity of other ions were constant. clinoptilolite: Am. Mineralogist, v. 56, p. 1724-1734. Contrasting mineral replacements have occurred at essentially 1971b, Stratigraphy, petrology, mineralogy, and metamorphism of the same stratigraphic horizon. They have not necessarily occurred mainly Triassic rocks, Hokonui Hills, Southland, New Zealand [Ph.D. coevally or at precisely the same temperature. Nevertheless the very thesis]: Dunedin, New Zealand, Univ. Otago, 406 p. existence of these replacements suggests the importance of varying 1972, Composition, optical properties, cell dimensions, and thermal ionic activities in the stratal waters. Surdam (1973) has also em- stability of some heulandite group zeolites: Am. Mineralogist, v. 57, p. phasized the importance of ionic equilibria in explaining the over- 1463-1493. lap of laumontite and prehnite in the Kamutsen Group of British 1974, Structure, stratigraphy and petrology of mainly Triassic rocks, Columbia. Hokonui Hills, Southland, New Zealand: New Zealand Jour. Geol- ogy and Geophysics, v. 17, p. 337-374. We believe that these replacements provide evidence for move- Bradley, W. E., 1954, X-ray diffraction criteria for the characterization of ments of stratal waters of varying composition through the chlorite material in sediments: Clays and Clay Minerals, pub. 327, sedimentary pile. Further evidence for such movements can be Natl. Acad. Sci. and Natl. Research Council, p. 324-334. found in widespread, though sparse, cross-cutting veins, shear Browne, P.R.L., and Ellis, A. J., 1970, The Ohaki-Broadlands hydrother- zones, and joints that contain laumontite, stilbite, chlorite, or cal- mal area, New Zealand: Mineralogy and related geochemistry: Am.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021 ZEOLITIC TRIASSIC TUFFS, NEW ZEALAND 173

Jour. Sci., v. 269, p. 97-131. Mineralog. u. Petrog. Mitt., v. 48, p. 539-654. Carmichael, I.S.E., 1963, The crystallization of feldspar in volcanic acid Nockolds, S. R., 1954, Average chemical compositions of some igneous liquids: Geol. Soc. London Quart. Jour., v. 119, p. 95-131. rocks: Geol. Soc. America Bull., v. 65, p. 1007-1032. Carroll, D., 1970, Clay minerals — A guide to their identification: Geol. Otálora, 1964, Zeolites and related minerals in Cretaceous rocks of east- Soc. America Spec. Paper 126, 80 p. central Puerto Rico: Am. Jour. Sci., v. 262, p. 726-734. Coombs, D. S., 1954, The nature and alteration of some Triassic sediments Perry, E., and Hower, J., 1970, Burial diagenesis in Gulf Coast pelitic sedi- from Southland, New Zealand: Royal Soc. New Zealand Trans., v. ments: Clays and Clay Minerals, v. 18, p. 165-177. 82, p. 65-109. Rad, U. V., and Rosch, H., 1972, Mineralogy and origin of clay miner- 1960, Lower grade mineral facies in New Zealand: Internat. Geol. als, silica, and authigenic silicates in Leg 14 sediments: Initial Repts., Cong., 21st, Copenhagen 1960, Norden Rept., pt. 13, p. 339-351. Deep Sea Drilling Project, v. XIV, Leg 14, p. 727-739. 1961, Some recent work on the lower grades of metamorphism: Au- Read, P. B., and Eisbacher, G. H., 1974, Regional zeolite alteration of Sus- stralian Jour. Sci., v. 24, p. 203-215. tut Group, north-central British Columbia: Geol. Assoc. 1965, Sedimentary analcime rocks and sodium-rich gneisses: Canada/Mineral Assoc. Canada, Program Abs. and Circ. (Newfound- Mineralogical Mag., v. 34, p. 144-158. land Mtg.), p. 72. 1971, The present status of the zeolite facies, in Molecular sieve zeo- Ross, C. S., and Hendricks, S. B., 1945, Minerals of the montmorillonite lites, 1: Advances in Chemistry Ser., no. 101, Am. Chem. Soc., p. group: U. S. Geol. Survey Prof. Paper 205-B, 79 p. 317-327. Seki, Y., 1969, Facies series in low-grade metamorphism: Geol. Soc. Japan Coombs, D. S., and Whetten, J. J., 1967, Composition of analcime from Jour., v. 75, p. 255-266. sedimentary and burial metamorphic rocks: Geol. Soc. America Bull., Seki, Y., Oki, T., Matsuda, T., Mikami, K., and Okumüra, K., 1969, v. 78, p. 269-282. Metamorphism in the Tanzawa Mountains, central Japan: Japanese Coombs, D. S., Ellis, A. J., Fyfe, W. S., and Taylor, A. M., 1959, The zeolite Assoc. Mineralogists, Petrologists and Econ. Geologists Jour., v. 61, p. facies with comments on the interpretation of hydrothermal syntheses: 1-75. Geochim. et Cosmochim. Acta, v. 17, p. 53-107. Sheppard, R. A., and Gude, A. J., Ill, 1969, Diagenesis of tuffs in the Deer, W. A., Howie, R. A., and Zussman, J., 1963, Rock-forming minerals, Barstow Formation, Mud Hills, San Bernadino County, California: Vol. 3, in Sheet silicates: New York, John Wiley & Sons, Inc., 270 p. U.S. Geol. Survey Prof. Paper 634, 35 p. Dickinson, W. R., 1962, Petrology and diagenesis of Jurassic andesitic 1973, Zeolites and associated authigenic silicate minerals in tuffaceous strata in central Oregon: Am. Jour. Sci., v. 260, p. 481-500. rocks of the Big Sandy Formation, Mohave County, Arizona: U.S. Eugster, H. P., 1970, Thermal and ionic equilibria among muscovite, and Geol. Survey Prof. Paper 830, 36 p. K-feldspar and aluminosilicate assemblages: Fortschr. Mineralogie, v. Slemmons, D. B., 1962, Determination of volcanic and plutonic plagio- 47, p. 106-123. clases using a three or four-axis univeral stage: Geol. Soc. America Fyfe, W. S., Turner, F. J., and Verhoogen, J., 1958, Metamorphic reactions Spec. Paper 69, 64 p. and : Geol. Soc. America Mem. 73, 259 p. Surdam, R. C., 1973, Low-grade metamorphism of tuffaceous rocks in the Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals and equilibria: Karmutsen Group, Vancouver Island, British Columbia: Geol. Soc. New York, Harper and Row, 450 p. America Bull., v. 84, p. 1911-1922.

Griffin, J. J., Windom, H., and Goldberg, E. D., 1968, The distribution of Thompson, A. B., 1971, PC02 in low-grade metamorphism; zeolite, car- clay minerals in the world oceans: Deep-Sea Research, v. 15, p. 433- bonate, clay mineral, prehnite relations in the system

459. Ca0-Al203-Si02-C02-H20: Contr. Mineralogy and Petrology, v. Helgeson, H. C., 1967, Solution chemistry and metamorphism, in Abelson, 33, p. 145-161. P. H., ed,, Researches in geochemistry, Vol. 2: New York, John Wiley Tuttle, O. F., andBowen, N. L., 1958, Origin of in the light of experi-

& Sons, Inc., p. 362-404. mental studies in the system NaAlSi308-KAlSi308-Si02-H20: Geol. Jones, B. F., Rettig, S. L., and Eugster, H. P., 1967, Silica in alkaline brines: Soc. America Mem. 74, 153 p.1 Science, v. 158, p. 1310-1314. Wise, W. S., and Eugster, H. P., 1964, Celadonite: Synthesis, thermal stabil- Liou, J. G., 1971a, Analcime equilibria: Lithos, v. 4, p. 389-402. ity and occurrence: Am. Mineralogist, v. 49, p. 1031-1083. 1971b, P-T stabilities of laumontite, wairakite, lawsonite, and related Zen, E-an, 1959, Mineralogy and petrology of marine bottom sediment

minerals in the system CaAl2Si20g-Si02-H20: Jour. Petrology, v. 12, samples off the coast of Peru and Chile: Jour. Sed. Petrology, v. 29, p. p. 379-411. 513-539. 1971c, Stilbite-laumontite equilibrium: Contr. Mineralogy and Petrol- 1961, The zeolite facies: An interpretation: Am. Jour. Sci., v. 259, p. ogy, v. 31, p. 171-177. 401^09. Martini, J., and Vuagnat, M., 1968, Étude pétrographique des Grès de MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 1, 1974 Taveyanne entre Arve et Giffre (Haute-Savoie, France): Schweizer. REVISED MANUSCRIPT RECEIVED JULY 10, 1974

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/2/163/3428962/i0016-7606-86-2-163.pdf by guest on 28 September 2021