Abstract

Dr M. J. Le Bas said he welcomed this contribution on these unusual pyroclastic rocks which he had the good fortune to stumble on 20 years ago. This paper is important because it gives data on the products of an unusual type of volcanism, involving magmas of low viscosity (c. 5 × 10−2 poise) and low density (2.2 kg m−3). These figures are vastly different from those of normal (silicate) magmas. Do the authors distinguish any features in the bedded tephra which are peculiar to such low viscosity and low density magma?

The authors interpret the lath-shaped multi-granular calcite pseudomorphs as secondary after nyerereite, in the knowledge that lath-shaped primary nyerereite occurs in the 1960 natrocarbonatite lava from the carbonatite volcano of Oldoinyo Lengai in N Tanzania. Dr Le Bas emphasized that very similar pseudomorphs can be obtained after melilite, and also after quench calcite. Some of the illustrations shown during the presentation of the paper are exactly similar to undoubted pseudomorphs after melilites currently being studied from W Kenya where all transitions can be seen. The sutured margins, peg structure and median line characteristics of melilite so frequently seen when replaced by cebollite, are not always preserved when replaced by calcite. Therefore their absence cannot be used to deny the former presence of melilite. Nobody so far has experimentally replaced nyerereite by calcite, and any critical textural or chemical clues have yet to be recorded. The speaker therefore cannot agree that the calcite pseudomorphs are necessarily after nyerereite. In

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