Textural evidence and other considerations indicate the following paragenetic sequence of reactions in marine Triassic tuff beds of rhyolitic to andesitic parentage that are scattered through a 4.8 to 8.5 km thickness of the Murihiku Supergroup, Hokonui Hills, Southland, New Zealand: (1) Glass → montmorillonite ± illite; (2) glass →heulandite + chlorite and celadonite; and (3) heulandite → laumontite, or prehnite, or calcite or analcime, or albite.
Chemical analyses of the altered tuff indicate that Ca and Na ions have been relatively mobile. Heulandite and laumontite altered tuffs are Ca-enriched, whereas analcime tuff is Na-enriched relative to unaltered volcanic rocks. Heulandite, chlorite, and celadonite have been analyzed by electron microprobe. Heulandite with high Si/Al ratio, sometimes in the clinoptilolite range, is associated with calcium-poor pyroclastic feldspar, whereas 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 Al 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 waters played a more significant role than depth of burial in controlling distribution of the diagenetic and very low grade metamorphic phases in the Hokonui Hills.