| Summary: | Field evidence from exposed plutonic rocks indicates that mafic-felsic magma interaction is an important process during the construction and evolution of magma chambers. The exhumed, ~140 Ma, Halfmoon Pluton of Stewart Island, New Zealand is characterized by a sequence of mingled mafic sheets and enclaves preserved within an intermediate-felsic host, and provides a unique opportunity to directly investigate the physico-chemical processes that operate within an arc setting. Interpretation of mingling structures and textures in the field, in combination with extensive petrographic, geochemical and geochronological data, allow for conclusions to be reached regarding the nature of the mafic-felsic magma interactions, and the physical, chemical and thermal processes responsible for the generation and evolution of the calc-alkaline magmas. Detailed documentation and interpretation of mafic-felsic magma mingling structures and textures reveal that the Halfmoon Pluton formed incrementally as the result of episodic replenishments of mafic magma emplaced onto the floor of an aggrading intermediate-felsic magma chamber. Physico-chemical processes identified include fractional crystallization and accumulation of a plagioclase – hornblende – apatite – zircon mineral assemblage, episodic replenishment by hot, wet basaltic magmas, magmatic flow and compaction. Early amphibole and apatite crystallization played an important role in the compositional diversity within the Halfmoon Pluton. Variations in the style of magma mingling preserved within the magmatic “stratigraphy” indicate that processes operating within the chamber varied in space and time. Variations in mineral zoning and composition within hornblende indicate that the Halfmoon Pluton crystallized within a magma in which melt composition fluctuated in response to repeated mafic magma replenishments, fractionation, crystal settling and convection. Mineral assemblages, chemical characteristics, isotopic data and geochronological evidence indicate that the amphibole-rich calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting, most probably an island-arc. Data are consistent with a model whereby ‘wet’ amphibole-rich basaltic magmas pond at the crust-mantle interface and episodically rise, inject and mingle with an overlying intermediate-felsic magma chamber that itself represents the fractionated product of the mantle melts.
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