Construction of a calc-alkaline pluton by repeated injection of mafic sheets into an intermediate-felsic magma chamber - an example from New Zealand

A mafic-felsic layered intrusion of calc-alkaline affinity (Bungaree Intrusives, Stewart Island) was emplaced along the New Zealand section of the convergent margin of Gondwana at ~140 Ma. This pluton is characterized by a sequence of mafic magma replenishments that mixed and mingled with an intermediate-felsic host. Structures and textures preserved within the mingled rocks provide information on the role of coeval mafic melts in the generation and evolution of calc-alkaline granitic magmas. The rock sequence represents a stratigraphic record of magmatic processes that operated during pluton construction within a subduction-related setting. Physical processes identified from detailed field and textural observations include magma mingling and mixing, crystal accumulation, compaction and magmatic flow. Fractional crystallization of the host produced intermediate cumulates and more evolved felsic compositions towards the top of the chamber. Both mafic and felsic rocks are hornblende-bearing. Hornblendes from within the felsic rocks reveal large fluctuations in mineral chemistry, indicative of a protracted history of episodic heating as a result of the repeated injection of mafic magmas. Transects across zircon crystals also exhibit variations in chemistry consistent with episodic thermal events. Textural, mineralogical and geochemical data indicate that the Bungaree Intrusives comprised at least two adjacent magma pods that each formed incrementally as the result of multiple replenishments of mafic melt into an aggrading magma chamber of evolving intermediate-felsic composition.