Pumpellyites in two low grade metamorphic terranes north of Newcastle, New South Wales, Australia

Microprobe analyses of pumpellyites from rocks of variable chemistry formed under similar metamorphic conditions in two Palaeozoic, low grade metamorphic terranes show that they have an extreme range in composition (FeO*=0.9–22.96) and that Fe2+⇌Mg2+ and Fe3+⇌Al3+ are the dominant substitutions. A less extreme variation in composition of pumpellyites has been noted in samples taken from a metamorphosed differentiated metadolerite. On an A1-Fe*-Mg diagram, these pumpellyites extend through the fields of high pressure to low pressure terranes, indicating that pumpellyite compositions should be used with caution when determining metamorphic conditions.

Bulk chemical composition of the host rock does not appear to be a controlling factor in determining pumpellyite compositions. Rather, intensity of alteration, particularly of opaque mineral phases, fluid chemistry and variation in oxidation potential are considered to be more important variables. Coexisting epidote and composition of the precursor mineral also appear to be important in some rocks.