Volcanic greywackes from the upper Devonian baldwin formation, Tamworth‐Barraba district, New South Wales

The Baldwin Formation consists of interbedded greywackes and mudstones deposited in the New England Eugeosyncline during Late Devonian times. The greywackes consist essentially of andesitic detritus and they may be termed volcanic greywackes. They contain very little detrital quartz or sedimentary rock fragments. Chemical analyses of ten greywackes are presented. These show that the greywackes have an andesitic composition, differing only in their relatively high sodium contents. The high sodium values may be due to reaction between the detritus and sea water. Burial metamorphism of the greywackes has produced minerals characteristic of the prehnite‐pumpellyite metagreywacke facies. Secondary minerals include albite, chlorite, calcite, prehnite, pumpellyite and epidote.