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The ~140 Ma Halfmoon Pluton of Stewart Island, New Zealand, provides direct evidence for a number of physico-chemical processes that operate at depth within active arc settings. It is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate-felsic host-rocks. Way-up structures and textures are consistent with a younging direction to the south, and allow a south-dipping magma chamber 'stratigraphy' to be constructed.The similarity in modal mineralogy, geochemistry, and isotopic signature between the various components, and their proximity in space and time, indicates that the system can be considered as a single composite pluton with discrete portions active at different times. Interpretation of mingling structures and textures in the field, combined with detailed geochemical and geochronological analyses, has allowed the identification of several physical and chemical processes operating within the chamber including: (1) multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth; (2) the development of mineral fabrics (e.g. aligned plagioclase and hornblende crystals) and alignment of mafic enclaves caused by processes of crystal accumulation, magmatic flow and compaction; (3) shortening of the chamber during crystallization as a result of magmatic loading; (4) fractional crystallization of the intermediate-felsic host magma that was interrupted periodically by mafic magma intrusion; (5) incremental assembly of the pluton from the base up; (6) limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate-felsic host-rocks. Variations in morphology, chemistry and texture of the mingled mafic rocks relative to their position within the magmatic 'stratigraphy' indicate that processes operating within the chamber varied in both space and time. Mineral assemblages, chemical characteristics, primitive isotopic signatures