Field and Geochemical Constraints on Mafic-Felsic Interactions, and Processes in High-level Arc Magma Chambers: an Example from the Halfmoon Pluton, New Zealand

The similar to 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 and a lack of zircon inheritance indicate that the amphibole-rich, calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials. A model of pluton construction and evolution is presented that shows that the Halfmoon Pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate-felsic composition. Data are consistent with a model whereby hydrous amphibole-rich basaltic magmas ponded at the crust-mantle interface and episodically rose, injected and mingled with an overlying intermediate-felsic magma chamber that itself represented the fractionated product of the same basaltic mantle melts.