4. Destruction of analcime and albitization of plagioclase

These two metamorphic processes, observed with increasing depth of burial, are quite distinct and can proceed independently, but as one has probably facilitated the other and as their chemical effects seem to be interconnected, they will be treated together.

Even in the Upper Triassic analcimized tuffs, embayment of analcime by quartz and albite has already set in. The following equation shows that a simple molecular addition of silica to analcime with simultaneous loss of water approximates to a volume for volume replacement, although for this condition to be strictly fulfilled rather less silica need be added and a little Na and Al removed.

NaAlSi₂O₆.H₂O analcime + SiO₂ = NaAlSi₃O₈ albite + H₂O

220.1 gm., 97.9 cc. 60 gm. 262.1 gm., 100.4 cc.

Some or perhaps all of the authigenic albite of the Upper Triassic replaces earlier-formed analcime by the addition of silica, and the fact that the analcime in these rocks is partly replaced by quartz also implies that further Na and Al must have been liberated for potential deposition in albite elsewhere. It is found that when analcime is abundant in a rock, plagioclase is unalbitized or incompletely so. The soda-content of the analcime rocks is high (e.g., 6.13% in 9051) and so albitization of the plagioclase could be accomplished without any increase in the bulk soda content of the rock. Raw (1943) suggested that the early formation of analcime in palagonite tuffs from Jamaica is responsible for their soda-rich nature, and he describes how, with increasing metamorphism, albite is formed from the analcime.

With increasing temperature under hydrothermal conditions, there must be a tendency towards the break-down of lime-bearing plagioclase, but simple metamorphic break-down to albite and a lime-bearing silicate such as occurs during the early stages of regional metamorphism is not generally accompanied by any increase in the soda-content of the rocks concerned. Thus Turner (1948, p. 117) has pointed out that two greywackes from the Naseby Subdivision of North Otago contain 4.01% and 3.79% Na₂O whereas the average soda-content of twelve corresponding quartz-albite-muscovite-chlorite schists is 3.50% Na₂O. In the North Range on the other hand the albitic lithic tuff 8873 has a much higher soda-content (5.38%) and lower potash (0.92%) than the corresponding little-altered tuff 8871 (3.17% Na₂O, 2.79% K₂O).

The above mechanism of early formation of analcime by reaction between glass and saline waters, followed by destruction of the analcime and albitization of andesine at depth, seems competent to explain the soda-rich nature of many of the North Range rocks. Lime and alumina were released during the albitization of andesine-labradorite. Some of the alumina has been retained in sericite inclusions in the albite whereas the rest of the alumina and the lime have contributed to the formation of prehnite, pumpellyite, epidote, calcite, laumontite and sphene. In some cases one or more or these minerals has crystallized in place as inclusions in the albitized plagioclase, but more often their components have been cleared from the crystal under the influence of solutions enriched in