Mineral Reactions in Zeolitic Triassic Tuff, 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 mineralogy 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 -^heulandite + chlorite and celadonite; and (3) heulandite —» posits (Sheppard and Gude, 1973), zeolite assemblages have been laumontite, or prehnite, 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 clinoptilolite 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 cleavage, 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.