Experimental data suggest that some igneous mafic inclusions are formed at the interface between underlying mafic magma and more silicic magma in a reservoir by a process of gas exsolution and vesiculation. We report cm-scale plume-like structures exposed over several m2 at a candidate interface in a large composite pluton from NW Palmer Land, Antarctic Peninsula and suggest that, as well as representing ‘frozen’ mafic inclusion formation, plume-like structures of this type can be used as way-up criteria and may be of particular value in the interpretation of palaeomagnetic data.