AUTOMETASOMATISM IN THE LOWER SPILITES OF THE BUILTH VOLCANIC SERIES

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.