Laminar flow, immiscibility and segregated fractionation in the Mt Townsend lamprophyric tephrite dyke, New South Wales: Does flow change activities in the melt?

Abstract

A vertical 1 m‐wide ocellar dyke on the eastern slopes of Mt Townsend, New South Wales, is a member of a dispersed Iamprophyric dyke swarm within the Carboniferous Snowy Mountains Batholith. The dyke has a bulk composition of primitive melanephelinite, but crystallised as a monchiquite due to high levels of magmatic CO2 and H2O. Despite extreme variations in crystal morphology and relative phase abundances, compositions of silicate minerals are relatively constant across the dyke, and from host to ocelli. Whole‐rock analyses show that the dyke has a constant bulk composition, precluding lateral transport of components. More than ten thin flow zones have a dramatic increase in the volume of ocelli (5 to 35%). In the dyke core these are replaced by a pronounced clustering of minerals. The chemical constraints indicate that the ocelli‐rich zones are not caused by a temperature profile, a fractionation profile, flow migration of ocelli, multiple injection, magma retraction, or mixing of related or unrelated m...