Abstract

The Lower Spilite formation of the Builth Volcanic Series is made up of numerous thin flows of strongly porphyritic and amygdaloidal lava. Less metasomatized specimens are composed of albite-oligoclase and chlorite with smaller amounts of pyroxene and sphene. Strongly metasomatized examples show conversion of the aibite-oligoclase to albite and replacement of chlorite, including that of the amygdales, by albite plus or minus quartz. Chemical data show that these rocks have gained Na and Si and lost Al, Ca, Mg, Fe and H₂O.

Rarely, individual sacs can be extracted from the rock. Their margins are richer in Mg, Fe and H₂O than their cores and an example is described in which a Ca–Mg–Fe metasomatism has caused replacement of plagioclase by pumpellyite.

In explanation of the observations, immiscibility of two liquid phases is suggested: one the silicate residual magmatic liquid, the other a liquid enriched in Mg, Fe, H₂O and probably Ca and CO₂. The amygdales arise by the separation and subsequent crystallization of this second liquid. Segregation of this phase within the magma can account for most of the variation observed in these lavas, the margins of the sacs being relatively enriched in the second phase and the strongly meta-somatized lavas correspondingly impoverished.